diff --git a/builtin/fetch.c b/builtin/fetch.c
index d4d573b985..47708451bc 100644
--- a/builtin/fetch.c
+++ b/builtin/fetch.c
@@ -941,7 +941,7 @@ static int prune_refs(struct refspec *refs, int ref_count, struct ref *ref_map,
 		for (ref = stale_refs; ref; ref = ref->next)
 			string_list_append(&refnames, ref->name);
 
-		result = delete_refs(&refnames, 0);
+		result = delete_refs("fetch: prune", &refnames, 0);
 		string_list_clear(&refnames, 0);
 	}
 
diff --git a/builtin/remote.c b/builtin/remote.c
index 9054e2858e..f1a88fe265 100644
--- a/builtin/remote.c
+++ b/builtin/remote.c
@@ -786,7 +786,7 @@ static int rm(int argc, const char **argv)
 	strbuf_release(&buf);
 
 	if (!result)
-		result = delete_refs(&branches, REF_NODEREF);
+		result = delete_refs("remote: remove", &branches, REF_NODEREF);
 	string_list_clear(&branches, 0);
 
 	if (skipped.nr) {
@@ -1304,7 +1304,7 @@ static int prune_remote(const char *remote, int dry_run)
 	string_list_sort(&refs_to_prune);
 
 	if (!dry_run)
-		result |= delete_refs(&refs_to_prune, 0);
+		result |= delete_refs("remote: prune", &refs_to_prune, 0);
 
 	for_each_string_list_item(item, &states.stale) {
 		const char *refname = item->util;
diff --git a/lockfile.h b/lockfile.h
index 7b715f9e77..572064939c 100644
--- a/lockfile.h
+++ b/lockfile.h
@@ -175,6 +175,14 @@ static inline int hold_lock_file_for_update(
 	return hold_lock_file_for_update_timeout(lk, path, flags, 0);
 }
 
+/*
+ * Return a nonzero value iff `lk` is currently locked.
+ */
+static inline int is_lock_file_locked(struct lock_file *lk)
+{
+	return is_tempfile_active(&lk->tempfile);
+}
+
 /*
  * Append an appropriate error message to `buf` following the failure
  * of `hold_lock_file_for_update()` to lock `path`. `err` should be the
diff --git a/ref-filter.c b/ref-filter.c
index af5c0edb48..ab32bc9c31 100644
--- a/ref-filter.c
+++ b/ref-filter.c
@@ -1671,6 +1671,68 @@ static int filter_pattern_match(struct ref_filter *filter, const char *refname)
 	return match_pattern(filter, refname);
 }
 
+/*
+ * Find the longest prefix of pattern we can pass to
+ * `for_each_fullref_in()`, namely the part of pattern preceding the
+ * first glob character. (Note that `for_each_fullref_in()` is
+ * perfectly happy working with a prefix that doesn't end at a
+ * pathname component boundary.)
+ */
+static void find_longest_prefix(struct strbuf *out, const char *pattern)
+{
+	const char *p;
+
+	for (p = pattern; *p && !is_glob_special(*p); p++)
+		;
+
+	strbuf_add(out, pattern, p - pattern);
+}
+
+/*
+ * This is the same as for_each_fullref_in(), but it tries to iterate
+ * only over the patterns we'll care about. Note that it _doesn't_ do a full
+ * pattern match, so the callback still has to match each ref individually.
+ */
+static int for_each_fullref_in_pattern(struct ref_filter *filter,
+				       each_ref_fn cb,
+				       void *cb_data,
+				       int broken)
+{
+	struct strbuf prefix = STRBUF_INIT;
+	int ret;
+
+	if (!filter->match_as_path) {
+		/*
+		 * in this case, the patterns are applied after
+		 * prefixes like "refs/heads/" etc. are stripped off,
+		 * so we have to look at everything:
+		 */
+		return for_each_fullref_in("", cb, cb_data, broken);
+	}
+
+	if (!filter->name_patterns[0]) {
+		/* no patterns; we have to look at everything */
+		return for_each_fullref_in("", cb, cb_data, broken);
+	}
+
+	if (filter->name_patterns[1]) {
+		/*
+		 * multiple patterns; in theory this could still work as long
+		 * as the patterns are disjoint. We'd just make multiple calls
+		 * to for_each_ref(). But if they're not disjoint, we'd end up
+		 * reporting the same ref multiple times. So let's punt on that
+		 * for now.
+		 */
+		return for_each_fullref_in("", cb, cb_data, broken);
+	}
+
+	find_longest_prefix(&prefix, filter->name_patterns[0]);
+
+	ret = for_each_fullref_in(prefix.buf, cb, cb_data, broken);
+	strbuf_release(&prefix);
+	return ret;
+}
+
 /*
  * Given a ref (sha1, refname), check if the ref belongs to the array
  * of sha1s. If the given ref is a tag, check if the given tag points
@@ -1917,7 +1979,7 @@ int filter_refs(struct ref_array *array, struct ref_filter *filter, unsigned int
 		else if (filter->kind == FILTER_REFS_TAGS)
 			ret = for_each_fullref_in("refs/tags/", ref_filter_handler, &ref_cbdata, broken);
 		else if (filter->kind & FILTER_REFS_ALL)
-			ret = for_each_fullref_in("", ref_filter_handler, &ref_cbdata, broken);
+			ret = for_each_fullref_in_pattern(filter, ref_filter_handler, &ref_cbdata, broken);
 		if (!ret && (filter->kind & FILTER_REFS_DETACHED_HEAD))
 			head_ref(ref_filter_handler, &ref_cbdata);
 	}
diff --git a/refs.c b/refs.c
index 8af9641aa1..f0685c9251 100644
--- a/refs.c
+++ b/refs.c
@@ -848,11 +848,24 @@ struct ref_transaction *ref_transaction_begin(struct strbuf *err)
 
