merge(third_party/git): Merge squashed git subtree at v2.23.0 r/373

Merge commit '1b593e1ea4d2af0f6444d9a7788d5d99abd6fde5' as 'third_party/git'

1 files changed, 1660 insertions, 0 deletions

diff --git a/third_party/git/refs/packed-backend.c b/third_party/git/refs/packed-backend.c
new file mode 100644
index 000000000000..c01c7f5901a6
--- /dev/null
+++ b/third_party/git/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+ * `get_snapshot()` is called; if the existing snapshot is obsolete, a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
+};