Squashed 'third_party/git/' content from commit cb71568594

git-subtree-dir: third_party/git
git-subtree-split: cb715685942260375e1eb8153b0768a376e4ece7

7 files changed, 6805 insertions, 0 deletions

diff --git a/refs/files-backend.c b/refs/files-backend.c
new file mode 100644
index 000000000000..d60767ab739f
--- /dev/null
+++ b/refs/files-backend.c
@@ -0,0 +1,3201 @@
+#include "../cache.h"
+#include "../config.h"
+#include "../refs.h"
+#include "refs-internal.h"
+#include "ref-cache.h"
+#include "packed-backend.h"
+#include "../iterator.h"
+#include "../dir-iterator.h"
+#include "../lockfile.h"
+#include "../object.h"
+#include "../dir.h"
+#include "../chdir-notify.h"
+#include "worktree.h"
+
+/*
+ * This backend uses the following flags in `ref_update::flags` for
+ * internal bookkeeping purposes. Their numerical values must not
+ * conflict with REF_NO_DEREF, REF_FORCE_CREATE_REFLOG, REF_HAVE_NEW,
+ * REF_HAVE_OLD, or REF_IS_PRUNING, which are also stored in
+ * `ref_update::flags`.
+ */
+
+/*
+ * Used as a flag in ref_update::flags when a loose ref is being
+ * pruned. This flag must only be used when REF_NO_DEREF is set.
+ */
+#define REF_IS_PRUNING (1 << 4)
+
+/*
+ * Flag passed to lock_ref_sha1_basic() telling it to tolerate broken
+ * refs (i.e., because the reference is about to be deleted anyway).
+ */
+#define REF_DELETING (1 << 5)
+
+/*
+ * Used as a flag in ref_update::flags when the lockfile needs to be
+ * committed.
+ */
+#define REF_NEEDS_COMMIT (1 << 6)
+
+/*
+ * Used as a flag in ref_update::flags when we want to log a ref
+ * update but not actually perform it.  This is used when a symbolic
+ * ref update is split up.
+ */
+#define REF_LOG_ONLY (1 << 7)
+
+/*
+ * Used as a flag in ref_update::flags when the ref_update was via an
+ * update to HEAD.
+ */
+#define REF_UPDATE_VIA_HEAD (1 << 8)
+
+/*
+ * Used as a flag in ref_update::flags when the loose reference has
+ * been deleted.
+ */
+#define REF_DELETED_LOOSE (1 << 9)
+
+struct ref_lock {
+	char *ref_name;
+	struct lock_file lk;
+	struct object_id old_oid;
+};
+
+struct files_ref_store {
+	struct ref_store base;
+	unsigned int store_flags;
+
+	char *gitdir;
+	char *gitcommondir;
+
+	struct ref_cache *loose;
+
+	struct ref_store *packed_ref_store;
+};
+
+static void clear_loose_ref_cache(struct files_ref_store *refs)
+{
+	if (refs->loose) {
+		free_ref_cache(refs->loose);
+		refs->loose = NULL;
+	}
+}
+
+/*
+ * Create a new submodule ref cache and add it to the internal
+ * set of caches.
+ */
+static struct ref_store *files_ref_store_create(const char *gitdir,
+						unsigned int flags)
+{
+	struct files_ref_store *refs = xcalloc(1, sizeof(*refs));
+	struct ref_store *ref_store = (struct ref_store *)refs;
+	struct strbuf sb = STRBUF_INIT;
+
+	base_ref_store_init(ref_store, &refs_be_files);
+	refs->store_flags = flags;
+
+	refs->gitdir = xstrdup(gitdir);
+	get_common_dir_noenv(&sb, gitdir);
+	refs->gitcommondir = strbuf_detach(&sb, NULL);
+	strbuf_addf(&sb, "%s/packed-refs", refs->gitcommondir);
+	refs->packed_ref_store = packed_ref_store_create(sb.buf, flags);
+	strbuf_release(&sb);
+
+	chdir_notify_reparent("files-backend $GIT_DIR",
+			      &refs->gitdir);
+	chdir_notify_reparent("files-backend $GIT_COMMONDIR",
+			      &refs->gitcommondir);
+
+	return ref_store;
+}
+
+/*
+ * Die if refs is not the main ref store. caller is used in any
+ * necessary error messages.
+ */
+static void files_assert_main_repository(struct files_ref_store *refs,
+					 const char *caller)
+{
+	if (refs->store_flags & REF_STORE_MAIN)
+		return;
+
+	BUG("operation %s only allowed for main ref store", caller);
+}
+
+/*
+ * Downcast ref_store to files_ref_store. Die if ref_store is not a
+ * files_ref_store. required_flags is compared with ref_store's
+ * store_flags to ensure the ref_store has all required capabilities.
+ * "caller" is used in any necessary error messages.
+ */
+static struct files_ref_store *files_downcast(struct ref_store *ref_store,
+					      unsigned int required_flags,
+					      const char *caller)
+{
+	struct files_ref_store *refs;
+
+	if (ref_store->be != &refs_be_files)
+		BUG("ref_store is type \"%s\" not \"files\" in %s",
+		    ref_store->be->name, caller);
+
+	refs = (struct files_ref_store *)ref_store;
+
+	if ((refs->store_flags & required_flags) != required_flags)
+		BUG("operation %s requires abilities 0x%x, but only have 0x%x",
+		    caller, required_flags, refs->store_flags);
+
+	return refs;
+}
+
+static void files_reflog_path_other_worktrees(struct files_ref_store *refs,
+					      struct strbuf *sb,
+					      const char *refname)
+{
+	const char *real_ref;
+	const char *worktree_name;
+	int length;
+
+	if (parse_worktree_ref(refname, &worktree_name, &length, &real_ref))
+		BUG("refname %s is not a other-worktree ref", refname);
+
+	if (worktree_name)
+		strbuf_addf(sb, "%s/worktrees/%.*s/logs/%s", refs->gitcommondir,
+			    length, worktree_name, real_ref);
+	else
+		strbuf_addf(sb, "%s/logs/%s", refs->gitcommondir,
+			    real_ref);
+}
+
+static void files_reflog_path(struct files_ref_store *refs,
+			      struct strbuf *sb,
+			      const char *refname)
+{
+	switch (ref_type(refname)) {
+	case REF_TYPE_PER_WORKTREE:
+	case REF_TYPE_PSEUDOREF:
+		strbuf_addf(sb, "%s/logs/%s", refs->gitdir, refname);
+		break;
+	case REF_TYPE_OTHER_PSEUDOREF:
+	case REF_TYPE_MAIN_PSEUDOREF:
+		files_reflog_path_other_worktrees(refs, sb, refname);
+		break;
+	case REF_TYPE_NORMAL:
+		strbuf_addf(sb, "%s/logs/%s", refs->gitcommondir, refname);
+		break;
+	default:
+		BUG("unknown ref type %d of ref %s",
+		    ref_type(refname), refname);
+	}
+}
+
+static void files_ref_path(struct files_ref_store *refs,
+			   struct strbuf *sb,
+			   const char *refname)
+{
+	switch (ref_type(refname)) {
+	case REF_TYPE_PER_WORKTREE:
+	case REF_TYPE_PSEUDOREF:
+		strbuf_addf(sb, "%s/%s", refs->gitdir, refname);
+		break;
+	case REF_TYPE_MAIN_PSEUDOREF:
+		if (!skip_prefix(refname, "main-worktree/", &refname))
+			BUG("ref %s is not a main pseudoref", refname);
+		/* fallthrough */
+	case REF_TYPE_OTHER_PSEUDOREF:
+	case REF_TYPE_NORMAL:
+		strbuf_addf(sb, "%s/%s", refs->gitcommondir, refname);
+		break;
+	default:
+		BUG("unknown ref type %d of ref %s",
+		    ref_type(refname), refname);
+	}
+}
+
+/*
+ * Manually add refs/bisect, refs/rewritten and refs/worktree, which, being
+ * per-worktree, might not appear in the directory listing for
+ * refs/ in the main repo.
+ */
+static void add_per_worktree_entries_to_dir(struct ref_dir *dir, const char *dirname)
+{
+	const char *prefixes[] = { "refs/bisect/", "refs/worktree/", "refs/rewritten/" };
+	int ip;
+
+	if (strcmp(dirname, "refs/"))
+		return;
+
+	for (ip = 0; ip < ARRAY_SIZE(prefixes); ip++) {
+		const char *prefix = prefixes[ip];
+		int prefix_len = strlen(prefix);
+		struct ref_entry *child_entry;
+		int pos;
+
+		pos = search_ref_dir(dir, prefix, prefix_len);
+		if (pos >= 0)
+			continue;
+		child_entry = create_dir_entry(dir->cache, prefix, prefix_len, 1);
+		add_entry_to_dir(dir, child_entry);
+	}
+}
+
+/*
+ * Read the loose references from the namespace dirname into dir
+ * (without recursing).  dirname must end with '/'.  dir must be the
+ * directory entry corresponding to dirname.
+ */
+static void loose_fill_ref_dir(struct ref_store *ref_store,
+			       struct ref_dir *dir, const char *dirname)
+{
+	struct files_ref_store *refs =
+		files_downcast(ref_store, REF_STORE_READ, "fill_ref_dir");
+	DIR *d;
+	struct dirent *de;
+	int dirnamelen = strlen(dirname);
+	struct strbuf refname;
+	struct strbuf path = STRBUF_INIT;
+	size_t path_baselen;
+
+	files_ref_path(refs, &path, dirname);
+	path_baselen = path.len;
+
+	d = opendir(path.buf);
+	if (!d) {
+		strbuf_release(&path);
+		return;
+	}
+
+	strbuf_init(&refname, dirnamelen + 257);
+	strbuf_add(&refname, dirname, dirnamelen);
+
+	while ((de = readdir(d)) != NULL) {
+		struct object_id oid;
+		struct stat st;
+		int flag;
+
+		if (de->d_name[0] == '.')
+			continue;
+		if (ends_with(de->d_name, ".lock"))
+			continue;
+		strbuf_addstr(&refname, de->d_name);
+		strbuf_addstr(&path, de->d_name);
+		if (stat(path.buf, &st) < 0) {
+			; /* silently ignore */
+		} else if (S_ISDIR(st.st_mode)) {
+			strbuf_addch(&refname, '/');
+			add_entry_to_dir(dir,
+					 create_dir_entry(dir->cache, refname.buf,
+							  refname.len, 1));
+		} else {
+			if (!refs_resolve_ref_unsafe(&refs->base,
+						     refname.buf,
+						     RESOLVE_REF_READING,
+						     &oid, &flag)) {
+				oidclr(&oid);
+				flag |= REF_ISBROKEN;
+			} else if (is_null_oid(&oid)) {
+				/*
+				 * It is so astronomically unlikely
+				 * that null_oid is the OID of an
+				 * actual object that we consider its
+				 * appearance in a loose reference
+				 * file to be repo corruption
+				 * (probably due to a software bug).
+				 */
+				flag |= REF_ISBROKEN;
+			}
+
+			if (check_refname_format(refname.buf,
+						 REFNAME_ALLOW_ONELEVEL)) {
+				if (!refname_is_safe(refname.buf))
+					die("loose refname is dangerous: %s", refname.buf);
+				oidclr(&oid);
+				flag |= REF_BAD_NAME | REF_ISBROKEN;
+			}
+			add_entry_to_dir(dir,
+					 create_ref_entry(refname.buf, &oid, flag));
+		}
+		strbuf_setlen(&refname, dirnamelen);
+		strbuf_setlen(&path, path_baselen);
+	}
+	strbuf_release(&refname);
+	strbuf_release(&path);
+	closedir(d);
+
+	add_per_worktree_entries_to_dir(dir, dirname);
+}
+
+static struct ref_cache *get_loose_ref_cache(struct files_ref_store *refs)
+{
+	if (!refs->loose) {
+		/*
+		 * Mark the top-level directory complete because we
+		 * are about to read the only subdirectory that can
+		 * hold references:
+		 */
+		refs->loose = create_ref_cache(&refs->base, loose_fill_ref_dir);
+
+		/* We're going to fill the top level ourselves: */
+		refs->loose->root->flag &= ~REF_INCOMPLETE;
+
+		/*
+		 * Add an incomplete entry for "refs/" (to be filled
+		 * lazily):
+		 */
+		add_entry_to_dir(get_ref_dir(refs->loose->root),
+				 create_dir_entry(refs->loose, "refs/", 5, 1));
+	}
+	return refs->loose;
+}
+
+static int files_read_raw_ref(struct ref_store *ref_store,
+			      const char *refname, struct object_id *oid,
+			      struct strbuf *referent, unsigned int *type)
+{
+	struct files_ref_store *refs =
+		files_downcast(ref_store, REF_STORE_READ, "read_raw_ref");
+	struct strbuf sb_contents = STRBUF_INIT;
+	struct strbuf sb_path = STRBUF_INIT;
+	const char *path;
+	const char *buf;
+	const char *p;
+	struct stat st;
+	int fd;
+	int ret = -1;
+	int save_errno;
+	int remaining_retries = 3;
+
+	*type = 0;
+	strbuf_reset(&sb_path);
+
+	files_ref_path(refs, &sb_path, refname);
+
+	path = sb_path.buf;
+
+stat_ref:
+	/*
+	 * We might have to loop back here to avoid a race
+	 * condition: first we lstat() the file, then we try
+	 * to read it as a link or as a file.  But if somebody
+	 * changes the type of the file (file <-> directory
+	 * <-> symlink) between the lstat() and reading, then
+	 * we don't want to report that as an error but rather
+	 * try again starting with the lstat().
+	 *
+	 * We'll keep a count of the retries, though, just to avoid
+	 * any confusing situation sending us into an infinite loop.
+	 */
+
+	if (remaining_retries-- <= 0)
+		goto out;
+
+	if (lstat(path, &st) < 0) {
+		if (errno != ENOENT)
+			goto out;
+		if (refs_read_raw_ref(refs->packed_ref_store, refname,
+				      oid, referent, type)) {
+			errno = ENOENT;
+			goto out;
+		}
+		ret = 0;
+		goto out;
+	}
+
+	/* Follow "normalized" - ie "refs/.." symlinks by hand */
+	if (S_ISLNK(st.st_mode)) {
+		strbuf_reset(&sb_contents);
+		if (strbuf_readlink(&sb_contents, path, st.st_size) < 0) {
+			if (errno == ENOENT || errno == EINVAL)
+				/* inconsistent with lstat; retry */
+				goto stat_ref;
+			else
+				goto out;
+		}
+		if (starts_with(sb_contents.buf, "refs/") &&
+		    !check_refname_format(sb_contents.buf, 0)) {
+			strbuf_swap(&sb_contents, referent);
+			*type |= REF_ISSYMREF;
+			ret = 0;
+			goto out;
+		}
+		/*
+		 * It doesn't look like a refname; fall through to just
+		 * treating it like a non-symlink, and reading whatever it
+		 * points to.
+		 */
+	}
+
+	/* Is it a directory? */
+	if (S_ISDIR(st.st_mode)) {
+		/*
+		 * Even though there is a directory where the loose
+		 * ref is supposed to be, there could still be a
+		 * packed ref:
+		 */
+		if (refs_read_raw_ref(refs->packed_ref_store, refname,
+				      oid, referent, type)) {
+			errno = EISDIR;
+			goto out;
+		}
+		ret = 0;
+		goto out;
+	}
+
+	/*
+	 * Anything else, just open it and try to use it as
+	 * a ref
+	 */
+	fd = open(path, O_RDONLY);
+	if (fd < 0) {
+		if (errno == ENOENT && !S_ISLNK(st.st_mode))
+			/* inconsistent with lstat; retry */
+			goto stat_ref;
+		else
+			goto out;
+	}
+	strbuf_reset(&sb_contents);
+	if (strbuf_read(&sb_contents, fd, 256) < 0) {
+		int save_errno = errno;
+		close(fd);
+		errno = save_errno;
+		goto out;
+	}
+	close(fd);
+	strbuf_rtrim(&sb_contents);
+	buf = sb_contents.buf;
+	if (starts_with(buf, "ref:")) {
+		buf += 4;
+		while (isspace(*buf))
+			buf++;
+
+		strbuf_reset(referent);
+		strbuf_addstr(referent, buf);
+		*type |= REF_ISSYMREF;
+		ret = 0;
+		goto out;
+	}
+
+	/*
+	 * Please note that FETCH_HEAD has additional
+	 * data after the sha.
+	 */
+	if (parse_oid_hex(buf, oid, &p) ||
+	    (*p != '\0' && !isspace(*p))) {
+		*type |= REF_ISBROKEN;
+		errno = EINVAL;
+		goto out;
+	}
+
+	ret = 0;
+
+out:
+	save_errno = errno;
+	strbuf_release(&sb_path);
+	strbuf_release(&sb_contents);
+	errno = save_errno;
+	return ret;
+}
+
+static void unlock_ref(struct ref_lock *lock)
+{
+	rollback_lock_file(&lock->lk);
+	free(lock->ref_name);
+	free(lock);
+}
+
+/*
+ * Lock refname, without following symrefs, and set *lock_p to point
+ * at a newly-allocated lock object. Fill in lock->old_oid, referent,
+ * and type similarly to read_raw_ref().
+ *
+ * The caller must verify that refname is a "safe" reference name (in
+ * the sense of refname_is_safe()) before calling this function.
+ *
+ * If the reference doesn't already exist, verify that refname doesn't
+ * have a D/F conflict with any existing references. extras and skip
+ * are passed to refs_verify_refname_available() for this check.
+ *
+ * If mustexist is not set and the reference is not found or is
+ * broken, lock the reference anyway but clear old_oid.
+ *
+ * Return 0 on success. On failure, write an error message to err and
+ * return TRANSACTION_NAME_CONFLICT or TRANSACTION_GENERIC_ERROR.
+ *
+ * Implementation note: This function is basically
+ *
+ *     lock reference
+ *     read_raw_ref()
+ *
+ * but it includes a lot more code to
+ * - Deal with possible races with other processes
+ * - Avoid calling refs_verify_refname_available() when it can be
+ *   avoided, namely if we were successfully able to read the ref
+ * - Generate informative error messages in the case of failure
+ */
+static int lock_raw_ref(struct files_ref_store *refs,
+			const char *refname, int mustexist,
+			const struct string_list *extras,
+			const struct string_list *skip,
+			struct ref_lock **lock_p,
+			struct strbuf *referent,
+			unsigned int *type,
+			struct strbuf *err)
+{
+	struct ref_lock *lock;
+	struct strbuf ref_file = STRBUF_INIT;
+	int attempts_remaining = 3;
+	int ret = TRANSACTION_GENERIC_ERROR;
+
+	assert(err);
+	files_assert_main_repository(refs, "lock_raw_ref");
+
+	*type = 0;
+
+	/* First lock the file so it can't change out from under us. */
+
+	*lock_p = lock = xcalloc(1, sizeof(*lock));
+
+	lock->ref_name = xstrdup(refname);
+	files_ref_path(refs, &ref_file, refname);
+
+retry:
+	switch (safe_create_leading_directories(ref_file.buf)) {
+	case SCLD_OK:
+		break; /* success */
+	case SCLD_EXISTS:
+		/*
+		 * Suppose refname is "refs/foo/bar". We just failed
+		 * to create the containing directory, "refs/foo",
+		 * because there was a non-directory in the way. This
+		 * indicates a D/F conflict, probably because of
+		 * another reference such as "refs/foo". There is no
+		 * reason to expect this error to be transitory.
+		 */
+		if (refs_verify_refname_available(&refs->base, refname,
+						  extras, skip, err)) {
+			if (mustexist) {
+				/*
+				 * To the user the relevant error is
+				 * that the "mustexist" reference is
+				 * missing:
+				 */
+				strbuf_reset(err);
+				strbuf_addf(err, "unable to resolve reference '%s'",
+					    refname);
+			} else {
+				/*
+				 * The error message set by
+				 * refs_verify_refname_available() is
+				 * OK.
+				 */
+				ret = TRANSACTION_NAME_CONFLICT;
+			}
+		} else {
+			/*
+			 * The file that is in the way isn't a loose
+			 * reference. Report it as a low-level
+			 * failure.
+			 */
+			strbuf_addf(err, "unable to create lock file %s.lock; "
+				    "non-directory in the way",
+				    ref_file.buf);
+		}
+		goto error_return;
+	case SCLD_VANISHED:
+		/* Maybe another process was tidying up. Try again. */
+		if (--attempts_remaining > 0)
+			goto retry;
+		/* fall through */
+	default:
+		strbuf_addf(err, "unable to create directory for %s",
+			    ref_file.buf);
+		goto error_return;
+	}
+
+	if (hold_lock_file_for_update_timeout(
+			    &lock->lk, ref_file.buf, LOCK_NO_DEREF,
+			    get_files_ref_lock_timeout_ms()) < 0) {
+		if (errno == ENOENT && --attempts_remaining > 0) {
+			/*
+			 * Maybe somebody just deleted one of the
+			 * directories leading to ref_file.  Try
+			 * again:
+			 */
+			goto retry;
+		} else {
+			unable_to_lock_message(ref_file.buf, errno, err);
+			goto error_return;
+		}
+	}
+
+	/*
+	 * Now we hold the lock and can read the reference without
+	 * fear that its value will change.
+	 */
+
+	if (files_read_raw_ref(&refs->base, refname,
+			       &lock->old_oid, referent, type)) {
+		if (errno == ENOENT) {
+			if (mustexist) {
+				/* Garden variety missing reference. */
+				strbuf_addf(err, "unable to resolve reference '%s'",
+					    refname);
+				goto error_return;
+			} else {
+				/*
+				 * Reference is missing, but that's OK. We
+				 * know that there is not a conflict with
+				 * another loose reference because
+				 * (supposing that we are trying to lock
+				 * reference "refs/foo/bar"):
+				 *
+				 * - We were successfully able to create
+				 *   the lockfile refs/foo/bar.lock, so we
+				 *   know there cannot be a loose reference
+				 *   named "refs/foo".
+				 *
+				 * - We got ENOENT and not EISDIR, so we
+				 *   know that there cannot be a loose
+				 *   reference named "refs/foo/bar/baz".
+				 */
+			}
+		} else if (errno == EISDIR) {
+			/*
+			 * There is a directory in the way. It might have
+			 * contained references that have been deleted. If
+			 * we don't require that the reference already
+			 * exists, try to remove the directory so that it
+			 * doesn't cause trouble when we want to rename the
+			 * lockfile into place later.
+			 */
+			if (mustexist) {
+				/* Garden variety missing reference. */
+				strbuf_addf(err, "unable to resolve reference '%s'",
+					    refname);
+				goto error_return;
+			} else if (remove_dir_recursively(&ref_file,
+							  REMOVE_DIR_EMPTY_ONLY)) {
+				if (refs_verify_refname_available(
+						    &refs->base, refname,
+						    extras, skip, err)) {
+					/*
+					 * The error message set by
+					 * verify_refname_available() is OK.
+					 */
+					ret = TRANSACTION_NAME_CONFLICT;
+					goto error_return;
+				} else {
+					/*
+					 * We can't delete the directory,
+					 * but we also don't know of any
+					 * references that it should
+					 * contain.
+					 */
+					strbuf_addf(err, "there is a non-empty directory '%s' "
+						    "blocking reference '%s'",
+						    ref_file.buf, refname);
+					goto error_return;
+				}
+			}
+		} else if (errno == EINVAL && (*type & REF_ISBROKEN)) {
+			strbuf_addf(err, "unable to resolve reference '%s': "
+				    "reference broken", refname);
+			goto error_return;
+		} else {
+			strbuf_addf(err, "unable to resolve reference '%s': %s",
+				    refname, strerror(errno));
+			goto error_return;
+		}
+
+		/*
+		 * If the ref did not exist and we are creating it,
+		 * make sure there is no existing packed ref that
+		 * conflicts with refname:
+		 */
+		if (refs_verify_refname_available(
+				    refs->packed_ref_store, refname,
+				    extras, skip, err))
+			goto error_return;
+	}
+
+	ret = 0;
+	goto out;
+
+error_return:
+	unlock_ref(lock);
+	*lock_p = NULL;
+
+out:
+	strbuf_release(&ref_file);
+	return ret;
+}
+
+struct files_ref_iterator {
+	struct ref_iterator base;
+
+	struct ref_iterator *iter0;
+	unsigned int flags;
+};
+
+static int files_ref_iterator_advance(struct ref_iterator *ref_iterator)
+{
+	struct files_ref_iterator *iter =
+		(struct files_ref_iterator *)ref_iterator;
+	int ok;
+
+	while ((ok = ref_iterator_advance(iter->iter0)) == ITER_OK) {
+		if (iter->flags & DO_FOR_EACH_PER_WORKTREE_ONLY &&
+		    ref_type(iter->iter0->refname) != REF_TYPE_PER_WORKTREE)
+			continue;
+
+		if (!(iter->flags & DO_FOR_EACH_INCLUDE_BROKEN) &&
+		    !ref_resolves_to_object(iter->iter0->refname,
+					    iter->iter0->oid,
+					    iter->iter0->flags))
+			continue;
+
+		iter->base.refname = iter->iter0->refname;
+		iter->base.oid = iter->iter0->oid;
+		iter->base.flags = iter->iter0->flags;
+		return ITER_OK;
+	}
+
+	iter->iter0 = NULL;
+	if (ref_iterator_abort(ref_iterator) != ITER_DONE)
+		ok = ITER_ERROR;
+
+	return ok;
+}
+
+static int files_ref_iterator_peel(struct ref_iterator *ref_iterator,
+				   struct object_id *peeled)
+{
+	struct files_ref_iterator *iter =
+		(struct files_ref_iterator *)ref_iterator;
+
+	return ref_iterator_peel(iter->iter0, peeled);
+}
+
+static int files_ref_iterator_abort(struct ref_iterator *ref_iterator)
+{
+	struct files_ref_iterator *iter =
+		(struct files_ref_iterator *)ref_iterator;
+	int ok = ITER_DONE;
+
+	if (iter->iter0)
+		ok = ref_iterator_abort(iter->iter0);
+
+	base_ref_iterator_free(ref_iterator);
+	return ok;
+}
+
+static struct ref_iterator_vtable files_ref_iterator_vtable = {
+	files_ref_iterator_advance,
+	files_ref_iterator_peel,
+	files_ref_iterator_abort
+};
+
+static struct ref_iterator *files_ref_iterator_begin(
+		struct ref_store *ref_store,
+		const char *prefix, unsigned int flags)
+{
+	struct files_ref_store *refs;
+	struct ref_iterator *loose_iter, *packed_iter, *overlay_iter;
+	struct files_ref_iterator *iter;
+	struct ref_iterator *ref_iterator;
+	unsigned int required_flags = REF_STORE_READ;
+
+	if (!(flags & DO_FOR_EACH_INCLUDE_BROKEN))
+		required_flags |= REF_STORE_ODB;
+
+	refs = files_downcast(ref_store, required_flags, "ref_iterator_begin");
+
+	/*
+	 * We must make sure that all loose refs are read before
+	 * accessing the packed-refs file; this avoids a race
+	 * condition if loose refs are migrated to the packed-refs
+	 * file by a simultaneous process, but our in-memory view is
+	 * from before the migration. We ensure this as follows:
+	 * First, we call start the loose refs iteration with its
+	 * `prime_ref` argument set to true. This causes the loose
+	 * references in the subtree to be pre-read into the cache.
+	 * (If they've already been read, that's OK; we only need to
+	 * guarantee that they're read before the packed refs, not
+	 * *how much* before.) After that, we call
+	 * packed_ref_iterator_begin(), which internally checks
+	 * whether the packed-ref cache is up to date with what is on
+	 * disk, and re-reads it if not.
+	 */
+
+	loose_iter = cache_ref_iterator_begin(get_loose_ref_cache(refs),
+					      prefix, 1);
+
+	/*
+	 * The packed-refs file might contain broken references, for
+	 * example an old version of a reference that points at an
+	 * object that has since been garbage-collected. This is OK as
+	 * long as there is a corresponding loose reference that
+	 * overrides it, and we don't want to emit an error message in
+	 * this case. So ask the packed_ref_store for all of its
+	 * references, and (if needed) do our own check for broken
+	 * ones in files_ref_iterator_advance(), after we have merged
+	 * the packed and loose references.
+	 */
+	packed_iter = refs_ref_iterator_begin(
+			refs->packed_ref_store, prefix, 0,
+			DO_FOR_EACH_INCLUDE_BROKEN);
+
+	overlay_iter = overlay_ref_iterator_begin(loose_iter, packed_iter);
+
+	iter = xcalloc(1, sizeof(*iter));
+	ref_iterator = &iter->base;
+	base_ref_iterator_init(ref_iterator, &files_ref_iterator_vtable,
+			       overlay_iter->ordered);
+	iter->iter0 = overlay_iter;
+	iter->flags = flags;
+
+	return ref_iterator;
+}
+
+/*
+ * Verify that the reference locked by lock has the value old_oid
+ * (unless it is NULL).  Fail if the reference doesn't exist and
+ * mustexist is set. Return 0 on success. On error, write an error
+ * message to err, set errno, and return a negative value.