 void ref_transaction_free(struct ref_transaction *transaction)
 {
-	int i;
+	size_t i;
 
 	if (!transaction)
 		return;
 
+	switch (transaction->state) {
+	case REF_TRANSACTION_OPEN:
+	case REF_TRANSACTION_CLOSED:
+		/* OK */
+		break;
+	case REF_TRANSACTION_PREPARED:
+		die("BUG: free called on a prepared reference transaction");
+		break;
+	default:
+		die("BUG: unexpected reference transaction state");
+		break;
+	}
+
 	for (i = 0; i < transaction->nr; i++) {
 		free(transaction->updates[i]->msg);
 		free(transaction->updates[i]);
@@ -1246,8 +1259,19 @@ struct ref_iterator *refs_ref_iterator_begin(
 {
 	struct ref_iterator *iter;
 
+	if (ref_paranoia < 0)
+		ref_paranoia = git_env_bool("GIT_REF_PARANOIA", 0);
+	if (ref_paranoia)
+		flags |= DO_FOR_EACH_INCLUDE_BROKEN;
+
 	iter = refs->be->iterator_begin(refs, prefix, flags);
-	iter = prefix_ref_iterator_begin(iter, prefix, trim);
+
+	/*
+	 * `iterator_begin()` already takes care of prefix, but we
+	 * might need to do some trimming:
+	 */
+	if (trim)
+		iter = prefix_ref_iterator_begin(iter, "", trim);
 
 	return iter;
 }
@@ -1683,18 +1707,108 @@ int create_symref(const char *ref_target, const char *refs_heads_master,
 				  refs_heads_master, logmsg);
 }
 
-int ref_transaction_commit(struct ref_transaction *transaction,
-			   struct strbuf *err)
+int ref_update_reject_duplicates(struct string_list *refnames,
+				 struct strbuf *err)
+{
+	size_t i, n = refnames->nr;
+
+	assert(err);
+
+	for (i = 1; i < n; i++) {
+		int cmp = strcmp(refnames->items[i - 1].string,
+				 refnames->items[i].string);
+
+		if (!cmp) {
+			strbuf_addf(err,
+				    "multiple updates for ref '%s' not allowed.",
+				    refnames->items[i].string);
+			return 1;
+		} else if (cmp > 0) {
+			die("BUG: ref_update_reject_duplicates() received unsorted list");
+		}
+	}
+	return 0;
+}
+
+int ref_transaction_prepare(struct ref_transaction *transaction,
+			    struct strbuf *err)
 {
 	struct ref_store *refs = transaction->ref_store;
 
+	switch (transaction->state) {
+	case REF_TRANSACTION_OPEN:
+		/* Good. */
+		break;
+	case REF_TRANSACTION_PREPARED:
+		die("BUG: prepare called twice on reference transaction");
+		break;
+	case REF_TRANSACTION_CLOSED:
+		die("BUG: prepare called on a closed reference transaction");
+		break;
+	default:
+		die("BUG: unexpected reference transaction state");
+		break;
+	}
+
 	if (getenv(GIT_QUARANTINE_ENVIRONMENT)) {
 		strbuf_addstr(err,
 			      _("ref updates forbidden inside quarantine environment"));
 		return -1;
 	}
 
