Merge branch 'mh/packed-ref-store'

The "ref-store" code reorganization continues.

* mh/packed-ref-store: (32 commits)
  files-backend: cheapen refname_available check when locking refs
  packed_ref_store: handle a packed-refs file that is a symlink
  read_packed_refs(): die if `packed-refs` contains bogus data
  t3210: add some tests of bogus packed-refs file contents
  repack_without_refs(): don't lock or unlock the packed refs
  commit_packed_refs(): remove call to `packed_refs_unlock()`
  clear_packed_ref_cache(): don't protest if the lock is held
  packed_refs_unlock(), packed_refs_is_locked(): new functions
  packed_refs_lock(): report errors via a `struct strbuf *err`
  packed_refs_lock(): function renamed from lock_packed_refs()
  commit_packed_refs(): use a staging file separate from the lockfile
  commit_packed_refs(): report errors rather than dying
  packed_ref_store: make class into a subclass of `ref_store`
  packed-backend: new module for handling packed references
  packed_read_raw_ref(): new function, replacing `resolve_packed_ref()`
  packed_ref_store: support iteration
  packed_peel_ref(): new function, extracted from `files_peel_ref()`
  repack_without_refs(): take a `packed_ref_store *` parameter
  get_packed_ref(): take a `packed_ref_store *` parameter
  rollback_packed_refs(): take a `packed_ref_store *` parameter
  ...

4 files changed, 995 insertions, 564 deletions

diff --git a/refs/files-backend.c b/refs/files-backend.c
index 0404f2c233..5cca55510b 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -3,6 +3,7 @@
 #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"
@@ -16,39 +17,6 @@ struct ref_lock {
 };
 
 /*
- * 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.
- */
-static int ref_resolves_to_object(const char *refname,
-				  const struct object_id *oid,
-				  unsigned int flags)
-{
-	if (flags & REF_ISBROKEN)
-		return 0;
-	if (!has_sha1_file(oid->hash)) {
-		error("%s does not point to a valid object!", refname);
-		return 0;
-	}
-	return 1;
-}
-
-struct packed_ref_cache {
-	struct ref_cache *cache;
-
-	/*
-	 * Count of references to the data structure in this instance,
-	 * including the pointer from files_ref_store::packed if any.
-	 * The data will not be freed as long as the reference count
-	 * is nonzero.
-	 */
-	unsigned int referrers;
-
-	/* The metadata from when this packed-refs cache was read */
-	struct stat_validity validity;
-};
-
-/*
  * Future: need to be in "struct repository"
  * when doing a full libification.
  */
@@ -58,54 +26,12 @@ struct files_ref_store {
 
 	char *gitdir;
 	char *gitcommondir;
-	char *packed_refs_path;
 
 	struct ref_cache *loose;
-	struct packed_ref_cache *packed;
 
-	/*
-	 * Lock used for the "packed-refs" file. Note that this (and
-	 * thus the enclosing `files_ref_store`) must not be freed.
-	 */
-	struct lock_file packed_refs_lock;
+	struct ref_store *packed_ref_store;
 };
 
-/*
- * Increment the reference count of *packed_refs.
- */
-static void acquire_packed_ref_cache(struct packed_ref_cache *packed_refs)
-{
-	packed_refs->referrers++;
-}
-
-/*
- * Decrease the reference count of *packed_refs.  If it goes to zero,
- * free *packed_refs and return true; otherwise return false.
- */
-static int release_packed_ref_cache(struct packed_ref_cache *packed_refs)
-{
-	if (!--packed_refs->referrers) {
-		free_ref_cache(packed_refs->cache);
-		stat_validity_clear(&packed_refs->validity);
-		free(packed_refs);
-		return 1;
-	} else {
-		return 0;
-	}
-}
-
-static void clear_packed_ref_cache(struct files_ref_store *refs)
-{
-	if (refs->packed) {
-		struct packed_ref_cache *packed_refs = refs->packed;
-
-		if (is_lock_file_locked(&refs->packed_refs_lock))
-			die("BUG: packed-ref cache cleared while locked");
-		refs->packed = NULL;
-		release_packed_ref_cache(packed_refs);
-	}
-}
-
 static void clear_loose_ref_cache(struct files_ref_store *refs)
 {
 	if (refs->loose) {
@@ -132,7 +58,8 @@ static struct ref_store *files_ref_store_create(const char *gitdir,
 	get_common_dir_noenv(&sb, gitdir);
 	refs->gitcommondir = strbuf_detach(&sb, NULL);
 	strbuf_addf(&sb, "%s/packed-refs", refs->gitcommondir);
-	refs->packed_refs_path = strbuf_detach(&sb, NULL);
+	refs->packed_ref_store = packed_ref_store_create(sb.buf, flags);
+	strbuf_release(&sb);
 
 	return ref_store;
 }
@@ -175,156 +102,6 @@ static struct files_ref_store *files_downcast(struct ref_store *ref_store,
 	return refs;
 }
 
