5 files changed, 357 insertions, 159 deletions

diff --git a/refs/files-backend.c b/refs/files-backend.c
index cb1f528cbe..0404f2c233 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -1,4 +1,5 @@
 #include "../cache.h"
+#include "../config.h"
 #include "../refs.h"
 #include "refs-internal.h"
 #include "ref-cache.h"
@@ -43,15 +44,6 @@ struct packed_ref_cache {
 	 */
 	unsigned int referrers;
 
-	/*
-	 * Iff the packed-refs file associated with this instance is
-	 * currently locked for writing, this points at the associated
-	 * lock (which is owned by somebody else).  The referrer count
-	 * is also incremented when the file is locked and decremented
-	 * when it is unlocked.
-	 */
-	struct lock_file *lock;
-
 	/* The metadata from when this packed-refs cache was read */
 	struct stat_validity validity;
 };
@@ -70,10 +62,13 @@ struct files_ref_store {
 
 	struct ref_cache *loose;
 	struct packed_ref_cache *packed;
-};
 
-/* Lock used for the main packed-refs file: */
-static struct lock_file packlock;
+	/*
+	 * 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;
+};
 
 /*
  * Increment the reference count of *packed_refs.
@@ -104,8 +99,8 @@ static void clear_packed_ref_cache(struct files_ref_store *refs)
 	if (refs->packed) {
 		struct packed_ref_cache *packed_refs = refs->packed;
 
-		if (packed_refs->lock)
-			die("internal error: packed-ref cache cleared while locked");
+		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);
 	}
@@ -215,7 +210,9 @@ static const char *parse_ref_line(struct strbuf *line, struct object_id *oid)
 }
 
 /*
- * Read f, which is a packed-refs file, into dir.
+ * 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:
@@ -241,12 +238,36 @@ static const char *parse_ref_line(struct strbuf *line, struct object_id *oid)
  *      compatibility with older clients, but we do not require it
  *      (i.e., "peeled" is a no-op if "fully-peeled" is set).
  */
-static void read_packed_refs(FILE *f, struct ref_dir *dir)
+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;
@@ -271,7 +292,7 @@ static void read_packed_refs(FILE *f, struct ref_dir *dir)
 				oidclr(&oid);
 				flag |= REF_BAD_NAME | REF_ISBROKEN;
 			}
-			last = create_ref_entry(refname, &oid, flag, 0);
+			last = create_ref_entry(refname, &oid, flag);
 			if (peeled == PEELED_FULLY ||
 			    (peeled == PEELED_TAGS && starts_with(refname, "refs/tags/")))
 				last->flag |= REF_KNOWS_PEELED;
@@ -293,7 +314,10 @@ static void read_packed_refs(FILE *f, struct ref_dir *dir)
 		}
 	}
 
+	fclose(f);
 	strbuf_release(&line);
+
+	return packed_refs;
 }
 
 static const char *files_packed_refs_path(struct files_ref_store *refs)
@@ -347,31 +371,35 @@ 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 it if necessary.
+ * 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 (refs->packed &&
-	    !stat_validity_check(&refs->packed->validity, packed_refs_file))
-		clear_packed_ref_cache(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);
 
-	if (!refs->packed) {
-		FILE *f;
-
-		refs->packed = xcalloc(1, sizeof(*refs->packed));
-		acquire_packed_ref_cache(refs->packed);
-		refs->packed->cache = create_ref_cache(&refs->base, NULL);
-		refs->packed->cache->root->flag &= ~REF_INCOMPLETE;
-		f = fopen(packed_refs_file, "r");
-		if (f) {
-			stat_validity_update(&refs->packed->validity, fileno(f));
-			read_packed_refs(f, get_ref_dir(refs->packed->cache->root));
-			fclose(f);
-		}
-	}
 	return refs->packed;
 }
 
@@ -396,10 +424,14 @@ static void add_packed_ref(struct files_ref_store *refs,
 {
 	struct packed_ref_cache *packed_ref_cache = get_packed_ref_cache(refs);
 
-	if (!packed_ref_cache->lock)
-		die("internal error: packed refs not locked");
+	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, 1));
+		      create_ref_entry(refname, oid, REF_ISPACKED));
 }
 