+ */
+static int verify_lock(struct ref_store *ref_store, struct ref_lock *lock,
+		       const struct object_id *old_oid, int mustexist,
+		       struct strbuf *err)
+{
+	assert(err);
+
+	if (refs_read_ref_full(ref_store, lock->ref_name,
+			       mustexist ? RESOLVE_REF_READING : 0,
+			       &lock->old_oid, NULL)) {
+		if (old_oid) {
+			int save_errno = errno;
+			strbuf_addf(err, "can't verify ref '%s'", lock->ref_name);
+			errno = save_errno;
+			return -1;
+		} else {
+			oidclr(&lock->old_oid);
+			return 0;
+		}
+	}
+	if (old_oid && !oideq(&lock->old_oid, old_oid)) {
+		strbuf_addf(err, "ref '%s' is at %s but expected %s",
+			    lock->ref_name,
+			    oid_to_hex(&lock->old_oid),
+			    oid_to_hex(old_oid));
+		errno = EBUSY;
+		return -1;
+	}
+	return 0;
+}
+
+static int remove_empty_directories(struct strbuf *path)
+{
+	/*
+	 * we want to create a file but there is a directory there;
+	 * if that is an empty directory (or a directory that contains
+	 * only empty directories), remove them.
+	 */
+	return remove_dir_recursively(path, REMOVE_DIR_EMPTY_ONLY);
+}
+
+static int create_reflock(const char *path, void *cb)
+{
+	struct lock_file *lk = cb;
+
+	return hold_lock_file_for_update_timeout(
+			lk, path, LOCK_NO_DEREF,
+			get_files_ref_lock_timeout_ms()) < 0 ? -1 : 0;
+}
+
+/*
+ * Locks a ref returning the lock on success and NULL on failure.
+ * On failure errno is set to something meaningful.
+ */
+static struct ref_lock *lock_ref_oid_basic(struct files_ref_store *refs,
+					   const char *refname,
+					   const struct object_id *old_oid,
+					   const struct string_list *extras,
+					   const struct string_list *skip,
+					   unsigned int flags, int *type,
+					   struct strbuf *err)
+{
+	struct strbuf ref_file = STRBUF_INIT;
+	struct ref_lock *lock;
+	int last_errno = 0;
+	int mustexist = (old_oid && !is_null_oid(old_oid));
+	int resolve_flags = RESOLVE_REF_NO_RECURSE;
+	int resolved;
+
+	files_assert_main_repository(refs, "lock_ref_oid_basic");
+	assert(err);
+
+	lock = xcalloc(1, sizeof(struct ref_lock));
+
+	if (mustexist)
+		resolve_flags |= RESOLVE_REF_READING;
+	if (flags & REF_DELETING)
+		resolve_flags |= RESOLVE_REF_ALLOW_BAD_NAME;
+
+	files_ref_path(refs, &ref_file, refname);
+	resolved = !!refs_resolve_ref_unsafe(&refs->base,
+					     refname, resolve_flags,
+					     &lock->old_oid, type);
+	if (!resolved && errno == EISDIR) {
+		/*
+		 * we are trying to lock foo but we used to
+		 * have foo/bar which now does not exist;
+		 * it is normal for the empty directory 'foo'
+		 * to remain.
+		 */
+		if (remove_empty_directories(&ref_file)) {
+			last_errno = errno;
+			if (!refs_verify_refname_available(
+					    &refs->base,
+					    refname, extras, skip, err))
+				strbuf_addf(err, "there are still refs under '%s'",
+					    refname);
+			goto error_return;
+		}
+		resolved = !!refs_resolve_ref_unsafe(&refs->base,
+						     refname, resolve_flags,
+						     &lock->old_oid, type);
+	}
+	if (!resolved) {
+		last_errno = errno;
+		if (last_errno != ENOTDIR ||
+		    !refs_verify_refname_available(&refs->base, refname,
+						   extras, skip, err))
+			strbuf_addf(err, "unable to resolve reference '%s': %s",
+				    refname, strerror(last_errno));
+
+		goto error_return;
+	}
+
+	/*
+	 * If the ref did not exist and we are creating it, make sure
+	 * there is no existing packed ref whose name begins with our
+	 * refname, nor a packed ref whose name is a proper prefix of
+	 * our refname.
+	 */
+	if (is_null_oid(&lock->old_oid) &&
+	    refs_verify_refname_available(refs->packed_ref_store, refname,
+					  extras, skip, err)) {
+		last_errno = ENOTDIR;
+		goto error_return;
+	}
+
+	lock->ref_name = xstrdup(refname);
+
+	if (raceproof_create_file(ref_file.buf, create_reflock, &lock->lk)) {
+		last_errno = errno;
+		unable_to_lock_message(ref_file.buf, errno, err);
+		goto error_return;
+	}
+
+	if (verify_lock(&refs->base, lock, old_oid, mustexist, err)) {
+		last_errno = errno;
+		goto error_return;
+	}
+	goto out;
+
+ error_return:
+	unlock_ref(lock);
+	lock = NULL;
+
+ out:
+	strbuf_release(&ref_file);
+	errno = last_errno;
+	return lock;
+}
+
+struct ref_to_prune {
+	struct ref_to_prune *next;
+	struct object_id oid;
+	char name[FLEX_ARRAY];
+};
+
+enum {
+	REMOVE_EMPTY_PARENTS_REF = 0x01,
+	REMOVE_EMPTY_PARENTS_REFLOG = 0x02
+};
+
+/*
+ * Remove empty parent directories associated with the specified
+ * reference and/or its reflog, but spare [logs/]refs/ and immediate
+ * subdirs. flags is a combination of REMOVE_EMPTY_PARENTS_REF and/or
+ * REMOVE_EMPTY_PARENTS_REFLOG.
+ */
+static void try_remove_empty_parents(struct files_ref_store *refs,
+				     const char *refname,
+				     unsigned int flags)
+{
+	struct strbuf buf = STRBUF_INIT;
+	struct strbuf sb = STRBUF_INIT;
+	char *p, *q;
+	int i;
+
+	strbuf_addstr(&buf, refname);
+	p = buf.buf;
+	for (i = 0; i < 2; i++) { /* refs/{heads,tags,...}/ */
+		while (*p && *p != '/')
+			p++;
+		/* tolerate duplicate slashes; see check_refname_format() */
+		while (*p == '/')
+			p++;
+	}
+	q = buf.buf + buf.len;
+	while (flags & (REMOVE_EMPTY_PARENTS_REF | REMOVE_EMPTY_PARENTS_REFLOG)) {
+		while (q > p && *q != '/')
+			q--;
+		while (q > p && *(q-1) == '/')
+			q--;
+		if (q == p)
+			break;
+		strbuf_setlen(&buf, q - buf.buf);
+
+		strbuf_reset(&sb);
+		files_ref_path(refs, &sb, buf.buf);
+		if ((flags & REMOVE_EMPTY_PARENTS_REF) && rmdir(sb.buf))
+			flags &= ~REMOVE_EMPTY_PARENTS_REF;
+
+		strbuf_reset(&sb);
+		files_reflog_path(refs, &sb, buf.buf);
+		if ((flags & REMOVE_EMPTY_PARENTS_REFLOG) && rmdir(sb.buf))
+			flags &= ~REMOVE_EMPTY_PARENTS_REFLOG;
+	}
+	strbuf_release(&buf);
+	strbuf_release(&sb);
+}
+
+/* make sure nobody touched the ref, and unlink */
+static void prune_ref(struct files_ref_store *refs, struct ref_to_prune *r)
+{
+	struct ref_transaction *transaction;
+	struct strbuf err = STRBUF_INIT;
+	int ret = -1;
+
+	if (check_refname_format(r->name, 0))
+		return;
+
+	transaction = ref_store_transaction_begin(&refs->base, &err);
+	if (!transaction)
+		goto cleanup;
+	ref_transaction_add_update(
+			transaction, r->name,
+			REF_NO_DEREF | REF_HAVE_NEW | REF_HAVE_OLD | REF_IS_PRUNING,
+			&null_oid, &r->oid, NULL);
+	if (ref_transaction_commit(transaction, &err))
+		goto cleanup;
+
+	ret = 0;
+
+cleanup:
+	if (ret)
+		error("%s", err.buf);
+	strbuf_release(&err);
+	ref_transaction_free(transaction);
+	return;
+}
+
+/*
+ * Prune the loose versions of the references in the linked list
+ * `*refs_to_prune`, freeing the entries in the list as we go.
+ */
+static void prune_refs(struct files_ref_store *refs, struct ref_to_prune **refs_to_prune)
+{
+	while (*refs_to_prune) {
+		struct ref_to_prune *r = *refs_to_prune;
+		*refs_to_prune = r->next;
+		prune_ref(refs, r);
+		free(r);
+	}
+}
+
+/*
+ * Return true if the specified reference should be packed.
+ */
+static int should_pack_ref(const char *refname,
+			   const struct object_id *oid, unsigned int ref_flags,
+			   unsigned int pack_flags)
+{
+	/* Do not pack per-worktree refs: */
+	if (ref_type(refname) != REF_TYPE_NORMAL)
+		return 0;
+
+	/* Do not pack non-tags unless PACK_REFS_ALL is set: */
+	if (!(pack_flags & PACK_REFS_ALL) && !starts_with(refname, "refs/tags/"))
+		return 0;
+
+	/* Do not pack symbolic refs: */
+	if (ref_flags & REF_ISSYMREF)
+		return 0;
+
+	/* Do not pack broken refs: */
+	if (!ref_resolves_to_object(refname, oid, ref_flags))
+		return 0;
+
+	return 1;
+}
+
+static int files_pack_refs(struct ref_store *ref_store, unsigned int flags)
+{
+	struct files_ref_store *refs =
+		files_downcast(ref_store, REF_STORE_WRITE | REF_STORE_ODB,
+			       "pack_refs");
+	struct ref_iterator *iter;
+	int ok;
+	struct ref_to_prune *refs_to_prune = NULL;
+	struct strbuf err = STRBUF_INIT;
+	struct ref_transaction *transaction;
+
+	transaction = ref_store_transaction_begin(refs->packed_ref_store, &err);
+	if (!transaction)
+		return -1;
+
+	packed_refs_lock(refs->packed_ref_store, LOCK_DIE_ON_ERROR, &err);
+
+	iter = cache_ref_iterator_begin(get_loose_ref_cache(refs), NULL, 0);
+	while ((ok = ref_iterator_advance(iter)) == ITER_OK) {
+		/*
+		 * If the loose reference can be packed, add an entry
+		 * in the packed ref cache. If the reference should be
+		 * pruned, also add it to refs_to_prune.
+		 */
+		if (!should_pack_ref(iter->refname, iter->oid, iter->flags,
+				     flags))
+			continue;
+
+		/*
+		 * Add a reference creation for this reference to the
+		 * packed-refs transaction:
+		 */
+		if (ref_transaction_update(transaction, iter->refname,
+					   iter->oid, NULL,
+					   REF_NO_DEREF, NULL, &err))
+			die("failure preparing to create packed reference %s: %s",
+			    iter->refname, err.buf);
+
+		/* Schedule the loose reference for pruning if requested. */
+		if ((flags & PACK_REFS_PRUNE)) {
+			struct ref_to_prune *n;
+			FLEX_ALLOC_STR(n, name, iter->refname);
+			oidcpy(&n->oid, iter->oid);
+			n->next = refs_to_prune;
+			refs_to_prune = n;
+		}
+	}
+	if (ok != ITER_DONE)
+		die("error while iterating over references");
+
+	if (ref_transaction_commit(transaction, &err))
+		die("unable to write new packed-refs: %s", err.buf);
+
+	ref_transaction_free(transaction);
+
+	packed_refs_unlock(refs->packed_ref_store);
+
+	prune_refs(refs, &refs_to_prune);
+	strbuf_release(&err);
+	return 0;
+}
+
+static int files_delete_refs(struct ref_store *ref_store, const char *msg,
+			     struct string_list *refnames, unsigned int flags)
+{
+	struct files_ref_store *refs =
+		files_downcast(ref_store, REF_STORE_WRITE, "delete_refs");
+	struct strbuf err = STRBUF_INIT;
+	int i, result = 0;
+
+	if (!refnames->nr)
+		return 0;
+
+	if (packed_refs_lock(refs->packed_ref_store, 0, &err))
+		goto error;
+
+	if (refs_delete_refs(refs->packed_ref_store, msg, refnames, flags)) {
+		packed_refs_unlock(refs->packed_ref_store);
+		goto error;
+	}
+
+	packed_refs_unlock(refs->packed_ref_store);
+
+	for (i = 0; i < refnames->nr; i++) {
+		const char *refname = refnames->items[i].string;
+
+		if (refs_delete_ref(&refs->base, msg, refname, NULL, flags))
+			result |= error(_("could not remove reference %s"), refname);
+	}
+
+	strbuf_release(&err);
+	return result;
+
+error:
+	/*
+	 * If we failed to rewrite the packed-refs file, then it is
+	 * unsafe to try to remove loose refs, because doing so might
+	 * expose an obsolete packed value for a reference that might
+	 * even point at an object that has been garbage collected.
+	 */
+	if (refnames->nr == 1)
+		error(_("could not delete reference %s: %s"),
+		      refnames->items[0].string, err.buf);
+	else
+		error(_("could not delete references: %s"), err.buf);
+
+	strbuf_release(&err);
+	return -1;
+}
+
+/*
+ * People using contrib's git-new-workdir have .git/logs/refs ->
+ * /some/other/path/.git/logs/refs, and that may live on another device.
+ *
+ * IOW, to avoid cross device rename errors, the temporary renamed log must
+ * live into logs/refs.
+ */
+#define TMP_RENAMED_LOG  "refs/.tmp-renamed-log"
+
+struct rename_cb {
+	const char *tmp_renamed_log;
+	int true_errno;
+};
+
+static int rename_tmp_log_callback(const char *path, void *cb_data)
+{
+	struct rename_cb *cb = cb_data;
+
+	if (rename(cb->tmp_renamed_log, path)) {
+		/*
+		 * rename(a, b) when b is an existing directory ought
+		 * to result in ISDIR, but Solaris 5.8 gives ENOTDIR.
+		 * Sheesh. Record the true errno for error reporting,
+		 * but report EISDIR to raceproof_create_file() so
+		 * that it knows to retry.
+		 */
+		cb->true_errno = errno;
+		if (errno == ENOTDIR)
+			errno = EISDIR;
+		return -1;
+	} else {
+		return 0;
+	}
+}
+
+static int rename_tmp_log(struct files_ref_store *refs, const char *newrefname)
+{
+	struct strbuf path = STRBUF_INIT;
+	struct strbuf tmp = STRBUF_INIT;
+	struct rename_cb cb;
+	int ret;
+
+	files_reflog_path(refs, &path, newrefname);
+	files_reflog_path(refs, &tmp, TMP_RENAMED_LOG);
+	cb.tmp_renamed_log = tmp.buf;
+	ret = raceproof_create_file(path.buf, rename_tmp_log_callback, &cb);
+	if (ret) {
+		if (errno == EISDIR)
+			error("directory not empty: %s", path.buf);
+		else
+			error("unable to move logfile %s to %s: %s",
+			      tmp.buf, path.buf,
+			      strerror(cb.true_errno));
+	}
+
+	strbuf_release(&path);
+	strbuf_release(&tmp);
+	return ret;
+}
+
+static int write_ref_to_lockfile(struct ref_lock *lock,
+				 const struct object_id *oid, struct strbuf *err);
+static int commit_ref_update(struct files_ref_store *refs,
+			     struct ref_lock *lock,
+			     const struct object_id *oid, const char *logmsg,
+			     struct strbuf *err);
+
+static int files_copy_or_rename_ref(struct ref_store *ref_store,
+			    const char *oldrefname, const char *newrefname,
+			    const char *logmsg, int copy)
+{
+	struct files_ref_store *refs =
+		files_downcast(ref_store, REF_STORE_WRITE, "rename_ref");
+	struct object_id oid, orig_oid;
+	int flag = 0, logmoved = 0;
+	struct ref_lock *lock;
+	struct stat loginfo;
+	struct strbuf sb_oldref = STRBUF_INIT;
+	struct strbuf sb_newref = STRBUF_INIT;
+	struct strbuf tmp_renamed_log = STRBUF_INIT;
+	int log, ret;
+	struct strbuf err = STRBUF_INIT;
+
+	files_reflog_path(refs, &sb_oldref, oldrefname);
+	files_reflog_path(refs, &sb_newref, newrefname);
+	files_reflog_path(refs, &tmp_renamed_log, TMP_RENAMED_LOG);
+
+	log = !lstat(sb_oldref.buf, &loginfo);
+	if (log && S_ISLNK(loginfo.st_mode)) {
+		ret = error("reflog for %s is a symlink", oldrefname);
+		goto out;
+	}
+
+	if (!refs_resolve_ref_unsafe(&refs->base, oldrefname,
+				     RESOLVE_REF_READING | RESOLVE_REF_NO_RECURSE,
+				&orig_oid, &flag)) {
+		ret = error("refname %s not found", oldrefname);
+		goto out;
+	}
+
+	if (flag & REF_ISSYMREF) {
+		if (copy)
+			ret = error("refname %s is a symbolic ref, copying it is not supported",
+				    oldrefname);
+		else
+			ret = error("refname %s is a symbolic ref, renaming it is not supported",
+				    oldrefname);
+		goto out;
+	}
+	if (!refs_rename_ref_available(&refs->base, oldrefname, newrefname)) {
+		ret = 1;
+		goto out;
+	}
+
+	if (!copy && log && rename(sb_oldref.buf, tmp_renamed_log.buf)) {
+		ret = error("unable to move logfile logs/%s to logs/"TMP_RENAMED_LOG": %s",
+			    oldrefname, strerror(errno));
+		goto out;
+	}
+
+	if (copy && log && copy_file(tmp_renamed_log.buf, sb_oldref.buf, 0644)) {
+		ret = error("unable to copy logfile logs/%s to logs/"TMP_RENAMED_LOG": %s",
+			    oldrefname, strerror(errno));
+		goto out;
+	}
+
+	if (!copy && refs_delete_ref(&refs->base, logmsg, oldrefname,
+			    &orig_oid, REF_NO_DEREF)) {
+		error("unable to delete old %s", oldrefname);
+		goto rollback;
+	}
+
+	/*
+	 * Since we are doing a shallow lookup, oid is not the
+	 * correct value to pass to delete_ref as old_oid. But that
+	 * doesn't matter, because an old_oid check wouldn't add to
+	 * the safety anyway; we want to delete the reference whatever
+	 * its current value.
+	 */
+	if (!copy && !refs_read_ref_full(&refs->base, newrefname,
+				RESOLVE_REF_READING | RESOLVE_REF_NO_RECURSE,
+				&oid, NULL) &&
+	    refs_delete_ref(&refs->base, NULL, newrefname,
+			    NULL, REF_NO_DEREF)) {
+		if (errno == EISDIR) {
+			struct strbuf path = STRBUF_INIT;
+			int result;
+
+			files_ref_path(refs, &path, newrefname);
+			result = remove_empty_directories(&path);
+			strbuf_release(&path);
+
+			if (result) {
+				error("Directory not empty: %s", newrefname);
+				goto rollback;
+			}
+		} else {
+			error("unable to delete existing %s", newrefname);
+			goto rollback;
+		}
+	}
+
+	if (log && rename_tmp_log(refs, newrefname))
+		goto rollback;
+
+	logmoved = log;
+
+	lock = lock_ref_oid_basic(refs, newrefname, NULL, NULL, NULL,
+				  REF_NO_DEREF, NULL, &err);
+	if (!lock) {
+		if (copy)
+			error("unable to copy '%s' to '%s': %s", oldrefname, newrefname, err.buf);
+		else
+			error("unable to rename '%s' to '%s': %s", oldrefname, newrefname, err.buf);
+		strbuf_release(&err);
+		goto rollback;
+	}
+	oidcpy(&lock->old_oid, &orig_oid);
+
+	if (write_ref_to_lockfile(lock, &orig_oid, &err) ||
+	    commit_ref_update(refs, lock, &orig_oid, logmsg, &err)) {
+		error("unable to write current sha1 into %s: %s", newrefname, err.buf);
+		strbuf_release(&err);
+		goto rollback;
+	}
+
+	ret = 0;
+	goto out;
+
+ rollback:
+	lock = lock_ref_oid_basic(refs, oldrefname, NULL, NULL, NULL,
+				  REF_NO_DEREF, NULL, &err);
+	if (!lock) {
+		error("unable to lock %s for rollback: %s", oldrefname, err.buf);
+		strbuf_release(&err);
+		goto rollbacklog;
+	}
+
+	flag = log_all_ref_updates;
+	log_all_ref_updates = LOG_REFS_NONE;
+	if (write_ref_to_lockfile(lock, &orig_oid, &err) ||
+	    commit_ref_update(refs, lock, &orig_oid, NULL, &err)) {
+		error("unable to write current sha1 into %s: %s", oldrefname, err.buf);
+		strbuf_release(&err);
+	}
+	log_all_ref_updates = flag;
+
+ rollbacklog:
+	if (logmoved && rename(sb_newref.buf, sb_oldref.buf))
+		error("unable to restore logfile %s from %s: %s",
+			oldrefname, newrefname, strerror(errno));
+	if (!logmoved && log &&
+	    rename(tmp_renamed_log.buf, sb_oldref.buf))
+		error("unable to restore logfile %s from logs/"TMP_RENAMED_LOG": %s",
+			oldrefname, strerror(errno));
+	ret = 1;
+ out:
+	strbuf_release(&sb_newref);
+	strbuf_release(&sb_oldref);
+	strbuf_release(&tmp_renamed_log);
+
+	return ret;
+}
+
+static int files_rename_ref(struct ref_store *ref_store,
+			    const char *oldrefname, const char *newrefname,
+			    const char *logmsg)
+{
+	return files_copy_or_rename_ref(ref_store, oldrefname,
+				 newrefname, logmsg, 0);
+}
+
+static int files_copy_ref(struct ref_store *ref_store,
+			    const char *oldrefname, const char *newrefname,
+			    const char *logmsg)
+{
+	return files_copy_or_rename_ref(ref_store, oldrefname,
+				 newrefname, logmsg, 1);
+}
+
+static int close_ref_gently(struct ref_lock *lock)
+{
+	if (close_lock_file_gently(&lock->lk))
+		return -1;
+	return 0;
+}
+
+static int commit_ref(struct ref_lock *lock)
+{
+	char *path = get_locked_file_path(&lock->lk);
+	struct stat st;
+
+	if (!lstat(path, &st) && S_ISDIR(st.st_mode)) {
+		/*
+		 * There is a directory at the path we want to rename
+		 * the lockfile to. Hopefully it is empty; try to
+		 * delete it.
+		 */
+		size_t len = strlen(path);
+		struct strbuf sb_path = STRBUF_INIT;
+
+		strbuf_attach(&sb_path, path, len, len);
+
+		/*
+		 * If this fails, commit_lock_file() will also fail
+		 * and will report the problem.
+		 */
+		remove_empty_directories(&sb_path);
+		strbuf_release(&sb_path);
+	} else {
+		free(path);
+	}
+
+	if (commit_lock_file(&lock->lk))
+		return -1;
+	return 0;
+}
+
+static int open_or_create_logfile(const char *path, void *cb)
+{
+	int *fd = cb;
+
+	*fd = open(path, O_APPEND | O_WRONLY | O_CREAT, 0666);
+	return (*fd < 0) ? -1 : 0;
+}
+
+/*
+ * Create a reflog for a ref. If force_create = 0, only create the
+ * reflog for certain refs (those for which should_autocreate_reflog
+ * returns non-zero). Otherwise, create it regardless of the reference
+ * name. If the logfile already existed or was created, return 0 and
+ * set *logfd to the file descriptor opened for appending to the file.
+ * If no logfile exists and we decided not to create one, return 0 and
+ * set *logfd to -1. On failure, fill in *err, set *logfd to -1, and
+ * return -1.
+ */
+static int log_ref_setup(struct files_ref_store *refs,
+			 const char *refname, int force_create,
+			 int *logfd, struct strbuf *err)
+{
+	struct strbuf logfile_sb = STRBUF_INIT;
+	char *logfile;
+
+	files_reflog_path(refs, &logfile_sb, refname);
+	logfile = strbuf_detach(&logfile_sb, NULL);
+
+	if (force_create || should_autocreate_reflog(refname)) {
+		if (raceproof_create_file(logfile, open_or_create_logfile, logfd)) {
+			if (errno == ENOENT)
+				strbuf_addf(err, "unable to create directory for '%s': "
+					    "%s", logfile, strerror(errno));
+			else if (errno == EISDIR)
+				strbuf_addf(err, "there are still logs under '%s'",
+					    logfile);
+			else
+				strbuf_addf(err, "unable to append to '%s': %s",
+					    logfile, strerror(errno));
+
+			goto error;
+		}
+	} else {
+		*logfd = open(logfile, O_APPEND | O_WRONLY, 0666);
+		if (*logfd < 0) {
+			if (errno == ENOENT || errno == EISDIR) {
+				/*
+				 * The logfile doesn't already exist,
+				 * but that is not an error; it only
+				 * means that we won't write log
+				 * entries to it.
+				 */
+				;
+			} else {
+				strbuf_addf(err, "unable to append to '%s': %s",
+					    logfile, strerror(errno));
+				goto error;
+			}
+		}
+	}
+
+	if (*logfd >= 0)
+		adjust_shared_perm(logfile);
+
+	free(logfile);
+	return 0;
+
+error:
+	free(logfile);
+	return -1;
+}
+
+static int files_create_reflog(struct ref_store *ref_store,
+			       const char *refname, int force_create,
+			       struct strbuf *err)
+{
+	struct files_ref_store *refs =
+		files_downcast(ref_store, REF_STORE_WRITE, "create_reflog");
+	int fd;
+
+	if (log_ref_setup(refs, refname, force_create, &fd, err))
+		return -1;
+
+	if (fd >= 0)
+		close(fd);
+
+	return 0;
+}
+
+static int log_ref_write_fd(int fd, const struct object_id *old_oid,
+			    const struct object_id *new_oid,
+			    const char *committer, const char *msg)
+{
+	struct strbuf sb = STRBUF_INIT;
+	int ret = 0;
+
+	strbuf_addf(&sb, "%s %s %s", oid_to_hex(old_oid), oid_to_hex(new_oid), committer);
+	if (msg && *msg)
+		copy_reflog_msg(&sb, msg);
+	strbuf_addch(&sb, '\n');
+	if (write_in_full(fd, sb.buf, sb.len) < 0)
+		ret = -1;
+	strbuf_release(&sb);
+	return ret;
+}
+
+static int files_log_ref_write(struct files_ref_store *refs,
+			       const char *refname, const struct object_id *old_oid,
+			       const struct object_id *new_oid, const char *msg,
+			       int flags, struct strbuf *err)
+{
+	int logfd, result;
+
+	if (log_all_ref_updates == LOG_REFS_UNSET)
+		log_all_ref_updates = is_bare_repository() ? LOG_REFS_NONE : LOG_REFS_NORMAL;
+
+	result = log_ref_setup(refs, refname,
+			       flags & REF_FORCE_CREATE_REFLOG,
+			       &logfd, err);
+
+	if (result)
+		return result;
+
+	if (logfd < 0)
+		return 0;
+	result = log_ref_write_fd(logfd, old_oid, new_oid,
+				  git_committer_info(0), msg);
+	if (result) {
+		struct strbuf sb = STRBUF_INIT;
+		int save_errno = errno;
+
+		files_reflog_path(refs, &sb, refname);
+		strbuf_addf(err, "unable to append to '%s': %s",
+			    sb.buf, strerror(save_errno));
+		strbuf_release(&sb);
+		close(logfd);
+		return -1;
+	}
+	if (close(logfd)) {
+		struct strbuf sb = STRBUF_INIT;
+		int save_errno = errno;
+
+		files_reflog_path(refs, &sb, refname);
+		strbuf_addf(err, "unable to append to '%s': %s",
+			    sb.buf, strerror(save_errno));
+		strbuf_release(&sb);
+		return -1;
+	}
+	return 0;
+}
+
+/*
+ * Write oid into the open lockfile, then close the lockfile. On
+ * errors, rollback the lockfile, fill in *err and return -1.
