tracking branches: add advice to ambiguous refspec error

The error "not tracking: ambiguous information for ref" is raised
when we are evaluating what tracking information to set on a branch,
and find that the ref to be added as tracking branch is mapped
under multiple remotes' fetch refspecs.

This can easily happen when a user copy-pastes a remote definition
in their git config, and forgets to change the tracking path.

Add advice in this situation, explicitly highlighting which remotes
are involved and suggesting how to correct the situation. Also
update a test to explicitly expect that advice.

Signed-off-by: Tao Klerks <tao@klerks.biz>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config/advice.txt

@@ -4,6 +4,10 @@ advice.*::
 	can tell Git that you do not need help by setting these to 'false':
 +
 --
+	ambiguousFetchRefspec::
+		Advice shown when fetch refspec for multiple remotes map to
+		the same remote-tracking branch namespace and causes branch
+		tracking set-up to fail.
 	fetchShowForcedUpdates::
 		Advice shown when linkgit:git-fetch[1] takes a long time
 		to calculate forced updates after ref updates, or to warn

advice.c

@@ -39,6 +39,7 @@ static struct {
 	[ADVICE_ADD_EMPTY_PATHSPEC]			= { "addEmptyPathspec", 1 },
 	[ADVICE_ADD_IGNORED_FILE]			= { "addIgnoredFile", 1 },
 	[ADVICE_AM_WORK_DIR] 				= { "amWorkDir", 1 },
+	[ADVICE_AMBIGUOUS_FETCH_REFSPEC]		= { "ambiguousFetchRefspec", 1 },
 	[ADVICE_CHECKOUT_AMBIGUOUS_REMOTE_BRANCH_NAME] 	= { "checkoutAmbiguousRemoteBranchName", 1 },
 	[ADVICE_COMMIT_BEFORE_MERGE]			= { "commitBeforeMerge", 1 },
 	[ADVICE_DETACHED_HEAD]				= { "detachedHead", 1 },

advice.h

@@ -17,6 +17,7 @@ struct string_list;
 	ADVICE_ADD_EMPTY_PATHSPEC,
 	ADVICE_ADD_IGNORED_FILE,
 	ADVICE_AM_WORK_DIR,
+	ADVICE_AMBIGUOUS_FETCH_REFSPEC,
 	ADVICE_CHECKOUT_AMBIGUOUS_REMOTE_BRANCH_NAME,
 	ADVICE_COMMIT_BEFORE_MERGE,
 	ADVICE_DETACHED_HEAD,

branch.c

@@ -18,17 +18,31 @@ struct tracking {
 	int matches;
 };
 
+struct find_tracked_branch_cb {
+	struct tracking *tracking;
+	struct string_list ambiguous_remotes;
+};
+
 static int find_tracked_branch(struct remote *remote, void *priv)
 {
-	struct tracking *tracking = priv;
+	struct find_tracked_branch_cb *ftb = priv;
+	struct tracking *tracking = ftb->tracking;
 
 	if (!remote_find_tracking(remote, &tracking->spec)) {
-		if (++tracking->matches == 1) {
+		switch (++tracking->matches) {
+		case 1:
 			string_list_append(tracking->srcs, tracking->spec.src);
 			tracking->remote = remote->name;
-		} else {
+			break;
+		case 2:
+			/* there are at least two remotes; backfill the first one */
+			string_list_append(&ftb->ambiguous_remotes, tracking->remote);
+			/* fall through */
+		default:
+			string_list_append(&ftb->ambiguous_remotes, remote->name);
 			free(tracking->spec.src);
 			string_list_clear(tracking->srcs, 0);
+		break;
 		}
 		tracking->spec.src = NULL;
 	}
@@ -232,12 +246,16 @@ static void setup_tracking(const char *new_ref, const char *orig_ref,
 	struct tracking tracking;
 	struct string_list tracking_srcs = STRING_LIST_INIT_DUP;
 	int config_flags = quiet ? 0 : BRANCH_CONFIG_VERBOSE;
+	struct find_tracked_branch_cb ftb_cb = {
+		.tracking = &tracking,
+		.ambiguous_remotes = STRING_LIST_INIT_DUP,
+	};
 
 	memset(&tracking, 0, sizeof(tracking));
 	tracking.spec.dst = (char *)orig_ref;
 	tracking.srcs = &tracking_srcs;
 	if (track != BRANCH_TRACK_INHERIT)
-		for_each_remote(find_tracked_branch, &tracking);
+		for_each_remote(find_tracked_branch, &ftb_cb);
 	else if (inherit_tracking(&tracking, orig_ref))
 		goto cleanup;
 