 /*
@@ -476,7 +508,7 @@ static void loose_fill_ref_dir(struct ref_store *ref_store,
 				flag |= REF_BAD_NAME | REF_ISBROKEN;
 			}
 			add_entry_to_dir(dir,
-					 create_ref_entry(refname.buf, &oid, flag, 0));
+					 create_ref_entry(refname.buf, &oid, flag));
 		}
 		strbuf_setlen(&refname, dirnamelen);
 		strbuf_setlen(&path, path_baselen);
@@ -1057,15 +1089,12 @@ static struct ref_iterator *files_ref_iterator_begin(
 	struct ref_iterator *loose_iter, *packed_iter;
 	struct files_ref_iterator *iter;
 	struct ref_iterator *ref_iterator;
+	unsigned int required_flags = REF_STORE_READ;
 
-	if (ref_paranoia < 0)
-		ref_paranoia = git_env_bool("GIT_REF_PARANOIA", 0);
-	if (ref_paranoia)
-		flags |= DO_FOR_EACH_INCLUDE_BROKEN;
+	if (!(flags & DO_FOR_EACH_INCLUDE_BROKEN))
+		required_flags |= REF_STORE_ODB;
 
-	refs = files_downcast(ref_store,
-			      REF_STORE_READ | (ref_paranoia ? 0 : REF_STORE_ODB),
-			      "ref_iterator_begin");
+	refs = files_downcast(ref_store, required_flags, "ref_iterator_begin");
 
 	iter = xcalloc(1, sizeof(*iter));
 	ref_iterator = &iter->base;
@@ -1290,17 +1319,21 @@ static int lock_packed_refs(struct files_ref_store *refs, int flags)
 	}
 
 	if (hold_lock_file_for_update_timeout(
-			    &packlock, files_packed_refs_path(refs),
+			    &refs->packed_refs_lock, files_packed_refs_path(refs),
 			    flags, timeout_value) < 0)
 		return -1;
+
 	/*
-	 * Get the current packed-refs while holding the lock.  If the
-	 * packed-refs file has been modified since we last read it,
-	 * this will automatically invalidate the cache and re-read
-	 * the packed-refs file.
+	 * 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);
-	packed_ref_cache->lock = &packlock;
 	/* Increment the reference count to prevent it from being freed: */
 	acquire_packed_ref_cache(packed_ref_cache);
 	return 0;
@@ -1323,10 +1356,10 @@ static int commit_packed_refs(struct files_ref_store *refs)
 
 	files_assert_main_repository(refs, "commit_packed_refs");
 
-	if (!packed_ref_cache->lock)
-		die("internal error: packed-refs not locked");
+	if (!is_lock_file_locked(&refs->packed_refs_lock))
+		die("BUG: packed-refs not locked");
 
-	out = fdopen_lock_file(packed_ref_cache->lock, "w");
+	out = fdopen_lock_file(&refs->packed_refs_lock, "w");
 	if (!out)
 		die_errno("unable to fdopen packed-refs descriptor");
 
@@ -1344,11 +1377,10 @@ static int commit_packed_refs(struct files_ref_store *refs)
 	if (ok != ITER_DONE)
 		die("error while iterating over references");
 
-	if (commit_lock_file(packed_ref_cache->lock)) {
+	if (commit_lock_file(&refs->packed_refs_lock)) {
 		save_errno = errno;
 		error = -1;
 	}
-	packed_ref_cache->lock = NULL;
 	release_packed_ref_cache(packed_ref_cache);
 	errno = save_errno;
 	return error;
@@ -1366,10 +1398,9 @@ static void rollback_packed_refs(struct files_ref_store *refs)
 
 	files_assert_main_repository(refs, "rollback_packed_refs");
 
-	if (!packed_ref_cache->lock)
-		die("internal error: packed-refs not locked");
-	rollback_lock_file(packed_ref_cache->lock);
-	packed_ref_cache->lock = NULL;
+	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);
 }
@@ -1464,6 +1495,32 @@ static void prune_refs(struct files_ref_store *refs, struct ref_to_prune *r)
 	}
 }
 
