path: root/third_party/nix/src/libstore/optimise-store.cc

#include <errno.h>
#include <stdio.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>
#include <cstdlib>
#include <cstring>
#include <regex>
#include "globals.hh"
#include "glog/logging.h"
#include "local-store.hh"
#include "util.hh"

namespace nix {

static void makeWritable(const Path& path) {
  struct stat st;
  if (lstat(path.c_str(), &st))
    throw SysError(format("getting attributes of path '%1%'") % path);
  if (chmod(path.c_str(), st.st_mode | S_IWUSR) == -1)
    throw SysError(format("changing writability of '%1%'") % path);
}

struct MakeReadOnly {
  Path path;
  MakeReadOnly(const Path& path) : path(path) {}
  ~MakeReadOnly() {
    try {
      /* This will make the path read-only. */
      if (path != "") canonicaliseTimestampAndPermissions(path);
    } catch (...) {
      ignoreException();
    }
  }
};

LocalStore::InodeHash LocalStore::loadInodeHash() {
  DLOG(INFO) << "loading hash inodes in memory";
  InodeHash inodeHash;

  AutoCloseDir dir(opendir(linksDir.c_str()));
  if (!dir) {
    throw SysError(format("opening directory '%1%'") % linksDir);
  }

  struct dirent* dirent;
  while (errno = 0, dirent = readdir(dir.get())) { /* sic */
    checkInterrupt();
    // We don't care if we hit non-hash files, anything goes
    inodeHash.insert(dirent->d_ino);
  }
  if (errno) {
    throw SysError(format("reading directory '%1%'") % linksDir);
  }

  DLOG(INFO) << "loaded " << inodeHash.size() << " hash inodes";

  return inodeHash;
}

Strings LocalStore::readDirectoryIgnoringInodes(const Path& path,
                                                const InodeHash& inodeHash) {
  Strings names;

  AutoCloseDir dir(opendir(path.c_str()));
  if (!dir) {
    throw SysError(format("opening directory '%1%'") % path);
  }

  struct dirent* dirent;
  while (errno = 0, dirent = readdir(dir.get())) { /* sic */
    checkInterrupt();

    if (inodeHash.count(dirent->d_ino)) {
      DLOG(WARNING) << dirent->d_name << " is already linked";
      continue;
    }

    string name = dirent->d_name;
    if (name == "." || name == "..") continue;
    names.push_back(name);
  }
  if (errno) {
    throw SysError(format("reading directory '%1%'") % path);
  }

  return names;
}

void LocalStore::optimisePath_(OptimiseStats& stats, const Path& path,
                               InodeHash& inodeHash) {
  checkInterrupt();

  struct stat st;
  if (lstat(path.c_str(), &st))
    throw SysError(format("getting attributes of path '%1%'") % path);

#if __APPLE__
  /* HFS/macOS has some undocumented security feature disabling hardlinking for
     special files within .app dirs. *.app/Contents/PkgInfo and
     *.app/Contents/Resources/\*.lproj seem to be the only paths affected. See
     https://github.com/NixOS/nix/issues/1443 for more discussion. */

  if (std::regex_search(path, std::regex("\\.app/Contents/.+$"))) {
    debug(format("'%1%' is not allowed to be linked in macOS") % path);
    return;
  }
#endif

  if (S_ISDIR(st.st_mode)) {
    Strings names = readDirectoryIgnoringInodes(path, inodeHash);
    for (auto& i : names) optimisePath_(stats, path + "/" + i, inodeHash);
    return;
  }

  /* We can hard link regular files and maybe symlinks. */
  if (!S_ISREG(st.st_mode)
#if CAN_LINK_SYMLINK
      && !S_ISLNK(st.st_mode)
#endif
  )
    return;

  /* Sometimes SNAFUs can cause files in the Nix store to be
     modified, in particular when running programs as root under
     NixOS (example: $fontconfig/var/cache being modified).  Skip
     those files.  FIXME: check the modification time. */
  if (S_ISREG(st.st_mode) && (st.st_mode & S_IWUSR)) {
    LOG(WARNING) << "skipping suspicious writable file '" << path << "'";
    return;
  }

  /* This can still happen on top-level files. */
  if (st.st_nlink > 1 && inodeHash.count(st.st_ino)) {
    DLOG(INFO) << path << " is already linked, with " << (st.st_nlink - 2)
               << " other file(s)";
    return;
  }

  /* Hash the file.  Note that hashPath() returns the hash over the
     NAR serialisation, which includes the execute bit on the file.
     Thus, executable and non-executable files with the same
     contents *won't* be linked (which is good because otherwise the
     permissions would be screwed up).