@@ -252,9 +270,39 @@ static void setup_tracking(const char *new_ref, const char *orig_ref,
 			goto cleanup;
 		}
 
-	if (tracking.matches > 1)
-		die(_("not tracking: ambiguous information for ref %s"),
-		    orig_ref);
+	if (tracking.matches > 1) {
+		int status = die_message(_("not tracking: ambiguous information for ref '%s'"),
+					    orig_ref);
+		if (advice_enabled(ADVICE_AMBIGUOUS_FETCH_REFSPEC)) {
+			struct strbuf remotes_advice = STRBUF_INIT;
+			struct string_list_item *item;
+
+			for_each_string_list_item(item, &ftb_cb.ambiguous_remotes)
+				/*
+				 * TRANSLATORS: This is a line listing a remote with duplicate
+				 * refspecs in the advice message below. For RTL languages you'll
+				 * probably want to swap the "%s" and leading "  " space around.
+				 */
+				strbuf_addf(&remotes_advice, _("  %s\n"), item->string);
+
+			/*
+			 * TRANSLATORS: The second argument is a \n-delimited list of
+			 * duplicate refspecs, composed above.
+			 */
+			advise(_("There are multiple remotes whose fetch refspecs map to the remote\n"
+				 "tracking ref '%s':\n"
+				 "%s"
+				 "\n"
+				 "This is typically a configuration error.\n"
+				 "\n"
+				 "To support setting up tracking branches, ensure that\n"
+				 "different remotes' fetch refspecs map into different\n"
+				 "tracking namespaces."), orig_ref,
+			       remotes_advice.buf);
+			strbuf_release(&remotes_advice);
+		}
+		exit(status);
+	}
 
 	if (tracking.srcs->nr < 1)
 		string_list_append(tracking.srcs, orig_ref);
@@ -264,6 +312,7 @@ static void setup_tracking(const char *new_ref, const char *orig_ref,
 
 cleanup:
 	string_list_clear(&tracking_srcs, 0);
+	string_list_clear(&ftb_cb.ambiguous_remotes, 0);
 }
 
 int read_branch_desc(struct strbuf *buf, const char *branch_name)

t/t3200-branch.sh

@@ -1039,13 +1039,27 @@ test_expect_success 'checkout -b with -l makes reflog when core.logAllRefUpdates
 	git rev-parse --verify gamma@{0}
 '
 
-test_expect_success 'avoid ambiguous track' '
+test_expect_success 'avoid ambiguous track and advise' '
 	git config branch.autosetupmerge true &&
 	git config remote.ambi1.url lalala &&
 	git config remote.ambi1.fetch refs/heads/lalala:refs/heads/main &&
 	git config remote.ambi2.url lilili &&
 	git config remote.ambi2.fetch refs/heads/lilili:refs/heads/main &&
-	test_must_fail git branch all1 main &&
+	cat <<-EOF >expected &&
+	fatal: not tracking: ambiguous information for ref '\''refs/heads/main'\''
+	hint: There are multiple remotes whose fetch refspecs map to the remote
+	hint: tracking ref '\''refs/heads/main'\'':
+	hint:   ambi1
+	hint:   ambi2
+	hint: ''
+	hint: This is typically a configuration error.
+	hint: ''
+	hint: To support setting up tracking branches, ensure that
+	hint: different remotes'\'' fetch refspecs map into different
+	hint: tracking namespaces.
+	EOF
+	test_must_fail git branch all1 main 2>actual &&
+	test_cmp expected actual &&
 	test -z "$(git config branch.all1.merge)"
 '