-	return refs->be->transaction_commit(refs, transaction, err);
+	return refs->be->transaction_prepare(refs, transaction, err);
+}
+
+int ref_transaction_abort(struct ref_transaction *transaction,
+			  struct strbuf *err)
+{
+	struct ref_store *refs = transaction->ref_store;
+	int ret = 0;
+
+	switch (transaction->state) {
+	case REF_TRANSACTION_OPEN:
+		/* No need to abort explicitly. */
+		break;
+	case REF_TRANSACTION_PREPARED:
+		ret = refs->be->transaction_abort(refs, transaction, err);
+		break;
+	case REF_TRANSACTION_CLOSED:
+		die("BUG: abort called on a closed reference transaction");
+		break;
+	default:
+		die("BUG: unexpected reference transaction state");
+		break;
+	}
+
+	ref_transaction_free(transaction);
+	return ret;
+}
+
+int ref_transaction_commit(struct ref_transaction *transaction,
+			   struct strbuf *err)
+{
+	struct ref_store *refs = transaction->ref_store;
+	int ret;
+
+	switch (transaction->state) {
+	case REF_TRANSACTION_OPEN:
+		/* Need to prepare first. */
+		ret = ref_transaction_prepare(transaction, err);
+		if (ret)
+			return ret;
+		break;
+	case REF_TRANSACTION_PREPARED:
+		/* Fall through to finish. */
+		break;
+	case REF_TRANSACTION_CLOSED:
+		die("BUG: commit called on a closed reference transaction");
+		break;
+	default:
+		die("BUG: unexpected reference transaction state");
+		break;
+	}
+
+	return refs->be->transaction_finish(refs, transaction, err);
 }
 
 int refs_verify_refname_available(struct ref_store *refs,
@@ -1896,15 +2010,16 @@ int initial_ref_transaction_commit(struct ref_transaction *transaction,
 	return refs->be->initial_transaction_commit(refs, transaction, err);
 }
 
-int refs_delete_refs(struct ref_store *refs, struct string_list *refnames,
-		     unsigned int flags)
+int refs_delete_refs(struct ref_store *refs, const char *msg,
+		     struct string_list *refnames, unsigned int flags)
 {
-	return refs->be->delete_refs(refs, refnames, flags);
+	return refs->be->delete_refs(refs, msg, refnames, flags);
 }
 
-int delete_refs(struct string_list *refnames, unsigned int flags)
+int delete_refs(const char *msg, struct string_list *refnames,
+		unsigned int flags)
 {
-	return refs_delete_refs(get_main_ref_store(), refnames, flags);
+	return refs_delete_refs(get_main_ref_store(), msg, refnames, flags);
 }
 
 int refs_rename_ref(struct ref_store *refs, const char *oldref,
diff --git a/refs.h b/refs.h
index 685a979a0e..4be14c4b3c 100644
--- a/refs.h
+++ b/refs.h
@@ -143,30 +143,71 @@ int dwim_ref(const char *str, int len, unsigned char *sha1, char **ref);
 int dwim_log(const char *str, int len, unsigned char *sha1, char **ref);
 