     Also note that if `path' is a symlink, then we're hashing the
     contents of the symlink (i.e. the result of readlink()), not
     the contents of the target (which may not even exist). */
  Hash hash = hashPath(htSHA256, path).first;
  LOG(INFO) << path << " has hash " << hash.to_string();

  /* Check if this is a known hash. */
  Path linkPath = linksDir + "/" + hash.to_string(Base32, false);

retry:
  if (!pathExists(linkPath)) {
    /* Nope, create a hard link in the links directory. */
    if (link(path.c_str(), linkPath.c_str()) == 0) {
      inodeHash.insert(st.st_ino);
      return;
    }

    switch (errno) {
      case EEXIST:
        /* Fall through if another process created ‘linkPath’ before
           we did. */
        break;

      case ENOSPC:
        /* On ext4, that probably means the directory index is
           full.  When that happens, it's fine to ignore it: we
           just effectively disable deduplication of this
           file.  */
        LOG(WARNING) << "cannot link '" << linkPath << " to " << path << ": "
                     << strerror(errno);

        return;

      default:
        throw SysError("cannot link '%1%' to '%2%'", linkPath, path);
    }
  }

  /* Yes!  We've seen a file with the same contents.  Replace the
     current file with a hard link to that file. */
  struct stat stLink;
  if (lstat(linkPath.c_str(), &stLink))
    throw SysError(format("getting attributes of path '%1%'") % linkPath);

  if (st.st_ino == stLink.st_ino) {
    DLOG(INFO) << path << " is already linked to " << linkPath;
    return;
  }

  if (st.st_size != stLink.st_size) {
    LOG(WARNING) << "removing corrupted link '" << linkPath << "'";
    unlink(linkPath.c_str());
    goto retry;
  }

  DLOG(INFO) << "linking '" << path << "' to '" << linkPath << "'";

  /* Make the containing directory writable, but only if it's not
     the store itself (we don't want or need to mess with its
     permissions). */
  bool mustToggle = dirOf(path) != realStoreDir;
  if (mustToggle) makeWritable(dirOf(path));

  /* When we're done, make the directory read-only again and reset
     its timestamp back to 0. */
  MakeReadOnly makeReadOnly(mustToggle ? dirOf(path) : "");

  Path tempLink =
      (format("%1%/.tmp-link-%2%-%3%") % realStoreDir % getpid() % random())
          .str();

  if (link(linkPath.c_str(), tempLink.c_str()) == -1) {
    if (errno == EMLINK) {
      /* Too many links to the same file (>= 32000 on most file
         systems).  This is likely to happen with empty files.
         Just shrug and ignore. */
      if (st.st_size) {
        LOG(WARNING) << linkPath << " has maximum number of links";
      }
      return;
    }
    throw SysError("cannot link '%1%' to '%2%'", tempLink, linkPath);
  }

  /* Atomically replace the old file with the new hard link. */
  if (rename(tempLink.c_str(), path.c_str()) == -1) {
    if (unlink(tempLink.c_str()) == -1) {
      LOG(ERROR) << "unable to unlink '" << tempLink << "'";
    }
    if (errno == EMLINK) {
      /* Some filesystems generate too many links on the rename,
         rather than on the original link.  (Probably it
         temporarily increases the st_nlink field before
         decreasing it again.) */
      DLOG(WARNING) << "'" << linkPath
                    << "' has reached maximum number of links";
      return;
    }
    throw SysError(format("cannot rename '%1%' to '%2%'") % tempLink % path);
  }

  stats.filesLinked++;
  stats.bytesFreed += st.st_size;
  stats.blocksFreed += st.st_blocks;
}

void LocalStore::optimiseStore(OptimiseStats& stats) {
  PathSet paths = queryAllValidPaths();
  InodeHash inodeHash = loadInodeHash();

  uint64_t done = 0;

  for (auto& i : paths) {
    addTempRoot(i);
    if (!isValidPath(i)) continue; /* path was GC'ed, probably */
    {
      LOG(INFO) << "optimising path '" << i << "'";
      optimisePath_(stats, realStoreDir + "/" + baseNameOf(i), inodeHash);
    }
    done++;
  }
}

static string showBytes(unsigned long long bytes) {
  return (format("%.2f MiB") % (bytes / (1024.0 * 1024.0))).str();
}

void LocalStore::optimiseStore() {
  OptimiseStats stats;

  optimiseStore(stats);

  LOG(INFO) << showBytes(stats.bytesFreed) << " freed by hard-linking "
            << stats.filesLinked << " files";
}

void LocalStore::optimisePath(const Path& path) {
  OptimiseStats stats;
  InodeHash inodeHash;

  if (settings.autoOptimiseStore) {
    optimisePath_(stats, path, inodeHash);
  }
}

}  // namespace nix