Merge branch 'mh/ref-directory-file'

The ref API did not handle cases where 'refs/heads/xyzzy/frotz' is
removed at the same time as 'refs/heads/xyzzy' is added (or vice
versa) very well.

* mh/ref-directory-file:
  reflog_expire(): integrate lock_ref_sha1_basic() errors into ours
  ref_transaction_commit(): delete extra "the" from error message
  ref_transaction_commit(): provide better error messages
  rename_ref(): integrate lock_ref_sha1_basic() errors into ours
  lock_ref_sha1_basic(): improve diagnostics for ref D/F conflicts
  lock_ref_sha1_basic(): report errors via a "struct strbuf *err"
  verify_refname_available(): report errors via a "struct strbuf *err"
  verify_refname_available(): rename function
  refs: check for D/F conflicts among refs created in a transaction
  ref_transaction_commit(): use a string_list for detecting duplicates
  is_refname_available(): use dirname in first loop
  struct nonmatching_ref_data: store a refname instead of a ref_entry
  report_refname_conflict(): inline function
  entry_matches(): inline function
  is_refname_available(): convert local variable "dirname" to strbuf
  is_refname_available(): avoid shadowing "dir" variable
  is_refname_available(): revamp the comments
  t1404: new tests of ref D/F conflicts within transactions

3 files changed, 312 insertions, 118 deletions

diff --git a/refs.c b/refs.c
index 825a1f6847..f704ee285c 100644
--- a/refs.c
+++ b/refs.c
@@ -268,7 +268,7 @@ struct ref_dir {
  * presence of an empty subdirectory does not block the creation of a
  * similarly-named reference.  (The fact that reference names with the
  * same leading components can conflict *with each other* is a
- * separate issue that is regulated by is_refname_available().)
+ * separate issue that is regulated by verify_refname_available().)
  *
  * Please note that the name field contains the fully-qualified
  * reference (or subdirectory) name.  Space could be saved by only
@@ -844,121 +844,181 @@ static void prime_ref_dir(struct ref_dir *dir)
 	}
 }
 
-static int entry_matches(struct ref_entry *entry, const struct string_list *list)
-{
-	return list && string_list_has_string(list, entry->name);
-}
-
 struct nonmatching_ref_data {
 	const struct string_list *skip;
-	struct ref_entry *found;
+	const char *conflicting_refname;
 };
 
 static int nonmatching_ref_fn(struct ref_entry *entry, void *vdata)
 {
 	struct nonmatching_ref_data *data = vdata;
 
-	if (entry_matches(entry, data->skip))
+	if (data->skip && string_list_has_string(data->skip, entry->name))
 		return 0;
 
-	data->found = entry;
+	data->conflicting_refname = entry->name;
 	return 1;
 }
 
-static void report_refname_conflict(struct ref_entry *entry,
-				    const char *refname)
-{
-	error("'%s' exists; cannot create '%s'", entry->name, refname);
-}
-
 /*
- * Return true iff a reference named refname could be created without
- * conflicting with the name of an existing reference in dir.  If
- * skip is non-NULL, ignore potential conflicts with refs in skip
- * (e.g., because they are scheduled for deletion in the same
- * operation).
+ * Return 0 if a reference named refname could be created without
+ * conflicting with the name of an existing reference in dir.
+ * Otherwise, return a negative value and write an explanation to err.
+ * If extras is non-NULL, it is a list of additional refnames with
+ * which refname is not allowed to conflict. If skip is non-NULL,
+ * ignore potential conflicts with refs in skip (e.g., because they
+ * are scheduled for deletion in the same operation). Behavior is
+ * undefined if the same name is listed in both extras and skip.
  *
  * Two reference names conflict if one of them exactly matches the
- * leading components of the other; e.g., "foo/bar" conflicts with
- * both "foo" and with "foo/bar/baz" but not with "foo/bar" or
- * "foo/barbados".
+ * leading components of the other; e.g., "refs/foo/bar" conflicts
+ * with both "refs/foo" and with "refs/foo/bar/baz" but not with
+ * "refs/foo/bar" or "refs/foo/barbados".
  *
- * skip must be sorted.
+ * extras and skip must be sorted.
  */
-static int is_refname_available(const char *refname,
-				const struct string_list *skip,
-				struct ref_dir *dir)
+static int verify_refname_available(const char *refname,
+				    const struct string_list *extras,
+				    const struct string_list *skip,
+				    struct ref_dir *dir,
+				    struct strbuf *err)
 {
 	const char *slash;
-	size_t len;
 	int pos;
-	char *dirname;
+	struct strbuf dirname = STRBUF_INIT;
+	int ret = -1;
+
+	/*
+	 * For the sake of comments in this function, suppose that
+	 * refname is "refs/foo/bar".
+	 */
 