-/* The length of a peeled reference line in packed-refs, including EOL: */
-#define PEELED_LINE_LENGTH 42
-
-/*
- * The packed-refs header line that we write out.  Perhaps other
- * traits will be added later.  The trailing space is required.
- */
-static const char PACKED_REFS_HEADER[] =
-	"# pack-refs with: peeled fully-peeled \n";
-
-/*
- * Parse one line from a packed-refs file.  Write the SHA1 to sha1.
- * Return a pointer to the refname within the line (null-terminated),
- * or NULL if there was a problem.
- */
-static const char *parse_ref_line(struct strbuf *line, struct object_id *oid)
-{
-	const char *ref;
-
-	if (parse_oid_hex(line->buf, oid, &ref) < 0)
-		return NULL;
-	if (!isspace(*ref++))
-		return NULL;
-
-	if (isspace(*ref))
-		return NULL;
-
-	if (line->buf[line->len - 1] != '\n')
-		return NULL;
-	line->buf[--line->len] = 0;
-
-	return ref;
-}
-
-/*
- * Read from `packed_refs_file` into a newly-allocated
- * `packed_ref_cache` 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:
- *
- *   No traits:
- *
- *      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).
- */
-static struct packed_ref_cache *read_packed_refs(const char *packed_refs_file)
-{
-	FILE *f;
-	struct packed_ref_cache *packed_refs = xcalloc(1, sizeof(*packed_refs));
-	struct ref_entry *last = NULL;
-	struct strbuf line = STRBUF_INIT;
-	enum { PEELED_NONE, PEELED_TAGS, PEELED_FULLY } peeled = PEELED_NONE;
-	struct ref_dir *dir;
-
-	acquire_packed_ref_cache(packed_refs);
-	packed_refs->cache = create_ref_cache(NULL, NULL);
-	packed_refs->cache->root->flag &= ~REF_INCOMPLETE;
-
-	f = fopen(packed_refs_file, "r");
-	if (!f) {
-		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.
-			 */
-			return packed_refs;
-		} else {
-			die_errno("couldn't read %s", packed_refs_file);
-		}
-	}
-
-	stat_validity_update(&packed_refs->validity, fileno(f));
-
-	dir = get_ref_dir(packed_refs->cache->root);
-	while (strbuf_getwholeline(&line, f, '\n') != EOF) {
-		struct object_id oid;
-		const char *refname;
-		const char *traits;
-
-		if (skip_prefix(line.buf, "# pack-refs with:", &traits)) {
-			if (strstr(traits, " fully-peeled "))
-				peeled = PEELED_FULLY;
-			else if (strstr(traits, " peeled "))
-				peeled = PEELED_TAGS;
-			/* perhaps other traits later as well */
-			continue;
-		}
-
-		refname = parse_ref_line(&line, &oid);
-		if (refname) {
-			int flag = REF_ISPACKED;
-
-			if (check_refname_format(refname, REFNAME_ALLOW_ONELEVEL)) {
-				if (!refname_is_safe(refname))
-					die("packed refname is dangerous: %s", refname);
-				oidclr(&oid);
-				flag |= REF_BAD_NAME | REF_ISBROKEN;
-			}
-			last = create_ref_entry(refname, &oid, flag);
-			if (peeled == PEELED_FULLY ||
-			    (peeled == PEELED_TAGS && starts_with(refname, "refs/tags/")))
-				last->flag |= REF_KNOWS_PEELED;
-			add_ref_entry(dir, last);
-			continue;
-		}
-		if (last &&
-		    line.buf[0] == '^' &&
-		    line.len == PEELED_LINE_LENGTH &&
-		    line.buf[PEELED_LINE_LENGTH - 1] == '\n' &&
-		    !get_oid_hex(line.buf + 1, &oid)) {
-			oidcpy(&last->u.value.peeled, &oid);
-			/*
-			 * Regardless of what the file header said,
-			 * we definitely know the value of *this*
-			 * reference:
-			 */
-			last->flag |= REF_KNOWS_PEELED;
-		}
-	}
-
-	fclose(f);
-	strbuf_release(&line);
-
-	return packed_refs;
-}
-
-static const char *files_packed_refs_path(struct files_ref_store *refs)
-{
-	return refs->packed_refs_path;
-}
-
 static void files_reflog_path(struct files_ref_store *refs,
 			      struct strbuf *sb,
 			      const char *refname)
@@ -371,70 +148,6 @@ static void files_ref_path(struct files_ref_store *refs,
 }
 
 /*
- * Check that the packed refs cache (if any) still reflects the
- * contents of the file. If not, clear the cache.
- */
-static void validate_packed_ref_cache(struct files_ref_store *refs)
-{
-	if (refs->packed &&
-	    !stat_validity_check(&refs->packed->validity,
-				 files_packed_refs_path(refs)))
-		clear_packed_ref_cache(refs);
-}
-
-/*
- * Get the packed_ref_cache for the specified files_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()`.
- */
-static struct packed_ref_cache *get_packed_ref_cache(struct files_ref_store *refs)
-{
-	const char *packed_refs_file = files_packed_refs_path(refs);
-
-	if (!is_lock_file_locked(&refs->packed_refs_lock))
-		validate_packed_ref_cache(refs);
-
-	if (!refs->packed)
-		refs->packed = read_packed_refs(packed_refs_file);
-
-	return refs->packed;
-}
-
-static struct ref_dir *get_packed_ref_dir(struct packed_ref_cache *packed_ref_cache)
-{
-	return get_ref_dir(packed_ref_cache->cache->root);
-}
-
-static struct ref_dir *get_packed_refs(struct files_ref_store *refs)
-{
-	return get_packed_ref_dir(get_packed_ref_cache(refs));
-}
-
-/*
- * Add a reference to the in-memory packed reference cache.  This may
- * only be called while the packed-refs file is locked (see
- * lock_packed_refs()).  To actually write the packed-refs file, call
- * commit_packed_refs().
- */
-static void add_packed_ref(struct files_ref_store *refs,
-			   const char *refname, const struct object_id *oid)
-{
-	struct packed_ref_cache *packed_ref_cache = get_packed_ref_cache(refs);
-
-	if (!is_lock_file_locked(&refs->packed_refs_lock))
-		die("BUG: packed refs not locked");
-
-	if (check_refname_format(refname, REFNAME_ALLOW_ONELEVEL))
-		die("Reference has invalid format: '%s'", refname);
-
-	add_ref_entry(get_packed_ref_dir(packed_ref_cache),
-		      create_ref_entry(refname, oid, REF_ISPACKED));
-}
-
-/*
  * 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.
@@ -556,39 +269,6 @@ static struct ref_cache *get_loose_ref_cache(struct files_ref_store *refs)
 	return refs->loose;
 }
 