+/*
+ * Return true if the specified reference should be packed.
+ */
+static int should_pack_ref(const char *refname,
+			   const struct object_id *oid, unsigned int ref_flags,
+			   unsigned int pack_flags)
+{
+	/* Do not pack per-worktree refs: */
+	if (ref_type(refname) != REF_TYPE_NORMAL)
+		return 0;
+
+	/* Do not pack non-tags unless PACK_REFS_ALL is set: */
+	if (!(pack_flags & PACK_REFS_ALL) && !starts_with(refname, "refs/tags/"))
+		return 0;
+
+	/* Do not pack symbolic refs: */
+	if (ref_flags & REF_ISSYMREF)
+		return 0;
+
+	/* Do not pack broken refs: */
+	if (!ref_resolves_to_object(refname, oid, ref_flags))
+		return 0;
+
+	return 1;
+}
+
 static int files_pack_refs(struct ref_store *ref_store, unsigned int flags)
 {
 	struct files_ref_store *refs =
@@ -1485,21 +1542,9 @@ static int files_pack_refs(struct ref_store *ref_store, unsigned int flags)
 		 * pruned, also add it to refs_to_prune.
 		 */
 		struct ref_entry *packed_entry;
-		int is_tag_ref = starts_with(iter->refname, "refs/tags/");
-
-		/* Do not pack per-worktree refs: */
-		if (ref_type(iter->refname) != REF_TYPE_NORMAL)
-			continue;
-
-		/* ALWAYS pack tags */
-		if (!(flags & PACK_REFS_ALL) && !is_tag_ref)
-			continue;
 
-		/* Do not pack symbolic or broken refs: */
-		if (iter->flags & REF_ISSYMREF)
-			continue;
-
-		if (!ref_resolves_to_object(iter->refname, iter->oid, iter->flags))
+		if (!should_pack_ref(iter->refname, iter->oid, iter->flags,
+				     flags))
 			continue;
 
 		/*
@@ -1515,7 +1560,7 @@ static int files_pack_refs(struct ref_store *ref_store, unsigned int flags)
 			oidcpy(&packed_entry->u.value.oid, iter->oid);
 		} else {
 			packed_entry = create_ref_entry(iter->refname, iter->oid,
-							REF_ISPACKED, 0);
+							REF_ISPACKED);
 			add_ref_entry(packed_refs, packed_entry);
 		}
 		oidclr(&packed_entry->u.value.peeled);
@@ -1595,7 +1640,7 @@ static int repack_without_refs(struct files_ref_store *refs,
 	return ret;
 }
 
-static int files_delete_refs(struct ref_store *ref_store,
+static int files_delete_refs(struct ref_store *ref_store, const char *msg,
 			     struct string_list *refnames, unsigned int flags)
 {
 	struct files_ref_store *refs =
@@ -1627,7 +1672,7 @@ static int files_delete_refs(struct ref_store *ref_store,
 	for (i = 0; i < refnames->nr; i++) {
 		const char *refname = refnames->items[i].string;
 
-		if (refs_delete_ref(&refs->base, NULL, refname, NULL, flags))
+		if (refs_delete_ref(&refs->base, msg, refname, NULL, flags))
 			result |= error(_("could not remove reference %s"), refname);
 	}
 
@@ -2521,23 +2566,6 @@ static struct ref_iterator *files_reflog_iterator_begin(struct ref_store *ref_st
 	return ref_iterator;
 }
 
-static int ref_update_reject_duplicates(struct string_list *refnames,
-					struct strbuf *err)
-{
-	int i, n = refnames->nr;
-
-	assert(err);
-
-	for (i = 1; i < n; i++)
-		if (!strcmp(refnames->items[i - 1].string, refnames->items[i].string)) {
-			strbuf_addf(err,
-				    "multiple updates for ref '%s' not allowed.",
-				    refnames->items[i].string);
-			return 1;
-		}
-	return 0;
-}
-
 /*
  * If update is a direct update of head_ref (the reference pointed to
  * by HEAD), then add an extra REF_LOG_ONLY update for HEAD.
@@ -2843,31 +2871,45 @@ static int lock_ref_for_update(struct files_ref_store *refs,
 	return 0;
 }
 