+	assert(err);
+
+	strbuf_grow(&dirname, strlen(refname) + 1);
 	for (slash = strchr(refname, '/'); slash; slash = strchr(slash + 1, '/')) {
+		/* Expand dirname to the new prefix, not including the trailing slash: */
+		strbuf_add(&dirname, refname + dirname.len, slash - refname - dirname.len);
+
 		/*
-		 * We are still at a leading dir of the refname; we are
-		 * looking for a conflict with a leaf entry.
-		 *
-		 * If we find one, we still must make sure it is
-		 * not in "skip".
+		 * We are still at a leading dir of the refname (e.g.,
+		 * "refs/foo"; if there is a reference with that name,
+		 * it is a conflict, *unless* it is in skip.
 		 */
-		pos = search_ref_dir(dir, refname, slash - refname);
-		if (pos >= 0) {
-			struct ref_entry *entry = dir->entries[pos];
-			if (entry_matches(entry, skip))
-				return 1;
-			report_refname_conflict(entry, refname);
-			return 0;
+		if (dir) {
+			pos = search_ref_dir(dir, dirname.buf, dirname.len);
+			if (pos >= 0 &&
+			    (!skip || !string_list_has_string(skip, dirname.buf))) {
+				/*
+				 * We found a reference whose name is
+				 * a proper prefix of refname; e.g.,
+				 * "refs/foo", and is not in skip.
+				 */
+				strbuf_addf(err, "'%s' exists; cannot create '%s'",
+					    dirname.buf, refname);
+				goto cleanup;
+			}
 		}
 
+		if (extras && string_list_has_string(extras, dirname.buf) &&
+		    (!skip || !string_list_has_string(skip, dirname.buf))) {
+			strbuf_addf(err, "cannot process '%s' and '%s' at the same time",
+				    refname, dirname.buf);
+			goto cleanup;
+		}
 
 		/*
 		 * Otherwise, we can try to continue our search with
-		 * the next component; if we come up empty, we know
-		 * there is nothing under this whole prefix.
+		 * the next component. So try to look up the
+		 * directory, e.g., "refs/foo/". If we come up empty,
+		 * we know there is nothing under this whole prefix,
+		 * but even in that case we still have to continue the
+		 * search for conflicts with extras.
 		 */
-		pos = search_ref_dir(dir, refname, slash + 1 - refname);
-		if (pos < 0)
-			return 1;
-
-		dir = get_ref_dir(dir->entries[pos]);
+		strbuf_addch(&dirname, '/');
+		if (dir) {
+			pos = search_ref_dir(dir, dirname.buf, dirname.len);
+			if (pos < 0) {
+				/*
+				 * There was no directory "refs/foo/",
+				 * so there is nothing under this
+				 * whole prefix. So there is no need
+				 * to continue looking for conflicting
+				 * references. But we need to continue
+				 * looking for conflicting extras.
+				 */
+				dir = NULL;
+			} else {
+				dir = get_ref_dir(dir->entries[pos]);
+			}
+		}
 	}
 
