packed-backend: new module for handling packed references

Now that the interface between `files_ref_store` and
`packed_ref_store` is relatively narrow, move the latter into a new
module, "refs/packed-backend.h" and "refs/packed-backend.c". It still
doesn't quite implement the `ref_store` interface, but it will soon.

This commit moves code around and adjusts its visibility, but doesn't
change anything.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

6 files changed, 685 insertions, 639 deletions

diff --git a/Makefile b/Makefile
index e35542e63..aff9c6d7e 100644
--- a/Makefile
+++ b/Makefile
@@ -817,6 +817,7 @@ LIB_OBJS += reflog-walk.o
 LIB_OBJS += refs.o
 LIB_OBJS += refs/files-backend.o
 LIB_OBJS += refs/iterator.o
+LIB_OBJS += refs/packed-backend.o
 LIB_OBJS += refs/ref-cache.o
 LIB_OBJS += ref-filter.o
 LIB_OBJS += remote.o
diff --git a/refs.c b/refs.c
index 32177969f..5880c1237 100644
--- a/refs.c
+++ b/refs.c
@@ -173,6 +173,24 @@ int refname_is_safe(const char *refname)
 	return 1;
 }
 
+/*
+ * 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)
+{
+	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;
+}
+
 char *refs_resolve_refdup(struct ref_store *refs,
 			  const char *refname, int resolve_flags,
 			  unsigned char *sha1, int *flags)
diff --git a/refs/files-backend.c b/refs/files-backend.c
index 346794cf7..7df974779 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -2,6 +2,7 @@
 #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"
@@ -15,85 +16,6 @@ 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;
-};
-
-/*
- * A container for `packed-refs`-related data. It is not (yet) a
- * `ref_store`.
- */
-struct packed_ref_store {
-	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;
-};
-
-static struct packed_ref_store *packed_ref_store_create(
-		const char *path, unsigned int store_flags)
-{
-	struct packed_ref_store *refs = xcalloc(1, sizeof(*refs));
-
-	refs->store_flags = store_flags;
-	refs->path = xstrdup(path);
-	return refs;
-}
-
-/*
- * 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);
-}
-
-/*
  * Future: need to be in "struct repository"
  * when doing a full libification.
  */
@@ -109,42 +31,6 @@ struct files_ref_store {
 	struct packed_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 packed_ref_store *refs)
-{
-	if (refs->cache) {
-		struct packed_ref_cache *cache = refs->cache;
-
-		if (is_lock_file_locked(&refs->lock))
-			die("BUG: packed-ref cache cleared while locked");
-		refs->cache = NULL;
-		release_packed_ref_cache(cache);
-	}
-}
-
 static void clear_loose_ref_cache(struct files_ref_store *refs)
 {
 	if (refs->loose) {
@@ -215,151 +101,6 @@ 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 void files_reflog_path(struct files_ref_store *refs,
 			      struct strbuf *sb,
 			      const char *refname)
@@ -406,77 +147,6 @@ 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 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 lock_packed_refs()). To actually write the packed-refs file,
- * call commit_packed_refs().
- */
-static void add_packed_ref(struct packed_ref_store *refs,
-			   const char *refname, const struct object_id *oid)
-{
-	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);
-	}
-}
-
-/*
  * 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.
@@ -598,35 +268,6 @@ 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 packed_ref_store *refs,
-					const char *refname)
-{
-	return find_ref_entry(get_packed_refs(refs), refname);
-}
-
-static int packed_read_raw_ref(struct packed_ref_store *refs,
-			       const char *refname, unsigned char *sha1,
-			       struct strbuf *referent, unsigned int *type)
-{
-	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 files_read_raw_ref(struct ref_store *ref_store,
 			      const char *refname, unsigned char *sha1,
 			      struct strbuf *referent, unsigned int *type)
@@ -1011,18 +652,6 @@ out:
 	return ret;
 }
 
-static int packed_peel_ref(struct packed_ref_store *refs,
-			   const char *refname, unsigned char *sha1)
-{
-	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;
-}
-
 static int files_peel_ref(struct ref_store *ref_store,
 			  const char *refname, unsigned char *sha1)
 {
@@ -1060,99 +689,6 @@ static int files_peel_ref(struct ref_store *ref_store,
 	return peel_object(base, sha1);
 }
 
-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 packed_ref_store *refs,
-		const char *prefix, unsigned int flags)
-{
-	struct packed_ref_iterator *iter;
-	struct ref_iterator *ref_iterator;
-
-	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;
-}
-
 struct files_ref_iterator {
 	struct ref_iterator base;
 