-/*
- * Return the ref_entry for the given refname from the packed
- * references.  If it does not exist, return NULL.
- */
-static struct ref_entry *get_packed_ref(struct files_ref_store *refs,
-					const char *refname)
-{
-	return find_ref_entry(get_packed_refs(refs), refname);
-}
-
-/*
- * A loose ref file doesn't exist; check for a packed ref.
- */
-static int resolve_packed_ref(struct files_ref_store *refs,
-			      const char *refname,
-			      unsigned char *sha1, unsigned int *flags)
-{
-	struct ref_entry *entry;
-
-	/*
-	 * The loose reference file does not exist; check for a packed
-	 * reference.
-	 */
-	entry = get_packed_ref(refs, refname);
-	if (entry) {
-		hashcpy(sha1, entry->u.value.oid.hash);
-		*flags |= REF_ISPACKED;
-		return 0;
-	}
-	/* refname is not a packed reference. */
-	return -1;
-}
-
 static int files_read_raw_ref(struct ref_store *ref_store,
 			      const char *refname, unsigned char *sha1,
 			      struct strbuf *referent, unsigned int *type)
@@ -632,7 +312,8 @@ stat_ref:
 	if (lstat(path, &st) < 0) {
 		if (errno != ENOENT)
 			goto out;
-		if (resolve_packed_ref(refs, refname, sha1, type)) {
+		if (refs_read_raw_ref(refs->packed_ref_store, refname,
+				      sha1, referent, type)) {
 			errno = ENOENT;
 			goto out;
 		}
@@ -671,7 +352,8 @@ stat_ref:
 		 * ref is supposed to be, there could still be a
 		 * packed ref:
 		 */
-		if (resolve_packed_ref(refs, refname, sha1, type)) {
+		if (refs_read_raw_ref(refs->packed_ref_store, refname,
+				      sha1, referent, type)) {
 			errno = EISDIR;
 			goto out;
 		}
@@ -950,11 +632,11 @@ retry:
 
 		/*
 		 * If the ref did not exist and we are creating it,
-		 * make sure there is no existing ref that conflicts
-		 * with refname:
+		 * make sure there is no existing packed ref that
+		 * conflicts with refname:
 		 */
 		if (refs_verify_refname_available(
-				    &refs->base, refname,
+				    refs->packed_ref_store, refname,
 				    extras, skip, err))
 			goto error_return;
 	}
@@ -1001,15 +683,9 @@ static int files_peel_ref(struct ref_store *ref_store,
 	 * be expensive and (b) loose references anyway usually do not
 	 * have REF_KNOWS_PEELED.
 	 */
-	if (flag & REF_ISPACKED) {
-		struct ref_entry *r = get_packed_ref(refs, refname);
-		if (r) {
-			if (peel_entry(r, 0))
-				return -1;
-			hashcpy(sha1, r->u.value.peeled.hash);
-			return 0;
-		}
-	}
+	if (flag & REF_ISPACKED &&
+	    !refs_peel_ref(refs->packed_ref_store, refname, sha1))
+		return 0;
 
 	return peel_object(base, sha1);
 }
@@ -1017,7 +693,6 @@ static int files_peel_ref(struct ref_store *ref_store,
 struct files_ref_iterator {
 	struct ref_iterator base;
 
-	struct packed_ref_cache *packed_ref_cache;
 	struct ref_iterator *iter0;
 	unsigned int flags;
 };
@@ -1070,7 +745,6 @@ static int files_ref_iterator_abort(struct ref_iterator *ref_iterator)
 	if (iter->iter0)
 		ok = ref_iterator_abort(iter->iter0);
 
-	release_packed_ref_cache(iter->packed_ref_cache);
 	base_ref_iterator_free(ref_iterator);
 	return ok;
 }
@@ -1112,18 +786,28 @@ static struct ref_iterator *files_ref_iterator_begin(
 	 * (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
-	 * get_packed_ref_cache(), which internally checks whether the
-	 * packed-ref cache is up to date with what is on disk, and
-	 * re-reads it if not.
+	 * 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);
 
-	iter->packed_ref_cache = get_packed_ref_cache(refs);
-	acquire_packed_ref_cache(iter->packed_ref_cache);
-	packed_iter = cache_ref_iterator_begin(iter->packed_ref_cache->cache,
-					       prefix, 0);
+	/*
+	 * 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);
 
 	iter->iter0 = overlay_ref_iterator_begin(loose_iter, packed_iter);
 	iter->flags = flags;
@@ -1255,7 +939,7 @@ static struct ref_lock *lock_ref_sha1_basic(struct files_ref_store *refs,
 	 * our refname.
 	 */
 	if (is_null_oid(&lock->old_oid) &&
-	    refs_verify_refname_available(&refs->base, refname,
+	    refs_verify_refname_available(refs->packed_ref_store, refname,
 					  extras, skip, err)) {
 		last_errno = ENOTDIR;
 		goto error_return;
@@ -1287,124 +971,6 @@ static struct ref_lock *lock_ref_sha1_basic(struct files_ref_store *refs,
 	return lock;
 }
 