 	/*
-	 * We are at the leaf of our refname; we want to
-	 * make sure there are no directories which match it.
+	 * We are at the leaf of our refname (e.g., "refs/foo/bar").
+	 * There is no point in searching for a reference with that
+	 * name, because a refname isn't considered to conflict with
+	 * itself. But we still need to check for references whose
+	 * names are in the "refs/foo/bar/" namespace, because they
+	 * *do* conflict.
 	 */
-	len = strlen(refname);
-	dirname = xmallocz(len + 1);
-	sprintf(dirname, "%s/", refname);
-	pos = search_ref_dir(dir, dirname, len + 1);
-	free(dirname);
+	strbuf_addstr(&dirname, refname + dirname.len);
+	strbuf_addch(&dirname, '/');
+
+	if (dir) {
+		pos = search_ref_dir(dir, dirname.buf, dirname.len);
+
+		if (pos >= 0) {
+			/*
+			 * We found a directory named "$refname/"
+			 * (e.g., "refs/foo/bar/"). It is a problem
+			 * iff it contains any ref that is not in
+			 * "skip".
+			 */
+			struct nonmatching_ref_data data;
+
+			data.skip = skip;
+			data.conflicting_refname = NULL;
+			dir = get_ref_dir(dir->entries[pos]);
+			sort_ref_dir(dir);
+			if (do_for_each_entry_in_dir(dir, 0, nonmatching_ref_fn, &data)) {
+				strbuf_addf(err, "'%s' exists; cannot create '%s'",
+					    data.conflicting_refname, refname);
+				goto cleanup;
+			}
+		}
+	}
 
-	if (pos >= 0) {
+	if (extras) {
 		/*
-		 * We found a directory named "refname". It is a
-		 * problem iff it contains any ref that is not
-		 * in "skip".
+		 * Check for entries in extras that start with
+		 * "$refname/". We do that by looking for the place
+		 * where "$refname/" would be inserted in extras. If
+		 * there is an entry at that position that starts with
+		 * "$refname/" and is not in skip, then we have a
+		 * conflict.
 		 */
-		struct ref_entry *entry = dir->entries[pos];
-		struct ref_dir *dir = get_ref_dir(entry);
-		struct nonmatching_ref_data data;
+		for (pos = string_list_find_insert_index(extras, dirname.buf, 0);
+		     pos < extras->nr; pos++) {
+			const char *extra_refname = extras->items[pos].string;
 
-		data.skip = skip;
-		sort_ref_dir(dir);
-		if (!do_for_each_entry_in_dir(dir, 0, nonmatching_ref_fn, &data))
-			return 1;
+			if (!starts_with(extra_refname, dirname.buf))
+				break;
 
-		report_refname_conflict(data.found, refname);
-		return 0;
+			if (!skip || !string_list_has_string(skip, extra_refname)) {
+				strbuf_addf(err, "cannot process '%s' and '%s' at the same time",
+					    refname, extra_refname);
+				goto cleanup;
+			}
+		}
 	}
 
-	/*
-	 * There is no point in searching for another leaf
-	 * node which matches it; such an entry would be the
-	 * ref we are looking for, not a conflict.
-	 */
-	return 1;
+	/* No conflicts were found */
+	ret = 0;
+
+cleanup:
+	strbuf_release(&dirname);
+	return ret;
 }
 
 struct packed_ref_cache {
@@ -2294,8 +2354,10 @@ int dwim_log(const char *str, int len, unsigned char *sha1, char **log)
  */
 static struct ref_lock *lock_ref_sha1_basic(const char *refname,
 					    const unsigned char *old_sha1,
+					    const struct string_list *extras,
 					    const struct string_list *skip,
-					    unsigned int flags, int *type_p)
+					    unsigned int flags, int *type_p,
+					    struct strbuf *err)
 {
 	const char *ref_file;
 	const char *orig_refname = refname;
@@ -2306,6 +2368,8 @@ static struct ref_lock *lock_ref_sha1_basic(const char *refname,
 	int resolve_flags = 0;
 	int attempts_remaining = 3;
 
+	assert(err);
+
 	lock = xcalloc(1, sizeof(struct ref_lock));
 