+ */
+static int write_ref_to_lockfile(struct ref_lock *lock,
+				 const struct object_id *oid, struct strbuf *err)
+{
+	static char term = '\n';
+	struct object *o;
+	int fd;
+
+	o = parse_object(the_repository, oid);
+	if (!o) {
+		strbuf_addf(err,
+			    "trying to write ref '%s' with nonexistent object %s",
+			    lock->ref_name, oid_to_hex(oid));
+		unlock_ref(lock);
+		return -1;
+	}
+	if (o->type != OBJ_COMMIT && is_branch(lock->ref_name)) {
+		strbuf_addf(err,
+			    "trying to write non-commit object %s to branch '%s'",
+			    oid_to_hex(oid), lock->ref_name);
+		unlock_ref(lock);
+		return -1;
+	}
+	fd = get_lock_file_fd(&lock->lk);
+	if (write_in_full(fd, oid_to_hex(oid), the_hash_algo->hexsz) < 0 ||
+	    write_in_full(fd, &term, 1) < 0 ||
+	    close_ref_gently(lock) < 0) {
+		strbuf_addf(err,
+			    "couldn't write '%s'", get_lock_file_path(&lock->lk));
+		unlock_ref(lock);
+		return -1;
+	}
+	return 0;
+}
+
+/*
+ * Commit a change to a loose reference that has already been written
+ * to the loose reference lockfile. Also update the reflogs if
+ * necessary, using the specified lockmsg (which can be NULL).
+ */
+static int commit_ref_update(struct files_ref_store *refs,
+			     struct ref_lock *lock,
+			     const struct object_id *oid, const char *logmsg,
+			     struct strbuf *err)
+{
+	files_assert_main_repository(refs, "commit_ref_update");
+
+	clear_loose_ref_cache(refs);
+	if (files_log_ref_write(refs, lock->ref_name,
+				&lock->old_oid, oid,
+				logmsg, 0, err)) {
+		char *old_msg = strbuf_detach(err, NULL);
+		strbuf_addf(err, "cannot update the ref '%s': %s",
+			    lock->ref_name, old_msg);
+		free(old_msg);
+		unlock_ref(lock);
+		return -1;
+	}
+
+	if (strcmp(lock->ref_name, "HEAD") != 0) {
+		/*
+		 * Special hack: If a branch is updated directly and HEAD
+		 * points to it (may happen on the remote side of a push
+		 * for example) then logically the HEAD reflog should be
+		 * updated too.
+		 * A generic solution implies reverse symref information,
+		 * but finding all symrefs pointing to the given branch
+		 * would be rather costly for this rare event (the direct
+		 * update of a branch) to be worth it.  So let's cheat and
+		 * check with HEAD only which should cover 99% of all usage
+		 * scenarios (even 100% of the default ones).
+		 */
+		int head_flag;
+		const char *head_ref;
+
+		head_ref = refs_resolve_ref_unsafe(&refs->base, "HEAD",
+						   RESOLVE_REF_READING,
+						   NULL, &head_flag);
+		if (head_ref && (head_flag & REF_ISSYMREF) &&
+		    !strcmp(head_ref, lock->ref_name)) {
+			struct strbuf log_err = STRBUF_INIT;
+			if (files_log_ref_write(refs, "HEAD",
+						&lock->old_oid, oid,
+						logmsg, 0, &log_err)) {
+				error("%s", log_err.buf);
+				strbuf_release(&log_err);
+			}
+		}
+	}
+
+	if (commit_ref(lock)) {
+		strbuf_addf(err, "couldn't set '%s'", lock->ref_name);
+		unlock_ref(lock);
+		return -1;
+	}
+
+	unlock_ref(lock);
+	return 0;
+}
+
+static int create_ref_symlink(struct ref_lock *lock, const char *target)
+{
+	int ret = -1;
+#ifndef NO_SYMLINK_HEAD
+	char *ref_path = get_locked_file_path(&lock->lk);
+	unlink(ref_path);
+	ret = symlink(target, ref_path);
+	free(ref_path);
+
+	if (ret)
+		fprintf(stderr, "no symlink - falling back to symbolic ref\n");
+#endif
+	return ret;
+}
+
+static void update_symref_reflog(struct files_ref_store *refs,
+				 struct ref_lock *lock, const char *refname,
+				 const char *target, const char *logmsg)
+{
+	struct strbuf err = STRBUF_INIT;
+	struct object_id new_oid;
+	if (logmsg &&
+	    !refs_read_ref_full(&refs->base, target,
+				RESOLVE_REF_READING, &new_oid, NULL) &&
+	    files_log_ref_write(refs, refname, &lock->old_oid,
+				&new_oid, logmsg, 0, &err)) {
+		error("%s", err.buf);
+		strbuf_release(&err);
+	}
+}
+
+static int create_symref_locked(struct files_ref_store *refs,
+				struct ref_lock *lock, const char *refname,
+				const char *target, const char *logmsg)
+{
+	if (prefer_symlink_refs && !create_ref_symlink(lock, target)) {
+		update_symref_reflog(refs, lock, refname, target, logmsg);
+		return 0;
+	}
+
+	if (!fdopen_lock_file(&lock->lk, "w"))
+		return error("unable to fdopen %s: %s",
+			     lock->lk.tempfile->filename.buf, strerror(errno));
+
+	update_symref_reflog(refs, lock, refname, target, logmsg);
+
+	/* no error check; commit_ref will check ferror */
+	fprintf(lock->lk.tempfile->fp, "ref: %s\n", target);
+	if (commit_ref(lock) < 0)
+		return error("unable to write symref for %s: %s", refname,
+			     strerror(errno));
+	return 0;
+}
+
+static int files_create_symref(struct ref_store *ref_store,
+			       const char *refname, const char *target,
+			       const char *logmsg)
+{
+	struct files_ref_store *refs =
+		files_downcast(ref_store, REF_STORE_WRITE, "create_symref");
+	struct strbuf err = STRBUF_INIT;
+	struct ref_lock *lock;
+	int ret;
+
+	lock = lock_ref_oid_basic(refs, refname, NULL,
+				  NULL, NULL, REF_NO_DEREF, NULL,
+				  &err);
+	if (!lock) {
+		error("%s", err.buf);
+		strbuf_release(&err);
+		return -1;
+	}
+
+	ret = create_symref_locked(refs, lock, refname, target, logmsg);
+	unlock_ref(lock);
+	return ret;
+}
+
+static int files_reflog_exists(struct ref_store *ref_store,
+			       const char *refname)
+{
+	struct files_ref_store *refs =
+		files_downcast(ref_store, REF_STORE_READ, "reflog_exists");
+	struct strbuf sb = STRBUF_INIT;
+	struct stat st;
+	int ret;
+
+	files_reflog_path(refs, &sb, refname);
+	ret = !lstat(sb.buf, &st) && S_ISREG(st.st_mode);
+	strbuf_release(&sb);
+	return ret;
+}
+
+static int files_delete_reflog(struct ref_store *ref_store,
+			       const char *refname)
+{
+	struct files_ref_store *refs =
+		files_downcast(ref_store, REF_STORE_WRITE, "delete_reflog");
+	struct strbuf sb = STRBUF_INIT;
+	int ret;
+
+	files_reflog_path(refs, &sb, refname);
+	ret = remove_path(sb.buf);
+	strbuf_release(&sb);
+	return ret;
+}
+
+static int show_one_reflog_ent(struct strbuf *sb, each_reflog_ent_fn fn, void *cb_data)
+{
+	struct object_id ooid, noid;
+	char *email_end, *message;
+	timestamp_t timestamp;
+	int tz;
+	const char *p = sb->buf;
+
+	/* old SP new SP name <email> SP time TAB msg LF */
+	if (!sb->len || sb->buf[sb->len - 1] != '\n' ||
+	    parse_oid_hex(p, &ooid, &p) || *p++ != ' ' ||
+	    parse_oid_hex(p, &noid, &p) || *p++ != ' ' ||
+	    !(email_end = strchr(p, '>')) ||
+	    email_end[1] != ' ' ||
+	    !(timestamp = parse_timestamp(email_end + 2, &message, 10)) ||
+	    !message || message[0] != ' ' ||
+	    (message[1] != '+' && message[1] != '-') ||
+	    !isdigit(message[2]) || !isdigit(message[3]) ||
+	    !isdigit(message[4]) || !isdigit(message[5]))
+		return 0; /* corrupt? */
+	email_end[1] = '\0';
+	tz = strtol(message + 1, NULL, 10);
+	if (message[6] != '\t')
+		message += 6;
+	else
+		message += 7;
+	return fn(&ooid, &noid, p, timestamp, tz, message, cb_data);
+}
+
+static char *find_beginning_of_line(char *bob, char *scan)
+{
+	while (bob < scan && *(--scan) != '\n')
+		; /* keep scanning backwards */
+	/*
+	 * Return either beginning of the buffer, or LF at the end of
+	 * the previous line.
+	 */
+	return scan;
+}
+
+static int files_for_each_reflog_ent_reverse(struct ref_store *ref_store,
+					     const char *refname,
+					     each_reflog_ent_fn fn,
+					     void *cb_data)
+{
+	struct files_ref_store *refs =
+		files_downcast(ref_store, REF_STORE_READ,
+			       "for_each_reflog_ent_reverse");
+	struct strbuf sb = STRBUF_INIT;
+	FILE *logfp;
+	long pos;
+	int ret = 0, at_tail = 1;
+
+	files_reflog_path(refs, &sb, refname);
+	logfp = fopen(sb.buf, "r");
+	strbuf_release(&sb);
+	if (!logfp)
+		return -1;
+
+	/* Jump to the end */
+	if (fseek(logfp, 0, SEEK_END) < 0)
+		ret = error("cannot seek back reflog for %s: %s",
+			    refname, strerror(errno));
+	pos = ftell(logfp);
+	while (!ret && 0 < pos) {
+		int cnt;
+		size_t nread;
+		char buf[BUFSIZ];
+		char *endp, *scanp;
+
+		/* Fill next block from the end */
+		cnt = (sizeof(buf) < pos) ? sizeof(buf) : pos;
+		if (fseek(logfp, pos - cnt, SEEK_SET)) {
+			ret = error("cannot seek back reflog for %s: %s",
+				    refname, strerror(errno));
+			break;
+		}
+		nread = fread(buf, cnt, 1, logfp);
+		if (nread != 1) {
+			ret = error("cannot read %d bytes from reflog for %s: %s",
+				    cnt, refname, strerror(errno));
+			break;
+		}
+		pos -= cnt;
+
+		scanp = endp = buf + cnt;
+		if (at_tail && scanp[-1] == '\n')
+			/* Looking at the final LF at the end of the file */
+			scanp--;
+		at_tail = 0;
+
+		while (buf < scanp) {
+			/*
+			 * terminating LF of the previous line, or the beginning
+			 * of the buffer.
+			 */
+			char *bp;
+
+			bp = find_beginning_of_line(buf, scanp);
+
+			if (*bp == '\n') {
+				/*
+				 * The newline is the end of the previous line,
+				 * so we know we have complete line starting
+				 * at (bp + 1). Prefix it onto any prior data
+				 * we collected for the line and process it.
+				 */
+				strbuf_splice(&sb, 0, 0, bp + 1, endp - (bp + 1));
+				scanp = bp;
+				endp = bp + 1;
+				ret = show_one_reflog_ent(&sb, fn, cb_data);
+				strbuf_reset(&sb);
+				if (ret)
+					break;
+			} else if (!pos) {
+				/*
+				 * We are at the start of the buffer, and the
+				 * start of the file; there is no previous
+				 * line, and we have everything for this one.
+				 * Process it, and we can end the loop.
+				 */
+				strbuf_splice(&sb, 0, 0, buf, endp - buf);
+				ret = show_one_reflog_ent(&sb, fn, cb_data);
+				strbuf_reset(&sb);
+				break;
+			}
+
+			if (bp == buf) {
+				/*
+				 * We are at the start of the buffer, and there
+				 * is more file to read backwards. Which means
+				 * we are in the middle of a line. Note that we
+				 * may get here even if *bp was a newline; that
+				 * just means we are at the exact end of the
+				 * previous line, rather than some spot in the
+				 * middle.
+				 *
+				 * Save away what we have to be combined with
+				 * the data from the next read.
+				 */
+				strbuf_splice(&sb, 0, 0, buf, endp - buf);
+				break;
+			}
+		}
+
+	}
+	if (!ret && sb.len)
+		BUG("reverse reflog parser had leftover data");
+
+	fclose(logfp);
+	strbuf_release(&sb);
+	return ret;
+}
+
+static int files_for_each_reflog_ent(struct ref_store *ref_store,
+				     const char *refname,
+				     each_reflog_ent_fn fn, void *cb_data)
+{
+	struct files_ref_store *refs =
+		files_downcast(ref_store, REF_STORE_READ,
+			       "for_each_reflog_ent");
+	FILE *logfp;
+	struct strbuf sb = STRBUF_INIT;
+	int ret = 0;
+
+	files_reflog_path(refs, &sb, refname);
+	logfp = fopen(sb.buf, "r");
+	strbuf_release(&sb);
+	if (!logfp)
+		return -1;
+
+	while (!ret && !strbuf_getwholeline(&sb, logfp, '\n'))
+		ret = show_one_reflog_ent(&sb, fn, cb_data);
+	fclose(logfp);
+	strbuf_release(&sb);
+	return ret;
+}
+
+struct files_reflog_iterator {
+	struct ref_iterator base;
+
+	struct ref_store *ref_store;
+	struct dir_iterator *dir_iterator;
+	struct object_id oid;
+};
+
+static int files_reflog_iterator_advance(struct ref_iterator *ref_iterator)
+{
+	struct files_reflog_iterator *iter =
+		(struct files_reflog_iterator *)ref_iterator;
+	struct dir_iterator *diter = iter->dir_iterator;
+	int ok;
+
+	while ((ok = dir_iterator_advance(diter)) == ITER_OK) {
+		int flags;
+
+		if (!S_ISREG(diter->st.st_mode))
+			continue;
+		if (diter->basename[0] == '.')
+			continue;
+		if (ends_with(diter->basename, ".lock"))
+			continue;
+
+		if (refs_read_ref_full(iter->ref_store,
+				       diter->relative_path, 0,
+				       &iter->oid, &flags)) {
+			error("bad ref for %s", diter->path.buf);
+			continue;
+		}
+
+		iter->base.refname = diter->relative_path;
+		iter->base.oid = &iter->oid;
+		iter->base.flags = flags;
+		return ITER_OK;
+	}
+
+	iter->dir_iterator = NULL;
+	if (ref_iterator_abort(ref_iterator) == ITER_ERROR)
+		ok = ITER_ERROR;
+	return ok;
+}
+
+static int files_reflog_iterator_peel(struct ref_iterator *ref_iterator,
+				   struct object_id *peeled)
+{
+	BUG("ref_iterator_peel() called for reflog_iterator");
+}
+
+static int files_reflog_iterator_abort(struct ref_iterator *ref_iterator)
+{
+	struct files_reflog_iterator *iter =
+		(struct files_reflog_iterator *)ref_iterator;
+	int ok = ITER_DONE;
+
+	if (iter->dir_iterator)
+		ok = dir_iterator_abort(iter->dir_iterator);
+
+	base_ref_iterator_free(ref_iterator);
+	return ok;
+}
+
+static struct ref_iterator_vtable files_reflog_iterator_vtable = {
+	files_reflog_iterator_advance,
+	files_reflog_iterator_peel,
+	files_reflog_iterator_abort
+};
+
+static struct ref_iterator *reflog_iterator_begin(struct ref_store *ref_store,
+						  const char *gitdir)
+{
+	struct dir_iterator *diter;
+	struct files_reflog_iterator *iter;
+	struct ref_iterator *ref_iterator;
+	struct strbuf sb = STRBUF_INIT;
+
+	strbuf_addf(&sb, "%s/logs", gitdir);
+
+	diter = dir_iterator_begin(sb.buf, 0);
+	if (!diter) {
+		strbuf_release(&sb);
+		return empty_ref_iterator_begin();
+	}
+
+	iter = xcalloc(1, sizeof(*iter));
+	ref_iterator = &iter->base;
+
+	base_ref_iterator_init(ref_iterator, &files_reflog_iterator_vtable, 0);
+	iter->dir_iterator = diter;
+	iter->ref_store = ref_store;
+	strbuf_release(&sb);
+
+	return ref_iterator;
+}
+
+static enum iterator_selection reflog_iterator_select(
+	struct ref_iterator *iter_worktree,
+	struct ref_iterator *iter_common,
+	void *cb_data)
+{
+	if (iter_worktree) {
+		/*
+		 * We're a bit loose here. We probably should ignore
+		 * common refs if they are accidentally added as
+		 * per-worktree refs.
+		 */
+		return ITER_SELECT_0;
+	} else if (iter_common) {
+		if (ref_type(iter_common->refname) == REF_TYPE_NORMAL)
+			return ITER_SELECT_1;
+
+		/*
+		 * The main ref store may contain main worktree's
+		 * per-worktree refs, which should be ignored
+		 */
+		return ITER_SKIP_1;
+	} else
+		return ITER_DONE;
+}
+
+static struct ref_iterator *files_reflog_iterator_begin(struct ref_store *ref_store)
+{
+	struct files_ref_store *refs =
+		files_downcast(ref_store, REF_STORE_READ,
+			       "reflog_iterator_begin");
+
+	if (!strcmp(refs->gitdir, refs->gitcommondir)) {
+		return reflog_iterator_begin(ref_store, refs->gitcommondir);
+	} else {
+		return merge_ref_iterator_begin(
+			0,
+			reflog_iterator_begin(ref_store, refs->gitdir),
+			reflog_iterator_begin(ref_store, refs->gitcommondir),
+			reflog_iterator_select, refs);
+	}
+}
+
+/*
+ * If update is a direct update of head_ref (the reference pointed to
+ * by HEAD), then add an extra REF_LOG_ONLY update for HEAD.
+ */
+static int split_head_update(struct ref_update *update,
+			     struct ref_transaction *transaction,
+			     const char *head_ref,
+			     struct string_list *affected_refnames,
+			     struct strbuf *err)
+{
+	struct string_list_item *item;
+	struct ref_update *new_update;
+
+	if ((update->flags & REF_LOG_ONLY) ||
+	    (update->flags & REF_IS_PRUNING) ||
+	    (update->flags & REF_UPDATE_VIA_HEAD))
+		return 0;
+
+	if (strcmp(update->refname, head_ref))
+		return 0;
+
+	/*
+	 * First make sure that HEAD is not already in the
+	 * transaction. This check is O(lg N) in the transaction
+	 * size, but it happens at most once per transaction.
+	 */
+	if (string_list_has_string(affected_refnames, "HEAD")) {
+		/* An entry already existed */
+		strbuf_addf(err,
+			    "multiple updates for 'HEAD' (including one "
+			    "via its referent '%s') are not allowed",
+			    update->refname);
+		return TRANSACTION_NAME_CONFLICT;
+	}
+
+	new_update = ref_transaction_add_update(
+			transaction, "HEAD",
+			update->flags | REF_LOG_ONLY | REF_NO_DEREF,
+			&update->new_oid, &update->old_oid,
+			update->msg);
+
+	/*
+	 * Add "HEAD". This insertion is O(N) in the transaction
+	 * size, but it happens at most once per transaction.
+	 * Add new_update->refname instead of a literal "HEAD".
+	 */
+	if (strcmp(new_update->refname, "HEAD"))
+		BUG("%s unexpectedly not 'HEAD'", new_update->refname);
+	item = string_list_insert(affected_refnames, new_update->refname);
+	item->util = new_update;
+
+	return 0;
+}
+
+/*
+ * update is for a symref that points at referent and doesn't have
+ * REF_NO_DEREF set. Split it into two updates:
+ * - The original update, but with REF_LOG_ONLY and REF_NO_DEREF set
+ * - A new, separate update for the referent reference
+ * Note that the new update will itself be subject to splitting when
+ * the iteration gets to it.
+ */
+static int split_symref_update(struct ref_update *update,
+			       const char *referent,
+			       struct ref_transaction *transaction,
+			       struct string_list *affected_refnames,
+			       struct strbuf *err)
+{
+	struct string_list_item *item;
+	struct ref_update *new_update;
+	unsigned int new_flags;
+
+	/*
+	 * First make sure that referent is not already in the
+	 * transaction. This check is O(lg N) in the transaction
+	 * size, but it happens at most once per symref in a
+	 * transaction.
+	 */
+	if (string_list_has_string(affected_refnames, referent)) {
+		/* An entry already exists */
+		strbuf_addf(err,
+			    "multiple updates for '%s' (including one "
+			    "via symref '%s') are not allowed",
+			    referent, update->refname);
+		return TRANSACTION_NAME_CONFLICT;
+	}
+
+	new_flags = update->flags;
+	if (!strcmp(update->refname, "HEAD")) {
+		/*
+		 * Record that the new update came via HEAD, so that
+		 * when we process it, split_head_update() doesn't try
+		 * to add another reflog update for HEAD. Note that
+		 * this bit will be propagated if the new_update
+		 * itself needs to be split.
+		 */
+		new_flags |= REF_UPDATE_VIA_HEAD;
+	}
+
+	new_update = ref_transaction_add_update(
+			transaction, referent, new_flags,
+			&update->new_oid, &update->old_oid,
+			update->msg);
+
+	new_update->parent_update = update;
+
+	/*
+	 * Change the symbolic ref update to log only. Also, it
+	 * doesn't need to check its old OID value, as that will be
+	 * done when new_update is processed.
+	 */
+	update->flags |= REF_LOG_ONLY | REF_NO_DEREF;
+	update->flags &= ~REF_HAVE_OLD;
+
+	/*
+	 * Add the referent. This insertion is O(N) in the transaction
+	 * size, but it happens at most once per symref in a
+	 * transaction. Make sure to add new_update->refname, which will
+	 * be valid as long as affected_refnames is in use, and NOT
+	 * referent, which might soon be freed by our caller.
+	 */
+	item = string_list_insert(affected_refnames, new_update->refname);
+	if (item->util)
+		BUG("%s unexpectedly found in affected_refnames",
+		    new_update->refname);
+	item->util = new_update;
+
+	return 0;
+}
+
+/*
+ * Return the refname under which update was originally requested.
+ */
+static const char *original_update_refname(struct ref_update *update)
+{
+	while (update->parent_update)
+		update = update->parent_update;
+
+	return update->refname;
+}
+
+/*
+ * Check whether the REF_HAVE_OLD and old_oid values stored in update
+ * are consistent with oid, which is the reference's current value. If
+ * everything is OK, return 0; otherwise, write an error message to
+ * err and return -1.
+ */
+static int check_old_oid(struct ref_update *update, struct object_id *oid,
+			 struct strbuf *err)
+{
+	if (!(update->flags & REF_HAVE_OLD) ||
+		   oideq(oid, &update->old_oid))
+		return 0;
+
+	if (is_null_oid(&update->old_oid))
+		strbuf_addf(err, "cannot lock ref '%s': "
+			    "reference already exists",
+			    original_update_refname(update));
+	else if (is_null_oid(oid))
+		strbuf_addf(err, "cannot lock ref '%s': "
+			    "reference is missing but expected %s",
+			    original_update_refname(update),
+			    oid_to_hex(&update->old_oid));
+	else
+		strbuf_addf(err, "cannot lock ref '%s': "
+			    "is at %s but expected %s",
+			    original_update_refname(update),
+			    oid_to_hex(oid),
+			    oid_to_hex(&update->old_oid));
+
+	return -1;
+}
+
+/*
+ * Prepare for carrying out update:
+ * - Lock the reference referred to by update.
+ * - Read the reference under lock.
+ * - Check that its old OID value (if specified) is correct, and in
+ *   any case record it in update->lock->old_oid for later use when
+ *   writing the reflog.
+ * - If it is a symref update without REF_NO_DEREF, split it up into a
+ *   REF_LOG_ONLY update of the symref and add a separate update for
+ *   the referent to transaction.
+ * - If it is an update of head_ref, add a corresponding REF_LOG_ONLY
+ *   update of HEAD.
+ */
+static int lock_ref_for_update(struct files_ref_store *refs,
+			       struct ref_update *update,
+			       struct ref_transaction *transaction,
+			       const char *head_ref,
+			       struct string_list *affected_refnames,
+			       struct strbuf *err)
+{
+	struct strbuf referent = STRBUF_INIT;
+	int mustexist = (update->flags & REF_HAVE_OLD) &&
+		!is_null_oid(&update->old_oid);
+	int ret = 0;
+	struct ref_lock *lock;
+
+	files_assert_main_repository(refs, "lock_ref_for_update");
+
+	if ((update->flags & REF_HAVE_NEW) && is_null_oid(&update->new_oid))
+		update->flags |= REF_DELETING;
+
+	if (head_ref) {
+		ret = split_head_update(update, transaction, head_ref,
+					affected_refnames, err);
+		if (ret)
+			goto out;
+	}
+
+	ret = lock_raw_ref(refs, update->refname, mustexist,
+			   affected_refnames, NULL,
+			   &lock, &referent,
+			   &update->type, err);
+	if (ret) {
+		char *reason;
+
+		reason = strbuf_detach(err, NULL);
+		strbuf_addf(err, "cannot lock ref '%s': %s",
+			    original_update_refname(update), reason);
+		free(reason);
+		goto out;
+	}
+
+	update->backend_data = lock;
+
+	if (update->type & REF_ISSYMREF) {
+		if (update->flags & REF_NO_DEREF) {
+			/*
+			 * We won't be reading the referent as part of
+			 * the transaction, so we have to read it here
+			 * to record and possibly check old_oid:
+			 */
+			if (refs_read_ref_full(&refs->base,
+					       referent.buf, 0,
+					       &lock->old_oid, NULL)) {
+				if (update->flags & REF_HAVE_OLD) {
+					strbuf_addf(err, "cannot lock ref '%s': "
+						    "error reading reference",
+						    original_update_refname(update));
+					ret = TRANSACTION_GENERIC_ERROR;
+					goto out;
+				}
+			} else if (check_old_oid(update, &lock->old_oid, err)) {
+				ret = TRANSACTION_GENERIC_ERROR;
+				goto out;
+			}
+		} else {
+			/*
+			 * Create a new update for the reference this
+			 * symref is pointing at. Also, we will record
+			 * and verify old_oid for this update as part
+			 * of processing the split-off update, so we
+			 * don't have to do it here.
+			 */
+			ret = split_symref_update(update,
+						  referent.buf, transaction,
+						  affected_refnames, err);
+			if (ret)
+				goto out;
+		}
+	} else {
+		struct ref_update *parent_update;
+
+		if (check_old_oid(update, &lock->old_oid, err)) {
+			ret = TRANSACTION_GENERIC_ERROR;
+			goto out;
+		}
+
+		/*
+		 * If this update is happening indirectly because of a
+		 * symref update, record the old OID in the parent
+		 * update:
+		 */
+		for (parent_update = update->parent_update;
+		     parent_update;
+		     parent_update = parent_update->parent_update) {
+			struct ref_lock *parent_lock = parent_update->backend_data;
+			oidcpy(&parent_lock->old_oid, &lock->old_oid);
+		}
+	}
+
+	if ((update->flags & REF_HAVE_NEW) &&
+	    !(update->flags & REF_DELETING) &&
+	    !(update->flags & REF_LOG_ONLY)) {
+		if (!(update->type & REF_ISSYMREF) &&
+		    oideq(&lock->old_oid, &update->new_oid)) {
+			/*
+			 * The reference already has the desired
+			 * value, so we don't need to write it.
+			 */
+		} else if (write_ref_to_lockfile(lock, &update->new_oid,
+						 err)) {
+			char *write_err = strbuf_detach(err, NULL);
+
+			/*
+			 * The lock was freed upon failure of
+			 * write_ref_to_lockfile():
+			 */
+			update->backend_data = NULL;
+			strbuf_addf(err,
+				    "cannot update ref '%s': %s",
+				    update->refname, write_err);
+			free(write_err);
+			ret = TRANSACTION_GENERIC_ERROR;
+			goto out;
+		} else {
+			update->flags |= REF_NEEDS_COMMIT;
+		}
+	}
+	if (!(update->flags & REF_NEEDS_COMMIT)) {
+		/*
+		 * We didn't call write_ref_to_lockfile(), so
+		 * the lockfile is still open. Close it to
+		 * free up the file descriptor:
+		 */
+		if (close_ref_gently(lock)) {
+			strbuf_addf(err, "couldn't close '%s.lock'",
+				    update->refname);
+			ret = TRANSACTION_GENERIC_ERROR;
+			goto out;
+		}
+	}
+
+out:
+	strbuf_release(&referent);
+	return ret;
+}
+
+struct files_transaction_backend_data {
+	struct ref_transaction *packed_transaction;
+	int packed_refs_locked;
+};
+
+/*
+ * Unlock any references in `transaction` that are still locked, and
+ * mark the transaction closed.
