checkout: don't revert file on ambiguous tracking branches

For easier understanding, here are the existing good scenarios:

  1) Have *no* file 'foo', *no* local branch 'foo' and a *single*
     remote branch 'foo'
  2) `git checkout foo` will create local branch foo, see [1]

  and

  1) Have *a* file 'foo', *no* local branch 'foo' and a *single*
     remote branch 'foo'
  2) `git checkout foo` will complain, see [3]

This patch prevents the following scenario:

  1) Have *a* file 'foo', *no* local branch 'foo' and *multiple*
     remote branches 'foo'
  2) `git checkout foo` will successfully... revert contents of
     file `foo`!

That is, adding another remote suddenly changes behavior significantly,
which is a surprise at best and could go unnoticed by user at worst.
Please see [3] which gives some real world complaints.

To my understanding, fix in [3] overlooked the case of multiple remotes,
and the whole behavior of falling back to reverting file was never
intended:

  [1] introduces the unexpected behavior. Before, there was fallback
  from not-a-ref to pathspec. This is reasonable fallback. After, there
  is another fallback from ambiguous-remote to pathspec. I understand
  that it was a copy&paste oversight.

  [2] noticed the unexpected behavior but chose to semi-document it
  instead of forbidding, because the goal of the patch series was
  focused on something else.

  [3] adds `die()` when there is ambiguity between branch and file. The
  case of multiple tracking branches is seemingly overlooked.

The new behavior: if there is no local branch and multiple remote
candidates, just die() and don't try reverting file whether it
exists (prevents surprise) or not (improves error message).

[1] Commit 70c9ac2f ("DWIM "git checkout frotz" to "git checkout -b frotz origin/frotz"" 2009-10-18)
    https://public-inbox.org/git/7vaazpxha4.fsf_-_@alter.siamese.dyndns.org/
[2] Commit ad8d5104 ("checkout: add advice for ambiguous "checkout <branch>"", 2018-06-05)
    https://public-inbox.org/git/20180502105452.17583-1-avarab@gmail.com/
[3] Commit be4908f1 ("checkout: disambiguate dwim tracking branches and local files", 2018-11-13)
    https://public-inbox.org/git/20181110120707.25846-1-pclouds@gmail.com/

Signed-off-by: Alexandr Miloslavskiy <alexandr.miloslavskiy@syntevo.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/checkout.c

@@ -1115,10 +1115,10 @@ static void setup_new_branch_info_and_source_tree(
 
 static const char *parse_remote_branch(const char *arg,
 				       struct object_id *rev,
-				       int could_be_checkout_paths,
-				       int *dwim_remotes_matched)
+				       int could_be_checkout_paths)
 {
-	const char *remote = unique_tracking_name(arg, rev, dwim_remotes_matched);
+	int num_matches = 0;
+	const char *remote = unique_tracking_name(arg, rev, &num_matches);
 
 	if (remote && could_be_checkout_paths) {
 		die(_("'%s' could be both a local file and a tracking branch.\n"
@@ -1126,6 +1126,22 @@ static const char *parse_remote_branch(const char *arg,
 		    arg);
 	}
 
+	if (!remote && num_matches > 1) {
+	    if (advice_checkout_ambiguous_remote_branch_name) {
+		    advise(_("If you meant to check out a remote tracking branch on, e.g. 'origin',\n"
+			     "you can do so by fully qualifying the name with the --track option:\n"
+			     "\n"
+			     "    git checkout --track origin/<name>\n"
+			     "\n"
+			     "If you'd like to always have checkouts of an ambiguous <name> prefer\n"
+			     "one remote, e.g. the 'origin' remote, consider setting\n"
+			     "checkout.defaultRemote=origin in your config."));
+	    }
+
+	    die(_("'%s' matched multiple (%d) remote tracking branches"),
+		arg, num_matches);
+	}
+
 	return remote;
 }
 