 	if (mustexist)
@@ -2327,7 +2391,12 @@ static struct ref_lock *lock_ref_sha1_basic(const char *refname,
 		ref_file = git_path("%s", orig_refname);
 		if (remove_empty_directories(ref_file)) {
 			last_errno = errno;
-			error("there are still refs under '%s'", orig_refname);
+
+			if (!verify_refname_available(orig_refname, extras, skip,
+						      get_loose_refs(&ref_cache), err))
+				strbuf_addf(err, "there are still refs under '%s'",
+					    orig_refname);
+
 			goto error_return;
 		}
 		refname = resolve_ref_unsafe(orig_refname, resolve_flags,
@@ -2337,8 +2406,12 @@ static struct ref_lock *lock_ref_sha1_basic(const char *refname,
 	    *type_p = type;
 	if (!refname) {
 		last_errno = errno;
-		error("unable to resolve reference %s: %s",
-			orig_refname, strerror(errno));
+		if (last_errno != ENOTDIR ||
+		    !verify_refname_available(orig_refname, extras, skip,
+					      get_loose_refs(&ref_cache), err))
+			strbuf_addf(err, "unable to resolve reference %s: %s",
+				    orig_refname, strerror(last_errno));
+
 		goto error_return;
 	}
 	/*
@@ -2348,7 +2421,8 @@ static struct ref_lock *lock_ref_sha1_basic(const char *refname,
 	 * our refname.
 	 */
 	if (is_null_sha1(lock->old_sha1) &&
-	     !is_refname_available(refname, skip, get_packed_refs(&ref_cache))) {
+	    verify_refname_available(refname, extras, skip,
+				     get_packed_refs(&ref_cache), err)) {
 		last_errno = ENOTDIR;
 		goto error_return;
 	}
@@ -2374,7 +2448,7 @@ static struct ref_lock *lock_ref_sha1_basic(const char *refname,
 		/* fall through */
 	default:
 		last_errno = errno;
-		error("unable to create directory for %s", ref_file);
+		strbuf_addf(err, "unable to create directory for %s", ref_file);
 		goto error_return;
 	}
 
@@ -2388,10 +2462,7 @@ static struct ref_lock *lock_ref_sha1_basic(const char *refname,
 			 */
 			goto retry;
 		else {
-			struct strbuf err = STRBUF_INIT;
-			unable_to_lock_message(ref_file, errno, &err);
-			error("%s", err.buf);
-			strbuf_release(&err);
+			unable_to_lock_message(ref_file, errno, err);
 			goto error_return;
 		}
 	}
@@ -2785,12 +2856,19 @@ static int rename_tmp_log(const char *newrefname)
 static int rename_ref_available(const char *oldname, const char *newname)
 {
 	struct string_list skip = STRING_LIST_INIT_NODUP;
+	struct strbuf err = STRBUF_INIT;
 	int ret;
 
 	string_list_insert(&skip, oldname);
-	ret = is_refname_available(newname, &skip, get_packed_refs(&ref_cache))
-	    && is_refname_available(newname, &skip, get_loose_refs(&ref_cache));
+	ret = !verify_refname_available(newname, NULL, &skip,
+					get_packed_refs(&ref_cache), &err)
+		&& !verify_refname_available(newname, NULL, &skip,
+					     get_loose_refs(&ref_cache), &err);
+	if (!ret)
+		error("%s", err.buf);
+
 	string_list_clear(&skip, 0);
+	strbuf_release(&err);
 	return ret;
 }
 
@@ -2806,6 +2884,7 @@ int rename_ref(const char *oldrefname, const char *newrefname, const char *logms
 	struct stat loginfo;
 	int log = !lstat(git_path("logs/%s", oldrefname), &loginfo);
 	const char *symref = NULL;
+	struct strbuf err = STRBUF_INIT;
 
 	if (log && S_ISLNK(loginfo.st_mode))
 		return error("reflog for %s is a symlink", oldrefname);
@@ -2848,9 +2927,10 @@ int rename_ref(const char *oldrefname, const char *newrefname, const char *logms
 
 	logmoved = log;
 
-	lock = lock_ref_sha1_basic(newrefname, NULL, NULL, 0, NULL);
+	lock = lock_ref_sha1_basic(newrefname, NULL, NULL, NULL, 0, NULL, &err);
 	if (!lock) {
-		error("unable to lock %s for update", newrefname);
+		error("unable to rename '%s' to '%s': %s", oldrefname, newrefname, err.buf);
+		strbuf_release(&err);
 		goto rollback;
 	}
 	hashcpy(lock->old_sha1, orig_sha1);
@@ -2864,9 +2944,10 @@ int rename_ref(const char *oldrefname, const char *newrefname, const char *logms
 	return 0;
 
  rollback:
-	lock = lock_ref_sha1_basic(oldrefname, NULL, NULL, 0, NULL);
+	lock = lock_ref_sha1_basic(oldrefname, NULL, NULL, NULL, 0, NULL, &err);
 	if (!lock) {
-		error("unable to lock %s for rollback", oldrefname);
+		error("unable to lock %s for rollback: %s", oldrefname, err.buf);
+		strbuf_release(&err);
 		goto rollbacklog;
 	}
 