+ */
+static void files_transaction_cleanup(struct files_ref_store *refs,
+				      struct ref_transaction *transaction)
+{
+	size_t i;
+	struct files_transaction_backend_data *backend_data =
+		transaction->backend_data;
+	struct strbuf err = STRBUF_INIT;
+
+	for (i = 0; i < transaction->nr; i++) {
+		struct ref_update *update = transaction->updates[i];
+		struct ref_lock *lock = update->backend_data;
+
+		if (lock) {
+			unlock_ref(lock);
+			update->backend_data = NULL;
+		}
+	}
+
+	if (backend_data->packed_transaction &&
+	    ref_transaction_abort(backend_data->packed_transaction, &err)) {
+		error("error aborting transaction: %s", err.buf);
+		strbuf_release(&err);
+	}
+
+	if (backend_data->packed_refs_locked)
+		packed_refs_unlock(refs->packed_ref_store);
+
+	free(backend_data);
+
+	transaction->state = REF_TRANSACTION_CLOSED;
+}
+
+static int files_transaction_prepare(struct ref_store *ref_store,
+				     struct ref_transaction *transaction,
+				     struct strbuf *err)
+{
+	struct files_ref_store *refs =
+		files_downcast(ref_store, REF_STORE_WRITE,
+			       "ref_transaction_prepare");
+	size_t i;
+	int ret = 0;
+	struct string_list affected_refnames = STRING_LIST_INIT_NODUP;
+	char *head_ref = NULL;
+	int head_type;
+	struct files_transaction_backend_data *backend_data;
+	struct ref_transaction *packed_transaction = NULL;
+
+	assert(err);
+
+	if (!transaction->nr)
+		goto cleanup;
+
+	backend_data = xcalloc(1, sizeof(*backend_data));
+	transaction->backend_data = backend_data;
+
+	/*
+	 * Fail if a refname appears more than once in the
+	 * transaction. (If we end up splitting up any updates using
+	 * split_symref_update() or split_head_update(), those
+	 * functions will check that the new updates don't have the
+	 * same refname as any existing ones.) Also fail if any of the
+	 * updates use REF_IS_PRUNING without REF_NO_DEREF.
+	 */
+	for (i = 0; i < transaction->nr; i++) {
+		struct ref_update *update = transaction->updates[i];
+		struct string_list_item *item =
+			string_list_append(&affected_refnames, update->refname);
+
+		if ((update->flags & REF_IS_PRUNING) &&
+		    !(update->flags & REF_NO_DEREF))
+			BUG("REF_IS_PRUNING set without REF_NO_DEREF");
+
+		/*
+		 * We store a pointer to update in item->util, but at
+		 * the moment we never use the value of this field
+		 * except to check whether it is non-NULL.
+		 */
+		item->util = update;
+	}
+	string_list_sort(&affected_refnames);
+	if (ref_update_reject_duplicates(&affected_refnames, err)) {
+		ret = TRANSACTION_GENERIC_ERROR;
+		goto cleanup;
+	}
+
+	/*
+	 * Special hack: If a branch is updated directly and HEAD
+	 * points to it (may happen on the remote side of a push
+	 * for example) then logically the HEAD reflog should be
+	 * updated too.
+	 *
+	 * A generic solution would require reverse symref lookups,
+	 * but finding all symrefs pointing to a given branch would be
+	 * rather costly for this rare event (the direct update of a
+	 * branch) to be worth it. So let's cheat and check with HEAD
+	 * only, which should cover 99% of all usage scenarios (even
+	 * 100% of the default ones).
+	 *
+	 * So if HEAD is a symbolic reference, then record the name of
+	 * the reference that it points to. If we see an update of
+	 * head_ref within the transaction, then split_head_update()
+	 * arranges for the reflog of HEAD to be updated, too.
+	 */
+	head_ref = refs_resolve_refdup(ref_store, "HEAD",
+				       RESOLVE_REF_NO_RECURSE,
+				       NULL, &head_type);
+
+	if (head_ref && !(head_type & REF_ISSYMREF)) {
+		FREE_AND_NULL(head_ref);
+	}
+
+	/*
+	 * Acquire all locks, verify old values if provided, check
+	 * that new values are valid, and write new values to the
+	 * lockfiles, ready to be activated. Only keep one lockfile
+	 * open at a time to avoid running out of file descriptors.
+	 * Note that lock_ref_for_update() might append more updates
+	 * to the transaction.
+	 */
+	for (i = 0; i < transaction->nr; i++) {
+		struct ref_update *update = transaction->updates[i];
+
+		ret = lock_ref_for_update(refs, update, transaction,
+					  head_ref, &affected_refnames, err);
+		if (ret)
+			goto cleanup;
+
+		if (update->flags & REF_DELETING &&
+		    !(update->flags & REF_LOG_ONLY) &&
+		    !(update->flags & REF_IS_PRUNING)) {
+			/*
+			 * This reference has to be deleted from
+			 * packed-refs if it exists there.
+			 */
+			if (!packed_transaction) {
+				packed_transaction = ref_store_transaction_begin(
+						refs->packed_ref_store, err);
+				if (!packed_transaction) {
+					ret = TRANSACTION_GENERIC_ERROR;
+					goto cleanup;
+				}
+
+				backend_data->packed_transaction =
+					packed_transaction;
+			}
+
+			ref_transaction_add_update(
+					packed_transaction, update->refname,
+					REF_HAVE_NEW | REF_NO_DEREF,
+					&update->new_oid, NULL,
+					NULL);
+		}
+	}
+
+	if (packed_transaction) {
+		if (packed_refs_lock(refs->packed_ref_store, 0, err)) {
+			ret = TRANSACTION_GENERIC_ERROR;
+			goto cleanup;
+		}
+		backend_data->packed_refs_locked = 1;
+
+		if (is_packed_transaction_needed(refs->packed_ref_store,
+						 packed_transaction)) {
+			ret = ref_transaction_prepare(packed_transaction, err);
+			/*
+			 * A failure during the prepare step will abort
+			 * itself, but not free. Do that now, and disconnect
+			 * from the files_transaction so it does not try to
+			 * abort us when we hit the cleanup code below.
+			 */
+			if (ret) {
+				ref_transaction_free(packed_transaction);
+				backend_data->packed_transaction = NULL;
+			}
+		} else {
+			/*
+			 * We can skip rewriting the `packed-refs`
+			 * file. But we do need to leave it locked, so
+			 * that somebody else doesn't pack a reference
+			 * that we are trying to delete.
+			 *
+			 * We need to disconnect our transaction from
+			 * backend_data, since the abort (whether successful or
+			 * not) will free it.
+			 */
+			backend_data->packed_transaction = NULL;
+			if (ref_transaction_abort(packed_transaction, err)) {
+				ret = TRANSACTION_GENERIC_ERROR;
+				goto cleanup;
+			}
+		}
+	}
+
+cleanup:
+	free(head_ref);
+	string_list_clear(&affected_refnames, 0);
+
+	if (ret)
+		files_transaction_cleanup(refs, transaction);
+	else
+		transaction->state = REF_TRANSACTION_PREPARED;
+
+	return ret;
+}
+
+static int files_transaction_finish(struct ref_store *ref_store,
+				    struct ref_transaction *transaction,
+				    struct strbuf *err)
+{
+	struct files_ref_store *refs =
+		files_downcast(ref_store, 0, "ref_transaction_finish");
+	size_t i;
+	int ret = 0;
+	struct strbuf sb = STRBUF_INIT;
+	struct files_transaction_backend_data *backend_data;
+	struct ref_transaction *packed_transaction;
+
+
+	assert(err);
+
+	if (!transaction->nr) {
+		transaction->state = REF_TRANSACTION_CLOSED;
+		return 0;
+	}
+
+	backend_data = transaction->backend_data;
+	packed_transaction = backend_data->packed_transaction;
+
+	/* Perform updates first so live commits remain referenced */
+	for (i = 0; i < transaction->nr; i++) {
+		struct ref_update *update = transaction->updates[i];
+		struct ref_lock *lock = update->backend_data;
+
+		if (update->flags & REF_NEEDS_COMMIT ||
+		    update->flags & REF_LOG_ONLY) {
+			if (files_log_ref_write(refs,
+						lock->ref_name,
+						&lock->old_oid,
+						&update->new_oid,
+						update->msg, update->flags,
+						err)) {
+				char *old_msg = strbuf_detach(err, NULL);
+
+				strbuf_addf(err, "cannot update the ref '%s': %s",
+					    lock->ref_name, old_msg);
+				free(old_msg);
+				unlock_ref(lock);
+				update->backend_data = NULL;
+				ret = TRANSACTION_GENERIC_ERROR;
+				goto cleanup;
+			}
+		}
+		if (update->flags & REF_NEEDS_COMMIT) {
+			clear_loose_ref_cache(refs);
+			if (commit_ref(lock)) {
+				strbuf_addf(err, "couldn't set '%s'", lock->ref_name);
+				unlock_ref(lock);
+				update->backend_data = NULL;
+				ret = TRANSACTION_GENERIC_ERROR;
+				goto cleanup;
+			}
+		}
+	}
+
+	/*
+	 * Now that updates are safely completed, we can perform
+	 * deletes. First delete the reflogs of any references that
+	 * will be deleted, since (in the unexpected event of an
+	 * error) leaving a reference without a reflog is less bad
+	 * than leaving a reflog without a reference (the latter is a
+	 * mildly invalid repository state):
+	 */
+	for (i = 0; i < transaction->nr; i++) {
+		struct ref_update *update = transaction->updates[i];
+		if (update->flags & REF_DELETING &&
+		    !(update->flags & REF_LOG_ONLY) &&
+		    !(update->flags & REF_IS_PRUNING)) {
+			strbuf_reset(&sb);
+			files_reflog_path(refs, &sb, update->refname);
+			if (!unlink_or_warn(sb.buf))
+				try_remove_empty_parents(refs, update->refname,
+							 REMOVE_EMPTY_PARENTS_REFLOG);
+		}
+	}
+
+	/*
+	 * Perform deletes now that updates are safely completed.
+	 *
+	 * First delete any packed versions of the references, while
+	 * retaining the packed-refs lock:
+	 */
+	if (packed_transaction) {
+		ret = ref_transaction_commit(packed_transaction, err);
+		ref_transaction_free(packed_transaction);
+		packed_transaction = NULL;
+		backend_data->packed_transaction = NULL;
+		if (ret)
+			goto cleanup;
+	}
+
+	/* Now delete the loose versions of the references: */
+	for (i = 0; i < transaction->nr; i++) {
+		struct ref_update *update = transaction->updates[i];
+		struct ref_lock *lock = update->backend_data;
+
+		if (update->flags & REF_DELETING &&
+		    !(update->flags & REF_LOG_ONLY)) {
+			if (!(update->type & REF_ISPACKED) ||
+			    update->type & REF_ISSYMREF) {
+				/* It is a loose reference. */
+				strbuf_reset(&sb);
+				files_ref_path(refs, &sb, lock->ref_name);
+				if (unlink_or_msg(sb.buf, err)) {
+					ret = TRANSACTION_GENERIC_ERROR;
+					goto cleanup;
+				}
+				update->flags |= REF_DELETED_LOOSE;
+			}
+		}
+	}
+
+	clear_loose_ref_cache(refs);
+
+cleanup:
+	files_transaction_cleanup(refs, transaction);
+
+	for (i = 0; i < transaction->nr; i++) {
+		struct ref_update *update = transaction->updates[i];
+
+		if (update->flags & REF_DELETED_LOOSE) {
+			/*
+			 * The loose reference was deleted. Delete any
+			 * empty parent directories. (Note that this
+			 * can only work because we have already
+			 * removed the lockfile.)
+			 */
+			try_remove_empty_parents(refs, update->refname,
+						 REMOVE_EMPTY_PARENTS_REF);
+		}
+	}
+
+	strbuf_release(&sb);
+	return ret;
+}
+
+static int files_transaction_abort(struct ref_store *ref_store,
+				   struct ref_transaction *transaction,
+				   struct strbuf *err)
+{
+	struct files_ref_store *refs =
+		files_downcast(ref_store, 0, "ref_transaction_abort");
+
+	files_transaction_cleanup(refs, transaction);
+	return 0;
+}
+
+static int ref_present(const char *refname,
+		       const struct object_id *oid, int flags, void *cb_data)
+{
+	struct string_list *affected_refnames = cb_data;
+
+	return string_list_has_string(affected_refnames, refname);
+}
+
+static int files_initial_transaction_commit(struct ref_store *ref_store,
+					    struct ref_transaction *transaction,
+					    struct strbuf *err)
+{
+	struct files_ref_store *refs =
+		files_downcast(ref_store, REF_STORE_WRITE,
+			       "initial_ref_transaction_commit");
+	size_t i;
+	int ret = 0;
+	struct string_list affected_refnames = STRING_LIST_INIT_NODUP;
+	struct ref_transaction *packed_transaction = NULL;
+
+	assert(err);
+
+	if (transaction->state != REF_TRANSACTION_OPEN)
+		BUG("commit called for transaction that is not open");
+
+	/* Fail if a refname appears more than once in the transaction: */
+	for (i = 0; i < transaction->nr; i++)
+		string_list_append(&affected_refnames,
+				   transaction->updates[i]->refname);
+	string_list_sort(&affected_refnames);
+	if (ref_update_reject_duplicates(&affected_refnames, err)) {
+		ret = TRANSACTION_GENERIC_ERROR;
+		goto cleanup;
+	}
+
+	/*
+	 * It's really undefined to call this function in an active
+	 * repository or when there are existing references: we are
+	 * only locking and changing packed-refs, so (1) any
+	 * simultaneous processes might try to change a reference at
+	 * the same time we do, and (2) any existing loose versions of
+	 * the references that we are setting would have precedence
+	 * over our values. But some remote helpers create the remote
+	 * "HEAD" and "master" branches before calling this function,
+	 * so here we really only check that none of the references
+	 * that we are creating already exists.
+	 */
+	if (refs_for_each_rawref(&refs->base, ref_present,
+				 &affected_refnames))
+		BUG("initial ref transaction called with existing refs");
+
+	packed_transaction = ref_store_transaction_begin(refs->packed_ref_store, err);
+	if (!packed_transaction) {
+		ret = TRANSACTION_GENERIC_ERROR;
+		goto cleanup;
+	}
+
+	for (i = 0; i < transaction->nr; i++) {
+		struct ref_update *update = transaction->updates[i];
+
+		if ((update->flags & REF_HAVE_OLD) &&
+		    !is_null_oid(&update->old_oid))
+			BUG("initial ref transaction with old_sha1 set");
+		if (refs_verify_refname_available(&refs->base, update->refname,
+						  &affected_refnames, NULL,
+						  err)) {
+			ret = TRANSACTION_NAME_CONFLICT;
+			goto cleanup;
+		}
+
+		/*
+		 * Add a reference creation for this reference to the
+		 * packed-refs transaction:
+		 */
+		ref_transaction_add_update(packed_transaction, update->refname,
+					   update->flags & ~REF_HAVE_OLD,
+					   &update->new_oid, &update->old_oid,
+					   NULL);
+	}
+
+	if (packed_refs_lock(refs->packed_ref_store, 0, err)) {
+		ret = TRANSACTION_GENERIC_ERROR;
+		goto cleanup;
+	}
+
+	if (initial_ref_transaction_commit(packed_transaction, err)) {
+		ret = TRANSACTION_GENERIC_ERROR;
+	}
+
+	packed_refs_unlock(refs->packed_ref_store);
+cleanup:
+	if (packed_transaction)
+		ref_transaction_free(packed_transaction);
+	transaction->state = REF_TRANSACTION_CLOSED;
+	string_list_clear(&affected_refnames, 0);
+	return ret;
+}
+
+struct expire_reflog_cb {
+	unsigned int flags;
+	reflog_expiry_should_prune_fn *should_prune_fn;
+	void *policy_cb;
+	FILE *newlog;
+	struct object_id last_kept_oid;
+};
+
+static int expire_reflog_ent(struct object_id *ooid, struct object_id *noid,
+			     const char *email, timestamp_t timestamp, int tz,
+			     const char *message, void *cb_data)
+{
+	struct expire_reflog_cb *cb = cb_data;
+	struct expire_reflog_policy_cb *policy_cb = cb->policy_cb;
+
+	if (cb->flags & EXPIRE_REFLOGS_REWRITE)
+		ooid = &cb->last_kept_oid;
+
+	if ((*cb->should_prune_fn)(ooid, noid, email, timestamp, tz,
+				   message, policy_cb)) {
+		if (!cb->newlog)
+			printf("would prune %s", message);
+		else if (cb->flags & EXPIRE_REFLOGS_VERBOSE)
+			printf("prune %s", message);
+	} else {
+		if (cb->newlog) {
+			fprintf(cb->newlog, "%s %s %s %"PRItime" %+05d\t%s",
+				oid_to_hex(ooid), oid_to_hex(noid),
+				email, timestamp, tz, message);
+			oidcpy(&cb->last_kept_oid, noid);
+		}
+		if (cb->flags & EXPIRE_REFLOGS_VERBOSE)
+			printf("keep %s", message);
+	}
+	return 0;
+}
+
+static int files_reflog_expire(struct ref_store *ref_store,
+			       const char *refname, const struct object_id *oid,
+			       unsigned int flags,
+			       reflog_expiry_prepare_fn prepare_fn,
+			       reflog_expiry_should_prune_fn should_prune_fn,
+			       reflog_expiry_cleanup_fn cleanup_fn,
+			       void *policy_cb_data)
+{
+	struct files_ref_store *refs =
+		files_downcast(ref_store, REF_STORE_WRITE, "reflog_expire");
+	struct lock_file reflog_lock = LOCK_INIT;
+	struct expire_reflog_cb cb;
+	struct ref_lock *lock;
+	struct strbuf log_file_sb = STRBUF_INIT;
+	char *log_file;
+	int status = 0;
+	int type;
+	struct strbuf err = STRBUF_INIT;
+
+	memset(&cb, 0, sizeof(cb));
+	cb.flags = flags;
+	cb.policy_cb = policy_cb_data;
+	cb.should_prune_fn = should_prune_fn;
+
+	/*
+	 * The reflog file is locked by holding the lock on the
+	 * reference itself, plus we might need to update the
+	 * reference if --updateref was specified:
+	 */
+	lock = lock_ref_oid_basic(refs, refname, oid,
+				  NULL, NULL, REF_NO_DEREF,
+				  &type, &err);
+	if (!lock) {
+		error("cannot lock ref '%s': %s", refname, err.buf);
+		strbuf_release(&err);
+		return -1;
+	}
+	if (!refs_reflog_exists(ref_store, refname)) {
+		unlock_ref(lock);
+		return 0;
+	}
+
+	files_reflog_path(refs, &log_file_sb, refname);
+	log_file = strbuf_detach(&log_file_sb, NULL);
+	if (!(flags & EXPIRE_REFLOGS_DRY_RUN)) {
+		/*
+		 * Even though holding $GIT_DIR/logs/$reflog.lock has
+		 * no locking implications, we use the lock_file
+		 * machinery here anyway because it does a lot of the
+		 * work we need, including cleaning up if the program
+		 * exits unexpectedly.
+		 */
+		if (hold_lock_file_for_update(&reflog_lock, log_file, 0) < 0) {
+			struct strbuf err = STRBUF_INIT;
+			unable_to_lock_message(log_file, errno, &err);
+			error("%s", err.buf);
+			strbuf_release(&err);
+			goto failure;
+		}
+		cb.newlog = fdopen_lock_file(&reflog_lock, "w");
+		if (!cb.newlog) {
+			error("cannot fdopen %s (%s)",
+			      get_lock_file_path(&reflog_lock), strerror(errno));
+			goto failure;
+		}
+	}
+
+	(*prepare_fn)(refname, oid, cb.policy_cb);
+	refs_for_each_reflog_ent(ref_store, refname, expire_reflog_ent, &cb);
+	(*cleanup_fn)(cb.policy_cb);
+
+	if (!(flags & EXPIRE_REFLOGS_DRY_RUN)) {
+		/*
+		 * It doesn't make sense to adjust a reference pointed
+		 * to by a symbolic ref based on expiring entries in
+		 * the symbolic reference's reflog. Nor can we update
+		 * a reference if there are no remaining reflog
+		 * entries.
+		 */
+		int update = (flags & EXPIRE_REFLOGS_UPDATE_REF) &&
+			!(type & REF_ISSYMREF) &&
+			!is_null_oid(&cb.last_kept_oid);
+
+		if (close_lock_file_gently(&reflog_lock)) {
+			status |= error("couldn't write %s: %s", log_file,
+					strerror(errno));
+			rollback_lock_file(&reflog_lock);
+		} else if (update &&
+			   (write_in_full(get_lock_file_fd(&lock->lk),
+				oid_to_hex(&cb.last_kept_oid), the_hash_algo->hexsz) < 0 ||
+			    write_str_in_full(get_lock_file_fd(&lock->lk), "\n") < 0 ||
+			    close_ref_gently(lock) < 0)) {
+			status |= error("couldn't write %s",
+					get_lock_file_path(&lock->lk));
+			rollback_lock_file(&reflog_lock);
+		} else if (commit_lock_file(&reflog_lock)) {
+			status |= error("unable to write reflog '%s' (%s)",
+					log_file, strerror(errno));
+		} else if (update && commit_ref(lock)) {
+			status |= error("couldn't set %s", lock->ref_name);
+		}
+	}
+	free(log_file);
+	unlock_ref(lock);
+	return status;
+
+ failure:
+	rollback_lock_file(&reflog_lock);
+	free(log_file);
+	unlock_ref(lock);
+	return -1;
+}
+
+static int files_init_db(struct ref_store *ref_store, struct strbuf *err)
+{
+	struct files_ref_store *refs =
+		files_downcast(ref_store, REF_STORE_WRITE, "init_db");
+	struct strbuf sb = STRBUF_INIT;
+
+	/*
+	 * Create .git/refs/{heads,tags}
+	 */
+	files_ref_path(refs, &sb, "refs/heads");
+	safe_create_dir(sb.buf, 1);
+
+	strbuf_reset(&sb);
+	files_ref_path(refs, &sb, "refs/tags");
+	safe_create_dir(sb.buf, 1);
+
+	strbuf_release(&sb);
+	return 0;
+}
+
+struct ref_storage_be refs_be_files = {
+	NULL,
+	"files",
+	files_ref_store_create,
+	files_init_db,
+	files_transaction_prepare,
+	files_transaction_finish,
+	files_transaction_abort,
+	files_initial_transaction_commit,
+
+	files_pack_refs,
+	files_create_symref,
+	files_delete_refs,
+	files_rename_ref,
+	files_copy_ref,
+
+	files_ref_iterator_begin,
+	files_read_raw_ref,
+
+	files_reflog_iterator_begin,
+	files_for_each_reflog_ent,
+	files_for_each_reflog_ent_reverse,
+	files_reflog_exists,
+	files_create_reflog,
+	files_delete_reflog,
+	files_reflog_expire
+};
diff --git a/refs/iterator.c b/refs/iterator.c
new file mode 100644
index 000000000000..629e00a122a7
--- /dev/null
+++ b/refs/iterator.c
@@ -0,0 +1,435 @@
+/*
+ * Generic reference iterator infrastructure. See refs-internal.h for
+ * documentation about the design and use of reference iterators.
+ */
+
+#include "cache.h"
+#include "refs.h"
+#include "refs/refs-internal.h"
+#include "iterator.h"
+
+int ref_iterator_advance(struct ref_iterator *ref_iterator)
+{
+	return ref_iterator->vtable->advance(ref_iterator);
+}
+
+int ref_iterator_peel(struct ref_iterator *ref_iterator,
+		      struct object_id *peeled)
+{
+	return ref_iterator->vtable->peel(ref_iterator, peeled);
+}
+
+int ref_iterator_abort(struct ref_iterator *ref_iterator)
+{
+	return ref_iterator->vtable->abort(ref_iterator);
+}
+
+void base_ref_iterator_init(struct ref_iterator *iter,
+			    struct ref_iterator_vtable *vtable,
+			    int ordered)
+{
+	iter->vtable = vtable;
+	iter->ordered = !!ordered;
+	iter->refname = NULL;
+	iter->oid = NULL;
+	iter->flags = 0;
+}
+
+void base_ref_iterator_free(struct ref_iterator *iter)
+{
+	/* Help make use-after-free bugs fail quickly: */
+	iter->vtable = NULL;
+	free(iter);
+}
+
+struct empty_ref_iterator {
+	struct ref_iterator base;
+};
+
+static int empty_ref_iterator_advance(struct ref_iterator *ref_iterator)
+{
+	return ref_iterator_abort(ref_iterator);
+}
+
+static int empty_ref_iterator_peel(struct ref_iterator *ref_iterator,
+				   struct object_id *peeled)
+{
+	BUG("peel called for empty iterator");
+}
+
+static int empty_ref_iterator_abort(struct ref_iterator *ref_iterator)
+{
+	base_ref_iterator_free(ref_iterator);
+	return ITER_DONE;
+}
+
+static struct ref_iterator_vtable empty_ref_iterator_vtable = {
+	empty_ref_iterator_advance,
+	empty_ref_iterator_peel,
+	empty_ref_iterator_abort
+};
+
+struct ref_iterator *empty_ref_iterator_begin(void)
+{
+	struct empty_ref_iterator *iter = xcalloc(1, sizeof(*iter));
+	struct ref_iterator *ref_iterator = &iter->base;
+
+	base_ref_iterator_init(ref_iterator, &empty_ref_iterator_vtable, 1);
+	return ref_iterator;
+}
+
+int is_empty_ref_iterator(struct ref_iterator *ref_iterator)
+{
+	return ref_iterator->vtable == &empty_ref_iterator_vtable;
+}
+
+struct merge_ref_iterator {
+	struct ref_iterator base;
+
+	struct ref_iterator *iter0, *iter1;
+
+	ref_iterator_select_fn *select;
+	void *cb_data;
+
+	/*
+	 * A pointer to iter0 or iter1 (whichever is supplying the
+	 * current value), or NULL if advance has not yet been called.
+	 */
+	struct ref_iterator **current;
+};
+
+static int merge_ref_iterator_advance(struct ref_iterator *ref_iterator)
+{
+	struct merge_ref_iterator *iter =
+		(struct merge_ref_iterator *)ref_iterator;
+	int ok;
+
+	if (!iter->current) {
+		/* Initialize: advance both iterators to their first entries */
+		if ((ok = ref_iterator_advance(iter->iter0)) != ITER_OK) {
+			iter->iter0 = NULL;
+			if (ok == ITER_ERROR)
+				goto error;
+		}
+		if ((ok = ref_iterator_advance(iter->iter1)) != ITER_OK) {
+			iter->iter1 = NULL;
+			if (ok == ITER_ERROR)
+				goto error;
+		}
+	} else {
+		/*
+		 * Advance the current iterator past the just-used
+		 * entry:
+		 */
+		if ((ok = ref_iterator_advance(*iter->current)) != ITER_OK) {
+			*iter->current = NULL;
+			if (ok == ITER_ERROR)
+				goto error;
+		}
+	}
+
+	/* Loop until we find an entry that we can yield. */
+	while (1) {
+		struct ref_iterator **secondary;
+		enum iterator_selection selection =
+			iter->select(iter->iter0, iter->iter1, iter->cb_data);
+
+		if (selection == ITER_SELECT_DONE) {
+			return ref_iterator_abort(ref_iterator);
+		} else if (selection == ITER_SELECT_ERROR) {
+			ref_iterator_abort(ref_iterator);
+			return ITER_ERROR;
+		}
+
+		if ((selection & ITER_CURRENT_SELECTION_MASK) == 0) {
+			iter->current = &iter->iter0;
+			secondary = &iter->iter1;
+		} else {
+			iter->current = &iter->iter1;
+			secondary = &iter->iter0;
+		}
+
+		if (selection & ITER_SKIP_SECONDARY) {
+			if ((ok = ref_iterator_advance(*secondary)) != ITER_OK) {
+				*secondary = NULL;
+				if (ok == ITER_ERROR)
+					goto error;
+			}
+		}
+
+		if (selection & ITER_YIELD_CURRENT) {
+			iter->base.refname = (*iter->current)->refname;
+			iter->base.oid = (*iter->current)->oid;
+			iter->base.flags = (*iter->current)->flags;
+			return ITER_OK;
+		}
+	}
+
+error:
+	ref_iterator_abort(ref_iterator);
+	return ITER_ERROR;
+}
+
+static int merge_ref_iterator_peel(struct ref_iterator *ref_iterator,
+				   struct object_id *peeled)
+{
+	struct merge_ref_iterator *iter =
+		(struct merge_ref_iterator *)ref_iterator;
+
+	if (!iter->current) {
+		BUG("peel called before advance for merge iterator");
+	}
+	return ref_iterator_peel(*iter->current, peeled);
+}
+
+static int merge_ref_iterator_abort(struct ref_iterator *ref_iterator)
+{
+	struct merge_ref_iterator *iter =
+		(struct merge_ref_iterator *)ref_iterator;
+	int ok = ITER_DONE;
+
+	if (iter->iter0) {
+		if (ref_iterator_abort(iter->iter0) != ITER_DONE)
+			ok = ITER_ERROR;
+	}
+	if (iter->iter1) {
+		if (ref_iterator_abort(iter->iter1) != ITER_DONE)
+			ok = ITER_ERROR;
+	}
+	base_ref_iterator_free(ref_iterator);
+	return ok;
+}
+
+static struct ref_iterator_vtable merge_ref_iterator_vtable = {
+	merge_ref_iterator_advance,
+	merge_ref_iterator_peel,
+	merge_ref_iterator_abort
+};
+
+struct ref_iterator *merge_ref_iterator_begin(
+		int ordered,
+		struct ref_iterator *iter0, struct ref_iterator *iter1,
+		ref_iterator_select_fn *select, void *cb_data)
+{
+	struct merge_ref_iterator *iter = xcalloc(1, sizeof(*iter));
+	struct ref_iterator *ref_iterator = &iter->base;
+
+	/*
+	 * We can't do the same kind of is_empty_ref_iterator()-style
+	 * optimization here as overlay_ref_iterator_begin() does,
+	 * because we don't know the semantics of the select function.