 /*
- * A ref_transaction represents a collection of ref updates
- * that should succeed or fail together.
+ * A ref_transaction represents a collection of reference updates that
+ * should succeed or fail together.
  *
  * Calling sequence
  * ----------------
+ *
  * - Allocate and initialize a `struct ref_transaction` by calling
  *   `ref_transaction_begin()`.
  *
- * - List intended ref updates by calling functions like
- *   `ref_transaction_update()` and `ref_transaction_create()`.
+ * - Specify the intended ref updates by calling one or more of the
+ *   following functions:
+ *   - `ref_transaction_update()`
+ *   - `ref_transaction_create()`
+ *   - `ref_transaction_delete()`
+ *   - `ref_transaction_verify()`
  *
- * - Call `ref_transaction_commit()` to execute the transaction.
- *   If this succeeds, the ref updates will have taken place and
- *   the transaction cannot be rolled back.
+ * - Then either:
  *
- * - Instead of `ref_transaction_commit`, use
- *   `initial_ref_transaction_commit()` if the ref database is known
- *   to be empty (e.g. during clone).  This is likely to be much
- *   faster.
+ *   - Optionally call `ref_transaction_prepare()` to prepare the
+ *     transaction. This locks all references, checks preconditions,
+ *     etc. but doesn't finalize anything. If this step fails, the
+ *     transaction has been closed and can only be freed. If this step
+ *     succeeds, then `ref_transaction_commit()` is almost certain to
+ *     succeed. However, you can still call `ref_transaction_abort()`
+ *     if you decide not to commit the transaction after all.
  *
- * - At any time call `ref_transaction_free()` to discard the
- *   transaction and free associated resources.  In particular,
- *   this rolls back the transaction if it has not been
- *   successfully committed.
+ *   - Call `ref_transaction_commit()` to execute the transaction,
+ *     make the changes permanent, and release all locks. If you
+ *     haven't already called `ref_transaction_prepare()`, then
+ *     `ref_transaction_commit()` calls it for you.
+ *
+ *   Or
+ *
+ *   - Call `initial_ref_transaction_commit()` if the ref database is
+ *     known to be empty and have no other writers (e.g. during
+ *     clone). This is likely to be much faster than
+ *     `ref_transaction_commit()`. `ref_transaction_prepare()` should
+ *     *not* be called before `initial_ref_transaction_commit()`.
+ *
+ * - Then finally, call `ref_transaction_free()` to free the
+ *   `ref_transaction` data structure.
+ *
+ * At any time before calling `ref_transaction_commit()`, you can call
+ * `ref_transaction_abort()` to abort the transaction, rollback any
+ * locks, and free any associated resources (including the
+ * `ref_transaction` data structure).
+ *
+ * Putting it all together, a complete reference update looks like
+ *
+ *         struct ref_transaction *transaction;
+ *         struct strbuf err = STRBUF_INIT;
+ *         int ret = 0;
+ *
+ *         transaction = ref_store_transaction_begin(refs, &err);
+ *         if (!transaction ||
+ *             ref_transaction_update(...) ||
+ *             ref_transaction_create(...) ||
+ *             ...etc... ||
+ *             ref_transaction_commit(transaction, &err)) {
+ *                 error("%s", err.buf);
+ *                 ret = -1;
+ *         }
+ *         ref_transaction_free(transaction);
+ *         strbuf_release(&err);
+ *         return ret;
  *
  * Error handling
  * --------------
@@ -183,8 +224,9 @@ int dwim_log(const char *str, int len, unsigned char *sha1, char **ref);
  * -------
  *
  * Note that no locks are taken, and no refs are read, until
- * `ref_transaction_commit` is called.  So `ref_transaction_verify`
- * won't report a verification failure until the commit is attempted.
+ * `ref_transaction_prepare()` or `ref_transaction_commit()` is
+ * called. So, for example, `ref_transaction_verify()` won't report a
+ * verification failure until the commit is attempted.
  */
 struct ref_transaction;
 
@@ -331,7 +373,8 @@ int reflog_exists(const char *refname);
  * verify that the current value of the reference is old_sha1 before
  * deleting it. If old_sha1 is NULL, delete the reference if it
  * exists, regardless of its old value. It is an error for old_sha1 to
- * be NULL_SHA1. flags is passed through to ref_transaction_delete().
+ * be NULL_SHA1. msg and flags are passed through to
+ * ref_transaction_delete().
  */
 int refs_delete_ref(struct ref_store *refs, const char *msg,
 		    const char *refname,
@@ -343,12 +386,13 @@ int delete_ref(const char *msg, const char *refname,
 /*
  * Delete the specified references. If there are any problems, emit
  * errors but attempt to keep going (i.e., the deletes are not done in
- * an all-or-nothing transaction). flags is passed through to
+ * an all-or-nothing transaction). msg and flags are passed through to
  * ref_transaction_delete().
  */
-int refs_delete_refs(struct ref_store *refs, struct string_list *refnames,
-		     unsigned int flags);
-int delete_refs(struct string_list *refnames, unsigned int flags);
+int refs_delete_refs(struct ref_store *refs, const char *msg,
+		     struct string_list *refnames, unsigned int flags);
+int delete_refs(const char *msg, struct string_list *refnames,
+		unsigned int flags);
 