@@ -3746,25 +3827,18 @@ int update_ref(const char *msg, const char *refname,
 	return 0;
 }
 
-static int ref_update_compare(const void *r1, const void *r2)
-{
-	const struct ref_update * const *u1 = r1;
-	const struct ref_update * const *u2 = r2;
-	return strcmp((*u1)->refname, (*u2)->refname);
-}
-
-static int ref_update_reject_duplicates(struct ref_update **updates, int n,
+static int ref_update_reject_duplicates(struct string_list *refnames,
 					struct strbuf *err)
 {
-	int i;
+	int i, n = refnames->nr;
 
 	assert(err);
 
 	for (i = 1; i < n; i++)
-		if (!strcmp(updates[i - 1]->refname, updates[i]->refname)) {
+		if (!strcmp(refnames->items[i - 1].string, refnames->items[i].string)) {
 			strbuf_addf(err,
 				    "Multiple updates for ref '%s' not allowed.",
-				    updates[i]->refname);
+				    refnames->items[i].string);
 			return 1;
 		}
 	return 0;
@@ -3778,6 +3852,7 @@ int ref_transaction_commit(struct ref_transaction *transaction,
 	struct ref_update **updates = transaction->updates;
 	struct string_list refs_to_delete = STRING_LIST_INIT_NODUP;
 	struct string_list_item *ref_to_delete;
+	struct string_list affected_refnames = STRING_LIST_INIT_NODUP;
 
 	assert(err);
 
@@ -3789,9 +3864,11 @@ int ref_transaction_commit(struct ref_transaction *transaction,
 		return 0;
 	}
 
-	/* Copy, sort, and reject duplicate refs */
-	qsort(updates, n, sizeof(*updates), ref_update_compare);
-	if (ref_update_reject_duplicates(updates, n, err)) {
+	/* Fail if a refname appears more than once in the transaction: */
+	for (i = 0; i < n; i++)
+		string_list_append(&affected_refnames, updates[i]->refname);
+	string_list_sort(&affected_refnames);
+	if (ref_update_reject_duplicates(&affected_refnames, err)) {
 		ret = TRANSACTION_GENERIC_ERROR;
 		goto cleanup;
 	}
@@ -3812,15 +3889,20 @@ int ref_transaction_commit(struct ref_transaction *transaction,
 				update->refname,
 				((update->flags & REF_HAVE_OLD) ?
 				 update->old_sha1 : NULL),
-				NULL,
+				&affected_refnames, NULL,
 				update->flags,
-				&update->type);
+				&update->type,
+				err);
 		if (!update->lock) {
+			char *reason;
+
 			ret = (errno == ENOTDIR)
 				? TRANSACTION_NAME_CONFLICT
 				: TRANSACTION_GENERIC_ERROR;
-			strbuf_addf(err, "Cannot lock the ref '%s'.",
-				    update->refname);
+			reason = strbuf_detach(err, NULL);
+			strbuf_addf(err, "Cannot lock ref '%s': %s",
+				    update->refname, reason);
+			free(reason);
 			goto cleanup;
 		}
 		if ((update->flags & REF_HAVE_NEW) &&
@@ -3913,6 +3995,7 @@ cleanup:
 		if (updates[i]->lock)
 			unlock_ref(updates[i]->lock);
 	string_list_clear(&refs_to_delete, 0);
+	string_list_clear(&affected_refnames, 0);
 	return ret;
 }
 
@@ -4102,6 +4185,7 @@ int reflog_expire(const char *refname, const unsigned char *sha1,
 	char *log_file;
 	int status = 0;
 	int type;
+	struct strbuf err = STRBUF_INIT;
 