-/*
- * 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.
- */
-static void write_packed_entry(FILE *fh, const char *refname,
-			       const unsigned char *sha1,
-			       const unsigned char *peeled)
-{
-	fprintf_or_die(fh, "%s %s\n", sha1_to_hex(sha1), refname);
-	if (peeled)
-		fprintf_or_die(fh, "^%s\n", sha1_to_hex(peeled));
-}
-
-/*
- * Lock the packed-refs file for writing. Flags is passed to
- * hold_lock_file_for_update(). Return 0 on success. On errors, set
- * errno appropriately and return a nonzero value.
- */
-static int lock_packed_refs(struct files_ref_store *refs, int flags)
-{
-	static int timeout_configured = 0;
-	static int timeout_value = 1000;
-	struct packed_ref_cache *packed_ref_cache;
-
-	files_assert_main_repository(refs, "lock_packed_refs");
-
-	if (!timeout_configured) {
-		git_config_get_int("core.packedrefstimeout", &timeout_value);
-		timeout_configured = 1;
-	}
-
-	if (hold_lock_file_for_update_timeout(
-			    &refs->packed_refs_lock, files_packed_refs_path(refs),
-			    flags, timeout_value) < 0)
-		return -1;
-
-	/*
-	 * Now that we hold the `packed-refs` lock, make sure that our
-	 * cache matches the current version of the file. Normally
-	 * `get_packed_ref_cache()` does that for us, but that
-	 * function assumes that when the file is locked, any existing
-	 * cache is still valid. We've just locked the file, but it
-	 * might have changed the moment *before* we locked it.
-	 */
-	validate_packed_ref_cache(refs);
-
-	packed_ref_cache = get_packed_ref_cache(refs);
-	/* Increment the reference count to prevent it from being freed: */
-	acquire_packed_ref_cache(packed_ref_cache);
-	return 0;
-}
-
-/*
- * Write the current version of the packed refs cache from memory to
- * disk. The packed-refs file must already be locked for writing (see
- * lock_packed_refs()). Return zero on success. On errors, set errno
- * and return a nonzero value
- */
-static int commit_packed_refs(struct files_ref_store *refs)
-{
-	struct packed_ref_cache *packed_ref_cache =
-		get_packed_ref_cache(refs);
-	int ok, error = 0;
-	int save_errno = 0;
-	FILE *out;
-	struct ref_iterator *iter;
-
-	files_assert_main_repository(refs, "commit_packed_refs");
-
-	if (!is_lock_file_locked(&refs->packed_refs_lock))
-		die("BUG: packed-refs not locked");
-
-	out = fdopen_lock_file(&refs->packed_refs_lock, "w");
-	if (!out)
-		die_errno("unable to fdopen packed-refs descriptor");
-
-	fprintf_or_die(out, "%s", PACKED_REFS_HEADER);
-
-	iter = cache_ref_iterator_begin(packed_ref_cache->cache, NULL, 0);
-	while ((ok = ref_iterator_advance(iter)) == ITER_OK) {
-		struct object_id peeled;
-		int peel_error = ref_iterator_peel(iter, &peeled);
-
-		write_packed_entry(out, iter->refname, iter->oid->hash,
-				   peel_error ? NULL : peeled.hash);
-	}
-
-	if (ok != ITER_DONE)
-		die("error while iterating over references");
-
-	if (commit_lock_file(&refs->packed_refs_lock)) {
-		save_errno = errno;
-		error = -1;
-	}
-	release_packed_ref_cache(packed_ref_cache);
-	errno = save_errno;
-	return error;
-}
-
-/*
- * Rollback the lockfile for the packed-refs file, and discard the
- * in-memory packed reference cache.  (The packed-refs file will be
- * read anew if it is needed again after this function is called.)
- */
-static void rollback_packed_refs(struct files_ref_store *refs)
-{
-	struct packed_ref_cache *packed_ref_cache =
-		get_packed_ref_cache(refs);
-
-	files_assert_main_repository(refs, "rollback_packed_refs");
-
-	if (!is_lock_file_locked(&refs->packed_refs_lock))
-		die("BUG: packed-refs not locked");
-	rollback_lock_file(&refs->packed_refs_lock);
-	release_packed_ref_cache(packed_ref_cache);
-	clear_packed_ref_cache(refs);
-}
-
 struct ref_to_prune {
 	struct ref_to_prune *next;
 	unsigned char sha1[20];
@@ -1527,12 +1093,11 @@ static int files_pack_refs(struct ref_store *ref_store, unsigned int flags)
 		files_downcast(ref_store, REF_STORE_WRITE | REF_STORE_ODB,
 			       "pack_refs");
 	struct ref_iterator *iter;
-	struct ref_dir *packed_refs;
 	int ok;
 	struct ref_to_prune *refs_to_prune = NULL;
+	struct strbuf err = STRBUF_INIT;
 
-	lock_packed_refs(refs, LOCK_DIE_ON_ERROR);
-	packed_refs = get_packed_refs(refs);
+	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) {
@@ -1541,8 +1106,6 @@ static int files_pack_refs(struct ref_store *ref_store, unsigned int flags)
 		 * in the packed ref cache. If the reference should be
 		 * pruned, also add it to refs_to_prune.
 		 */
-		struct ref_entry *packed_entry;
-
 		if (!should_pack_ref(iter->refname, iter->oid, iter->flags,
 				     flags))
 			continue;
@@ -1553,17 +1116,7 @@ static int files_pack_refs(struct ref_store *ref_store, unsigned int flags)
 		 * we don't copy the peeled status, because we want it
 		 * to be re-peeled.
 		 */
-		packed_entry = find_ref_entry(packed_refs, iter->refname);
-		if (packed_entry) {
-			/* Overwrite existing packed entry with info from loose entry */
-			packed_entry->flag = REF_ISPACKED;
-			oidcpy(&packed_entry->u.value.oid, iter->oid);
-		} else {
-			packed_entry = create_ref_entry(iter->refname, iter->oid,
-							REF_ISPACKED);
-			add_ref_entry(packed_refs, packed_entry);
-		}
-		oidclr(&packed_entry->u.value.peeled);
+		add_packed_ref(refs->packed_ref_store, iter->refname, iter->oid);
 
 		/* Schedule the loose reference for pruning if requested. */
 		if ((flags & PACK_REFS_PRUNE)) {
@@ -1577,69 +1130,15 @@ static int files_pack_refs(struct ref_store *ref_store, unsigned int flags)
 	if (ok != ITER_DONE)
 		die("error while iterating over references");
 
-	if (commit_packed_refs(refs))
-		die_errno("unable to overwrite old ref-pack file");
+	if (commit_packed_refs(refs->packed_ref_store, &err))
+		die("unable to overwrite old ref-pack file: %s", err.buf);
+	packed_refs_unlock(refs->packed_ref_store);
 