+	 * It might, for example, implement "intersect" by passing
+	 * references through only if they exist in both iterators.
+	 */
+
+	base_ref_iterator_init(ref_iterator, &merge_ref_iterator_vtable, ordered);
+	iter->iter0 = iter0;
+	iter->iter1 = iter1;
+	iter->select = select;
+	iter->cb_data = cb_data;
+	iter->current = NULL;
+	return ref_iterator;
+}
+
+/*
+ * A ref_iterator_select_fn that overlays the items from front on top
+ * of those from back (like loose refs over packed refs). See
+ * overlay_ref_iterator_begin().
+ */
+static enum iterator_selection overlay_iterator_select(
+		struct ref_iterator *front, struct ref_iterator *back,
+		void *cb_data)
+{
+	int cmp;
+
+	if (!back)
+		return front ? ITER_SELECT_0 : ITER_SELECT_DONE;
+	else if (!front)
+		return ITER_SELECT_1;
+
+	cmp = strcmp(front->refname, back->refname);
+
+	if (cmp < 0)
+		return ITER_SELECT_0;
+	else if (cmp > 0)
+		return ITER_SELECT_1;
+	else
+		return ITER_SELECT_0_SKIP_1;
+}
+
+struct ref_iterator *overlay_ref_iterator_begin(
+		struct ref_iterator *front, struct ref_iterator *back)
+{
+	/*
+	 * Optimization: if one of the iterators is empty, return the
+	 * other one rather than incurring the overhead of wrapping
+	 * them.
+	 */
+	if (is_empty_ref_iterator(front)) {
+		ref_iterator_abort(front);
+		return back;
+	} else if (is_empty_ref_iterator(back)) {
+		ref_iterator_abort(back);
+		return front;
+	} else if (!front->ordered || !back->ordered) {
+		BUG("overlay_ref_iterator requires ordered inputs");
+	}
+
+	return merge_ref_iterator_begin(1, front, back,
+					overlay_iterator_select, NULL);
+}
+
+struct prefix_ref_iterator {
+	struct ref_iterator base;
+
+	struct ref_iterator *iter0;
+	char *prefix;
+	int trim;
+};
+
+/* Return -1, 0, 1 if refname is before, inside, or after the prefix. */
+static int compare_prefix(const char *refname, const char *prefix)
+{
+	while (*prefix) {
+		if (*refname != *prefix)
+			return ((unsigned char)*refname < (unsigned char)*prefix) ? -1 : +1;
+
+		refname++;
+		prefix++;
+	}
+
+	return 0;
+}
+
+static int prefix_ref_iterator_advance(struct ref_iterator *ref_iterator)
+{
+	struct prefix_ref_iterator *iter =
+		(struct prefix_ref_iterator *)ref_iterator;
+	int ok;
+
+	while ((ok = ref_iterator_advance(iter->iter0)) == ITER_OK) {
+		int cmp = compare_prefix(iter->iter0->refname, iter->prefix);
+
+		if (cmp < 0)
+			continue;
+
+		if (cmp > 0) {
+			/*
+			 * If the source iterator is ordered, then we
+			 * can stop the iteration as soon as we see a
+			 * refname that comes after the prefix:
+			 */
+			if (iter->iter0->ordered) {
+				ok = ref_iterator_abort(iter->iter0);
+				break;
+			} else {
+				continue;
+			}
+		}
+
+		if (iter->trim) {
+			/*
+			 * It is nonsense to trim off characters that
+			 * you haven't already checked for via a
+			 * prefix check, whether via this
+			 * `prefix_ref_iterator` or upstream in
+			 * `iter0`). So if there wouldn't be at least
+			 * one character left in the refname after
+			 * trimming, report it as a bug:
+			 */
+			if (strlen(iter->iter0->refname) <= iter->trim)
+				BUG("attempt to trim too many characters");
+			iter->base.refname = iter->iter0->refname + iter->trim;
+		} else {
+			iter->base.refname = iter->iter0->refname;
+		}
+
+		iter->base.oid = iter->iter0->oid;
+		iter->base.flags = iter->iter0->flags;
+		return ITER_OK;
+	}
+
+	iter->iter0 = NULL;
+	if (ref_iterator_abort(ref_iterator) != ITER_DONE)
+		return ITER_ERROR;
+	return ok;
+}
+
+static int prefix_ref_iterator_peel(struct ref_iterator *ref_iterator,
+				    struct object_id *peeled)
+{
+	struct prefix_ref_iterator *iter =
+		(struct prefix_ref_iterator *)ref_iterator;
+
+	return ref_iterator_peel(iter->iter0, peeled);
+}
+
+static int prefix_ref_iterator_abort(struct ref_iterator *ref_iterator)
+{
+	struct prefix_ref_iterator *iter =
+		(struct prefix_ref_iterator *)ref_iterator;
+	int ok = ITER_DONE;
+
+	if (iter->iter0)
+		ok = ref_iterator_abort(iter->iter0);
+	free(iter->prefix);
+	base_ref_iterator_free(ref_iterator);
+	return ok;
+}
+
+static struct ref_iterator_vtable prefix_ref_iterator_vtable = {
+	prefix_ref_iterator_advance,
+	prefix_ref_iterator_peel,
+	prefix_ref_iterator_abort
+};
+
+struct ref_iterator *prefix_ref_iterator_begin(struct ref_iterator *iter0,
+					       const char *prefix,
+					       int trim)
+{
+	struct prefix_ref_iterator *iter;
+	struct ref_iterator *ref_iterator;
+
+	if (!*prefix && !trim)
+		return iter0; /* optimization: no need to wrap iterator */
+
+	iter = xcalloc(1, sizeof(*iter));
+	ref_iterator = &iter->base;
+
+	base_ref_iterator_init(ref_iterator, &prefix_ref_iterator_vtable, iter0->ordered);
+
+	iter->iter0 = iter0;
+	iter->prefix = xstrdup(prefix);
+	iter->trim = trim;
+
+	return ref_iterator;
+}
+
+struct ref_iterator *current_ref_iter = NULL;
+
+int do_for_each_repo_ref_iterator(struct repository *r, struct ref_iterator *iter,
+				  each_repo_ref_fn fn, void *cb_data)
+{
+	int retval = 0, ok;
+	struct ref_iterator *old_ref_iter = current_ref_iter;
+
+	current_ref_iter = iter;
+	while ((ok = ref_iterator_advance(iter)) == ITER_OK) {
+		retval = fn(r, iter->refname, iter->oid, iter->flags, cb_data);
+		if (retval) {
+			/*
+			 * If ref_iterator_abort() returns ITER_ERROR,
+			 * we ignore that error in deference to the
+			 * callback function's return value.
+			 */
+			ref_iterator_abort(iter);
+			goto out;
+		}
+	}
+
+out:
+	current_ref_iter = old_ref_iter;
+	if (ok == ITER_ERROR)
+		return -1;
+	return retval;
+}
diff --git a/refs/packed-backend.c b/refs/packed-backend.c
new file mode 100644
index 000000000000..c01c7f5901a6
--- /dev/null
+++ b/refs/packed-backend.c
@@ -0,0 +1,1660 @@
+#include "../cache.h"
+#include "../config.h"
+#include "../refs.h"
+#include "refs-internal.h"
+#include "packed-backend.h"
+#include "../iterator.h"
+#include "../lockfile.h"
+#include "../chdir-notify.h"
+
+enum mmap_strategy {
+	/*
+	 * Don't use mmap() at all for reading `packed-refs`.
+	 */
+	MMAP_NONE,
+
+	/*
+	 * Can use mmap() for reading `packed-refs`, but the file must
+	 * not remain mmapped. This is the usual option on Windows,
+	 * where you cannot rename a new version of a file onto a file
+	 * that is currently mmapped.
+	 */
+	MMAP_TEMPORARY,
+
+	/*
+	 * It is OK to leave the `packed-refs` file mmapped while
+	 * arbitrary other code is running.
+	 */
+	MMAP_OK
+};
+
+#if defined(NO_MMAP)
+static enum mmap_strategy mmap_strategy = MMAP_NONE;
+#elif defined(MMAP_PREVENTS_DELETE)
+static enum mmap_strategy mmap_strategy = MMAP_TEMPORARY;
+#else
+static enum mmap_strategy mmap_strategy = MMAP_OK;
+#endif
+
+struct packed_ref_store;
+
+/*
+ * A `snapshot` represents one snapshot of a `packed-refs` file.
+ *
+ * Normally, this will be a mmapped view of the contents of the
+ * `packed-refs` file at the time the snapshot was created. However,
+ * if the `packed-refs` file was not sorted, this might point at heap
+ * memory holding the contents of the `packed-refs` file with its
+ * records sorted by refname.
+ *
+ * `snapshot` instances are reference counted (via
+ * `acquire_snapshot()` and `release_snapshot()`). This is to prevent
+ * an instance from disappearing while an iterator is still iterating
+ * over it. Instances are garbage collected when their `referrers`
+ * count goes to zero.
+ *
+ * The most recent `snapshot`, if available, is referenced by the
+ * `packed_ref_store`. Its freshness is checked whenever
+ * `get_snapshot()` is called; if the existing snapshot is obsolete, a
+ * new snapshot is taken.
+ */
+struct snapshot {
+	/*
+	 * A back-pointer to the packed_ref_store with which this
+	 * snapshot is associated:
+	 */
+	struct packed_ref_store *refs;
+
+	/* Is the `packed-refs` file currently mmapped? */
+	int mmapped;
+
+	/*
+	 * The contents of the `packed-refs` file:
+	 *
+	 * - buf -- a pointer to the start of the memory
+	 * - start -- a pointer to the first byte of actual references
+	 *   (i.e., after the header line, if one is present)
+	 * - eof -- a pointer just past the end of the reference
+	 *   contents
+	 *
+	 * If the `packed-refs` file was already sorted, `buf` points
+	 * at the mmapped contents of the file. If not, it points at
+	 * heap-allocated memory containing the contents, sorted. If
+	 * there were no contents (e.g., because the file didn't
+	 * exist), `buf`, `start`, and `eof` are all NULL.
+	 */
+	char *buf, *start, *eof;
+
+	/*
+	 * What is the peeled state of the `packed-refs` file that
+	 * this snapshot represents? (This is usually determined from
+	 * the file's header.)
+	 */
+	enum { PEELED_NONE, PEELED_TAGS, PEELED_FULLY } peeled;
+
+	/*
+	 * Count of references to this instance, including the pointer
+	 * from `packed_ref_store::snapshot`, if any. The instance
+	 * will not be freed as long as the reference count is
+	 * nonzero.
+	 */
+	unsigned int referrers;
+
+	/*
+	 * The metadata of the `packed-refs` file from which this
+	 * snapshot was created, used to tell if the file has been
+	 * replaced since we read it.
+	 */
+	struct stat_validity validity;
+};
+
+/*
+ * A `ref_store` representing references stored in a `packed-refs`
+ * file. It implements the `ref_store` interface, though it has some
+ * limitations:
+ *
+ * - It cannot store symbolic references.
+ *
+ * - It cannot store reflogs.
+ *
+ * - It does not support reference renaming (though it could).
+ *
+ * On the other hand, it can be locked outside of a reference
+ * transaction. In that case, it remains locked even after the
+ * transaction is done and the new `packed-refs` file is activated.
+ */
+struct packed_ref_store {
+	struct ref_store base;
+
+	unsigned int store_flags;
+
+	/* The path of the "packed-refs" file: */
+	char *path;
+
+	/*
+	 * A snapshot of the values read from the `packed-refs` file,
+	 * if it might still be current; otherwise, NULL.
+	 */
+	struct snapshot *snapshot;
+
+	/*
+	 * Lock used for the "packed-refs" file. Note that this (and
+	 * thus the enclosing `packed_ref_store`) must not be freed.
+	 */
+	struct lock_file lock;
+
+	/*
+	 * Temporary file used when rewriting new contents to the
+	 * "packed-refs" file. Note that this (and thus the enclosing
+	 * `packed_ref_store`) must not be freed.
+	 */
+	struct tempfile *tempfile;
+};
+
+/*
+ * Increment the reference count of `*snapshot`.
+ */
+static void acquire_snapshot(struct snapshot *snapshot)
+{
+	snapshot->referrers++;
+}
+
+/*
+ * If the buffer in `snapshot` is active, then either munmap the
+ * memory and close the file, or free the memory. Then set the buffer
+ * pointers to NULL.
+ */
+static void clear_snapshot_buffer(struct snapshot *snapshot)
+{
+	if (snapshot->mmapped) {
+		if (munmap(snapshot->buf, snapshot->eof - snapshot->buf))
+			die_errno("error ummapping packed-refs file %s",
+				  snapshot->refs->path);
+		snapshot->mmapped = 0;
+	} else {
+		free(snapshot->buf);
+	}
+	snapshot->buf = snapshot->start = snapshot->eof = NULL;
+}
+
+/*
+ * Decrease the reference count of `*snapshot`. If it goes to zero,
+ * free `*snapshot` and return true; otherwise return false.
+ */
+static int release_snapshot(struct snapshot *snapshot)
+{
+	if (!--snapshot->referrers) {
+		stat_validity_clear(&snapshot->validity);
+		clear_snapshot_buffer(snapshot);
+		free(snapshot);
+		return 1;
+	} else {
+		return 0;
+	}
+}
+
+struct ref_store *packed_ref_store_create(const char *path,
+					  unsigned int store_flags)
+{
+	struct packed_ref_store *refs = xcalloc(1, sizeof(*refs));
+	struct ref_store *ref_store = (struct ref_store *)refs;
+
+	base_ref_store_init(ref_store, &refs_be_packed);
+	refs->store_flags = store_flags;
+
+	refs->path = xstrdup(path);
+	chdir_notify_reparent("packed-refs", &refs->path);
+
+	return ref_store;
+}
+
+/*
+ * Downcast `ref_store` to `packed_ref_store`. Die if `ref_store` is
+ * not a `packed_ref_store`. Also die if `packed_ref_store` doesn't
+ * support at least the flags specified in `required_flags`. `caller`
+ * is used in any necessary error messages.
+ */
+static struct packed_ref_store *packed_downcast(struct ref_store *ref_store,
+						unsigned int required_flags,
+						const char *caller)
+{
+	struct packed_ref_store *refs;
+
+	if (ref_store->be != &refs_be_packed)
+		BUG("ref_store is type \"%s\" not \"packed\" in %s",
+		    ref_store->be->name, caller);
+
+	refs = (struct packed_ref_store *)ref_store;
+
+	if ((refs->store_flags & required_flags) != required_flags)
+		BUG("unallowed operation (%s), requires %x, has %x\n",
+		    caller, required_flags, refs->store_flags);
+
+	return refs;
+}
+
+static void clear_snapshot(struct packed_ref_store *refs)
+{
+	if (refs->snapshot) {
+		struct snapshot *snapshot = refs->snapshot;
+
+		refs->snapshot = NULL;
+		release_snapshot(snapshot);
+	}
+}
+
+static NORETURN void die_unterminated_line(const char *path,
+					   const char *p, size_t len)
+{
+	if (len < 80)
+		die("unterminated line in %s: %.*s", path, (int)len, p);
+	else
+		die("unterminated line in %s: %.75s...", path, p);
+}
+
+static NORETURN void die_invalid_line(const char *path,
+				      const char *p, size_t len)
+{
+	const char *eol = memchr(p, '\n', len);
+
+	if (!eol)
+		die_unterminated_line(path, p, len);
+	else if (eol - p < 80)
+		die("unexpected line in %s: %.*s", path, (int)(eol - p), p);
+	else
+		die("unexpected line in %s: %.75s...", path, p);
+
+}
+
+struct snapshot_record {
+	const char *start;
+	size_t len;
+};
+
+static int cmp_packed_ref_records(const void *v1, const void *v2)
+{
+	const struct snapshot_record *e1 = v1, *e2 = v2;
+	const char *r1 = e1->start + the_hash_algo->hexsz + 1;
+	const char *r2 = e2->start + the_hash_algo->hexsz + 1;
+
+	while (1) {
+		if (*r1 == '\n')
+			return *r2 == '\n' ? 0 : -1;
+		if (*r1 != *r2) {
+			if (*r2 == '\n')
+				return 1;
+			else
+				return (unsigned char)*r1 < (unsigned char)*r2 ? -1 : +1;
+		}
+		r1++;
+		r2++;
+	}
+}
+
+/*
+ * Compare a snapshot record at `rec` to the specified NUL-terminated
+ * refname.
+ */
+static int cmp_record_to_refname(const char *rec, const char *refname)
+{
+	const char *r1 = rec + the_hash_algo->hexsz + 1;
+	const char *r2 = refname;
+
+	while (1) {
+		if (*r1 == '\n')
+			return *r2 ? -1 : 0;
+		if (!*r2)
+			return 1;
+		if (*r1 != *r2)
+			return (unsigned char)*r1 < (unsigned char)*r2 ? -1 : +1;
+		r1++;
+		r2++;
+	}
+}
+
+/*
+ * `snapshot->buf` is not known to be sorted. Check whether it is, and
+ * if not, sort it into new memory and munmap/free the old storage.
+ */
+static void sort_snapshot(struct snapshot *snapshot)
+{
+	struct snapshot_record *records = NULL;
+	size_t alloc = 0, nr = 0;
+	int sorted = 1;
+	const char *pos, *eof, *eol;
+	size_t len, i;
+	char *new_buffer, *dst;
+
+	pos = snapshot->start;
+	eof = snapshot->eof;
+
+	if (pos == eof)
+		return;
+
+	len = eof - pos;
+
+	/*
+	 * Initialize records based on a crude estimate of the number
+	 * of references in the file (we'll grow it below if needed):
+	 */
+	ALLOC_GROW(records, len / 80 + 20, alloc);
+
+	while (pos < eof) {
+		eol = memchr(pos, '\n', eof - pos);
+		if (!eol)
+			/* The safety check should prevent this. */
+			BUG("unterminated line found in packed-refs");
+		if (eol - pos < the_hash_algo->hexsz + 2)
+			die_invalid_line(snapshot->refs->path,
+					 pos, eof - pos);
+		eol++;
+		if (eol < eof && *eol == '^') {
+			/*
+			 * Keep any peeled line together with its
+			 * reference:
+			 */
+			const char *peeled_start = eol;
+
+			eol = memchr(peeled_start, '\n', eof - peeled_start);
+			if (!eol)
+				/* The safety check should prevent this. */
+				BUG("unterminated peeled line found in packed-refs");
+			eol++;
+		}
+
+		ALLOC_GROW(records, nr + 1, alloc);
+		records[nr].start = pos;
+		records[nr].len = eol - pos;
+		nr++;
+
+		if (sorted &&
+		    nr > 1 &&
+		    cmp_packed_ref_records(&records[nr - 2],
+					   &records[nr - 1]) >= 0)
+			sorted = 0;
+
+		pos = eol;
+	}
+
+	if (sorted)
+		goto cleanup;
+
+	/* We need to sort the memory. First we sort the records array: */
+	QSORT(records, nr, cmp_packed_ref_records);
+
+	/*
+	 * Allocate a new chunk of memory, and copy the old memory to
+	 * the new in the order indicated by `records` (not bothering
+	 * with the header line):
+	 */
+	new_buffer = xmalloc(len);
+	for (dst = new_buffer, i = 0; i < nr; i++) {
+		memcpy(dst, records[i].start, records[i].len);
+		dst += records[i].len;
+	}
+
+	/*
+	 * Now munmap the old buffer and use the sorted buffer in its
+	 * place:
+	 */
+	clear_snapshot_buffer(snapshot);
+	snapshot->buf = snapshot->start = new_buffer;
+	snapshot->eof = new_buffer + len;
+
+cleanup:
+	free(records);
+}
+
+/*
+ * Return a pointer to the start of the record that contains the
+ * character `*p` (which must be within the buffer). If no other
+ * record start is found, return `buf`.
+ */
+static const char *find_start_of_record(const char *buf, const char *p)
+{
+	while (p > buf && (p[-1] != '\n' || p[0] == '^'))
+		p--;
+	return p;
+}
+
+/*
+ * Return a pointer to the start of the record following the record
+ * that contains `*p`. If none is found before `end`, return `end`.
+ */
+static const char *find_end_of_record(const char *p, const char *end)
+{
+	while (++p < end && (p[-1] != '\n' || p[0] == '^'))
+		;
+	return p;
+}
+
+/*
+ * We want to be able to compare mmapped reference records quickly,
+ * without totally parsing them. We can do so because the records are
+ * LF-terminated, and the refname should start exactly (GIT_SHA1_HEXSZ
+ * + 1) bytes past the beginning of the record.
+ *
+ * But what if the `packed-refs` file contains garbage? We're willing
+ * to tolerate not detecting the problem, as long as we don't produce
+ * totally garbled output (we can't afford to check the integrity of
+ * the whole file during every Git invocation). But we do want to be
+ * sure that we never read past the end of the buffer in memory and
+ * perform an illegal memory access.
+ *
+ * Guarantee that minimum level of safety by verifying that the last
+ * record in the file is LF-terminated, and that it has at least
+ * (GIT_SHA1_HEXSZ + 1) characters before the LF. Die if either of
+ * these checks fails.
+ */
+static void verify_buffer_safe(struct snapshot *snapshot)
+{
+	const char *start = snapshot->start;
+	const char *eof = snapshot->eof;
+	const char *last_line;
+
+	if (start == eof)
+		return;
+
+	last_line = find_start_of_record(start, eof - 1);
+	if (*(eof - 1) != '\n' || eof - last_line < the_hash_algo->hexsz + 2)
+		die_invalid_line(snapshot->refs->path,
+				 last_line, eof - last_line);
+}
+
+#define SMALL_FILE_SIZE (32*1024)
+
+/*
+ * Depending on `mmap_strategy`, either mmap or read the contents of
+ * the `packed-refs` file into the snapshot. Return 1 if the file
+ * existed and was read, or 0 if the file was absent or empty. Die on
+ * errors.
+ */
+static int load_contents(struct snapshot *snapshot)
+{
+	int fd;
+	struct stat st;
+	size_t size;
+	ssize_t bytes_read;
+
+	fd = open(snapshot->refs->path, O_RDONLY);
+	if (fd < 0) {
+		if (errno == ENOENT) {
+			/*
+			 * This is OK; it just means that no
+			 * "packed-refs" file has been written yet,
+			 * which is equivalent to it being empty,
+			 * which is its state when initialized with
+			 * zeros.
+			 */
+			return 0;
+		} else {
+			die_errno("couldn't read %s", snapshot->refs->path);
+		}
+	}
+
+	stat_validity_update(&snapshot->validity, fd);
+
+	if (fstat(fd, &st) < 0)
+		die_errno("couldn't stat %s", snapshot->refs->path);
+	size = xsize_t(st.st_size);
+
+	if (!size) {
+		close(fd);
+		return 0;
+	} else if (mmap_strategy == MMAP_NONE || size <= SMALL_FILE_SIZE) {
+		snapshot->buf = xmalloc(size);
+		bytes_read = read_in_full(fd, snapshot->buf, size);
+		if (bytes_read < 0 || bytes_read != size)
+			die_errno("couldn't read %s", snapshot->refs->path);
+		snapshot->mmapped = 0;
+	} else {
+		snapshot->buf = xmmap(NULL, size, PROT_READ, MAP_PRIVATE, fd, 0);
+		snapshot->mmapped = 1;
+	}
+	close(fd);
+
+	snapshot->start = snapshot->buf;
+	snapshot->eof = snapshot->buf + size;
+
+	return 1;
+}
+
+/*
+ * Find the place in `snapshot->buf` where the start of the record for
+ * `refname` starts. If `mustexist` is true and the reference doesn't
+ * exist, then return NULL. If `mustexist` is false and the reference
+ * doesn't exist, then return the point where that reference would be
+ * inserted, or `snapshot->eof` (which might be NULL) if it would be
+ * inserted at the end of the file. In the latter mode, `refname`
+ * doesn't have to be a proper reference name; for example, one could
+ * search for "refs/replace/" to find the start of any replace
+ * references.
+ *
+ * The record is sought using a binary search, so `snapshot->buf` must
+ * be sorted.
+ */
+static const char *find_reference_location(struct snapshot *snapshot,
+					   const char *refname, int mustexist)
+{
+	/*
+	 * This is not *quite* a garden-variety binary search, because
+	 * the data we're searching is made up of records, and we
+	 * always need to find the beginning of a record to do a
+	 * comparison. A "record" here is one line for the reference
+	 * itself and zero or one peel lines that start with '^'. Our
+	 * loop invariant is described in the next two comments.
+	 */
+
+	/*
+	 * A pointer to the character at the start of a record whose
+	 * preceding records all have reference names that come
+	 * *before* `refname`.
+	 */
+	const char *lo = snapshot->start;
+
+	/*
+	 * A pointer to a the first character of a record whose
+	 * reference name comes *after* `refname`.
+	 */
+	const char *hi = snapshot->eof;
+
+	while (lo != hi) {
+		const char *mid, *rec;
+		int cmp;
+
+		mid = lo + (hi - lo) / 2;
+		rec = find_start_of_record(lo, mid);
+		cmp = cmp_record_to_refname(rec, refname);
+		if (cmp < 0) {
+			lo = find_end_of_record(mid, hi);
+		} else if (cmp > 0) {
+			hi = rec;
+		} else {
+			return rec;
+		}
+	}
+
+	if (mustexist)
+		return NULL;
+	else
+		return lo;
+}
+
+/*
+ * Create a newly-allocated `snapshot` of the `packed-refs` file in
+ * its current state and return it. The return value will already have
+ * its reference count incremented.
+ *
+ * A comment line of the form "# pack-refs with: " may contain zero or
+ * more traits. We interpret the traits as follows:
+ *
+ *   Neither `peeled` nor `fully-peeled`:
+ *
+ *      Probably no references are peeled. But if the file contains a
+ *      peeled value for a reference, we will use it.
+ *
+ *   `peeled`:
+ *
+ *      References under "refs/tags/", if they *can* be peeled, *are*
+ *      peeled in this file. References outside of "refs/tags/" are
+ *      probably not peeled even if they could have been, but if we find
+ *      a peeled value for such a reference we will use it.
+ *
+ *   `fully-peeled`:
+ *
+ *      All references in the file that can be peeled are peeled.
+ *      Inversely (and this is more important), any references in the
+ *      file for which no peeled value is recorded is not peelable. This
+ *      trait should typically be written alongside "peeled" for
+ *      compatibility with older clients, but we do not require it
+ *      (i.e., "peeled" is a no-op if "fully-peeled" is set).
+ *
+ *   `sorted`:
+ *
+ *      The references in this file are known to be sorted by refname.
+ */
+static struct snapshot *create_snapshot(struct packed_ref_store *refs)
+{
+	struct snapshot *snapshot = xcalloc(1, sizeof(*snapshot));
+	int sorted = 0;
+
+	snapshot->refs = refs;
+	acquire_snapshot(snapshot);
+	snapshot->peeled = PEELED_NONE;
+
+	if (!load_contents(snapshot))
+		return snapshot;
+
+	/* If the file has a header line, process it: */
+	if (snapshot->buf < snapshot->eof && *snapshot->buf == '#') {
+		char *tmp, *p, *eol;
+		struct string_list traits = STRING_LIST_INIT_NODUP;
+
+		eol = memchr(snapshot->buf, '\n',
+			     snapshot->eof - snapshot->buf);
+		if (!eol)
+			die_unterminated_line(refs->path,
+					      snapshot->buf,
+					      snapshot->eof - snapshot->buf);
+
+		tmp = xmemdupz(snapshot->buf, eol - snapshot->buf);
+
+		if (!skip_prefix(tmp, "# pack-refs with:", (const char **)&p))
+			die_invalid_line(refs->path,
+					 snapshot->buf,
+					 snapshot->eof - snapshot->buf);
+
+		string_list_split_in_place(&traits, p, ' ', -1);
+
+		if (unsorted_string_list_has_string(&traits, "fully-peeled"))
+			snapshot->peeled = PEELED_FULLY;
+		else if (unsorted_string_list_has_string(&traits, "peeled"))
+			snapshot->peeled = PEELED_TAGS;
+
+		sorted = unsorted_string_list_has_string(&traits, "sorted");
+
+		/* perhaps other traits later as well */
+
+		/* The "+ 1" is for the LF character. */
+		snapshot->start = eol + 1;
+
+		string_list_clear(&traits, 0);
+		free(tmp);
+	}
+
+	verify_buffer_safe(snapshot);
+
+	if (!sorted) {
+		sort_snapshot(snapshot);
+
+		/*
+		 * Reordering the records might have moved a short one
+		 * to the end of the buffer, so verify the buffer's
+		 * safety again:
+		 */
+		verify_buffer_safe(snapshot);
+	}
+
+	if (mmap_strategy != MMAP_OK && snapshot->mmapped) {
+		/*
+		 * We don't want to leave the file mmapped, so we are
+		 * forced to make a copy now:
+		 */
+		size_t size = snapshot->eof - snapshot->start;
+		char *buf_copy = xmalloc(size);
+
+		memcpy(buf_copy, snapshot->start, size);
+		clear_snapshot_buffer(snapshot);
+		snapshot->buf = snapshot->start = buf_copy;
+		snapshot->eof = buf_copy + size;
+	}
+
+	return snapshot;
+}
+
+/*
+ * Check that `refs->snapshot` (if present) still reflects the
+ * contents of the `packed-refs` file. If not, clear the snapshot.