@@ -1133,8 +1149,7 @@ static int parse_branchname_arg(int argc, const char **argv,
 				int dwim_new_local_branch_ok,
 				struct branch_info *new_branch_info,
 				struct checkout_opts *opts,
-				struct object_id *rev,
-				int *dwim_remotes_matched)
+				struct object_id *rev)
 {
 	const char **new_branch = &opts->new_branch;
 	int argcount = 0;
@@ -1240,8 +1255,7 @@ static int parse_branchname_arg(int argc, const char **argv,
 
 		if (recover_with_dwim) {
 			const char *remote = parse_remote_branch(arg, rev,
-								 could_be_checkout_paths,
-								 dwim_remotes_matched);
+								 could_be_checkout_paths);
 			if (remote) {
 				*new_branch = arg;
 				arg = remote;
@@ -1505,7 +1519,6 @@ static int checkout_main(int argc, const char **argv, const char *prefix,
 			 const char * const usagestr[])
 {
 	struct branch_info new_branch_info;
-	int dwim_remotes_matched = 0;
 	int parseopt_flags = 0;
 
 	memset(&new_branch_info, 0, sizeof(new_branch_info));
@@ -1613,8 +1626,7 @@ static int checkout_main(int argc, const char **argv, const char *prefix,
 			opts->track == BRANCH_TRACK_UNSPECIFIED &&
 			!opts->new_branch;
 		int n = parse_branchname_arg(argc, argv, dwim_ok,
-					     &new_branch_info, opts, &rev,
-					     &dwim_remotes_matched);
+					     &new_branch_info, opts, &rev);
 		argv += n;
 		argc -= n;
 	} else if (!opts->accept_ref && opts->from_treeish) {
@@ -1672,28 +1684,10 @@ static int checkout_main(int argc, const char **argv, const char *prefix,
 	}
 
 	UNLEAK(opts);
-	if (opts->patch_mode || opts->pathspec.nr) {
-		int ret = checkout_paths(opts, new_branch_info.name);
-		if (ret && dwim_remotes_matched > 1 &&
-		    advice_checkout_ambiguous_remote_branch_name)
-			advise(_("'%s' matched more than one remote tracking branch.\n"
-				 "We found %d remotes with a reference that matched. So we fell back\n"
-				 "on trying to resolve the argument as a path, but failed there too!\n"
-				 "\n"
-				 "If you meant to check out a remote tracking branch on, e.g. 'origin',\n"
-				 "you can do so by fully qualifying the name with the --track option:\n"
-				 "\n"
-				 "    git checkout --track origin/<name>\n"
-				 "\n"
-				 "If you'd like to always have checkouts of an ambiguous <name> prefer\n"
-				 "one remote, e.g. the 'origin' remote, consider setting\n"
-				 "checkout.defaultRemote=origin in your config."),
-			       argv[0],
-			       dwim_remotes_matched);
-		return ret;
-	} else {
+	if (opts->patch_mode || opts->pathspec.nr)
+		return checkout_paths(opts, new_branch_info.name);
+	else
 		return checkout_branch(opts, &new_branch_info);
-	}
 }
 
 int cmd_checkout(int argc, const char **argv, const char *prefix)

t/t2024-checkout-dwim.sh

@@ -37,7 +37,9 @@ test_expect_success 'setup' '
 		git checkout -b foo &&
 		test_commit a_foo &&
 		git checkout -b bar &&
-		test_commit a_bar
+		test_commit a_bar &&
+		git checkout -b ambiguous_branch_and_file &&
+		test_commit a_ambiguous_branch_and_file
 	) &&
 	git init repo_b &&
 	(
@@ -46,7 +48,9 @@ test_expect_success 'setup' '
 		git checkout -b foo &&
 		test_commit b_foo &&
 		git checkout -b baz &&
-		test_commit b_baz
+		test_commit b_baz &&
+		git checkout -b ambiguous_branch_and_file &&
+		test_commit b_ambiguous_branch_and_file
 	) &&
 	git remote add repo_a repo_a &&
 	git remote add repo_b repo_b &&
@@ -75,6 +79,26 @@ test_expect_success 'checkout of branch from multiple remotes fails #1' '
 	test_branch master
 '
 
+test_expect_success 'when arg matches multiple remotes, do not fallback to interpreting as pathspec' '
+	# create a file with name matching remote branch name
+	git checkout -b t_ambiguous_branch_and_file &&
+	>ambiguous_branch_and_file &&
+	git add ambiguous_branch_and_file &&
+	git commit -m "ambiguous_branch_and_file" &&
+
+	# modify file to verify that it will not be touched by checkout
+	test_when_finished "git checkout -- ambiguous_branch_and_file" &&
+	echo "file contents" >ambiguous_branch_and_file &&
+	cp ambiguous_branch_and_file expect &&
+
+	test_must_fail git checkout ambiguous_branch_and_file 2>err &&
+
+	test_i18ngrep "matched multiple (2) remote tracking branches" err &&
+
+	# file must not be altered
+	test_cmp expect ambiguous_branch_and_file
+'
+
 test_expect_success 'checkout of branch from multiple remotes fails with advice' '
 	git checkout -B master &&
 	test_might_fail git branch -D foo &&