 /** Delete a reflog */
 int refs_delete_reflog(struct ref_store *refs, const char *refname);
@@ -427,6 +471,19 @@ struct ref_transaction *ref_transaction_begin(struct strbuf *err);
  *
  *     refname -- the name of the reference to be affected.
  *
+ *     new_sha1 -- the SHA-1 that should be set to be the new value of
+ *         the reference. Some functions allow this parameter to be
+ *         NULL, meaning that the reference is not changed, or
+ *         null_sha1, meaning that the reference should be deleted. A
+ *         copy of this value is made in the transaction.
+ *
+ *     old_sha1 -- the SHA-1 value that the reference must have before
+ *         the update. Some functions allow this parameter to be NULL,
+ *         meaning that the old value of the reference is not checked,
+ *         or null_sha1, meaning that the reference must not exist
+ *         before the update. A copy of this value is made in the
+ *         transaction.
+ *
  *     flags -- flags affecting the update, passed to
  *         update_ref_lock(). Can be REF_NODEREF, which means that
  *         symbolic references should not be followed.
@@ -508,19 +565,47 @@ int ref_transaction_verify(struct ref_transaction *transaction,
 			   unsigned int flags,
 			   struct strbuf *err);
 
-/*
- * Commit all of the changes that have been queued in transaction, as
- * atomically as possible.
- *
- * Returns 0 for success, or one of the below error codes for errors.
- */
 /* Naming conflict (for example, the ref names A and A/B conflict). */
 #define TRANSACTION_NAME_CONFLICT -1
 /* All other errors. */
 #define TRANSACTION_GENERIC_ERROR -2
+
+/*
+ * Perform the preparatory stages of commiting `transaction`. Acquire
+ * any needed locks, check preconditions, etc.; basically, do as much
+ * as possible to ensure that the transaction will be able to go
+ * through, stopping just short of making any irrevocable or
+ * user-visible changes. The updates that this function prepares can
+ * be finished up by calling `ref_transaction_commit()` or rolled back
+ * by calling `ref_transaction_abort()`.
+ *
+ * On success, return 0 and leave the transaction in "prepared" state.
+ * On failure, abort the transaction, write an error message to `err`,
+ * and return one of the `TRANSACTION_*` constants.
+ *
+ * Callers who don't need such fine-grained control over commiting
+ * reference transactions should just call `ref_transaction_commit()`.
+ */
+int ref_transaction_prepare(struct ref_transaction *transaction,
+			    struct strbuf *err);
+
+/*
+ * Commit all of the changes that have been queued in transaction, as
+ * atomically as possible. On success, return 0 and leave the
+ * transaction in "closed" state. On failure, roll back the
+ * transaction, write an error message to `err`, and return one of the
+ * `TRANSACTION_*` constants
+ */
 int ref_transaction_commit(struct ref_transaction *transaction,
 			   struct strbuf *err);
 
+/*
+ * Abort `transaction`, which has been begun and possibly prepared,
+ * but not yet committed.
+ */
+int ref_transaction_abort(struct ref_transaction *transaction,
+			  struct strbuf *err);
+
 /*
  * Like ref_transaction_commit(), but optimized for creating
  * references when originally initializing a repository (e.g., by "git
@@ -536,7 +621,7 @@ int initial_ref_transaction_commit(struct ref_transaction *transaction,
 				   struct strbuf *err);
 
 /*
- * Free an existing transaction and all associated data.
+ * Free `*transaction` and all associated data.
  */
 void ref_transaction_free(struct ref_transaction *transaction);
 
diff --git a/refs/files-backend.c b/refs/files-backend.c
index cb1f528cbe..d8b3f73147 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -43,15 +43,6 @@ 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 +61,13 @@ 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 +98,8 @@ 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 +209,9 @@ 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 +237,36 @@ 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 +291,7 @@ 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 +313,10 @@ 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)
@@ -348,30 +371,24 @@ static void files_ref_path(struct files_ref_store *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 &&
+	    !is_lock_file_locked(&refs->packed_refs_lock) &&
 	    !stat_validity_check(&refs->packed->validity, packed_refs_file))
 		clear_packed_ref_cache(refs);
 
-	if (!refs->packed) {
-		FILE *f;
+	if (!refs->packed)
+		refs->packed = read_packed_refs(packed_refs_file);
 