-static int files_transaction_commit(struct ref_store *ref_store,
-				    struct ref_transaction *transaction,
-				    struct strbuf *err)
+/*
+ * Unlock any references in `transaction` that are still locked, and
+ * mark the transaction closed.
+ */
+static void files_transaction_cleanup(struct ref_transaction *transaction)
+{
+	size_t i;
+
+	for (i = 0; i < transaction->nr; i++) {
+		struct ref_update *update = transaction->updates[i];
+		struct ref_lock *lock = update->backend_data;
+
+		if (lock) {
+			unlock_ref(lock);
+			update->backend_data = NULL;
+		}
+	}
+
+	transaction->state = REF_TRANSACTION_CLOSED;
+}
+
+static int files_transaction_prepare(struct ref_store *ref_store,
+				     struct ref_transaction *transaction,
+				     struct strbuf *err)
 {
 	struct files_ref_store *refs =
 		files_downcast(ref_store, REF_STORE_WRITE,
-			       "ref_transaction_commit");
-	int ret = 0, i;
-	struct string_list refs_to_delete = STRING_LIST_INIT_NODUP;
-	struct string_list_item *ref_to_delete;
+			       "ref_transaction_prepare");
+	size_t i;
+	int ret = 0;
 	struct string_list affected_refnames = STRING_LIST_INIT_NODUP;
 	char *head_ref = NULL;
 	int head_type;
 	struct object_id head_oid;
-	struct strbuf sb = STRBUF_INIT;
 
 	assert(err);
 
-	if (transaction->state != REF_TRANSACTION_OPEN)
-		die("BUG: commit called for transaction that is not open");
-
-	if (!transaction->nr) {
-		transaction->state = REF_TRANSACTION_CLOSED;
-		return 0;
-	}
+	if (!transaction->nr)
+		goto cleanup;
 
 	/*
 	 * Fail if a refname appears more than once in the
@@ -2917,8 +2959,7 @@ static int files_transaction_commit(struct ref_store *ref_store,
 				       head_oid.hash, &head_type);
 
 	if (head_ref && !(head_type & REF_ISSYMREF)) {
-		free(head_ref);
-		head_ref = NULL;
+		FREE_AND_NULL(head_ref);
 	}
 
 	/*
@@ -2926,6 +2967,8 @@ static int files_transaction_commit(struct ref_store *ref_store,
 	 * that new values are valid, and write new values to the
 	 * lockfiles, ready to be activated. Only keep one lockfile
 	 * open at a time to avoid running out of file descriptors.
+	 * Note that lock_ref_for_update() might append more updates
+	 * to the transaction.
 	 */
 	for (i = 0; i < transaction->nr; i++) {
 		struct ref_update *update = transaction->updates[i];
@@ -2933,7 +2976,38 @@ static int files_transaction_commit(struct ref_store *ref_store,
 		ret = lock_ref_for_update(refs, update, transaction,
 					  head_ref, &affected_refnames, err);
 		if (ret)
-			goto cleanup;
+			break;
+	}
+
+cleanup:
+	free(head_ref);
+	string_list_clear(&affected_refnames, 0);
+
+	if (ret)
+		files_transaction_cleanup(transaction);
+	else
+		transaction->state = REF_TRANSACTION_PREPARED;
+
+	return ret;
+}
+
+static int files_transaction_finish(struct ref_store *ref_store,
+				    struct ref_transaction *transaction,
+				    struct strbuf *err)
+{
+	struct files_ref_store *refs =
+		files_downcast(ref_store, 0, "ref_transaction_finish");
+	size_t i;
+	int ret = 0;
+	struct string_list refs_to_delete = STRING_LIST_INIT_NODUP;
+	struct string_list_item *ref_to_delete;
+	struct strbuf sb = STRBUF_INIT;
+
+	assert(err);
+
+	if (!transaction->nr) {
+		transaction->state = REF_TRANSACTION_CLOSED;
+		return 0;
 	}
 