 	prune_refs(refs, refs_to_prune);
+	strbuf_release(&err);
 	return 0;
 }
 
-/*
- * Rewrite the packed-refs file, omitting any refs listed in
- * 'refnames'. On error, leave packed-refs unchanged, write an error
- * message to 'err', and return a nonzero value.
- *
- * The refs in 'refnames' needn't be sorted. `err` must not be NULL.
- */
-static int repack_without_refs(struct files_ref_store *refs,
-			       struct string_list *refnames, struct strbuf *err)
-{
-	struct ref_dir *packed;
-	struct string_list_item *refname;
-	int ret, needs_repacking = 0, removed = 0;
-
-	files_assert_main_repository(refs, "repack_without_refs");
-	assert(err);
-
-	/* Look for a packed ref */
-	for_each_string_list_item(refname, refnames) {
-		if (get_packed_ref(refs, refname->string)) {
-			needs_repacking = 1;
-			break;
-		}
-	}
-
-	/* Avoid locking if we have nothing to do */
-	if (!needs_repacking)
-		return 0; /* no refname exists in packed refs */
-
-	if (lock_packed_refs(refs, 0)) {
-		unable_to_lock_message(files_packed_refs_path(refs), errno, err);
-		return -1;
-	}
-	packed = get_packed_refs(refs);
-
-	/* Remove refnames from the cache */
-	for_each_string_list_item(refname, refnames)
-		if (remove_entry_from_dir(packed, refname->string) != -1)
-			removed = 1;
-	if (!removed) {
-		/*
-		 * All packed entries disappeared while we were
-		 * acquiring the lock.
-		 */
-		rollback_packed_refs(refs);
-		return 0;
-	}
-
-	/* Write what remains */
-	ret = commit_packed_refs(refs);
-	if (ret)
-		strbuf_addf(err, "unable to overwrite old ref-pack file: %s",
-			    strerror(errno));
-	return ret;
-}
-
 static int files_delete_refs(struct ref_store *ref_store, const char *msg,
 			     struct string_list *refnames, unsigned int flags)
 {
@@ -1651,24 +1150,16 @@ static int files_delete_refs(struct ref_store *ref_store, const char *msg,
 	if (!refnames->nr)
 		return 0;
 
-	result = repack_without_refs(refs, refnames, &err);
-	if (result) {
-		/*
-		 * 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);
+	if (packed_refs_lock(refs->packed_ref_store, 0, &err))
+		goto error;
 
-		goto out;
+	if (repack_without_refs(refs->packed_ref_store, refnames, &err)) {
+		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;
 
@@ -1676,9 +1167,24 @@ static int files_delete_refs(struct ref_store *ref_store, const char *msg,
 			result |= error(_("could not remove reference %s"), refname);
 	}
 
-out:
 	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;
 }
 
 /*
@@ -3070,11 +2576,19 @@ static int files_transaction_finish(struct ref_store *ref_store,
 		}
 	}
 
-	if (repack_without_refs(refs, &refs_to_delete, err)) {
+	if (packed_refs_lock(refs->packed_ref_store, 0, err)) {
 		ret = TRANSACTION_GENERIC_ERROR;
 		goto cleanup;
 	}
 
+	if (repack_without_refs(refs->packed_ref_store, &refs_to_delete, err)) {
+		ret = TRANSACTION_GENERIC_ERROR;
+		packed_refs_unlock(refs->packed_ref_store);
+		goto cleanup;
+	}
+
+	packed_refs_unlock(refs->packed_ref_store);
+
 	/* Delete the reflogs of any references that were deleted: */
 	for_each_string_list_item(ref_to_delete, &refs_to_delete) {
 		strbuf_reset(&sb);
@@ -3181,9 +2695,7 @@ static int files_initial_transaction_commit(struct ref_store *ref_store,
 		}
 	}
 
-	if (lock_packed_refs(refs, 0)) {
-		strbuf_addf(err, "unable to lock packed-refs file: %s",
-			    strerror(errno));
+	if (packed_refs_lock(refs->packed_ref_store, 0, err)) {
 		ret = TRANSACTION_GENERIC_ERROR;
 		goto cleanup;
 	}
@@ -3193,18 +2705,17 @@ static int files_initial_transaction_commit(struct ref_store *ref_store,
 
 		if ((update->flags & REF_HAVE_NEW) &&
 		    !is_null_oid(&update->new_oid))
-			add_packed_ref(refs, update->refname,
+			add_packed_ref(refs->packed_ref_store, update->refname,
 				       &update->new_oid);
 	}
 
-	if (commit_packed_refs(refs)) {
-		strbuf_addf(err, "unable to commit packed-refs file: %s",
-			    strerror(errno));
+	if (commit_packed_refs(refs->packed_ref_store, err)) {
 		ret = TRANSACTION_GENERIC_ERROR;
 		goto cleanup;
 	}
 