+ */
+static void validate_snapshot(struct packed_ref_store *refs)
+{
+	if (refs->snapshot &&
+	    !stat_validity_check(&refs->snapshot->validity, refs->path))
+		clear_snapshot(refs);
+}
+
+/*
+ * Get the `snapshot` for the specified packed_ref_store, creating and
+ * populating it if it hasn't been read before or if the file has been
+ * changed (according to its `validity` field) since it was last read.
+ * On the other hand, if we hold the lock, then assume that the file
+ * hasn't been changed out from under us, so skip the extra `stat()`
+ * call in `stat_validity_check()`. This function does *not* increase
+ * the snapshot's reference count on behalf of the caller.
+ */
+static struct snapshot *get_snapshot(struct packed_ref_store *refs)
+{
+	if (!is_lock_file_locked(&refs->lock))
+		validate_snapshot(refs);
+
+	if (!refs->snapshot)
+		refs->snapshot = create_snapshot(refs);
+
+	return refs->snapshot;
+}
+
+static int packed_read_raw_ref(struct ref_store *ref_store,
+			       const char *refname, struct object_id *oid,
+			       struct strbuf *referent, unsigned int *type)
+{
+	struct packed_ref_store *refs =
+		packed_downcast(ref_store, REF_STORE_READ, "read_raw_ref");
+	struct snapshot *snapshot = get_snapshot(refs);
+	const char *rec;
+
+	*type = 0;
+
+	rec = find_reference_location(snapshot, refname, 1);
+
+	if (!rec) {
+		/* refname is not a packed reference. */
+		errno = ENOENT;
+		return -1;
+	}
+
+	if (get_oid_hex(rec, oid))
+		die_invalid_line(refs->path, rec, snapshot->eof - rec);
+
+	*type = REF_ISPACKED;
+	return 0;
+}
+
+/*
+ * This value is set in `base.flags` if the peeled value of the
+ * current reference is known. In that case, `peeled` contains the
+ * correct peeled value for the reference, which might be `null_oid`
+ * if the reference is not a tag or if it is broken.
+ */
+#define REF_KNOWS_PEELED 0x40
+
+/*
+ * An iterator over a snapshot of a `packed-refs` file.
+ */
+struct packed_ref_iterator {
+	struct ref_iterator base;
+
+	struct snapshot *snapshot;
+
+	/* The current position in the snapshot's buffer: */
+	const char *pos;
+
+	/* The end of the part of the buffer that will be iterated over: */
+	const char *eof;
+
+	/* Scratch space for current values: */
+	struct object_id oid, peeled;
+	struct strbuf refname_buf;
+
+	unsigned int flags;
+};
+
+/*
+ * Move the iterator to the next record in the snapshot, without
+ * respect for whether the record is actually required by the current
+ * iteration. Adjust the fields in `iter` and return `ITER_OK` or
+ * `ITER_DONE`. This function does not free the iterator in the case
+ * of `ITER_DONE`.
+ */
+static int next_record(struct packed_ref_iterator *iter)
+{
+	const char *p = iter->pos, *eol;
+
+	strbuf_reset(&iter->refname_buf);
+
+	if (iter->pos == iter->eof)
+		return ITER_DONE;
+
+	iter->base.flags = REF_ISPACKED;
+
+	if (iter->eof - p < the_hash_algo->hexsz + 2 ||
+	    parse_oid_hex(p, &iter->oid, &p) ||
+	    !isspace(*p++))
+		die_invalid_line(iter->snapshot->refs->path,
+				 iter->pos, iter->eof - iter->pos);
+
+	eol = memchr(p, '\n', iter->eof - p);
+	if (!eol)
+		die_unterminated_line(iter->snapshot->refs->path,
+				      iter->pos, iter->eof - iter->pos);
+
+	strbuf_add(&iter->refname_buf, p, eol - p);
+	iter->base.refname = iter->refname_buf.buf;
+
+	if (check_refname_format(iter->base.refname, REFNAME_ALLOW_ONELEVEL)) {
+		if (!refname_is_safe(iter->base.refname))
+			die("packed refname is dangerous: %s",
+			    iter->base.refname);
+		oidclr(&iter->oid);
+		iter->base.flags |= REF_BAD_NAME | REF_ISBROKEN;
+	}
+	if (iter->snapshot->peeled == PEELED_FULLY ||
+	    (iter->snapshot->peeled == PEELED_TAGS &&
+	     starts_with(iter->base.refname, "refs/tags/")))
+		iter->base.flags |= REF_KNOWS_PEELED;
+
+	iter->pos = eol + 1;
+
+	if (iter->pos < iter->eof && *iter->pos == '^') {
+		p = iter->pos + 1;
+		if (iter->eof - p < the_hash_algo->hexsz + 1 ||
+		    parse_oid_hex(p, &iter->peeled, &p) ||
+		    *p++ != '\n')
+			die_invalid_line(iter->snapshot->refs->path,
+					 iter->pos, iter->eof - iter->pos);
+		iter->pos = p;
+
+		/*
+		 * Regardless of what the file header said, we
+		 * definitely know the value of *this* reference. But
+		 * we suppress it if the reference is broken:
+		 */
+		if ((iter->base.flags & REF_ISBROKEN)) {
+			oidclr(&iter->peeled);
+			iter->base.flags &= ~REF_KNOWS_PEELED;
+		} else {
+			iter->base.flags |= REF_KNOWS_PEELED;
+		}
+	} else {
+		oidclr(&iter->peeled);
+	}
+
+	return ITER_OK;
+}
+
+static int packed_ref_iterator_advance(struct ref_iterator *ref_iterator)
+{
+	struct packed_ref_iterator *iter =
+		(struct packed_ref_iterator *)ref_iterator;
+	int ok;
+
+	while ((ok = next_record(iter)) == ITER_OK) {
+		if (iter->flags & DO_FOR_EACH_PER_WORKTREE_ONLY &&
+		    ref_type(iter->base.refname) != REF_TYPE_PER_WORKTREE)
+			continue;
+
+		if (!(iter->flags & DO_FOR_EACH_INCLUDE_BROKEN) &&
+		    !ref_resolves_to_object(iter->base.refname, &iter->oid,
+					    iter->flags))
+			continue;
+
+		return ITER_OK;
+	}
+
+	if (ref_iterator_abort(ref_iterator) != ITER_DONE)
+		ok = ITER_ERROR;
+
+	return ok;
+}
+
+static int packed_ref_iterator_peel(struct ref_iterator *ref_iterator,
+				   struct object_id *peeled)
+{
+	struct packed_ref_iterator *iter =
+		(struct packed_ref_iterator *)ref_iterator;
+
+	if ((iter->base.flags & REF_KNOWS_PEELED)) {
+		oidcpy(peeled, &iter->peeled);
+		return is_null_oid(&iter->peeled) ? -1 : 0;
+	} else if ((iter->base.flags & (REF_ISBROKEN | REF_ISSYMREF))) {
+		return -1;
+	} else {
+		return !!peel_object(&iter->oid, peeled);
+	}
+}
+
+static int packed_ref_iterator_abort(struct ref_iterator *ref_iterator)
+{
+	struct packed_ref_iterator *iter =
+		(struct packed_ref_iterator *)ref_iterator;
+	int ok = ITER_DONE;
+
+	strbuf_release(&iter->refname_buf);
+	release_snapshot(iter->snapshot);
+	base_ref_iterator_free(ref_iterator);
+	return ok;
+}
+
+static struct ref_iterator_vtable packed_ref_iterator_vtable = {
+	packed_ref_iterator_advance,
+	packed_ref_iterator_peel,
+	packed_ref_iterator_abort
+};
+
+static struct ref_iterator *packed_ref_iterator_begin(
+		struct ref_store *ref_store,
+		const char *prefix, unsigned int flags)
+{
+	struct packed_ref_store *refs;
+	struct snapshot *snapshot;
+	const char *start;
+	struct packed_ref_iterator *iter;
+	struct ref_iterator *ref_iterator;
+	unsigned int required_flags = REF_STORE_READ;
+
+	if (!(flags & DO_FOR_EACH_INCLUDE_BROKEN))
+		required_flags |= REF_STORE_ODB;
+	refs = packed_downcast(ref_store, required_flags, "ref_iterator_begin");
+
+	/*
+	 * Note that `get_snapshot()` internally checks whether the
+	 * snapshot is up to date with what is on disk, and re-reads
+	 * it if not.
+	 */
+	snapshot = get_snapshot(refs);
+
+	if (prefix && *prefix)
+		start = find_reference_location(snapshot, prefix, 0);
+	else
+		start = snapshot->start;
+
+	if (start == snapshot->eof)
+		return empty_ref_iterator_begin();
+
+	iter = xcalloc(1, sizeof(*iter));
+	ref_iterator = &iter->base;
+	base_ref_iterator_init(ref_iterator, &packed_ref_iterator_vtable, 1);
+
+	iter->snapshot = snapshot;
+	acquire_snapshot(snapshot);
+
+	iter->pos = start;
+	iter->eof = snapshot->eof;
+	strbuf_init(&iter->refname_buf, 0);
+
+	iter->base.oid = &iter->oid;
+
+	iter->flags = flags;
+
+	if (prefix && *prefix)
+		/* Stop iteration after we've gone *past* prefix: */
+		ref_iterator = prefix_ref_iterator_begin(ref_iterator, prefix, 0);
+
+	return ref_iterator;
+}
+
+/*
+ * Write an entry to the packed-refs file for the specified refname.
+ * If peeled is non-NULL, write it as the entry's peeled value. On
+ * error, return a nonzero value and leave errno set at the value left
+ * by the failing call to `fprintf()`.
+ */
+static int write_packed_entry(FILE *fh, const char *refname,
+			      const struct object_id *oid,
+			      const struct object_id *peeled)
+{
+	if (fprintf(fh, "%s %s\n", oid_to_hex(oid), refname) < 0 ||
+	    (peeled && fprintf(fh, "^%s\n", oid_to_hex(peeled)) < 0))
+		return -1;
+
+	return 0;
+}
+
+int packed_refs_lock(struct ref_store *ref_store, int flags, struct strbuf *err)
+{
+	struct packed_ref_store *refs =
+		packed_downcast(ref_store, REF_STORE_WRITE | REF_STORE_MAIN,
+				"packed_refs_lock");
+	static int timeout_configured = 0;
+	static int timeout_value = 1000;
+
+	if (!timeout_configured) {
+		git_config_get_int("core.packedrefstimeout", &timeout_value);
+		timeout_configured = 1;
+	}
+
+	/*
+	 * Note that we close the lockfile immediately because we
+	 * don't write new content to it, but rather to a separate
+	 * tempfile.
+	 */
+	if (hold_lock_file_for_update_timeout(
+			    &refs->lock,
+			    refs->path,
+			    flags, timeout_value) < 0) {
+		unable_to_lock_message(refs->path, errno, err);
+		return -1;
+	}
+
+	if (close_lock_file_gently(&refs->lock)) {
+		strbuf_addf(err, "unable to close %s: %s", refs->path, strerror(errno));
+		rollback_lock_file(&refs->lock);
+		return -1;
+	}
+
+	/*
+	 * Now that we hold the `packed-refs` lock, make sure that our
+	 * snapshot matches the current version of the file. Normally
+	 * `get_snapshot()` does that for us, but that function
+	 * assumes that when the file is locked, any existing snapshot
+	 * is still valid. We've just locked the file, but it might
+	 * have changed the moment *before* we locked it.
+	 */
+	validate_snapshot(refs);
+
+	/*
+	 * Now make sure that the packed-refs file as it exists in the
+	 * locked state is loaded into the snapshot:
+	 */
+	get_snapshot(refs);
+	return 0;
+}
+
+void packed_refs_unlock(struct ref_store *ref_store)
+{
+	struct packed_ref_store *refs = packed_downcast(
+			ref_store,
+			REF_STORE_READ | REF_STORE_WRITE,
+			"packed_refs_unlock");
+
+	if (!is_lock_file_locked(&refs->lock))
+		BUG("packed_refs_unlock() called when not locked");
+	rollback_lock_file(&refs->lock);
+}
+
+int packed_refs_is_locked(struct ref_store *ref_store)
+{
+	struct packed_ref_store *refs = packed_downcast(
+			ref_store,
+			REF_STORE_READ | REF_STORE_WRITE,
+			"packed_refs_is_locked");
+
+	return is_lock_file_locked(&refs->lock);
+}
+
+/*
+ * The packed-refs header line that we write out. Perhaps other traits
+ * will be added later.
+ *
+ * Note that earlier versions of Git used to parse these traits by
+ * looking for " trait " in the line. For this reason, the space after
+ * the colon and the trailing space are required.
+ */
+static const char PACKED_REFS_HEADER[] =
+	"# pack-refs with: peeled fully-peeled sorted \n";
+
+static int packed_init_db(struct ref_store *ref_store, struct strbuf *err)
+{
+	/* Nothing to do. */
+	return 0;
+}
+
+/*
+ * Write the packed refs from the current snapshot to the packed-refs
+ * tempfile, incorporating any changes from `updates`. `updates` must
+ * be a sorted string list whose keys are the refnames and whose util
+ * values are `struct ref_update *`. On error, rollback the tempfile,
+ * write an error message to `err`, and return a nonzero value.
+ *
+ * The packfile must be locked before calling this function and will
+ * remain locked when it is done.
+ */
+static int write_with_updates(struct packed_ref_store *refs,
+			      struct string_list *updates,
+			      struct strbuf *err)
+{
+	struct ref_iterator *iter = NULL;
+	size_t i;
+	int ok;
+	FILE *out;
+	struct strbuf sb = STRBUF_INIT;
+	char *packed_refs_path;
+
+	if (!is_lock_file_locked(&refs->lock))
+		BUG("write_with_updates() called while unlocked");
+
+	/*
+	 * If packed-refs is a symlink, we want to overwrite the
+	 * symlinked-to file, not the symlink itself. Also, put the
+	 * staging file next to it:
+	 */
+	packed_refs_path = get_locked_file_path(&refs->lock);
+	strbuf_addf(&sb, "%s.new", packed_refs_path);
+	free(packed_refs_path);
+	refs->tempfile = create_tempfile(sb.buf);
+	if (!refs->tempfile) {
+		strbuf_addf(err, "unable to create file %s: %s",
+			    sb.buf, strerror(errno));
+		strbuf_release(&sb);
+		return -1;
+	}
+	strbuf_release(&sb);
+
+	out = fdopen_tempfile(refs->tempfile, "w");
+	if (!out) {
+		strbuf_addf(err, "unable to fdopen packed-refs tempfile: %s",
+			    strerror(errno));
+		goto error;
+	}
+
+	if (fprintf(out, "%s", PACKED_REFS_HEADER) < 0)
+		goto write_error;
+
+	/*
+	 * We iterate in parallel through the current list of refs and
+	 * the list of updates, processing an entry from at least one
+	 * of the lists each time through the loop. When the current
+	 * list of refs is exhausted, set iter to NULL. When the list
+	 * of updates is exhausted, leave i set to updates->nr.
+	 */
+	iter = packed_ref_iterator_begin(&refs->base, "",
+					 DO_FOR_EACH_INCLUDE_BROKEN);
+	if ((ok = ref_iterator_advance(iter)) != ITER_OK)
+		iter = NULL;
+
+	i = 0;
+
+	while (iter || i < updates->nr) {
+		struct ref_update *update = NULL;
+		int cmp;
+
+		if (i >= updates->nr) {
+			cmp = -1;
+		} else {
+			update = updates->items[i].util;
+
+			if (!iter)
+				cmp = +1;
+			else
+				cmp = strcmp(iter->refname, update->refname);
+		}
+
+		if (!cmp) {
+			/*
+			 * There is both an old value and an update
+			 * for this reference. Check the old value if
+			 * necessary:
+			 */
+			if ((update->flags & REF_HAVE_OLD)) {
+				if (is_null_oid(&update->old_oid)) {
+					strbuf_addf(err, "cannot update ref '%s': "
+						    "reference already exists",
+						    update->refname);
+					goto error;
+				} else if (!oideq(&update->old_oid, iter->oid)) {
+					strbuf_addf(err, "cannot update ref '%s': "
+						    "is at %s but expected %s",
+						    update->refname,
+						    oid_to_hex(iter->oid),
+						    oid_to_hex(&update->old_oid));
+					goto error;
+				}
+			}
+
+			/* Now figure out what to use for the new value: */
+			if ((update->flags & REF_HAVE_NEW)) {
+				/*
+				 * The update takes precedence. Skip
+				 * the iterator over the unneeded
+				 * value.
+				 */
+				if ((ok = ref_iterator_advance(iter)) != ITER_OK)
+					iter = NULL;
+				cmp = +1;
+			} else {
+				/*
+				 * The update doesn't actually want to
+				 * change anything. We're done with it.
+				 */
+				i++;
+				cmp = -1;
+			}
+		} else if (cmp > 0) {
+			/*
+			 * There is no old value but there is an
+			 * update for this reference. Make sure that
+			 * the update didn't expect an existing value:
+			 */
+			if ((update->flags & REF_HAVE_OLD) &&
+			    !is_null_oid(&update->old_oid)) {
+				strbuf_addf(err, "cannot update ref '%s': "
+					    "reference is missing but expected %s",
+					    update->refname,
+					    oid_to_hex(&update->old_oid));
+				goto error;
+			}
+		}
+
+		if (cmp < 0) {
+			/* Pass the old reference through. */
+
+			struct object_id peeled;
+			int peel_error = ref_iterator_peel(iter, &peeled);
+
+			if (write_packed_entry(out, iter->refname,
+					       iter->oid,
+					       peel_error ? NULL : &peeled))
+				goto write_error;
+
+			if ((ok = ref_iterator_advance(iter)) != ITER_OK)
+				iter = NULL;
+		} else if (is_null_oid(&update->new_oid)) {
+			/*
+			 * The update wants to delete the reference,
+			 * and the reference either didn't exist or we
+			 * have already skipped it. So we're done with
+			 * the update (and don't have to write
+			 * anything).
+			 */
+			i++;
+		} else {
+			struct object_id peeled;
+			int peel_error = peel_object(&update->new_oid,
+						     &peeled);
+
+			if (write_packed_entry(out, update->refname,
+					       &update->new_oid,
+					       peel_error ? NULL : &peeled))
+				goto write_error;
+
+			i++;
+		}
+	}
+
+	if (ok != ITER_DONE) {
+		strbuf_addstr(err, "unable to write packed-refs file: "
+			      "error iterating over old contents");
+		goto error;
+	}
+
+	if (close_tempfile_gently(refs->tempfile)) {
+		strbuf_addf(err, "error closing file %s: %s",
+			    get_tempfile_path(refs->tempfile),
+			    strerror(errno));
+		strbuf_release(&sb);
+		delete_tempfile(&refs->tempfile);
+		return -1;
+	}
+
+	return 0;
+
+write_error:
+	strbuf_addf(err, "error writing to %s: %s",
+		    get_tempfile_path(refs->tempfile), strerror(errno));
+
+error:
+	if (iter)
+		ref_iterator_abort(iter);
+
+	delete_tempfile(&refs->tempfile);
+	return -1;
+}
+
+int is_packed_transaction_needed(struct ref_store *ref_store,
+				 struct ref_transaction *transaction)
+{
+	struct packed_ref_store *refs = packed_downcast(
+			ref_store,
+			REF_STORE_READ,
+			"is_packed_transaction_needed");
+	struct strbuf referent = STRBUF_INIT;
+	size_t i;
+	int ret;
+
+	if (!is_lock_file_locked(&refs->lock))
+		BUG("is_packed_transaction_needed() called while unlocked");
+
+	/*
+	 * We're only going to bother returning false for the common,
+	 * trivial case that references are only being deleted, their
+	 * old values are not being checked, and the old `packed-refs`
+	 * file doesn't contain any of those reference(s). This gives
+	 * false positives for some other cases that could
+	 * theoretically be optimized away:
+	 *
+	 * 1. It could be that the old value is being verified without
+	 *    setting a new value. In this case, we could verify the
+	 *    old value here and skip the update if it agrees. If it
+	 *    disagrees, we could either let the update go through
+	 *    (the actual commit would re-detect and report the
+	 *    problem), or come up with a way of reporting such an
+	 *    error to *our* caller.
+	 *
+	 * 2. It could be that a new value is being set, but that it
+	 *    is identical to the current packed value of the
+	 *    reference.
+	 *
+	 * Neither of these cases will come up in the current code,
+	 * because the only caller of this function passes to it a
+	 * transaction that only includes `delete` updates with no
+	 * `old_id`. Even if that ever changes, false positives only
+	 * cause an optimization to be missed; they do not affect
+	 * correctness.
+	 */
+
+	/*
+	 * Start with the cheap checks that don't require old
+	 * reference values to be read:
+	 */
+	for (i = 0; i < transaction->nr; i++) {
+		struct ref_update *update = transaction->updates[i];
+
+		if (update->flags & REF_HAVE_OLD)
+			/* Have to check the old value -> needed. */
+			return 1;
+
+		if ((update->flags & REF_HAVE_NEW) && !is_null_oid(&update->new_oid))
+			/* Have to set a new value -> needed. */
+			return 1;
+	}
+
+	/*
+	 * The transaction isn't checking any old values nor is it
+	 * setting any nonzero new values, so it still might be able
+	 * to be skipped. Now do the more expensive check: the update
+	 * is needed if any of the updates is a delete, and the old
+	 * `packed-refs` file contains a value for that reference.
+	 */
+	ret = 0;
+	for (i = 0; i < transaction->nr; i++) {
+		struct ref_update *update = transaction->updates[i];
+		unsigned int type;
+		struct object_id oid;
+
+		if (!(update->flags & REF_HAVE_NEW))
+			/*
+			 * This reference isn't being deleted -> not
+			 * needed.
+			 */
+			continue;
+
+		if (!refs_read_raw_ref(ref_store, update->refname,
+				       &oid, &referent, &type) ||
+		    errno != ENOENT) {
+			/*
+			 * We have to actually delete that reference
+			 * -> this transaction is needed.
+			 */
+			ret = 1;
+			break;
+		}
+	}
+
+	strbuf_release(&referent);
+	return ret;
+}
+
+struct packed_transaction_backend_data {
+	/* True iff the transaction owns the packed-refs lock. */
+	int own_lock;
+
+	struct string_list updates;
+};
+
+static void packed_transaction_cleanup(struct packed_ref_store *refs,
+				       struct ref_transaction *transaction)
+{
+	struct packed_transaction_backend_data *data = transaction->backend_data;
+
+	if (data) {
+		string_list_clear(&data->updates, 0);
+
+		if (is_tempfile_active(refs->tempfile))
+			delete_tempfile(&refs->tempfile);
+
+		if (data->own_lock && is_lock_file_locked(&refs->lock)) {
+			packed_refs_unlock(&refs->base);
+			data->own_lock = 0;
+		}
+
+		free(data);
+		transaction->backend_data = NULL;
+	}
+
+	transaction->state = REF_TRANSACTION_CLOSED;
+}
+
+static int packed_transaction_prepare(struct ref_store *ref_store,
+				      struct ref_transaction *transaction,
+				      struct strbuf *err)
+{
+	struct packed_ref_store *refs = packed_downcast(
+			ref_store,
+			REF_STORE_READ | REF_STORE_WRITE | REF_STORE_ODB,
+			"ref_transaction_prepare");
+	struct packed_transaction_backend_data *data;
+	size_t i;
+	int ret = TRANSACTION_GENERIC_ERROR;
+
+	/*
+	 * Note that we *don't* skip transactions with zero updates,
+	 * because such a transaction might be executed for the side
+	 * effect of ensuring that all of the references are peeled or
+	 * ensuring that the `packed-refs` file is sorted. If the
+	 * caller wants to optimize away empty transactions, it should
+	 * do so itself.
+	 */
+
+	data = xcalloc(1, sizeof(*data));
+	string_list_init(&data->updates, 0);
+
+	transaction->backend_data = data;
+
+	/*
+	 * Stick the updates in a string list by refname so that we
+	 * can sort them:
+	 */
+	for (i = 0; i < transaction->nr; i++) {
+		struct ref_update *update = transaction->updates[i];
+		struct string_list_item *item =
+			string_list_append(&data->updates, update->refname);
+
+		/* Store a pointer to update in item->util: */
+		item->util = update;
+	}
+	string_list_sort(&data->updates);
+
+	if (ref_update_reject_duplicates(&data->updates, err))
+		goto failure;
+
+	if (!is_lock_file_locked(&refs->lock)) {
+		if (packed_refs_lock(ref_store, 0, err))
+			goto failure;
+		data->own_lock = 1;
+	}
+
+	if (write_with_updates(refs, &data->updates, err))
+		goto failure;
+
+	transaction->state = REF_TRANSACTION_PREPARED;
+	return 0;
+
+failure:
+	packed_transaction_cleanup(refs, transaction);
+	return ret;
+}
+
+static int packed_transaction_abort(struct ref_store *ref_store,
+				    struct ref_transaction *transaction,
+				    struct strbuf *err)
+{
+	struct packed_ref_store *refs = packed_downcast(
+			ref_store,
+			REF_STORE_READ | REF_STORE_WRITE | REF_STORE_ODB,
+			"ref_transaction_abort");
+
+	packed_transaction_cleanup(refs, transaction);
+	return 0;
+}
+
+static int packed_transaction_finish(struct ref_store *ref_store,
+				     struct ref_transaction *transaction,
+				     struct strbuf *err)
+{
+	struct packed_ref_store *refs = packed_downcast(
+			ref_store,
+			REF_STORE_READ | REF_STORE_WRITE | REF_STORE_ODB,
+			"ref_transaction_finish");
+	int ret = TRANSACTION_GENERIC_ERROR;
+	char *packed_refs_path;
+
+	clear_snapshot(refs);
+
+	packed_refs_path = get_locked_file_path(&refs->lock);
+	if (rename_tempfile(&refs->tempfile, packed_refs_path)) {
+		strbuf_addf(err, "error replacing %s: %s",
+			    refs->path, strerror(errno));
+		goto cleanup;
+	}
+
+	ret = 0;
+
+cleanup:
+	free(packed_refs_path);
+	packed_transaction_cleanup(refs, transaction);
+	return ret;
+}
+
+static int packed_initial_transaction_commit(struct ref_store *ref_store,
+					    struct ref_transaction *transaction,
+					    struct strbuf *err)
+{
+	return ref_transaction_commit(transaction, err);
+}
+
+static int packed_delete_refs(struct ref_store *ref_store, const char *msg,
+			     struct string_list *refnames, unsigned int flags)
+{
+	struct packed_ref_store *refs =
+		packed_downcast(ref_store, REF_STORE_WRITE, "delete_refs");
+	struct strbuf err = STRBUF_INIT;
+	struct ref_transaction *transaction;
+	struct string_list_item *item;
+	int ret;
+
+	(void)refs; /* We need the check above, but don't use the variable */
+
+	if (!refnames->nr)
+		return 0;
+
+	/*
+	 * Since we don't check the references' old_oids, the
+	 * individual updates can't fail, so we can pack all of the
+	 * updates into a single transaction.
+	 */
+
+	transaction = ref_store_transaction_begin(ref_store, &err);
+	if (!transaction)
+		return -1;
+
+	for_each_string_list_item(item, refnames) {
+		if (ref_transaction_delete(transaction, item->string, NULL,
+					   flags, msg, &err)) {
+			warning(_("could not delete reference %s: %s"),
+				item->string, err.buf);
+			strbuf_reset(&err);
+		}
+	}
+
+	ret = ref_transaction_commit(transaction, &err);
+
+	if (ret) {
+		if (refnames->nr == 1)
+			error(_("could not delete reference %s: %s"),
+			      refnames->items[0].string, err.buf);
+		else
+			error(_("could not delete references: %s"), err.buf);
+	}
+
+	ref_transaction_free(transaction);
+	strbuf_release(&err);
+	return ret;
+}
+
+static int packed_pack_refs(struct ref_store *ref_store, unsigned int flags)
+{
+	/*
+	 * Packed refs are already packed. It might be that loose refs
+	 * are packed *into* a packed refs store, but that is done by
+	 * updating the packed references via a transaction.