 	/* Perform updates first so live commits remain referenced */
@@ -3013,15 +3087,10 @@ static int files_transaction_commit(struct ref_store *ref_store,
 	clear_loose_ref_cache(refs);
 
 cleanup:
-	strbuf_release(&sb);
-	transaction->state = REF_TRANSACTION_CLOSED;
+	files_transaction_cleanup(transaction);
 
 	for (i = 0; i < transaction->nr; i++) {
 		struct ref_update *update = transaction->updates[i];
-		struct ref_lock *lock = update->backend_data;
-
-		if (lock)
-			unlock_ref(lock);
 
 		if (update->flags & REF_DELETED_LOOSE) {
 			/*
@@ -3035,13 +3104,19 @@ cleanup:
 		}
 	}
 
+	strbuf_release(&sb);
 	string_list_clear(&refs_to_delete, 0);
-	free(head_ref);
-	string_list_clear(&affected_refnames, 0);
-
 	return ret;
 }
 
+static int files_transaction_abort(struct ref_store *ref_store,
+				   struct ref_transaction *transaction,
+				   struct strbuf *err)
+{
+	files_transaction_cleanup(transaction);
+	return 0;
+}
+
 static int ref_present(const char *refname,
 		       const struct object_id *oid, int flags, void *cb_data)
 {
@@ -3057,7 +3132,8 @@ static int files_initial_transaction_commit(struct ref_store *ref_store,
 	struct files_ref_store *refs =
 		files_downcast(ref_store, REF_STORE_WRITE,
 			       "initial_ref_transaction_commit");
-	int ret = 0, i;
+	size_t i;
+	int ret = 0;
 	struct string_list affected_refnames = STRING_LIST_INIT_NODUP;
 
 	assert(err);
@@ -3311,7 +3387,9 @@ struct ref_storage_be refs_be_files = {
 	"files",
 	files_ref_store_create,
 	files_init_db,
-	files_transaction_commit,
+	files_transaction_prepare,
+	files_transaction_finish,
+	files_transaction_abort,
 	files_initial_transaction_commit,
 
 	files_pack_refs,
diff --git a/refs/iterator.c b/refs/iterator.c
index bce1f192f7..4cf449ef66 100644
--- a/refs/iterator.c
+++ b/refs/iterator.c
@@ -292,7 +292,23 @@ static int prefix_ref_iterator_advance(struct ref_iterator *ref_iterator)
 		if (!starts_with(iter->iter0->refname, iter->prefix))
 			continue;
 
-		iter->base.refname = iter->iter0->refname + iter->trim;
+		if (iter->trim) {
+			/*
+			 * It is nonsense to trim off characters that
+			 * you haven't already checked for via a
+			 * prefix check, whether via this
+			 * `prefix_ref_iterator` or upstream in
+			 * `iter0`). So if there wouldn't be at least
+			 * one character left in the refname after
+			 * trimming, report it as a bug:
+			 */
+			if (strlen(iter->iter0->refname) <= iter->trim)
+				die("BUG: attempt to trim too many characters");
+			iter->base.refname = iter->iter0->refname + iter->trim;
+		} else {
+			iter->base.refname = iter->iter0->refname;
+		}
+
 		iter->base.oid = iter->iter0->oid;
 		iter->base.flags = iter->iter0->flags;
 		return ITER_OK;
diff --git a/refs/ref-cache.c b/refs/ref-cache.c
index 6b11d9cd12..76bb723c86 100644
--- a/refs/ref-cache.c
+++ b/refs/ref-cache.c
@@ -32,14 +32,10 @@ struct ref_dir *get_ref_dir(struct ref_entry *entry)
 }
 
 struct ref_entry *create_ref_entry(const char *refname,
-				   const struct object_id *oid, int flag,
-				   int check_name)
+				   const struct object_id *oid, int flag)
 {
 	struct ref_entry *ref;
 
-	if (check_name &&
-	    check_refname_format(refname, REFNAME_ALLOW_ONELEVEL))
-		die("Reference has invalid format: '%s'", refname);
 	FLEX_ALLOC_STR(ref, name, refname);
 	oidcpy(&ref->u.value.oid, oid);
 	oidclr(&ref->u.value.peeled);
@@ -86,9 +82,8 @@ static void clear_ref_dir(struct ref_dir *dir)
 	int i;
 	for (i = 0; i < dir->nr; i++)
 		free_ref_entry(dir->entries[i]);
-	free(dir->entries);
+	FREE_AND_NULL(dir->entries);
 	dir->sorted = dir->nr = dir->alloc = 0;
-	dir->entries = NULL;
 }
 
 struct ref_entry *create_dir_entry(struct ref_cache *cache,
@@ -316,11 +311,42 @@ static void sort_ref_dir(struct ref_dir *dir)
 	dir->sorted = dir->nr = i;
 }
 