 	memset(&cb, 0, sizeof(cb));
 	cb.flags = flags;
@@ -4113,9 +4197,12 @@ int reflog_expire(const char *refname, const unsigned char *sha1,
 	 * reference itself, plus we might need to update the
 	 * reference if --updateref was specified:
 	 */
-	lock = lock_ref_sha1_basic(refname, sha1, NULL, 0, &type);
-	if (!lock)
-		return error("cannot lock ref '%s'", refname);
+	lock = lock_ref_sha1_basic(refname, sha1, NULL, NULL, 0, &type, &err);
+	if (!lock) {
+		error("cannot lock ref '%s': %s", refname, err.buf);
+		strbuf_release(&err);
+		return -1;
+	}
 	if (!reflog_exists(refname)) {
 		unlock_ref(lock);
 		return 0;
diff --git a/t/t1400-update-ref.sh b/t/t1400-update-ref.sh
index 636d3a167c..ba89f4c009 100755
--- a/t/t1400-update-ref.sh
+++ b/t/t1400-update-ref.sh
@@ -519,7 +519,7 @@ test_expect_success 'stdin create ref works with path with space to blob' '
 test_expect_success 'stdin update ref fails with wrong old value' '
 	echo "update $c $m $m~1" >stdin &&
 	test_must_fail git update-ref --stdin <stdin 2>err &&
-	grep "fatal: Cannot lock the ref '"'"'$c'"'"'" err &&
+	grep "fatal: Cannot lock ref '"'"'$c'"'"'" err &&
 	test_must_fail git rev-parse --verify -q $c
 '
 
@@ -555,7 +555,7 @@ test_expect_success 'stdin update ref works with right old value' '
 test_expect_success 'stdin delete ref fails with wrong old value' '
 	echo "delete $a $m~1" >stdin &&
 	test_must_fail git update-ref --stdin <stdin 2>err &&
-	grep "fatal: Cannot lock the ref '"'"'$a'"'"'" err &&
+	grep "fatal: Cannot lock ref '"'"'$a'"'"'" err &&
 	git rev-parse $m >expect &&
 	git rev-parse $a >actual &&
 	test_cmp expect actual
@@ -688,7 +688,7 @@ test_expect_success 'stdin update refs fails with wrong old value' '
 	update $c  ''
 	EOF
 	test_must_fail git update-ref --stdin <stdin 2>err &&
-	grep "fatal: Cannot lock the ref '"'"'$c'"'"'" err &&
+	grep "fatal: Cannot lock ref '"'"'$c'"'"'" err &&
 	git rev-parse $m >expect &&
 	git rev-parse $a >actual &&
 	test_cmp expect actual &&
@@ -883,7 +883,7 @@ test_expect_success 'stdin -z create ref works with path with space to blob' '
 test_expect_success 'stdin -z update ref fails with wrong old value' '
 	printf $F "update $c" "$m" "$m~1" >stdin &&
 	test_must_fail git update-ref -z --stdin <stdin 2>err &&
-	grep "fatal: Cannot lock the ref '"'"'$c'"'"'" err &&
+	grep "fatal: Cannot lock ref '"'"'$c'"'"'" err &&
 	test_must_fail git rev-parse --verify -q $c
 '
 