+	 */
+	return 0;
+}
+
+static int packed_create_symref(struct ref_store *ref_store,
+			       const char *refname, const char *target,
+			       const char *logmsg)
+{
+	BUG("packed reference store does not support symrefs");
+}
+
+static int packed_rename_ref(struct ref_store *ref_store,
+			    const char *oldrefname, const char *newrefname,
+			    const char *logmsg)
+{
+	BUG("packed reference store does not support renaming references");
+}
+
+static int packed_copy_ref(struct ref_store *ref_store,
+			   const char *oldrefname, const char *newrefname,
+			   const char *logmsg)
+{
+	BUG("packed reference store does not support copying references");
+}
+
+static struct ref_iterator *packed_reflog_iterator_begin(struct ref_store *ref_store)
+{
+	return empty_ref_iterator_begin();
+}
+
+static int packed_for_each_reflog_ent(struct ref_store *ref_store,
+				      const char *refname,
+				      each_reflog_ent_fn fn, void *cb_data)
+{
+	return 0;
+}
+
+static int packed_for_each_reflog_ent_reverse(struct ref_store *ref_store,
+					      const char *refname,
+					      each_reflog_ent_fn fn,
+					      void *cb_data)
+{
+	return 0;
+}
+
+static int packed_reflog_exists(struct ref_store *ref_store,
+			       const char *refname)
+{
+	return 0;
+}
+
+static int packed_create_reflog(struct ref_store *ref_store,
+			       const char *refname, int force_create,
+			       struct strbuf *err)
+{
+	BUG("packed reference store does not support reflogs");
+}
+
+static int packed_delete_reflog(struct ref_store *ref_store,
+			       const char *refname)
+{
+	return 0;
+}
+
+static int packed_reflog_expire(struct ref_store *ref_store,
+				const char *refname, const struct object_id *oid,
+				unsigned int flags,
+				reflog_expiry_prepare_fn prepare_fn,
+				reflog_expiry_should_prune_fn should_prune_fn,
+				reflog_expiry_cleanup_fn cleanup_fn,
+				void *policy_cb_data)
+{
+	return 0;
+}
+
+struct ref_storage_be refs_be_packed = {
+	NULL,
+	"packed",
+	packed_ref_store_create,
+	packed_init_db,
+	packed_transaction_prepare,
+	packed_transaction_finish,
+	packed_transaction_abort,
+	packed_initial_transaction_commit,
+
+	packed_pack_refs,
+	packed_create_symref,
+	packed_delete_refs,
+	packed_rename_ref,
+	packed_copy_ref,
+
+	packed_ref_iterator_begin,
+	packed_read_raw_ref,
+
+	packed_reflog_iterator_begin,
+	packed_for_each_reflog_ent,
+	packed_for_each_reflog_ent_reverse,
+	packed_reflog_exists,
+	packed_create_reflog,
+	packed_delete_reflog,
+	packed_reflog_expire
+};
diff --git a/refs/packed-backend.h b/refs/packed-backend.h
new file mode 100644
index 000000000000..a01a0aff9c77
--- /dev/null
+++ b/refs/packed-backend.h
@@ -0,0 +1,37 @@
+#ifndef REFS_PACKED_BACKEND_H
+#define REFS_PACKED_BACKEND_H
+
+struct ref_transaction;
+
+/*
+ * Support for storing references in a `packed-refs` file.
+ *
+ * Note that this backend doesn't check for D/F conflicts, because it
+ * doesn't care about them. But usually it should be wrapped in a
+ * `files_ref_store` that prevents D/F conflicts from being created,
+ * even among packed refs.
+ */
+
+struct ref_store *packed_ref_store_create(const char *path,
+					  unsigned int store_flags);
+
+/*
+ * Lock the packed-refs file for writing. Flags is passed to
+ * hold_lock_file_for_update(). Return 0 on success. On errors, write
+ * an error message to `err` and return a nonzero value.
+ */
+int packed_refs_lock(struct ref_store *ref_store, int flags, struct strbuf *err);
+
+void packed_refs_unlock(struct ref_store *ref_store);
+int packed_refs_is_locked(struct ref_store *ref_store);
+
+/*
+ * Return true if `transaction` really needs to be carried out against
+ * the specified packed_ref_store, or false if it can be skipped
+ * (i.e., because it is an obvious NOOP). `ref_store` must be locked
+ * before calling this function.
+ */
+int is_packed_transaction_needed(struct ref_store *ref_store,
+				 struct ref_transaction *transaction);
+
+#endif /* REFS_PACKED_BACKEND_H */
diff --git a/refs/ref-cache.c b/refs/ref-cache.c
new file mode 100644
index 000000000000..b7052f72e2f4
--- /dev/null
+++ b/refs/ref-cache.c
@@ -0,0 +1,551 @@
+#include "../cache.h"
+#include "../refs.h"
+#include "refs-internal.h"
+#include "ref-cache.h"
+#include "../iterator.h"
+
+void add_entry_to_dir(struct ref_dir *dir, struct ref_entry *entry)
+{
+	ALLOC_GROW(dir->entries, dir->nr + 1, dir->alloc);
+	dir->entries[dir->nr++] = entry;
+	/* optimize for the case that entries are added in order */
+	if (dir->nr == 1 ||
+	    (dir->nr == dir->sorted + 1 &&
+	     strcmp(dir->entries[dir->nr - 2]->name,
+		    dir->entries[dir->nr - 1]->name) < 0))
+		dir->sorted = dir->nr;
+}
+
+struct ref_dir *get_ref_dir(struct ref_entry *entry)
+{
+	struct ref_dir *dir;
+	assert(entry->flag & REF_DIR);
+	dir = &entry->u.subdir;
+	if (entry->flag & REF_INCOMPLETE) {
+		if (!dir->cache->fill_ref_dir)
+			BUG("incomplete ref_store without fill_ref_dir function");
+
+		dir->cache->fill_ref_dir(dir->cache->ref_store, dir, entry->name);
+		entry->flag &= ~REF_INCOMPLETE;
+	}
+	return dir;
+}
+
+struct ref_entry *create_ref_entry(const char *refname,
+				   const struct object_id *oid, int flag)
+{
+	struct ref_entry *ref;
+
+	FLEX_ALLOC_STR(ref, name, refname);
+	oidcpy(&ref->u.value.oid, oid);
+	ref->flag = flag;
+	return ref;
+}
+
+struct ref_cache *create_ref_cache(struct ref_store *refs,
+				   fill_ref_dir_fn *fill_ref_dir)
+{
+	struct ref_cache *ret = xcalloc(1, sizeof(*ret));
+
+	ret->ref_store = refs;
+	ret->fill_ref_dir = fill_ref_dir;
+	ret->root = create_dir_entry(ret, "", 0, 1);
+	return ret;
+}
+
+static void clear_ref_dir(struct ref_dir *dir);
+
+static void free_ref_entry(struct ref_entry *entry)
+{
+	if (entry->flag & REF_DIR) {
+		/*
+		 * Do not use get_ref_dir() here, as that might
+		 * trigger the reading of loose refs.
+		 */
+		clear_ref_dir(&entry->u.subdir);
+	}
+	free(entry);
+}
+
+void free_ref_cache(struct ref_cache *cache)
+{
+	free_ref_entry(cache->root);
+	free(cache);
+}
+
+/*
+ * Clear and free all entries in dir, recursively.
+ */
+static void clear_ref_dir(struct ref_dir *dir)
+{
+	int i;
+	for (i = 0; i < dir->nr; i++)
+		free_ref_entry(dir->entries[i]);
+	FREE_AND_NULL(dir->entries);
+	dir->sorted = dir->nr = dir->alloc = 0;
+}
+
+struct ref_entry *create_dir_entry(struct ref_cache *cache,
+				   const char *dirname, size_t len,
+				   int incomplete)
+{
+	struct ref_entry *direntry;
+
+	FLEX_ALLOC_MEM(direntry, name, dirname, len);
+	direntry->u.subdir.cache = cache;
+	direntry->flag = REF_DIR | (incomplete ? REF_INCOMPLETE : 0);
+	return direntry;
+}
+
+static int ref_entry_cmp(const void *a, const void *b)
+{
+	struct ref_entry *one = *(struct ref_entry **)a;
+	struct ref_entry *two = *(struct ref_entry **)b;
+	return strcmp(one->name, two->name);
+}
+
+static void sort_ref_dir(struct ref_dir *dir);
+
+struct string_slice {
+	size_t len;
+	const char *str;
+};
+
+static int ref_entry_cmp_sslice(const void *key_, const void *ent_)
+{
+	const struct string_slice *key = key_;
+	const struct ref_entry *ent = *(const struct ref_entry * const *)ent_;
+	int cmp = strncmp(key->str, ent->name, key->len);
+	if (cmp)
+		return cmp;
+	return '\0' - (unsigned char)ent->name[key->len];
+}
+
+int search_ref_dir(struct ref_dir *dir, const char *refname, size_t len)
+{
+	struct ref_entry **r;
+	struct string_slice key;
+
+	if (refname == NULL || !dir->nr)
+		return -1;
+
+	sort_ref_dir(dir);
+	key.len = len;
+	key.str = refname;
+	r = bsearch(&key, dir->entries, dir->nr, sizeof(*dir->entries),
+		    ref_entry_cmp_sslice);
+
+	if (r == NULL)
+		return -1;
+
+	return r - dir->entries;
+}
+
+/*
+ * Search for a directory entry directly within dir (without
+ * recursing).  Sort dir if necessary.  subdirname must be a directory
+ * name (i.e., end in '/').  If mkdir is set, then create the
+ * directory if it is missing; otherwise, return NULL if the desired
+ * directory cannot be found.  dir must already be complete.
+ */
+static struct ref_dir *search_for_subdir(struct ref_dir *dir,
+					 const char *subdirname, size_t len,
+					 int mkdir)
+{
+	int entry_index = search_ref_dir(dir, subdirname, len);
+	struct ref_entry *entry;
+	if (entry_index == -1) {
+		if (!mkdir)
+			return NULL;
+		/*
+		 * Since dir is complete, the absence of a subdir
+		 * means that the subdir really doesn't exist;
+		 * therefore, create an empty record for it but mark
+		 * the record complete.
+		 */
+		entry = create_dir_entry(dir->cache, subdirname, len, 0);
+		add_entry_to_dir(dir, entry);
+	} else {
+		entry = dir->entries[entry_index];
+	}
+	return get_ref_dir(entry);
+}
+
+/*
+ * If refname is a reference name, find the ref_dir within the dir
+ * tree that should hold refname. If refname is a directory name
+ * (i.e., it ends in '/'), then return that ref_dir itself. dir must
+ * represent the top-level directory and must already be complete.
+ * Sort ref_dirs and recurse into subdirectories as necessary. If
+ * mkdir is set, then create any missing directories; otherwise,
+ * return NULL if the desired directory cannot be found.
+ */
+static struct ref_dir *find_containing_dir(struct ref_dir *dir,
+					   const char *refname, int mkdir)
+{
+	const char *slash;
+	for (slash = strchr(refname, '/'); slash; slash = strchr(slash + 1, '/')) {
+		size_t dirnamelen = slash - refname + 1;
+		struct ref_dir *subdir;
+		subdir = search_for_subdir(dir, refname, dirnamelen, mkdir);
+		if (!subdir) {
+			dir = NULL;
+			break;
+		}
+		dir = subdir;
+	}
+
+	return dir;
+}
+
+struct ref_entry *find_ref_entry(struct ref_dir *dir, const char *refname)
+{
+	int entry_index;
+	struct ref_entry *entry;
+	dir = find_containing_dir(dir, refname, 0);
+	if (!dir)
+		return NULL;
+	entry_index = search_ref_dir(dir, refname, strlen(refname));
+	if (entry_index == -1)
+		return NULL;
+	entry = dir->entries[entry_index];
+	return (entry->flag & REF_DIR) ? NULL : entry;
+}
+
+int remove_entry_from_dir(struct ref_dir *dir, const char *refname)
+{
+	int refname_len = strlen(refname);
+	int entry_index;
+	struct ref_entry *entry;
+	int is_dir = refname[refname_len - 1] == '/';
+	if (is_dir) {
+		/*
+		 * refname represents a reference directory.  Remove
+		 * the trailing slash; otherwise we will get the
+		 * directory *representing* refname rather than the
+		 * one *containing* it.
+		 */
+		char *dirname = xmemdupz(refname, refname_len - 1);
+		dir = find_containing_dir(dir, dirname, 0);
+		free(dirname);
+	} else {
+		dir = find_containing_dir(dir, refname, 0);
+	}
+	if (!dir)
+		return -1;
+	entry_index = search_ref_dir(dir, refname, refname_len);
+	if (entry_index == -1)
+		return -1;
+	entry = dir->entries[entry_index];
+
+	MOVE_ARRAY(&dir->entries[entry_index],
+		   &dir->entries[entry_index + 1], dir->nr - entry_index - 1);
+	dir->nr--;
+	if (dir->sorted > entry_index)
+		dir->sorted--;
+	free_ref_entry(entry);
+	return dir->nr;
+}
+
+int add_ref_entry(struct ref_dir *dir, struct ref_entry *ref)
+{
+	dir = find_containing_dir(dir, ref->name, 1);
+	if (!dir)
+		return -1;
+	add_entry_to_dir(dir, ref);
+	return 0;
+}
+
+/*
+ * Emit a warning and return true iff ref1 and ref2 have the same name
+ * and the same oid. Die if they have the same name but different
+ * oids.
+ */
+static int is_dup_ref(const struct ref_entry *ref1, const struct ref_entry *ref2)
+{
+	if (strcmp(ref1->name, ref2->name))
+		return 0;
+
+	/* Duplicate name; make sure that they don't conflict: */
+
+	if ((ref1->flag & REF_DIR) || (ref2->flag & REF_DIR))
+		/* This is impossible by construction */
+		die("Reference directory conflict: %s", ref1->name);
+
+	if (!oideq(&ref1->u.value.oid, &ref2->u.value.oid))
+		die("Duplicated ref, and SHA1s don't match: %s", ref1->name);
+
+	warning("Duplicated ref: %s", ref1->name);
+	return 1;
+}
+
+/*
+ * Sort the entries in dir non-recursively (if they are not already
+ * sorted) and remove any duplicate entries.
+ */
+static void sort_ref_dir(struct ref_dir *dir)
+{
+	int i, j;
+	struct ref_entry *last = NULL;
+
+	/*
+	 * This check also prevents passing a zero-length array to qsort(),
+	 * which is a problem on some platforms.
+	 */
+	if (dir->sorted == dir->nr)
+		return;
+
+	QSORT(dir->entries, dir->nr, ref_entry_cmp);
+
+	/* Remove any duplicates: */
+	for (i = 0, j = 0; j < dir->nr; j++) {
+		struct ref_entry *entry = dir->entries[j];
+		if (last && is_dup_ref(last, entry))
+			free_ref_entry(entry);
+		else
+			last = dir->entries[i++] = entry;
+	}
+	dir->sorted = dir->nr = i;
+}
+
+enum prefix_state {
+	/* All refs within the directory would match prefix: */
+	PREFIX_CONTAINS_DIR,
+
+	/* Some, but not all, refs within the directory might match prefix: */
+	PREFIX_WITHIN_DIR,
+
+	/* No refs within the directory could possibly match prefix: */
+	PREFIX_EXCLUDES_DIR
+};
+
+/*
+ * Return a `prefix_state` constant describing the relationship
+ * between the directory with the specified `dirname` and `prefix`.
+ */
+static enum prefix_state overlaps_prefix(const char *dirname,
+					 const char *prefix)
+{
+	while (*prefix && *dirname == *prefix) {
+		dirname++;
+		prefix++;
+	}
+	if (!*prefix)
+		return PREFIX_CONTAINS_DIR;
+	else if (!*dirname)
+		return PREFIX_WITHIN_DIR;
+	else
+		return PREFIX_EXCLUDES_DIR;
+}
+
+/*
+ * Load all of the refs from `dir` (recursively) that could possibly
+ * contain references matching `prefix` into our in-memory cache. If
+ * `prefix` is NULL, prime unconditionally.
+ */
+static void prime_ref_dir(struct ref_dir *dir, const char *prefix)
+{
+	/*
+	 * The hard work of loading loose refs is done by get_ref_dir(), so we
+	 * just need to recurse through all of the sub-directories. We do not
+	 * even need to care about sorting, as traversal order does not matter
+	 * to us.
+	 */
+	int i;
+	for (i = 0; i < dir->nr; i++) {
+		struct ref_entry *entry = dir->entries[i];
+		if (!(entry->flag & REF_DIR)) {
+			/* Not a directory; no need to recurse. */
+		} else if (!prefix) {
+			/* Recurse in any case: */
+			prime_ref_dir(get_ref_dir(entry), NULL);
+		} else {
+			switch (overlaps_prefix(entry->name, prefix)) {
+			case PREFIX_CONTAINS_DIR:
+				/*
+				 * Recurse, and from here down we
+				 * don't have to check the prefix
+				 * anymore:
+				 */
+				prime_ref_dir(get_ref_dir(entry), NULL);
+				break;
+			case PREFIX_WITHIN_DIR:
+				prime_ref_dir(get_ref_dir(entry), prefix);
+				break;
+			case PREFIX_EXCLUDES_DIR:
+				/* No need to prime this directory. */
+				break;
+			}
+		}
+	}
+}
+
+/*
+ * A level in the reference hierarchy that is currently being iterated
+ * through.
+ */
+struct cache_ref_iterator_level {
+	/*
+	 * The ref_dir being iterated over at this level. The ref_dir
+	 * is sorted before being stored here.
+	 */
+	struct ref_dir *dir;
+
+	enum prefix_state prefix_state;
+
+	/*
+	 * The index of the current entry within dir (which might
+	 * itself be a directory). If index == -1, then the iteration
+	 * hasn't yet begun. If index == dir->nr, then the iteration
+	 * through this level is over.
+	 */
+	int index;
+};
+
+/*
+ * Represent an iteration through a ref_dir in the memory cache. The
+ * iteration recurses through subdirectories.
+ */
+struct cache_ref_iterator {
+	struct ref_iterator base;
+
+	/*
+	 * The number of levels currently on the stack. This is always
+	 * at least 1, because when it becomes zero the iteration is
+	 * ended and this struct is freed.
+	 */
+	size_t levels_nr;
+
+	/* The number of levels that have been allocated on the stack */
+	size_t levels_alloc;
+
+	/*
+	 * Only include references with this prefix in the iteration.
+	 * The prefix is matched textually, without regard for path
+	 * component boundaries.
+	 */
+	const char *prefix;
+
+	/*
+	 * A stack of levels. levels[0] is the uppermost level that is
+	 * being iterated over in this iteration. (This is not
+	 * necessary the top level in the references hierarchy. If we
+	 * are iterating through a subtree, then levels[0] will hold
+	 * the ref_dir for that subtree, and subsequent levels will go
+	 * on from there.)
+	 */
+	struct cache_ref_iterator_level *levels;
+};
+
+static int cache_ref_iterator_advance(struct ref_iterator *ref_iterator)
+{
+	struct cache_ref_iterator *iter =
+		(struct cache_ref_iterator *)ref_iterator;
+
+	while (1) {
+		struct cache_ref_iterator_level *level =
+			&iter->levels[iter->levels_nr - 1];
+		struct ref_dir *dir = level->dir;
+		struct ref_entry *entry;
+		enum prefix_state entry_prefix_state;
+
+		if (level->index == -1)
+			sort_ref_dir(dir);
+
+		if (++level->index == level->dir->nr) {
+			/* This level is exhausted; pop up a level */
+			if (--iter->levels_nr == 0)
+				return ref_iterator_abort(ref_iterator);
+
+			continue;
+		}
+
+		entry = dir->entries[level->index];
+
+		if (level->prefix_state == PREFIX_WITHIN_DIR) {
+			entry_prefix_state = overlaps_prefix(entry->name, iter->prefix);
+			if (entry_prefix_state == PREFIX_EXCLUDES_DIR)
+				continue;
+		} else {
+			entry_prefix_state = level->prefix_state;
+		}
+
+		if (entry->flag & REF_DIR) {
+			/* push down a level */
+			ALLOC_GROW(iter->levels, iter->levels_nr + 1,
+				   iter->levels_alloc);
+
+			level = &iter->levels[iter->levels_nr++];
+			level->dir = get_ref_dir(entry);
+			level->prefix_state = entry_prefix_state;
+			level->index = -1;
+		} else {
+			iter->base.refname = entry->name;
+			iter->base.oid = &entry->u.value.oid;
+			iter->base.flags = entry->flag;
+			return ITER_OK;
+		}
+	}
+}
+
+static int cache_ref_iterator_peel(struct ref_iterator *ref_iterator,
+				   struct object_id *peeled)
+{
+	return peel_object(ref_iterator->oid, peeled);
+}
+
+static int cache_ref_iterator_abort(struct ref_iterator *ref_iterator)
+{
+	struct cache_ref_iterator *iter =
+		(struct cache_ref_iterator *)ref_iterator;
+
+	free((char *)iter->prefix);
+	free(iter->levels);
+	base_ref_iterator_free(ref_iterator);
+	return ITER_DONE;
+}
+
+static struct ref_iterator_vtable cache_ref_iterator_vtable = {
+	cache_ref_iterator_advance,
+	cache_ref_iterator_peel,
+	cache_ref_iterator_abort
+};
+
+struct ref_iterator *cache_ref_iterator_begin(struct ref_cache *cache,
+					      const char *prefix,
+					      int prime_dir)
+{
+	struct ref_dir *dir;
+	struct cache_ref_iterator *iter;
+	struct ref_iterator *ref_iterator;
+	struct cache_ref_iterator_level *level;
+
+	dir = get_ref_dir(cache->root);
+	if (prefix && *prefix)
+		dir = find_containing_dir(dir, prefix, 0);
+	if (!dir)
+		/* There's nothing to iterate over. */
+		return empty_ref_iterator_begin();
+
+	if (prime_dir)
+		prime_ref_dir(dir, prefix);
+
+	iter = xcalloc(1, sizeof(*iter));
+	ref_iterator = &iter->base;
+	base_ref_iterator_init(ref_iterator, &cache_ref_iterator_vtable, 1);
+	ALLOC_GROW(iter->levels, 10, iter->levels_alloc);
+
+	iter->levels_nr = 1;
+	level = &iter->levels[0];
+	level->index = -1;
+	level->dir = dir;
+
+	if (prefix && *prefix) {
+		iter->prefix = xstrdup(prefix);
+		level->prefix_state = PREFIX_WITHIN_DIR;
+	} else {
+		level->prefix_state = PREFIX_CONTAINS_DIR;
+	}
+
+	return ref_iterator;
+}
diff --git a/refs/ref-cache.h b/refs/ref-cache.h
new file mode 100644
index 000000000000..3bfb89d2b343
--- /dev/null
+++ b/refs/ref-cache.h
@@ -0,0 +1,243 @@
+#ifndef REFS_REF_CACHE_H
+#define REFS_REF_CACHE_H
+
+#include "cache.h"
+
+struct ref_dir;
+struct ref_store;
+
+/*
+ * If this ref_cache is filled lazily, this function is used to load
+ * information into the specified ref_dir (shallow or deep, at the
+ * option of the ref_store). dirname includes a trailing slash.
+ */
+typedef void fill_ref_dir_fn(struct ref_store *ref_store,
+			     struct ref_dir *dir, const char *dirname);
+
+struct ref_cache {
+	struct ref_entry *root;
+
+	/* A pointer to the ref_store whose cache this is: */
+	struct ref_store *ref_store;
+
+	/*
+	 * Function used (if necessary) to lazily-fill cache. May be
+	 * NULL.
+	 */
+	fill_ref_dir_fn *fill_ref_dir;
+};
+
+/*
+ * Information used (along with the information in ref_entry) to
+ * describe a single cached reference.  This data structure only
+ * occurs embedded in a union in struct ref_entry, and only when
+ * (ref_entry->flag & REF_DIR) is zero.
+ */
+struct ref_value {
+	/*
+	 * The name of the object to which this reference resolves
+	 * (which may be a tag object).  If REF_ISBROKEN, this is
+	 * null.  If REF_ISSYMREF, then this is the name of the object
+	 * referred to by the last reference in the symlink chain.
+	 */
+	struct object_id oid;
+};
+
+/*
+ * Information used (along with the information in ref_entry) to
+ * describe a level in the hierarchy of references.  This data
+ * structure only occurs embedded in a union in struct ref_entry, and
+ * only when (ref_entry.flag & REF_DIR) is set.  In that case,
+ * (ref_entry.flag & REF_INCOMPLETE) determines whether the references
+ * in the directory have already been read:
+ *
+ *     (ref_entry.flag & REF_INCOMPLETE) unset -- a directory of loose
+ *         or packed references, already read.
+ *
+ *     (ref_entry.flag & REF_INCOMPLETE) set -- a directory of loose
+ *         references that hasn't been read yet (nor has any of its
+ *         subdirectories).
+ *
+ * Entries within a directory are stored within a growable array of
+ * pointers to ref_entries (entries, nr, alloc).  Entries 0 <= i <
+ * sorted are sorted by their component name in strcmp() order and the
+ * remaining entries are unsorted.
+ *
+ * Loose references are read lazily, one directory at a time.  When a
+ * directory of loose references is read, then all of the references
+ * in that directory are stored, and REF_INCOMPLETE stubs are created
+ * for any subdirectories, but the subdirectories themselves are not
+ * read.  The reading is triggered by get_ref_dir().
+ */
+struct ref_dir {
+	int nr, alloc;
+
+	/*
+	 * Entries with index 0 <= i < sorted are sorted by name.  New
+	 * entries are appended to the list unsorted, and are sorted
+	 * only when required; thus we avoid the need to sort the list
+	 * after the addition of every reference.
+	 */
+	int sorted;
+
+	/* The ref_cache containing this entry: */
+	struct ref_cache *cache;
+
+	struct ref_entry **entries;
+};
+
+/*
+ * Bit values for ref_entry::flag.  REF_ISSYMREF=0x01,
+ * REF_ISPACKED=0x02, REF_ISBROKEN=0x04 and REF_BAD_NAME=0x08 are
+ * public values; see refs.h.
+ */
+
+/* ref_entry represents a directory of references */
+#define REF_DIR 0x10
+
+/*
+ * Entry has not yet been read from disk (used only for REF_DIR
+ * entries representing loose references)
+ */
+#define REF_INCOMPLETE 0x20
+
+/*
+ * A ref_entry represents either a reference or a "subdirectory" of
+ * references.
+ *
+ * Each directory in the reference namespace is represented by a
+ * ref_entry with (flags & REF_DIR) set and containing a subdir member
+ * that holds the entries in that directory that have been read so
+ * far.  If (flags & REF_INCOMPLETE) is set, then the directory and
+ * its subdirectories haven't been read yet.  REF_INCOMPLETE is only
+ * used for loose reference directories.
+ *
+ * References are represented by a ref_entry with (flags & REF_DIR)
+ * unset and a value member that describes the reference's value.  The
+ * flag member is at the ref_entry level, but it is also needed to
+ * interpret the contents of the value field (in other words, a
+ * ref_value object is not very much use without the enclosing
+ * ref_entry).
+ *
+ * Reference names cannot end with slash and directories' names are
+ * always stored with a trailing slash (except for the top-level
+ * directory, which is always denoted by "").  This has two nice
+ * consequences: (1) when the entries in each subdir are sorted
+ * lexicographically by name (as they usually are), the references in
+ * a whole tree can be generated in lexicographic order by traversing
+ * the tree in left-to-right, depth-first order; (2) the names of
+ * references and subdirectories cannot conflict, and therefore the
+ * presence of an empty subdirectory does not block the creation of a
+ * similarly-named reference.  (The fact that reference names with the
+ * same leading components can conflict *with each other* is a
+ * separate issue that is regulated by refs_verify_refname_available().)
+ *
+ * Please note that the name field contains the fully-qualified
+ * reference (or subdirectory) name.  Space could be saved by only
+ * storing the relative names.  But that would require the full names
+ * to be generated on the fly when iterating in do_for_each_ref(), and
+ * would break callback functions, who have always been able to assume
+ * that the name strings that they are passed will not be freed during
+ * the iteration.
+ */
+struct ref_entry {
+	unsigned char flag; /* ISSYMREF? ISPACKED? */
+	union {
+		struct ref_value value; /* if not (flags&REF_DIR) */
+		struct ref_dir subdir; /* if (flags&REF_DIR) */
+	} u;
+	/*
+	 * The full name of the reference (e.g., "refs/heads/master")
+	 * or the full name of the directory with a trailing slash
+	 * (e.g., "refs/heads/"):
+	 */
+	char name[FLEX_ARRAY];
+};
+
+/*
+ * Return the index of the entry with the given refname from the
+ * ref_dir (non-recursively), sorting dir if necessary.  Return -1 if
+ * no such entry is found.  dir must already be complete.
+ */
+int search_ref_dir(struct ref_dir *dir, const char *refname, size_t len);
+
+struct ref_dir *get_ref_dir(struct ref_entry *entry);
+
+/*
+ * Create a struct ref_entry object for the specified dirname.