+enum prefix_state {
+	/* All refs within the directory would match prefix: */
+	PREFIX_CONTAINS_DIR,
+
+	/* Some, but not all, refs within the directory might match prefix: */
+	PREFIX_WITHIN_DIR,
+
+	/* No refs within the directory could possibly match prefix: */
+	PREFIX_EXCLUDES_DIR
+};
+
 /*
- * Load all of the refs from `dir` (recursively) into our in-memory
- * cache.
+ * Return a `prefix_state` constant describing the relationship
+ * between the directory with the specified `dirname` and `prefix`.
  */
-static void prime_ref_dir(struct ref_dir *dir)
+static enum prefix_state overlaps_prefix(const char *dirname,
+					 const char *prefix)
+{
+	while (*prefix && *dirname == *prefix) {
+		dirname++;
+		prefix++;
+	}
+	if (!*prefix)
+		return PREFIX_CONTAINS_DIR;
+	else if (!*dirname)
+		return PREFIX_WITHIN_DIR;
+	else
+		return PREFIX_EXCLUDES_DIR;
+}
+
+/*
+ * Load all of the refs from `dir` (recursively) that could possibly
+ * contain references matching `prefix` into our in-memory cache. If
+ * `prefix` is NULL, prime unconditionally.
+ */
+static void prime_ref_dir(struct ref_dir *dir, const char *prefix)
 {
 	/*
 	 * The hard work of loading loose refs is done by get_ref_dir(), so we
@@ -331,8 +357,29 @@ static void prime_ref_dir(struct ref_dir *dir)
 	int i;
 	for (i = 0; i < dir->nr; i++) {
 		struct ref_entry *entry = dir->entries[i];
-		if (entry->flag & REF_DIR)
-			prime_ref_dir(get_ref_dir(entry));
+		if (!(entry->flag & REF_DIR)) {
+			/* Not a directory; no need to recurse. */
+		} else if (!prefix) {
+			/* Recurse in any case: */
+			prime_ref_dir(get_ref_dir(entry), NULL);
+		} else {
+			switch (overlaps_prefix(entry->name, prefix)) {
+			case PREFIX_CONTAINS_DIR:
+				/*
+				 * Recurse, and from here down we
+				 * don't have to check the prefix
+				 * anymore:
+				 */
+				prime_ref_dir(get_ref_dir(entry), NULL);
+				break;
+			case PREFIX_WITHIN_DIR:
+				prime_ref_dir(get_ref_dir(entry), prefix);
+				break;
+			case PREFIX_EXCLUDES_DIR:
+				/* No need to prime this directory. */
+				break;
+			}
+		}
 	}
 }
 
@@ -347,6 +394,8 @@ struct cache_ref_iterator_level {
 	 */
 	struct ref_dir *dir;
 
+	enum prefix_state prefix_state;
+
 	/*
 	 * The index of the current entry within dir (which might
 	 * itself be a directory). If index == -1, then the iteration
@@ -374,6 +423,13 @@ struct cache_ref_iterator {
 	size_t levels_alloc;
 
 	/*
+	 * Only include references with this prefix in the iteration.
+	 * The prefix is matched textually, without regard for path
+	 * component boundaries.
+	 */
+	const char *prefix;
+
+	/*
 	 * A stack of levels. levels[0] is the uppermost level that is
 	 * being iterated over in this iteration. (This is not
 	 * necessary the top level in the references hierarchy. If we
@@ -394,6 +450,7 @@ static int cache_ref_iterator_advance(struct ref_iterator *ref_iterator)
 			&iter->levels[iter->levels_nr - 1];
 		struct ref_dir *dir = level->dir;
 		struct ref_entry *entry;
+		enum prefix_state entry_prefix_state;
 
 		if (level->index == -1)
 			sort_ref_dir(dir);
@@ -408,6 +465,14 @@ static int cache_ref_iterator_advance(struct ref_iterator *ref_iterator)
 
 		entry = dir->entries[level->index];
 