 cleanup:
+	packed_refs_unlock(refs->packed_ref_store);
 	transaction->state = REF_TRANSACTION_CLOSED;
 	string_list_clear(&affected_refnames, 0);
 	return ret;
diff --git a/refs/packed-backend.c b/refs/packed-backend.c
new file mode 100644
index 0000000000..412c85034f
--- /dev/null
+++ b/refs/packed-backend.c
@@ -0,0 +1,885 @@
+#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 "../lockfile.h"
+
+struct packed_ref_cache {
+	struct ref_cache *cache;
+
+	/*
+	 * Count of references to the data structure in this instance,
+	 * including the pointer from files_ref_store::packed if any.
+	 * The data will not be freed as long as the reference count
+	 * is nonzero.
+	 */
+	unsigned int referrers;
+
+	/* The metadata from when this packed-refs cache was read */
+	struct stat_validity validity;
+};
+
+/*
+ * Increment the reference count of *packed_refs.
+ */
+static void acquire_packed_ref_cache(struct packed_ref_cache *packed_refs)
+{
+	packed_refs->referrers++;
+}
+
+/*
+ * Decrease the reference count of *packed_refs.  If it goes to zero,
+ * free *packed_refs and return true; otherwise return false.
+ */
+static int release_packed_ref_cache(struct packed_ref_cache *packed_refs)
+{
+	if (!--packed_refs->referrers) {
+		free_ref_cache(packed_refs->cache);
+		stat_validity_clear(&packed_refs->validity);
+		free(packed_refs);
+		return 1;
+	} else {
+		return 0;
+	}
+}
+
+/*
+ * A container for `packed-refs`-related data. It is not (yet) a
+ * `ref_store`.
+ */
+struct packed_ref_store {
+	struct ref_store base;
+
+	unsigned int store_flags;
+
+	/* The path of the "packed-refs" file: */
+	char *path;
+
+	/*
+	 * A cache of the values read from the `packed-refs` file, if
+	 * it might still be current; otherwise, NULL.
+	 */
+	struct packed_ref_cache *cache;
+
+	/*
+	 * 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;
+};
+
+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);
+	return ref_store;
+}
+
+/*
+ * Die if refs is not the main ref store. caller is used in any
+ * necessary error messages.
+ */
+static void packed_assert_main_repository(struct packed_ref_store *refs,
+					  const char *caller)
+{
+	if (refs->store_flags & REF_STORE_MAIN)
+		return;
+
+	die("BUG: operation %s only allowed for main ref store", caller);
+}
+
+/*
+ * 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)
+		die("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)
+		die("BUG: unallowed operation (%s), requires %x, has %x\n",
+		    caller, required_flags, refs->store_flags);
+
+	return refs;
+}
+
+static void clear_packed_ref_cache(struct packed_ref_store *refs)
+{
+	if (refs->cache) {
+		struct packed_ref_cache *cache = refs->cache;
+
+		refs->cache = NULL;
+		release_packed_ref_cache(cache);
+	}
+}
+
+/* The length of a peeled reference line in packed-refs, including EOL: */
+#define PEELED_LINE_LENGTH 42
+
+/*
+ * Parse one line from a packed-refs file.  Write the SHA1 to sha1.
+ * Return a pointer to the refname within the line (null-terminated),
+ * or NULL if there was a problem.
+ */
+static const char *parse_ref_line(struct strbuf *line, struct object_id *oid)
+{
+	const char *ref;
+
+	if (parse_oid_hex(line->buf, oid, &ref) < 0)
+		return NULL;
+	if (!isspace(*ref++))
+		return NULL;
+
+	if (isspace(*ref))
+		return NULL;
+
+	if (line->buf[line->len - 1] != '\n')
+		return NULL;
+	line->buf[--line->len] = 0;
+
+	return ref;
+}
+
+/*
+ * Read from `packed_refs_file` into a newly-allocated
+ * `packed_ref_cache` 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:
+ *
+ *   No traits:
+ *
+ *      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).
+ */
+static struct packed_ref_cache *read_packed_refs(const char *packed_refs_file)
+{
+	FILE *f;
+	struct packed_ref_cache *packed_refs = xcalloc(1, sizeof(*packed_refs));
+	struct ref_entry *last = NULL;
+	struct strbuf line = STRBUF_INIT;
+	enum { PEELED_NONE, PEELED_TAGS, PEELED_FULLY } peeled = PEELED_NONE;
+	struct ref_dir *dir;
+
+	acquire_packed_ref_cache(packed_refs);
+	packed_refs->cache = create_ref_cache(NULL, NULL);
+	packed_refs->cache->root->flag &= ~REF_INCOMPLETE;
+
+	f = fopen(packed_refs_file, "r");
+	if (!f) {
+		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.
+			 */
+			return packed_refs;
+		} else {
+			die_errno("couldn't read %s", packed_refs_file);
+		}
+	}
+
+	stat_validity_update(&packed_refs->validity, fileno(f));
+
+	dir = get_ref_dir(packed_refs->cache->root);
+	while (strbuf_getwholeline(&line, f, '\n') != EOF) {
+		struct object_id oid;
+		const char *refname;
+		const char *traits;
+
+		if (!line.len || line.buf[line.len - 1] != '\n')
+			die("unterminated line in %s: %s", packed_refs_file, line.buf);
+
+		if (skip_prefix(line.buf, "# pack-refs with:", &traits)) {
+			if (strstr(traits, " fully-peeled "))
+				peeled = PEELED_FULLY;
+			else if (strstr(traits, " peeled "))
+				peeled = PEELED_TAGS;
+			/* perhaps other traits later as well */
+			continue;
+		}
+
+		refname = parse_ref_line(&line, &oid);
+		if (refname) {
+			int flag = REF_ISPACKED;
+
+			if (check_refname_format(refname, REFNAME_ALLOW_ONELEVEL)) {
+				if (!refname_is_safe(refname))
+					die("packed refname is dangerous: %s", refname);
+				oidclr(&oid);
+				flag |= REF_BAD_NAME | REF_ISBROKEN;
+			}
+			last = create_ref_entry(refname, &oid, flag);
+			if (peeled == PEELED_FULLY ||
+			    (peeled == PEELED_TAGS && starts_with(refname, "refs/tags/")))
+				last->flag |= REF_KNOWS_PEELED;
+			add_ref_entry(dir, last);
+		} else if (last &&
+		    line.buf[0] == '^' &&
+		    line.len == PEELED_LINE_LENGTH &&
+		    line.buf[PEELED_LINE_LENGTH - 1] == '\n' &&
+		    !get_oid_hex(line.buf + 1, &oid)) {
+			oidcpy(&last->u.value.peeled, &oid);
+			/*
+			 * Regardless of what the file header said,
+			 * we definitely know the value of *this*
+			 * reference:
+			 */
+			last->flag |= REF_KNOWS_PEELED;
+		} else {
+			strbuf_setlen(&line, line.len - 1);
+			die("unexpected line in %s: %s", packed_refs_file, line.buf);
+		}
+	}
