worktree add: treat multiple matches with --guess-remote as an error

When 'git worktree add <path>' is invoked without <commit-ish> and
with the --guess-remote option (or when worktree.guessRemote is set to
true), it tries to find a remote-tracking branch matching the basename
of <path>.

Currently, the behavior when multiple matches are found is the same as
when no match is found: it falls back to creating a branch from
HEAD. This has been the behavior since 71d6682d8c (worktree: add
--guess-remote option to add subcommand, 2017-11-29), when the option
was first introduced.

However, if the specified <path> matches any remote-tracking branch,
we infer that the user intended to use one of the remote-tracking
branches as the start-point rather than HEAD. So we abort the creation
of the branch and worktree when there are multiple matches, and
instruct the user to choose the start-point.

Signed-off-by: Yoichi NAKAYAMA <yoichi.nakayama@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config/worktree.adoc

@@ -5,8 +5,9 @@
 	set to true, `worktree add` tries to find a remote-tracking
 	branch whose name uniquely matches the new branch name.  If
 	such a branch exists, it is checked out and set as "upstream"
-	for the new branch.  If no such match can be found, it falls
-	back to creating a new branch from the current `HEAD`.
+	for the new branch.  If multiple matches are found, the command
+	fails.  If no such match can be found, it falls back to
+	creating a new branch from the current `HEAD`.
 
 `worktree.useRelativePaths`::
 	Link worktrees using relative paths (when "`true`") or absolute

Documentation/git-worktree.adoc

@@ -219,7 +219,9 @@ To remove a locked worktree, specify `--force` twice.
 	of creating a new branch from `HEAD`, if there exists a tracking
 	branch in exactly one remote matching the basename of _<path>_,
 	base the new branch on the remote-tracking branch, and mark
-	the remote-tracking branch as "upstream" from the new branch.
+	the remote-tracking branch as "upstream" from the new branch. If
+	there are multiple matches, the command fails. If there is no
+	match, the command falls back to creating a new branch from `HEAD`.
 +
 This can also be set up as the default behaviour by using the
 `worktree.guessRemote` config option.

builtin/worktree.c

@@ -782,7 +782,7 @@ static void advise_disambiguating_remotes(const char *path, const char *branch,
 	       branch, path, branch);
 }
 
-static char *dwim_branch(const char *path, char **new_branch)
+static char *dwim_branch(const struct add_opts *opts, const char *path, char **new_branch)
 {
 	int n;
 	int branch_exists;
@@ -800,7 +800,21 @@ static char *dwim_branch(const char *path, char **new_branch)
 	*new_branch = branchname;
 	if (guess_remote) {
 		struct object_id oid;
-		char *remote = unique_tracking_name(*new_branch, &oid, NULL, NULL);
+		char *remote;
+		int num_matches = 0;
+		struct string_list matched_remote_names = STRING_LIST_INIT_DUP;
+
+		remote = unique_tracking_name(*new_branch, &oid, &num_matches,
+					      &matched_remote_names);
+		if (!remote && num_matches > 1) {
+			if (!opts->quiet &&
+			    advice_enabled(ADVICE_CHECKOUT_AMBIGUOUS_REMOTE_BRANCH_NAME))
+				advise_disambiguating_remotes(path, *new_branch,
+							      &matched_remote_names);
+			die(_("'%s' matched multiple (%d) remote tracking branches"),
+			    *new_branch, num_matches);
+		}
+		string_list_clear(&matched_remote_names, 0);
 		return remote;
 	}
 	return NULL;
@@ -908,7 +922,7 @@ static int add(int ac, const char **av, const char *prefix,
 		opts.orphan = dwim_orphan(&opts, !!opt_track, 0);
 	} else if (ac < 2) {
 		/* DWIM: Guess branch name from path. */
-		char *s = dwim_branch(path, &new_branch_to_free);
+		char *s = dwim_branch(&opts, path, &new_branch_to_free);
 		if (s)
 			branch = branch_to_free = s;
 		new_branch = new_branch_to_free;

t/t2400-worktree-add.sh

@@ -669,6 +669,19 @@ test_expect_success 'git worktree add --guess-remote sets up tracking' '
 		test_cmp_rev refs/remotes/repo_a/foo refs/heads/foo
 	)
 '
+
+test_expect_success 'git worktree add --guess-remote fails if there are multiple matches' '
+	test_when_finished rm -rf repo_a repo_b foo &&
+	setup_remote_repo repo_a repo_b &&
+	(
+		cd repo_b &&
+		git remote add repo_a2 ../repo_a &&
+		git fetch repo_a2 &&
+		test_must_fail git worktree add --guess-remote ../foo 2>actual &&
+		test_grep "matched multiple (2) remote tracking branches" actual
+	)
+'
+
 test_expect_success 'git worktree add --guess-remote sets up tracking (quiet)' '
 	test_when_finished rm -rf repo_a repo_b foo &&
 	setup_remote_repo repo_a repo_b &&