+		if (level->prefix_state == PREFIX_WITHIN_DIR) {
+			entry_prefix_state = overlaps_prefix(entry->name, iter->prefix);
+			if (entry_prefix_state == PREFIX_EXCLUDES_DIR)
+				continue;
+		} else {
+			entry_prefix_state = level->prefix_state;
+		}
+
 		if (entry->flag & REF_DIR) {
 			/* push down a level */
 			ALLOC_GROW(iter->levels, iter->levels_nr + 1,
@@ -415,6 +480,7 @@ static int cache_ref_iterator_advance(struct ref_iterator *ref_iterator)
 
 			level = &iter->levels[iter->levels_nr++];
 			level->dir = get_ref_dir(entry);
+			level->prefix_state = entry_prefix_state;
 			level->index = -1;
 		} else {
 			iter->base.refname = entry->name;
@@ -475,6 +541,7 @@ static int cache_ref_iterator_abort(struct ref_iterator *ref_iterator)
 	struct cache_ref_iterator *iter =
 		(struct cache_ref_iterator *)ref_iterator;
 
+	free((char *)iter->prefix);
 	free(iter->levels);
 	base_ref_iterator_free(ref_iterator);
 	return ITER_DONE;
@@ -500,10 +567,10 @@ struct ref_iterator *cache_ref_iterator_begin(struct ref_cache *cache,
 		dir = find_containing_dir(dir, prefix, 0);
 	if (!dir)
 		/* There's nothing to iterate over. */
-		return  empty_ref_iterator_begin();
+		return empty_ref_iterator_begin();
 
 	if (prime_dir)
-		prime_ref_dir(dir);
+		prime_ref_dir(dir, prefix);
 
 	iter = xcalloc(1, sizeof(*iter));
 	ref_iterator = &iter->base;
@@ -515,9 +582,12 @@ struct ref_iterator *cache_ref_iterator_begin(struct ref_cache *cache,
 	level->index = -1;
 	level->dir = dir;
 
-	if (prefix && *prefix)
-		ref_iterator = prefix_ref_iterator_begin(ref_iterator,
-							 prefix, 0);
+	if (prefix && *prefix) {
+		iter->prefix = xstrdup(prefix);
+		level->prefix_state = PREFIX_WITHIN_DIR;
+	} else {
+		level->prefix_state = PREFIX_CONTAINS_DIR;
+	}
 
 	return ref_iterator;
 }
diff --git a/refs/ref-cache.h b/refs/ref-cache.h
index 1f65e2f9ed..794f000fd3 100644
--- a/refs/ref-cache.h
+++ b/refs/ref-cache.h
@@ -185,8 +185,7 @@ struct ref_entry *create_dir_entry(struct ref_cache *cache,
 				   int incomplete);
 
 struct ref_entry *create_ref_entry(const char *refname,
-				   const struct object_id *oid, int flag,
-				   int check_name);
+				   const struct object_id *oid, int flag);
 
 /*
  * Return a pointer to a new `ref_cache`. Its top-level starts out
@@ -194,7 +193,8 @@ struct ref_entry *create_ref_entry(const char *refname,
  * function called to fill in incomplete directories in the
  * `ref_cache` when they are accessed. If it is NULL, then the whole
  * `ref_cache` must be filled (including clearing its directories'
- * `REF_INCOMPLETE` bits) before it is used.
+ * `REF_INCOMPLETE` bits) before it is used, and `refs` can be NULL,
+ * too.
  */
 struct ref_cache *create_ref_cache(struct ref_store *refs,
 				   fill_ref_dir_fn *fill_ref_dir);
diff --git a/refs/refs-internal.h b/refs/refs-internal.h
index b6b291cf00..192f9f85c9 100644
--- a/refs/refs-internal.h
+++ b/refs/refs-internal.h
@@ -170,6 +170,14 @@ int refs_read_raw_ref(struct ref_store *ref_store,
 		      struct strbuf *referent, unsigned int *type);
 