+
+	fclose(f);
+	strbuf_release(&line);
+
+	return packed_refs;
+}
+
+/*
+ * Check that the packed refs cache (if any) still reflects the
+ * contents of the file. If not, clear the cache.
+ */
+static void validate_packed_ref_cache(struct packed_ref_store *refs)
+{
+	if (refs->cache &&
+	    !stat_validity_check(&refs->cache->validity, refs->path))
+		clear_packed_ref_cache(refs);
+}
+
+/*
+ * Get the packed_ref_cache 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()`.
+ */
+static struct packed_ref_cache *get_packed_ref_cache(struct packed_ref_store *refs)
+{
+	if (!is_lock_file_locked(&refs->lock))
+		validate_packed_ref_cache(refs);
+
+	if (!refs->cache)
+		refs->cache = read_packed_refs(refs->path);
+
+	return refs->cache;
+}
+
+static struct ref_dir *get_packed_ref_dir(struct packed_ref_cache *packed_ref_cache)
+{
+	return get_ref_dir(packed_ref_cache->cache->root);
+}
+
+static struct ref_dir *get_packed_refs(struct packed_ref_store *refs)
+{
+	return get_packed_ref_dir(get_packed_ref_cache(refs));
+}
+
+/*
+ * Add or overwrite a reference in the in-memory packed reference
+ * cache. This may only be called while the packed-refs file is locked
+ * (see packed_refs_lock()). To actually write the packed-refs file,
+ * call commit_packed_refs().
+ */
+void add_packed_ref(struct ref_store *ref_store,
+		    const char *refname, const struct object_id *oid)
+{
+	struct packed_ref_store *refs =
+		packed_downcast(ref_store, REF_STORE_WRITE,
+				"add_packed_ref");
+	struct ref_dir *packed_refs;
+	struct ref_entry *packed_entry;
+
+	if (!is_lock_file_locked(&refs->lock))
+		die("BUG: packed refs not locked");
+
+	if (check_refname_format(refname, REFNAME_ALLOW_ONELEVEL))
+		die("Reference has invalid format: '%s'", refname);
+
+	packed_refs = get_packed_refs(refs);
+	packed_entry = find_ref_entry(packed_refs, refname);
+	if (packed_entry) {
+		/* Overwrite the existing entry: */
+		oidcpy(&packed_entry->u.value.oid, oid);
+		packed_entry->flag = REF_ISPACKED;
+		oidclr(&packed_entry->u.value.peeled);
+	} else {
+		packed_entry = create_ref_entry(refname, oid, REF_ISPACKED);
+		add_ref_entry(packed_refs, packed_entry);
+	}
+}
+
+/*
+ * Return the ref_entry for the given refname from the packed
+ * references.  If it does not exist, return NULL.
+ */
+static struct ref_entry *get_packed_ref(struct packed_ref_store *refs,
+					const char *refname)
+{
+	return find_ref_entry(get_packed_refs(refs), refname);
+}
+
+static int packed_read_raw_ref(struct ref_store *ref_store,
+			       const char *refname, unsigned char *sha1,
+			       struct strbuf *referent, unsigned int *type)
+{
+	struct packed_ref_store *refs =
+		packed_downcast(ref_store, REF_STORE_READ, "read_raw_ref");
+
+	struct ref_entry *entry;
+
+	*type = 0;
+
+	entry = get_packed_ref(refs, refname);
+	if (!entry) {
+		errno = ENOENT;
+		return -1;
+	}
+
+	hashcpy(sha1, entry->u.value.oid.hash);
+	*type = REF_ISPACKED;
+	return 0;
+}
+
+static int packed_peel_ref(struct ref_store *ref_store,
+			   const char *refname, unsigned char *sha1)
+{
+	struct packed_ref_store *refs =
+		packed_downcast(ref_store, REF_STORE_READ | REF_STORE_ODB,
+				"peel_ref");
+	struct ref_entry *r = get_packed_ref(refs, refname);
+
+	if (!r || peel_entry(r, 0))
+		return -1;
+
+	hashcpy(sha1, r->u.value.peeled.hash);
+	return 0;
+}
+
+struct packed_ref_iterator {
+	struct ref_iterator base;
+
+	struct packed_ref_cache *cache;
+	struct ref_iterator *iter0;
+	unsigned int flags;
+};
+
+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 = 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 packed_ref_iterator_peel(struct ref_iterator *ref_iterator,
+				   struct object_id *peeled)
+{
+	struct packed_ref_iterator *iter =
+		(struct packed_ref_iterator *)ref_iterator;
+
+	return ref_iterator_peel(iter->iter0, 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;
+
+	if (iter->iter0)
+		ok = ref_iterator_abort(iter->iter0);
+
+	release_packed_ref_cache(iter->cache);
+	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 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");
+
+	iter = xcalloc(1, sizeof(*iter));
+	ref_iterator = &iter->base;
+	base_ref_iterator_init(ref_iterator, &packed_ref_iterator_vtable);
+
+	/*
+	 * Note that get_packed_ref_cache() internally checks whether
+	 * the packed-ref cache is up to date with what is on disk,
+	 * and re-reads it if not.
+	 */
+
+	iter->cache = get_packed_ref_cache(refs);
+	acquire_packed_ref_cache(iter->cache);
+	iter->iter0 = cache_ref_iterator_begin(iter->cache->cache, prefix, 0);
+
+	iter->flags = flags;
+
+	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 unsigned char *sha1,
+			      const unsigned char *peeled)
+{
+	if (fprintf(fh, "%s %s\n", sha1_to_hex(sha1), refname) < 0 ||
+	    (peeled && fprintf(fh, "^%s\n", sha1_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;
+	struct packed_ref_cache *packed_ref_cache;
+
+	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(&refs->lock)) {
+		strbuf_addf(err, "unable to close %s: %s", refs->path, strerror(errno));
+		return -1;
+	}
+
+	/*
+	 * Now that we hold the `packed-refs` lock, make sure that our
+	 * cache matches the current version of the file. Normally
+	 * `get_packed_ref_cache()` does that for us, but that
+	 * function assumes that when the file is locked, any existing
+	 * cache is still valid. We've just locked the file, but it
+	 * might have changed the moment *before* we locked it.
+	 */
+	validate_packed_ref_cache(refs);
+
+	packed_ref_cache = get_packed_ref_cache(refs);
+	/* Increment the reference count to prevent it from being freed: */
+	acquire_packed_ref_cache(packed_ref_cache);
+	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))
+		die("BUG: packed_refs_unlock() called when not locked");
+	rollback_lock_file(&refs->lock);
+	release_packed_ref_cache(refs->cache);
+}
+
+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.  The trailing space is required.
+ */
+static const char PACKED_REFS_HEADER[] =