@@ -1434,124 +970,6 @@ 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 packed_ref_store *refs, int flags)
-{
-	static int timeout_configured = 0;
-	static int timeout_value = 1000;
-	struct packed_ref_cache *packed_ref_cache;
-
-	packed_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->lock,
-			    refs->path,
-			    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 packed_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;
-
-	packed_assert_main_repository(refs, "commit_packed_refs");
-
-	if (!is_lock_file_locked(&refs->lock))
-		die("BUG: packed-refs not locked");
-
-	out = fdopen_lock_file(&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->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 packed_ref_store *refs)
-{
-	struct packed_ref_cache *packed_ref_cache = get_packed_ref_cache(refs);
-
-	packed_assert_main_repository(refs, "rollback_packed_refs");
-
-	if (!is_lock_file_locked(&refs->lock))
-		die("BUG: packed-refs not locked");
-	rollback_lock_file(&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];
@@ -1717,62 +1135,6 @@ static int files_pack_refs(struct ref_store *ref_store, unsigned int flags)
 	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 packed_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;
-
-	packed_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(refs->path, 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)
 {
diff --git a/refs/packed-backend.c b/refs/packed-backend.c
new file mode 100644
index 000000000..e468421a4
--- /dev/null
+++ b/refs/packed-backend.c
@@ -0,0 +1,623 @@
+#include "../cache.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 {
+	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;
+};
+
+struct packed_ref_store *packed_ref_store_create(
+		const char *path, unsigned int store_flags)
+{
+	struct packed_ref_store *refs = xcalloc(1, sizeof(*refs));
+
+	refs->store_flags = store_flags;
+	refs->path = xstrdup(path);
+	return refs;
+}
+
+/*
+ * 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);
+}
+
+static void clear_packed_ref_cache(struct packed_ref_store *refs)
+{
+	if (refs->cache) {
+		struct packed_ref_cache *cache = refs->cache;
+
+		if (is_lock_file_locked(&refs->lock))
+			die("BUG: packed-ref cache cleared while locked");
+		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 (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;
+}
+
+/*
+ * 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 lock_packed_refs()). To actually write the packed-refs file,
+ * call commit_packed_refs().
+ */
+void add_packed_ref(struct packed_ref_store *refs,
+		    const char *refname, const struct object_id *oid)
+{
+	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);
+}
+
+int packed_read_raw_ref(struct packed_ref_store *refs,
+			const char *refname, unsigned char *sha1,
+			struct strbuf *referent, unsigned int *type)
+{
+	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;
+}
+
+int packed_peel_ref(struct packed_ref_store *refs,
+		    const char *refname, unsigned char *sha1)
+{
+	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
+};
+
+struct ref_iterator *packed_ref_iterator_begin(
+		struct packed_ref_store *refs,
+		const char *prefix, unsigned int flags)
+{
+	struct packed_ref_iterator *iter;
+	struct ref_iterator *ref_iterator;
+
+	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.
+ */
+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));
+}
+
+int lock_packed_refs(struct packed_ref_store *refs, int flags)
+{
+	static int timeout_configured = 0;
+	static int timeout_value = 1000;
+	struct packed_ref_cache *packed_ref_cache;
+
+	packed_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->lock,
+			    refs->path,
+			    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;
+}
+
+/*
+ * 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
+ * lock_packed_refs()). Return zero on success. On errors, set errno
+ * and return a nonzero value.
+ */
+int commit_packed_refs(struct packed_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;
+
+	packed_assert_main_repository(refs, "commit_packed_refs");
+
+	if (!is_lock_file_locked(&refs->lock))
+		die("BUG: packed-refs not locked");
+
+	out = fdopen_lock_file(&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->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 packed_ref_store *refs)
+{
+	struct packed_ref_cache *packed_ref_cache = get_packed_ref_cache(refs);
+
+	packed_assert_main_repository(refs, "rollback_packed_refs");
+
+	if (!is_lock_file_locked(&refs->lock))
+		die("BUG: packed-refs not locked");
+	rollback_lock_file(&refs->lock);
+	release_packed_ref_cache(packed_ref_cache);
+	clear_packed_ref_cache(refs);
+}
+
+/*
+ * 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.
+ */
+int repack_without_refs(struct packed_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;
+
+	packed_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(refs->path, 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;
+}
diff --git a/refs/packed-backend.h b/refs/packed-backend.h
new file mode 100644
index 000000000..22e8817ac
--- /dev/null
+++ b/refs/packed-backend.h
@@ -0,0 +1,33 @@
+#ifndef REFS_PACKED_BACKEND_H
+#define REFS_PACKED_BACKEND_H
+
+struct packed_ref_store *packed_ref_store_create(
+		const char *path, unsigned int store_flags);
+
+int packed_read_raw_ref(struct packed_ref_store *refs,
+			const char *refname, unsigned char *sha1,
+			struct strbuf *referent, unsigned int *type);
+
+int packed_peel_ref(struct packed_ref_store *refs,
+		    const char *refname, unsigned char *sha1);
+
+struct ref_iterator *packed_ref_iterator_begin(
+		struct packed_ref_store *refs,
+		const char *prefix, unsigned int flags);
+
+/*
+ * 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.
+ */
+int lock_packed_refs(struct packed_ref_store *refs, int flags);
+
+void add_packed_ref(struct packed_ref_store *refs,
+		    const char *refname, const struct object_id *oid);
+
+int commit_packed_refs(struct packed_ref_store *refs);
+
+int repack_without_refs(struct packed_ref_store *refs,
+			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 192f9f85c..6f8f9f561 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,