 /*
+ * Write an error to `err` and return a nonzero value iff the same
+ * refname appears multiple times in `refnames`. `refnames` must be
+ * sorted on entry to this function.
+ */
+int ref_update_reject_duplicates(struct string_list *refnames,
+				 struct strbuf *err);
+
+/*
  * Add a ref_update with the specified properties to transaction, and
  * return a pointer to the new object. This function does not verify
  * that refname is well-formed. new_sha1 and old_sha1 are only
@@ -185,17 +193,27 @@ struct ref_update *ref_transaction_add_update(
 
 /*
  * Transaction states.
- * OPEN:   The transaction is in a valid state and can accept new updates.
- *         An OPEN transaction can be committed.
- * CLOSED: A closed transaction is no longer active and no other operations
- *         than free can be used on it in this state.
- *         A transaction can either become closed by successfully committing
- *         an active transaction or if there is a failure while building
- *         the transaction thus rendering it failed/inactive.
+ *
+ * OPEN:   The transaction is initialized and new updates can still be
+ *         added to it. An OPEN transaction can be prepared,
+ *         committed, freed, or aborted (freeing and aborting an open
+ *         transaction are equivalent).
+ *
+ * PREPARED: ref_transaction_prepare(), which locks all of the
+ *         references involved in the update and checks that the
+ *         update has no errors, has been called successfully for the
+ *         transaction. A PREPARED transaction can be committed or
+ *         aborted.
+ *
+ * CLOSED: The transaction is no longer active. A transaction becomes
+ *         CLOSED if there is a failure while building the transaction
+ *         or if a transaction is committed or aborted. A CLOSED
+ *         transaction can only be freed.
  */
 enum ref_transaction_state {
-	REF_TRANSACTION_OPEN   = 0,
-	REF_TRANSACTION_CLOSED = 1
+	REF_TRANSACTION_OPEN     = 0,
+	REF_TRANSACTION_PREPARED = 1,
+	REF_TRANSACTION_CLOSED   = 2
 };
 
 /*
@@ -497,6 +515,18 @@ typedef struct ref_store *ref_store_init_fn(const char *gitdir,
 
 typedef int ref_init_db_fn(struct ref_store *refs, struct strbuf *err);
 
+typedef int ref_transaction_prepare_fn(struct ref_store *refs,
+				       struct ref_transaction *transaction,
+				       struct strbuf *err);
+
+typedef int ref_transaction_finish_fn(struct ref_store *refs,
+				      struct ref_transaction *transaction,
+				      struct strbuf *err);
+
+typedef int ref_transaction_abort_fn(struct ref_store *refs,
+				     struct ref_transaction *transaction,
+				     struct strbuf *err);
+
 typedef int ref_transaction_commit_fn(struct ref_store *refs,
 				      struct ref_transaction *transaction,
 				      struct strbuf *err);
@@ -508,16 +538,17 @@ typedef int create_symref_fn(struct ref_store *ref_store,
 			     const char *ref_target,
 			     const char *refs_heads_master,
 			     const char *logmsg);
-typedef int delete_refs_fn(struct ref_store *ref_store,
+typedef int delete_refs_fn(struct ref_store *ref_store, const char *msg,
 			   struct string_list *refnames, unsigned int flags);
 typedef int rename_ref_fn(struct ref_store *ref_store,
 			  const char *oldref, const char *newref,
 			  const char *logmsg);
 
 /*
- * Iterate over the references in the specified ref_store that are
- * within find_containing_dir(prefix). If prefix is NULL or the empty
- * string, iterate over all references in the submodule.
+ * Iterate over the references in `ref_store` whose names start with
+ * `prefix`. `prefix` is matched as a literal string, without regard
+ * for path separators. If prefix is NULL or the empty string, iterate
+ * over all references in `ref_store`.
  */
 typedef struct ref_iterator *ref_iterator_begin_fn(
 		struct ref_store *ref_store,
@@ -599,7 +630,10 @@ struct ref_storage_be {
 	const char *name;
 	ref_store_init_fn *init;
 	ref_init_db_fn *init_db;
-	ref_transaction_commit_fn *transaction_commit;
+
+	ref_transaction_prepare_fn *transaction_prepare;
+	ref_transaction_finish_fn *transaction_finish;
+	ref_transaction_abort_fn *transaction_abort;
 	ref_transaction_commit_fn *initial_transaction_commit;
 
 	pack_refs_fn *pack_refs;