+	"# pack-refs with: peeled fully-peeled \n";
+
+/*
+ * Write the current version of the packed refs cache from memory to
+ * disk. The packed-refs file must already be locked for writing (see
+ * packed_refs_lock()). Return zero on success. On errors, rollback
+ * the lockfile, write an error message to `err`, and return a nonzero
+ * value.
+ */
+int commit_packed_refs(struct ref_store *ref_store, struct strbuf *err)
+{
+	struct packed_ref_store *refs =
+		packed_downcast(ref_store, REF_STORE_WRITE | REF_STORE_MAIN,
+				"commit_packed_refs");
+	struct packed_ref_cache *packed_ref_cache =
+		get_packed_ref_cache(refs);
+	int ok;
+	int ret = -1;
+	struct strbuf sb = STRBUF_INIT;
+	FILE *out;
+	struct ref_iterator *iter;
+	char *packed_refs_path;
+
+	if (!is_lock_file_locked(&refs->lock))
+		die("BUG: commit_packed_refs() called when 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);
+	if (create_tempfile(&refs->tempfile, sb.buf) < 0) {
+		strbuf_addf(err, "unable to create file %s: %s",
+			    sb.buf, strerror(errno));
+		strbuf_release(&sb);
+		goto out;
+	}
+	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) {
+		strbuf_addf(err, "error writing to %s: %s",
+			    get_tempfile_path(&refs->tempfile), strerror(errno));
+		goto error;
+	}
+
+	iter = cache_ref_iterator_begin(packed_ref_cache->cache, NULL, 0);
+	while ((ok = ref_iterator_advance(iter)) == ITER_OK) {
+		struct object_id peeled;
+		int peel_error = ref_iterator_peel(iter, &peeled);
+
+		if (write_packed_entry(out, iter->refname, iter->oid->hash,
+				       peel_error ? NULL : peeled.hash)) {
+			strbuf_addf(err, "error writing to %s: %s",
+				    get_tempfile_path(&refs->tempfile),
+				    strerror(errno));
+			ref_iterator_abort(iter);
+			goto error;
+		}
+	}
+
+	if (ok != ITER_DONE) {
+		strbuf_addf(err, "unable to rewrite packed-refs file: "
+			    "error iterating over old contents");
+		goto error;
+	}
+
+	if (rename_tempfile(&refs->tempfile, packed_refs_path)) {
+		strbuf_addf(err, "error replacing %s: %s",
+			    refs->path, strerror(errno));
+		goto out;
+	}
+
+	ret = 0;
+	goto out;
+
+error:
+	delete_tempfile(&refs->tempfile);
+
+out:
+	free(packed_refs_path);
+	return ret;
+}
+
+/*
+ * Rewrite the packed-refs file, omitting any refs listed in
+ * 'refnames'. On error, leave packed-refs unchanged, write an error
+ * message to 'err', and return a nonzero value. The packed refs lock
+ * must be held when calling this function; it will still be held when
+ * the function returns.
+ *
+ * The refs in 'refnames' needn't be sorted. `err` must not be NULL.
+ */
+int repack_without_refs(struct ref_store *ref_store,
+			struct string_list *refnames, struct strbuf *err)
+{
+	struct packed_ref_store *refs =
+		packed_downcast(ref_store, REF_STORE_WRITE | REF_STORE_MAIN,
+				"repack_without_refs");
+	struct ref_dir *packed;
+	struct string_list_item *refname;
+	int needs_repacking = 0, removed = 0;
+
+	packed_assert_main_repository(refs, "repack_without_refs");
+	assert(err);
+
+	if (!is_lock_file_locked(&refs->lock))
+		die("BUG: repack_without_refs called without holding lock");
+
+	/* Look for a packed ref */
+	for_each_string_list_item(refname, refnames) {
+		if (get_packed_ref(refs, refname->string)) {
+			needs_repacking = 1;
+			break;
+		}
+	}
+
+	/* Avoid locking if we have nothing to do */
+	if (!needs_repacking)
+		return 0; /* no refname exists in packed refs */
+
+	packed = get_packed_refs(refs);
+
+	/* Remove refnames from the cache */
+	for_each_string_list_item(refname, refnames)
+		if (remove_entry_from_dir(packed, refname->string) != -1)
+			removed = 1;
+	if (!removed) {
+		/*
+		 * All packed entries disappeared while we were
+		 * acquiring the lock.
+		 */
+		clear_packed_ref_cache(refs);
+		return 0;
+	}
+
+	/* Write what remains */
+	return commit_packed_refs(&refs->base, err);
+}
+
+static int packed_init_db(struct ref_store *ref_store, struct strbuf *err)
+{
+	/* Nothing to do. */
+	return 0;
+}
+
+static int packed_transaction_prepare(struct ref_store *ref_store,
+				      struct ref_transaction *transaction,
+				      struct strbuf *err)
+{
+	die("BUG: not implemented yet");
+}
+
+static int packed_transaction_abort(struct ref_store *ref_store,
+				    struct ref_transaction *transaction,
+				    struct strbuf *err)
+{
+	die("BUG: not implemented yet");
+}
+
+static int packed_transaction_finish(struct ref_store *ref_store,
+				     struct ref_transaction *transaction,
+				     struct strbuf *err)
+{
+	die("BUG: not implemented yet");
+}
+
+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)
+{
+	die("BUG: not implemented yet");
+}
+
+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)
+{
+	die("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)
+{
+	die("BUG: packed reference store does not support renaming 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)
+{
+	die("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 unsigned char *sha1,
+				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_peel_ref,
+	packed_create_symref,
+	packed_delete_refs,
+	packed_rename_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 0000000000..03b7c1de95
--- /dev/null
+++ b/refs/packed-backend.h
@@ -0,0 +1,25 @@
+#ifndef REFS_PACKED_BACKEND_H
+#define REFS_PACKED_BACKEND_H
+
+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);
+
+void add_packed_ref(struct ref_store *ref_store,
+		    const char *refname, const struct object_id *oid);
+
+int commit_packed_refs(struct ref_store *ref_store, struct strbuf *err);
+
+int repack_without_refs(struct ref_store *ref_store,
+			struct string_list *refnames, struct strbuf *err);
+
+#endif /* REFS_PACKED_BACKEND_H */
diff --git a/refs/refs-internal.h b/refs/refs-internal.h
index 192f9f85c9..4789106fc0 100644
--- a/refs/refs-internal.h
+++ b/refs/refs-internal.h
@@ -77,6 +77,15 @@
  */
 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,
@@ -655,6 +664,7 @@ struct ref_storage_be {
 };
 
 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