-		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 +413,14 @@ 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 +497,7 @@ 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 +1078,12 @@ 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 +1308,17 @@ 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.
+	 * Get the current packed-refs while holding the lock. It is
+	 * important that we call `get_packed_ref_cache()` before
+	 * setting `packed_ref_cache->lock`, because otherwise the
+	 * former will see that the file is locked and assume that the
+	 * cache can't be stale.
 	 */
 	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 +1341,10 @@ 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 +1362,10 @@ 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 +1383,9 @@ 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 +1480,32 @@ 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 +1527,9 @@ 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 +1545,7 @@ 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 +1625,7 @@ 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 +1657,7 @@ 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 +2551,6 @@ 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 +2856,45 @@ 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
@@ -2926,6 +2953,8 @@ 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 +2962,38 @@ 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 +3073,10 @@ 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 +3090,19 @@ static int files_transaction_commit(struct ref_store *ref_store,
 		}
 	}
 
+	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 +3118,8 @@ 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 +3373,9 @@ 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..af2fcb2c12 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);
@@ -316,11 +312,42 @@ 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 +358,29 @@ 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 +395,8 @@ 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
@@ -373,6 +423,13 @@ struct cache_ref_iterator {
 	/* The number of levels that have been allocated on the stack */
 	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
@@ -394,6 +451,7 @@ 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 +466,14 @@ 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 +481,7 @@ 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 +542,7 @@ 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 +568,10 @@ 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 +583,12 @@ 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
@@ -169,6 +169,14 @@ int refs_read_raw_ref(struct ref_store *ref_store,
 		      const char *refname, unsigned char *sha1,
 		      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
@@ -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;
diff --git a/t/helper/test-ref-store.c b/t/helper/test-ref-store.c
index fba85e7da5..05d8c4d8af 100644
--- a/t/helper/test-ref-store.c
+++ b/t/helper/test-ref-store.c
@@ -93,12 +93,13 @@ static int cmd_create_symref(struct ref_store *refs, const char **argv)
 static int cmd_delete_refs(struct ref_store *refs, const char **argv)
 {
 	unsigned int flags = arg_flags(*argv++, "flags");
+	const char *msg = *argv++;
 	struct string_list refnames = STRING_LIST_INIT_NODUP;
 
 	while (*argv)
 		string_list_append(&refnames, *argv++);
 
-	return refs_delete_refs(refs, &refnames, flags);
+	return refs_delete_refs(refs, msg, &refnames, flags);
 }
 
 static int cmd_rename_ref(struct ref_store *refs, const char **argv)
diff --git a/t/t1405-main-ref-store.sh b/t/t1405-main-ref-store.sh
index 490521f8cb..e8115df5ba 100755
--- a/t/t1405-main-ref-store.sh
+++ b/t/t1405-main-ref-store.sh
@@ -31,7 +31,7 @@ test_expect_success 'create_symref(FOO, refs/heads/master)' '
 test_expect_success 'delete_refs(FOO, refs/tags/new-tag)' '
 	git rev-parse FOO -- &&
 	git rev-parse refs/tags/new-tag -- &&
-	$RUN delete-refs 0 FOO refs/tags/new-tag &&
+	$RUN delete-refs 0 nothing FOO refs/tags/new-tag &&
 	test_must_fail git rev-parse FOO -- &&
 	test_must_fail git rev-parse refs/tags/new-tag --
 '
diff --git a/t/t1406-submodule-ref-store.sh b/t/t1406-submodule-ref-store.sh
index 13b5454c56..c32d4cc465 100755
--- a/t/t1406-submodule-ref-store.sh
+++ b/t/t1406-submodule-ref-store.sh
@@ -31,7 +31,7 @@ test_expect_success 'create_symref() not allowed' '
 '
 
 test_expect_success 'delete_refs() not allowed' '
-	test_must_fail $RUN delete-refs 0 FOO refs/tags/new-tag
+	test_must_fail $RUN delete-refs 0 nothing FOO refs/tags/new-tag
 '
 
 test_expect_success 'rename_refs() not allowed' '
diff --git a/t/t3600-rm.sh b/t/t3600-rm.sh
index 5f9913ba33..f8568f8841 100755
--- a/t/t3600-rm.sh
+++ b/t/t3600-rm.sh
@@ -97,9 +97,9 @@ test_expect_success FUNNYNAMES \
 embedded'"
 
 test_expect_success SANITY 'Test that "git rm -f" fails if its rm fails' '
+	test_when_finished "chmod 775 ." &&
 	chmod a-w . &&
-	test_must_fail git rm -f baz &&
-	chmod 775 .
+	test_must_fail git rm -f baz
 '
 
 test_expect_success \