@@ -899,7 +899,7 @@ test_expect_success 'stdin -z create ref fails when ref exists' '
 	git rev-parse "$c" >expect &&
 	printf $F "create $c" "$m~1" >stdin &&
 	test_must_fail git update-ref -z --stdin <stdin 2>err &&
-	grep "fatal: Cannot lock the ref '"'"'$c'"'"'" err &&
+	grep "fatal: Cannot lock ref '"'"'$c'"'"'" err &&
 	git rev-parse "$c" >actual &&
 	test_cmp expect actual
 '
@@ -930,7 +930,7 @@ test_expect_success 'stdin -z update ref works with right old value' '
 test_expect_success 'stdin -z delete ref fails with wrong old value' '
 	printf $F "delete $a" "$m~1" >stdin &&
 	test_must_fail git update-ref -z --stdin <stdin 2>err &&
-	grep "fatal: Cannot lock the ref '"'"'$a'"'"'" err &&
+	grep "fatal: Cannot lock ref '"'"'$a'"'"'" err &&
 	git rev-parse $m >expect &&
 	git rev-parse $a >actual &&
 	test_cmp expect actual
@@ -1045,7 +1045,7 @@ test_expect_success 'stdin -z update refs fails with wrong old value' '
 	git update-ref $c $m &&
 	printf $F "update $a" "$m" "$m" "update $b" "$m" "$m" "update $c" "$m" "$Z" >stdin &&
 	test_must_fail git update-ref -z --stdin <stdin 2>err &&
-	grep "fatal: Cannot lock the ref '"'"'$c'"'"'" err &&
+	grep "fatal: Cannot lock ref '"'"'$c'"'"'" err &&
 	git rev-parse $m >expect &&
 	git rev-parse $a >actual &&
 	test_cmp expect actual &&
diff --git a/t/t1404-update-ref-df-conflicts.sh b/t/t1404-update-ref-df-conflicts.sh
new file mode 100755
index 0000000000..66bafb5cf4
--- /dev/null
+++ b/t/t1404-update-ref-df-conflicts.sh
@@ -0,0 +1,107 @@
+#!/bin/sh
+
+test_description='Test git update-ref with D/F conflicts'
+. ./test-lib.sh
+
+test_update_rejected () {
+	prefix="$1" &&
+	before="$2" &&
+	pack="$3" &&
+	create="$4" &&
+	error="$5" &&
+	printf "create $prefix/%s $C\n" $before |
+	git update-ref --stdin &&
+	git for-each-ref $prefix >unchanged &&
+	if $pack
+	then
+		git pack-refs --all
+	fi &&
+	printf "create $prefix/%s $C\n" $create >input &&
+	test_must_fail git update-ref --stdin <input 2>output.err &&
+	grep -F "$error" output.err &&
+	git for-each-ref $prefix >actual &&
+	test_cmp unchanged actual
+}
+
+Q="'"
+
+test_expect_success 'setup' '
+
+	git commit --allow-empty -m Initial &&
+	C=$(git rev-parse HEAD)
+
+'
+
+test_expect_success 'existing loose ref is a simple prefix of new' '
+
+	prefix=refs/1l &&
+	test_update_rejected $prefix "a c e" false "b c/x d" \
+		"$Q$prefix/c$Q exists; cannot create $Q$prefix/c/x$Q"
+
+'
+
+test_expect_success 'existing packed ref is a simple prefix of new' '
+
+	prefix=refs/1p &&
+	test_update_rejected $prefix "a c e" true "b c/x d" \
+		"$Q$prefix/c$Q exists; cannot create $Q$prefix/c/x$Q"
+
+'
+
+test_expect_success 'existing loose ref is a deeper prefix of new' '
+
+	prefix=refs/2l &&
+	test_update_rejected $prefix "a c e" false "b c/x/y d" \
+		"$Q$prefix/c$Q exists; cannot create $Q$prefix/c/x/y$Q"
+
+'
+
+test_expect_success 'existing packed ref is a deeper prefix of new' '
+
+	prefix=refs/2p &&
+	test_update_rejected $prefix "a c e" true "b c/x/y d" \
+		"$Q$prefix/c$Q exists; cannot create $Q$prefix/c/x/y$Q"
+
+'
+
+test_expect_success 'new ref is a simple prefix of existing loose' '
+
+	prefix=refs/3l &&
+	test_update_rejected $prefix "a c/x e" false "b c d" \
+		"$Q$prefix/c/x$Q exists; cannot create $Q$prefix/c$Q"
+
+'
+
+test_expect_success 'new ref is a simple prefix of existing packed' '
+
+	prefix=refs/3p &&
+	test_update_rejected $prefix "a c/x e" true "b c d" \
+		"$Q$prefix/c/x$Q exists; cannot create $Q$prefix/c$Q"
+
+'
+
+test_expect_success 'new ref is a deeper prefix of existing loose' '
+
+	prefix=refs/4l &&
+	test_update_rejected $prefix "a c/x/y e" false "b c d" \
+		"$Q$prefix/c/x/y$Q exists; cannot create $Q$prefix/c$Q"
+
+'
+
+test_expect_success 'new ref is a deeper prefix of existing packed' '
+
+	prefix=refs/4p &&
+	test_update_rejected $prefix "a c/x/y e" true "b c d" \
+		"$Q$prefix/c/x/y$Q exists; cannot create $Q$prefix/c$Q"
+
+'
+
+test_expect_success 'one new ref is a simple prefix of another' '
+
+	prefix=refs/5 &&
+	test_update_rejected $prefix "a e" false "b c c/x d" \
+		"cannot process $Q$prefix/c$Q and $Q$prefix/c/x$Q at the same time"
+
+'
+
+test_done