+ * dirname is the name of the directory with a trailing slash (e.g.,
+ * "refs/heads/") or "" for the top-level directory.
+ */
+struct ref_entry *create_dir_entry(struct ref_cache *cache,
+				   const char *dirname, size_t len,
+				   int incomplete);
+
+struct ref_entry *create_ref_entry(const char *refname,
+				   const struct object_id *oid, int flag);
+
+/*
+ * Return a pointer to a new `ref_cache`. Its top-level starts out
+ * marked incomplete. If `fill_ref_dir` is non-NULL, it is the
+ * function called to fill in incomplete directories in the
+ * `ref_cache` when they are accessed. If it is NULL, then the whole
+ * `ref_cache` must be filled (including clearing its directories'
+ * `REF_INCOMPLETE` bits) before it is used, and `refs` can be NULL,
+ * too.
+ */
+struct ref_cache *create_ref_cache(struct ref_store *refs,
+				   fill_ref_dir_fn *fill_ref_dir);
+
+/*
+ * Free the `ref_cache` and all of its associated data.
+ */
+void free_ref_cache(struct ref_cache *cache);
+
+/*
+ * Add a ref_entry to the end of dir (unsorted).  Entry is always
+ * stored directly in dir; no recursion into subdirectories is
+ * done.
+ */
+void add_entry_to_dir(struct ref_dir *dir, struct ref_entry *entry);
+
+/*
+ * Remove the entry with the given name from dir, recursing into
+ * subdirectories as necessary.  If refname is the name of a directory
+ * (i.e., ends with '/'), then remove the directory and its contents.
+ * If the removal was successful, return the number of entries
+ * remaining in the directory entry that contained the deleted entry.
+ * If the name was not found, return -1.  Please note that this
+ * function only deletes the entry from the cache; it does not delete
+ * it from the filesystem or ensure that other cache entries (which
+ * might be symbolic references to the removed entry) are updated.
+ * Nor does it remove any containing dir entries that might be made
+ * empty by the removal.  dir must represent the top-level directory
+ * and must already be complete.
+ */
+int remove_entry_from_dir(struct ref_dir *dir, const char *refname);
+
+/*
+ * Add a ref_entry to the ref_dir (unsorted), recursing into
+ * subdirectories as necessary.  dir must represent the top-level
+ * directory.  Return 0 on success.
+ */
+int add_ref_entry(struct ref_dir *dir, struct ref_entry *ref);
+
+/*
+ * Find the value entry with the given name in dir, sorting ref_dirs
+ * and recursing into subdirectories as necessary.  If the name is not
+ * found or it corresponds to a directory entry, return NULL.
+ */
+struct ref_entry *find_ref_entry(struct ref_dir *dir, const char *refname);
+
+/*
+ * Start iterating over references in `cache`. If `prefix` is
+ * specified, only include references whose names start with that
+ * prefix. If `prime_dir` is true, then fill any incomplete
+ * directories before beginning the iteration. The output is ordered
+ * by refname.
+ */
+struct ref_iterator *cache_ref_iterator_begin(struct ref_cache *cache,
+					      const char *prefix,
+					      int prime_dir);
+
+#endif /* REFS_REF_CACHE_H */
diff --git a/refs/refs-internal.h b/refs/refs-internal.h
new file mode 100644
index 000000000000..f2d8c0123a77
--- /dev/null
+++ b/refs/refs-internal.h
@@ -0,0 +1,678 @@
+#ifndef REFS_REFS_INTERNAL_H
+#define REFS_REFS_INTERNAL_H
+
+#include "cache.h"
+#include "refs.h"
+#include "iterator.h"
+
+struct ref_transaction;
+
+/*
+ * Data structures and functions for the internal use of the refs
+ * module. Code outside of the refs module should use only the public
+ * functions defined in "refs.h", and should *not* include this file.
+ */
+
+/*
+ * The following flags can appear in `ref_update::flags`. Their
+ * numerical values must not conflict with those of REF_NO_DEREF and
+ * REF_FORCE_CREATE_REFLOG, which are also stored in
+ * `ref_update::flags`.
+ */
+
+/*
+ * The reference should be updated to new_oid.
+ */
+#define REF_HAVE_NEW (1 << 2)
+
+/*
+ * The current reference's value should be checked to make sure that
+ * it agrees with old_oid.
+ */
+#define REF_HAVE_OLD (1 << 3)
+
+/*
+ * Return the length of time to retry acquiring a loose reference lock
+ * before giving up, in milliseconds:
+ */
+long get_files_ref_lock_timeout_ms(void);
+
+/*
+ * Return true iff refname is minimally safe. "Safe" here means that
+ * deleting a loose reference by this name will not do any damage, for
+ * example by causing a file that is not a reference to be deleted.
+ * This function does not check that the reference name is legal; for
+ * that, use check_refname_format().
+ *
+ * A refname that starts with "refs/" is considered safe iff it
+ * doesn't contain any "." or ".." components or consecutive '/'
+ * characters, end with '/', or (on Windows) contain any '\'
+ * characters. Names that do not start with "refs/" are considered
+ * safe iff they consist entirely of upper case characters and '_'
+ * (like "HEAD" and "MERGE_HEAD" but not "config" or "FOO/BAR").
+ */
+int refname_is_safe(const char *refname);
+
+/*
+ * Helper function: return true if refname, which has the specified
+ * oid and flags, can be resolved to an object in the database. If the
+ * referred-to object does not exist, emit a warning and return false.
+ */
+int ref_resolves_to_object(const char *refname,
+			   const struct object_id *oid,
+			   unsigned int flags);
+
+enum peel_status {
+	/* object was peeled successfully: */
+	PEEL_PEELED = 0,
+
+	/*
+	 * object cannot be peeled because the named object (or an
+	 * object referred to by a tag in the peel chain), does not
+	 * exist.
+	 */
+	PEEL_INVALID = -1,
+
+	/* object cannot be peeled because it is not a tag: */
+	PEEL_NON_TAG = -2,
+
+	/* ref_entry contains no peeled value because it is a symref: */
+	PEEL_IS_SYMREF = -3,
+
+	/*
+	 * ref_entry cannot be peeled because it is broken (i.e., the
+	 * symbolic reference cannot even be resolved to an object
+	 * name):
+	 */
+	PEEL_BROKEN = -4
+};
+
+/*
+ * Peel the named object; i.e., if the object is a tag, resolve the
+ * tag recursively until a non-tag is found.  If successful, store the
+ * result to oid and return PEEL_PEELED.  If the object is not a tag
+ * or is not valid, return PEEL_NON_TAG or PEEL_INVALID, respectively,
+ * and leave oid unchanged.
+ */
+enum peel_status peel_object(const struct object_id *name, struct object_id *oid);
+
+/*
+ * Copy the reflog message msg to sb while cleaning up the whitespaces.
+ * Especially, convert LF to space, because reflog file is one line per entry.
+ */
+void copy_reflog_msg(struct strbuf *sb, const char *msg);
+
+/**
+ * Information needed for a single ref update. Set new_oid to the new
+ * value or to null_oid to delete the ref. To check the old value
+ * while the ref is locked, set (flags & REF_HAVE_OLD) and set old_oid
+ * to the old value, or to null_oid to ensure the ref does not exist
+ * before update.
+ */
+struct ref_update {
+	/*
+	 * If (flags & REF_HAVE_NEW), set the reference to this value
+	 * (or delete it, if `new_oid` is `null_oid`).
+	 */
+	struct object_id new_oid;
+
+	/*
+	 * If (flags & REF_HAVE_OLD), check that the reference
+	 * previously had this value (or didn't previously exist, if
+	 * `old_oid` is `null_oid`).
+	 */
+	struct object_id old_oid;
+
+	/*
+	 * One or more of REF_NO_DEREF, REF_FORCE_CREATE_REFLOG,
+	 * REF_HAVE_NEW, REF_HAVE_OLD, or backend-specific flags.
+	 */
+	unsigned int flags;
+
+	void *backend_data;
+	unsigned int type;
+	char *msg;
+
+	/*
+	 * If this ref_update was split off of a symref update via
+	 * split_symref_update(), then this member points at that
+	 * update. This is used for two purposes:
+	 * 1. When reporting errors, we report the refname under which
+	 *    the update was originally requested.
+	 * 2. When we read the old value of this reference, we
+	 *    propagate it back to its parent update for recording in
+	 *    the latter's reflog.
+	 */
+	struct ref_update *parent_update;
+
+	const char refname[FLEX_ARRAY];
+};
+
+int refs_read_raw_ref(struct ref_store *ref_store,
+		      const char *refname, struct object_id *oid,
+		      struct strbuf *referent, unsigned int *type);
+
+/*
+ * Write an error to `err` and return a nonzero value iff the same
+ * refname appears multiple times in `refnames`. `refnames` must be
+ * sorted on entry to this function.
+ */
+int ref_update_reject_duplicates(struct string_list *refnames,
+				 struct strbuf *err);
+
+/*
+ * Add a ref_update with the specified properties to transaction, and
+ * return a pointer to the new object. This function does not verify
+ * that refname is well-formed. new_oid and old_oid are only
+ * dereferenced if the REF_HAVE_NEW and REF_HAVE_OLD bits,
+ * respectively, are set in flags.
+ */
+struct ref_update *ref_transaction_add_update(
+		struct ref_transaction *transaction,
+		const char *refname, unsigned int flags,
+		const struct object_id *new_oid,
+		const struct object_id *old_oid,
+		const char *msg);
+
+/*
+ * Transaction states.
+ *
+ * OPEN:   The transaction is initialized and new updates can still be
+ *         added to it. An OPEN transaction can be prepared,
+ *         committed, freed, or aborted (freeing and aborting an open
+ *         transaction are equivalent).
+ *
+ * PREPARED: ref_transaction_prepare(), which locks all of the
+ *         references involved in the update and checks that the
+ *         update has no errors, has been called successfully for the
+ *         transaction. A PREPARED transaction can be committed or
+ *         aborted.
+ *
+ * CLOSED: The transaction is no longer active. A transaction becomes
+ *         CLOSED if there is a failure while building the transaction
+ *         or if a transaction is committed or aborted. A CLOSED
+ *         transaction can only be freed.
+ */
+enum ref_transaction_state {
+	REF_TRANSACTION_OPEN     = 0,
+	REF_TRANSACTION_PREPARED = 1,
+	REF_TRANSACTION_CLOSED   = 2
+};
+
+/*
+ * Data structure for holding a reference transaction, which can
+ * consist of checks and updates to multiple references, carried out
+ * as atomically as possible.  This structure is opaque to callers.
+ */
+struct ref_transaction {
+	struct ref_store *ref_store;
+	struct ref_update **updates;
+	size_t alloc;
+	size_t nr;
+	enum ref_transaction_state state;
+	void *backend_data;
+};
+
+/*
+ * Check for entries in extras that are within the specified
+ * directory, where dirname is a reference directory name including
+ * the trailing slash (e.g., "refs/heads/foo/"). Ignore any
+ * conflicting references that are found in skip. If there is a
+ * conflicting reference, return its name.
+ *
+ * extras and skip must be sorted lists of reference names. Either one
+ * can be NULL, signifying the empty list.
+ */
+const char *find_descendant_ref(const char *dirname,
+				const struct string_list *extras,
+				const struct string_list *skip);
+
+/*
+ * Check whether an attempt to rename old_refname to new_refname would
+ * cause a D/F conflict with any existing reference (other than
+ * possibly old_refname). If there would be a conflict, emit an error
+ * message and return false; otherwise, return true.
+ *
+ * Note that this function is not safe against all races with other
+ * processes (though rename_ref() catches some races that might get by
+ * this check).
+ */
+int refs_rename_ref_available(struct ref_store *refs,
+			      const char *old_refname,
+			      const char *new_refname);
+
+/* We allow "recursive" symbolic refs. Only within reason, though */
+#define SYMREF_MAXDEPTH 5
+
+/* Include broken references in a do_for_each_ref*() iteration: */
+#define DO_FOR_EACH_INCLUDE_BROKEN 0x01
+
+/*
+ * Reference iterators
+ *
+ * A reference iterator encapsulates the state of an in-progress
+ * iteration over references. Create an instance of `struct
+ * ref_iterator` via one of the functions in this module.
+ *
+ * A freshly-created ref_iterator doesn't yet point at a reference. To
+ * advance the iterator, call ref_iterator_advance(). If successful,
+ * this sets the iterator's refname, oid, and flags fields to describe
+ * the next reference and returns ITER_OK. The data pointed at by
+ * refname and oid belong to the iterator; if you want to retain them
+ * after calling ref_iterator_advance() again or calling
+ * ref_iterator_abort(), you must make a copy. When the iteration has
+ * been exhausted, ref_iterator_advance() releases any resources
+ * assocated with the iteration, frees the ref_iterator object, and
+ * returns ITER_DONE. If you want to abort the iteration early, call
+ * ref_iterator_abort(), which also frees the ref_iterator object and
+ * any associated resources. If there was an internal error advancing
+ * to the next entry, ref_iterator_advance() aborts the iteration,
+ * frees the ref_iterator, and returns ITER_ERROR.
+ *
+ * The reference currently being looked at can be peeled by calling
+ * ref_iterator_peel(). This function is often faster than peel_ref(),
+ * so it should be preferred when iterating over references.
+ *
+ * Putting it all together, a typical iteration looks like this:
+ *
+ *     int ok;
+ *     struct ref_iterator *iter = ...;
+ *
+ *     while ((ok = ref_iterator_advance(iter)) == ITER_OK) {
+ *             if (want_to_stop_iteration()) {
+ *                     ok = ref_iterator_abort(iter);
+ *                     break;
+ *             }
+ *
+ *             // Access information about the current reference:
+ *             if (!(iter->flags & REF_ISSYMREF))
+ *                     printf("%s is %s\n", iter->refname, oid_to_hex(iter->oid));
+ *
+ *             // If you need to peel the reference:
+ *             ref_iterator_peel(iter, &oid);
+ *     }
+ *
+ *     if (ok != ITER_DONE)
+ *             handle_error();
+ */
+struct ref_iterator {
+	struct ref_iterator_vtable *vtable;
+
+	/*
+	 * Does this `ref_iterator` iterate over references in order
+	 * by refname?
+	 */
+	unsigned int ordered : 1;
+
+	const char *refname;
+	const struct object_id *oid;
+	unsigned int flags;
+};
+
+/*
+ * Advance the iterator to the first or next item and return ITER_OK.
+ * If the iteration is exhausted, free the resources associated with
+ * the ref_iterator and return ITER_DONE. On errors, free the iterator
+ * resources and return ITER_ERROR. It is a bug to use ref_iterator or
+ * call this function again after it has returned ITER_DONE or
+ * ITER_ERROR.
+ */
+int ref_iterator_advance(struct ref_iterator *ref_iterator);
+
+/*
+ * If possible, peel the reference currently being viewed by the
+ * iterator. Return 0 on success.
+ */
+int ref_iterator_peel(struct ref_iterator *ref_iterator,
+		      struct object_id *peeled);
+
+/*
+ * End the iteration before it has been exhausted, freeing the
+ * reference iterator and any associated resources and returning
+ * ITER_DONE. If the abort itself failed, return ITER_ERROR.
+ */
+int ref_iterator_abort(struct ref_iterator *ref_iterator);
+
+/*
+ * An iterator over nothing (its first ref_iterator_advance() call
+ * returns ITER_DONE).
+ */
+struct ref_iterator *empty_ref_iterator_begin(void);
+
+/*
+ * Return true iff ref_iterator is an empty_ref_iterator.
+ */
+int is_empty_ref_iterator(struct ref_iterator *ref_iterator);
+
+/*
+ * Return an iterator that goes over each reference in `refs` for
+ * which the refname begins with prefix. If trim is non-zero, then
+ * trim that many characters off the beginning of each refname. flags
+ * can be DO_FOR_EACH_INCLUDE_BROKEN to include broken references in
+ * the iteration. The output is ordered by refname.
+ */
+struct ref_iterator *refs_ref_iterator_begin(
+		struct ref_store *refs,
+		const char *prefix, int trim, int flags);
+
+/*
+ * A callback function used to instruct merge_ref_iterator how to
+ * interleave the entries from iter0 and iter1. The function should
+ * return one of the constants defined in enum iterator_selection. It
+ * must not advance either of the iterators itself.
+ *
+ * The function must be prepared to handle the case that iter0 and/or
+ * iter1 is NULL, which indicates that the corresponding sub-iterator
+ * has been exhausted. Its return value must be consistent with the
+ * current states of the iterators; e.g., it must not return
+ * ITER_SKIP_1 if iter1 has already been exhausted.
+ */
+typedef enum iterator_selection ref_iterator_select_fn(
+		struct ref_iterator *iter0, struct ref_iterator *iter1,
+		void *cb_data);
+
+/*
+ * Iterate over the entries from iter0 and iter1, with the values
+ * interleaved as directed by the select function. The iterator takes
+ * ownership of iter0 and iter1 and frees them when the iteration is
+ * over. A derived class should set `ordered` to 1 or 0 based on
+ * whether it generates its output in order by reference name.
+ */
+struct ref_iterator *merge_ref_iterator_begin(
+		int ordered,
+		struct ref_iterator *iter0, struct ref_iterator *iter1,
+		ref_iterator_select_fn *select, void *cb_data);
+
+/*
+ * An iterator consisting of the union of the entries from front and
+ * back. If there are entries common to the two sub-iterators, use the
+ * one from front. Each iterator must iterate over its entries in
+ * strcmp() order by refname for this to work.
+ *
+ * The new iterator takes ownership of its arguments and frees them
+ * when the iteration is over. As a convenience to callers, if front
+ * or back is an empty_ref_iterator, then abort that one immediately
+ * and return the other iterator directly, without wrapping it.
+ */
+struct ref_iterator *overlay_ref_iterator_begin(
+		struct ref_iterator *front, struct ref_iterator *back);
+
+/*
+ * Wrap iter0, only letting through the references whose names start
+ * with prefix. If trim is set, set iter->refname to the name of the
+ * reference with that many characters trimmed off the front;
+ * otherwise set it to the full refname. The new iterator takes over
+ * ownership of iter0 and frees it when iteration is over. It makes
+ * its own copy of prefix.
+ *
+ * As an convenience to callers, if prefix is the empty string and
+ * trim is zero, this function returns iter0 directly, without
+ * wrapping it.
+ *
+ * The resulting ref_iterator is ordered if iter0 is.
+ */
+struct ref_iterator *prefix_ref_iterator_begin(struct ref_iterator *iter0,
+					       const char *prefix,
+					       int trim);
+
+/* Internal implementation of reference iteration: */
+
+/*
+ * Base class constructor for ref_iterators. Initialize the
+ * ref_iterator part of iter, setting its vtable pointer as specified.
+ * `ordered` should be set to 1 if the iterator will iterate over
+ * references in order by refname; otherwise it should be set to 0.
+ * This is meant to be called only by the initializers of derived
+ * classes.
+ */
+void base_ref_iterator_init(struct ref_iterator *iter,
+			    struct ref_iterator_vtable *vtable,
+			    int ordered);
+
+/*
+ * Base class destructor for ref_iterators. Destroy the ref_iterator
+ * part of iter and shallow-free the object. This is meant to be
+ * called only by the destructors of derived classes.
+ */
+void base_ref_iterator_free(struct ref_iterator *iter);
+
+/* Virtual function declarations for ref_iterators: */
+
+typedef int ref_iterator_advance_fn(struct ref_iterator *ref_iterator);
+
+typedef int ref_iterator_peel_fn(struct ref_iterator *ref_iterator,
+				 struct object_id *peeled);
+
+/*
+ * Implementations of this function should free any resources specific
+ * to the derived class, then call base_ref_iterator_free() to clean
+ * up and free the ref_iterator object.
+ */
+typedef int ref_iterator_abort_fn(struct ref_iterator *ref_iterator);
+
+struct ref_iterator_vtable {
+	ref_iterator_advance_fn *advance;
+	ref_iterator_peel_fn *peel;
+	ref_iterator_abort_fn *abort;
+};
+
+/*
+ * current_ref_iter is a performance hack: when iterating over
+ * references using the for_each_ref*() functions, current_ref_iter is
+ * set to the reference iterator before calling the callback function.
+ * If the callback function calls peel_ref(), then peel_ref() first
+ * checks whether the reference to be peeled is the one referred to by
+ * the iterator (it usually is) and if so, asks the iterator for the
+ * peeled version of the reference if it is available. This avoids a
+ * refname lookup in a common case. current_ref_iter is set to NULL
+ * when the iteration is over.
+ */
+extern struct ref_iterator *current_ref_iter;
+
+/*
+ * The common backend for the for_each_*ref* functions. Call fn for
+ * each reference in iter. If the iterator itself ever returns
+ * ITER_ERROR, return -1. If fn ever returns a non-zero value, stop
+ * the iteration and return that value. Otherwise, return 0. In any
+ * case, free the iterator when done. This function is basically an
+ * adapter between the callback style of reference iteration and the
+ * iterator style.
+ */
+int do_for_each_repo_ref_iterator(struct repository *r,
+				  struct ref_iterator *iter,
+				  each_repo_ref_fn fn, void *cb_data);
+
+/*
+ * Only include per-worktree refs in a do_for_each_ref*() iteration.
+ * Normally this will be used with a files ref_store, since that's
+ * where all reference backends will presumably store their
+ * per-worktree refs.
+ */
+#define DO_FOR_EACH_PER_WORKTREE_ONLY 0x02
+
+struct ref_store;
+
+/* refs backends */
+
+/* ref_store_init flags */
+#define REF_STORE_READ		(1 << 0)
+#define REF_STORE_WRITE		(1 << 1) /* can perform update operations */
+#define REF_STORE_ODB		(1 << 2) /* has access to object database */
+#define REF_STORE_MAIN		(1 << 3)
+#define REF_STORE_ALL_CAPS	(REF_STORE_READ | \
+				 REF_STORE_WRITE | \
+				 REF_STORE_ODB | \
+				 REF_STORE_MAIN)
+
+/*
+ * Initialize the ref_store for the specified gitdir. These functions
+ * should call base_ref_store_init() to initialize the shared part of
+ * the ref_store and to record the ref_store for later lookup.
+ */
+typedef struct ref_store *ref_store_init_fn(const char *gitdir,
+					    unsigned int flags);
+
+typedef int ref_init_db_fn(struct ref_store *refs, struct strbuf *err);
+
+typedef int ref_transaction_prepare_fn(struct ref_store *refs,
+				       struct ref_transaction *transaction,
+				       struct strbuf *err);
+
+typedef int ref_transaction_finish_fn(struct ref_store *refs,
+				      struct ref_transaction *transaction,
+				      struct strbuf *err);
+
+typedef int ref_transaction_abort_fn(struct ref_store *refs,
+				     struct ref_transaction *transaction,
+				     struct strbuf *err);
+
+typedef int ref_transaction_commit_fn(struct ref_store *refs,
+				      struct ref_transaction *transaction,
+				      struct strbuf *err);
+
+typedef int pack_refs_fn(struct ref_store *ref_store, unsigned int flags);
+typedef int create_symref_fn(struct ref_store *ref_store,
+			     const char *ref_target,
+			     const char *refs_heads_master,
+			     const char *logmsg);
+typedef int delete_refs_fn(struct ref_store *ref_store, const char *msg,
+			   struct string_list *refnames, unsigned int flags);
+typedef int rename_ref_fn(struct ref_store *ref_store,
+			  const char *oldref, const char *newref,
+			  const char *logmsg);
+typedef int copy_ref_fn(struct ref_store *ref_store,
+			  const char *oldref, const char *newref,
+			  const char *logmsg);
+
+/*
+ * Iterate over the references in `ref_store` whose names start with
+ * `prefix`. `prefix` is matched as a literal string, without regard
+ * for path separators. If prefix is NULL or the empty string, iterate
+ * over all references in `ref_store`. The output is ordered by
+ * refname.
+ */
+typedef struct ref_iterator *ref_iterator_begin_fn(
+		struct ref_store *ref_store,
+		const char *prefix, unsigned int flags);
+
+/* reflog functions */
+
+/*
+ * Iterate over the references in the specified ref_store that have a
+ * reflog. The refs are iterated over in arbitrary order.
+ */
+typedef struct ref_iterator *reflog_iterator_begin_fn(
+		struct ref_store *ref_store);
+
+typedef int for_each_reflog_ent_fn(struct ref_store *ref_store,
+				   const char *refname,
+				   each_reflog_ent_fn fn,
+				   void *cb_data);
+typedef int for_each_reflog_ent_reverse_fn(struct ref_store *ref_store,
+					   const char *refname,
+					   each_reflog_ent_fn fn,
+					   void *cb_data);
+typedef int reflog_exists_fn(struct ref_store *ref_store, const char *refname);
+typedef int create_reflog_fn(struct ref_store *ref_store, const char *refname,
+			     int force_create, struct strbuf *err);
+typedef int delete_reflog_fn(struct ref_store *ref_store, const char *refname);
+typedef int reflog_expire_fn(struct ref_store *ref_store,
+			     const char *refname, const struct object_id *oid,
+			     unsigned int flags,
+			     reflog_expiry_prepare_fn prepare_fn,
+			     reflog_expiry_should_prune_fn should_prune_fn,
+			     reflog_expiry_cleanup_fn cleanup_fn,
+			     void *policy_cb_data);
+
+/*
+ * Read a reference from the specified reference store, non-recursively.
+ * Set type to describe the reference, and:
+ *
+ * - If refname is the name of a normal reference, fill in oid
+ *   (leaving referent unchanged).
+ *
+ * - If refname is the name of a symbolic reference, write the full
+ *   name of the reference to which it refers (e.g.
+ *   "refs/heads/master") to referent and set the REF_ISSYMREF bit in
+ *   type (leaving oid unchanged). The caller is responsible for
+ *   validating that referent is a valid reference name.
+ *
+ * WARNING: refname might be used as part of a filename, so it is
+ * important from a security standpoint that it be safe in the sense
+ * of refname_is_safe(). Moreover, for symrefs this function sets
+ * referent to whatever the repository says, which might not be a
+ * properly-formatted or even safe reference name. NEITHER INPUT NOR
+ * OUTPUT REFERENCE NAMES ARE VALIDATED WITHIN THIS FUNCTION.
+ *
+ * Return 0 on success. If the ref doesn't exist, set errno to ENOENT
+ * and return -1. If the ref exists but is neither a symbolic ref nor
+ * an object ID, it is broken; set REF_ISBROKEN in type, set errno to
+ * EINVAL, and return -1. If there is another error reading the ref,
+ * set errno appropriately and return -1.
+ *
+ * Backend-specific flags might be set in type as well, regardless of
+ * outcome.
+ *
+ * It is OK for refname to point into referent. If so:
+ *
+ * - if the function succeeds with REF_ISSYMREF, referent will be
+ *   overwritten and the memory formerly pointed to by it might be
+ *   changed or even freed.
+ *
+ * - in all other cases, referent will be untouched, and therefore
+ *   refname will still be valid and unchanged.
+ */
+typedef int read_raw_ref_fn(struct ref_store *ref_store,
+			    const char *refname, struct object_id *oid,
+			    struct strbuf *referent, unsigned int *type);
+
+struct ref_storage_be {
+	struct ref_storage_be *next;
+	const char *name;
+	ref_store_init_fn *init;
+	ref_init_db_fn *init_db;
+
+	ref_transaction_prepare_fn *transaction_prepare;
+	ref_transaction_finish_fn *transaction_finish;
+	ref_transaction_abort_fn *transaction_abort;
+	ref_transaction_commit_fn *initial_transaction_commit;
+
+	pack_refs_fn *pack_refs;
+	create_symref_fn *create_symref;
+	delete_refs_fn *delete_refs;
+	rename_ref_fn *rename_ref;
+	copy_ref_fn *copy_ref;
+
+	ref_iterator_begin_fn *iterator_begin;
+	read_raw_ref_fn *read_raw_ref;
+
+	reflog_iterator_begin_fn *reflog_iterator_begin;
+	for_each_reflog_ent_fn *for_each_reflog_ent;
+	for_each_reflog_ent_reverse_fn *for_each_reflog_ent_reverse;
+	reflog_exists_fn *reflog_exists;
+	create_reflog_fn *create_reflog;
+	delete_reflog_fn *delete_reflog;
+	reflog_expire_fn *reflog_expire;
+};
+
+extern struct ref_storage_be refs_be_files;
+extern struct ref_storage_be refs_be_packed;
+
+/*
+ * A representation of the reference store for the main repository or
+ * a submodule. The ref_store instances for submodules are kept in a
+ * linked list.
+ */
+struct ref_store {
+	/* The backend describing this ref_store's storage scheme: */
+	const struct ref_storage_be *be;
+};
+
+/*
+ * Fill in the generic part of refs and add it to our collection of
+ * reference stores.
+ */
+void base_ref_store_init(struct ref_store *refs,
+			 const struct ref_storage_be *be);
+
+#endif /* REFS_REFS_INTERNAL_H */