[PATCH v6 2/3] branch: report invalid tracking branch as broken

@@ -423,9 +423,19 @@ static void fill_tracking_info(struct strbuf *stat, const char *branch_name,
 	char *ref = NULL;
 	struct branch *branch = branch_get(branch_name);
 	struct strbuf fancy = STRBUF_INIT;
+	int broken_upstream = 0;
 
-	if (!stat_tracking_info(branch, &ours, &theirs))
+	switch (stat_tracking_info(branch, &ours, &theirs)) {
+	case 0:
+		/* Not set upstream. */
 		return;
+	case -1:
+		/* Upstream is missing or invalid. */
+		broken_upstream = 1;
+		break;
+	default:
+		break;
+	}
 
 	if (show_upstream_ref) {
 		ref = shorten_unambiguous_ref(branch->merge[0]->dst, 0);

@@ -437,7 +447,10 @@ static void fill_tracking_info(struct strbuf *stat, const char *branch_name,
 			strbuf_addstr(&fancy, ref);
 	}
 
-	if (!ours && !theirs) {
+	if (broken_upstream) {
+		if (show_upstream_ref)
+			strbuf_addf(stat, _("[%s: broken]"), fancy.buf);
+	} else if (!ours && !theirs) {
 		if (show_upstream_ref)
 			strbuf_addf(stat, _("[%s]"), fancy.buf);
 	} else if (!ours) {

@@ -1729,8 +1729,11 @@ int ref_newer(const unsigned char *new_sha1, const unsigned char *old_sha1)
 }
 
 /*
- * Return false if cannot stat a tracking branch (not exist or invalid),
- * otherwise true.
+ * Compare a branch with its tracking branch, and save their differences
+ * (number of commits) in *num_ours and *num_theirs.
+ *
+ * Return 0 if branch has no upstream, -1 if upstream is missing or invalid,
+ * otherwise 1.
  */
 int stat_tracking_info(struct branch *branch, int *num_ours, int *num_theirs)
 {

@@ -1749,16 +1752,16 @@ int stat_tracking_info(struct branch *branch, int *num_ours, int *num_theirs)
 	/* Cannot stat if what we used to build on no longer exists */
 	base = branch->merge[0]->dst;
 	if (read_ref(base, sha1))
-		return 0;
+		return -1;
 	theirs = lookup_commit_reference(sha1);
 	if (!theirs)
-		return 0;
+		return -1;
 
 	if (read_ref(branch->refname, sha1))
-		return 0;
+		return -1;
 	ours = lookup_commit_reference(sha1);
 	if (!ours)
-		return 0;
+		return -1;
 
 	/* are we the same? */
 	if (theirs == ours) {

@@ -1808,17 +1811,33 @@ int format_tracking_info(struct branch *branch, struct strbuf *sb)
 {
 	int ours, theirs;
 	const char *base;
+	int broken_upstream = 0;
 
-	if (!stat_tracking_info(branch, &ours, &theirs))
-		return 0;
-
-	/* Nothing to report if neither side has changes. */
-	if (!ours && !theirs)
+	switch (stat_tracking_info(branch, &ours, &theirs)) {
+	case 0:
+		/* Not set upstream. */
 		return 0;
+	case -1:
+		/* Upstream is missing or invalid. */
+		broken_upstream = 1;
+		break;
+	default:
+		/* Nothing to report if neither side has changes. */
+		if (!ours && !theirs)
+			return 0;
+		break;
+	}
 
 	base = branch->merge[0]->dst;
 	base = shorten_unambiguous_ref(base, 0);
-	if (!theirs) {
+	if (broken_upstream) {
+		strbuf_addf(sb,
+			_("Your branch is based on a broken ref '%s'.\n"),
+			base);
+		if (advice_status_hints)
+			strbuf_addf(sb,
+				_("  (use \"git branch --unset-upstream\" to fixup)\n"));
+	} else if (!theirs) {
 		strbuf_addf(sb,
 			Q_("Your branch is ahead of '%s' by %d commit.\n",
 			   "Your branch is ahead of '%s' by %d commits.\n",

@@ -60,6 +60,7 @@ b1 origin/master: ahead 1, behind 1
 b2 origin/master: ahead 1, behind 1
 b3 origin/master: behind 1
 b4 origin/master: ahead 2
+b5 brokenbase: broken
 EOF
 
 test_expect_success 'branch -vv' '

@@ -71,7 +72,7 @@ test_expect_success 'branch -vv' '
 	test_i18ncmp expect actual
 '
 
-test_expect_success 'checkout' '
+test_expect_success 'checkout (diverged from upstream)' '
 	(
 		cd test && git checkout b1
 	) >actual &&

@@ -84,7 +85,15 @@ test_expect_success 'checkout with local tracked branch' '
 	test_i18ngrep "is ahead of" actual
 '
 
-test_expect_success 'status' '
+test_expect_success 'checkout (broken upstream)' '
+	(
+		cd test &&
+		git checkout b5
+	) >actual &&
+	test_i18ngrep "is based on a broken ref" actual
+'
+
+test_expect_success 'status (diverged from upstream)' '
 	(
 		cd test &&
 		git checkout b1 >/dev/null &&

@@ -94,6 +103,42 @@ test_expect_success 'status' '
 	test_i18ngrep "have 1 and 1 different" actual
 '
 
+test_expect_success 'status (broken upstream)' '
+	(
+		cd test &&
+		git checkout b5 >/dev/null &&
+		# reports nothing to commit
+		test_must_fail git commit --dry-run
+	) >actual &&
+	test_i18ngrep "is based on a broken ref" actual
+'
+
+cat >expect <<\EOF
+## b1...origin/master [ahead 1, behind 1]
+EOF
+
+test_expect_success 'status -s -b (diverged from upstream)' '
+	(
+		cd test &&
+		git checkout b1 >/dev/null &&
+		git status -s -b | head -1
+	) >actual &&
+	test_i18ncmp expect actual
+'
+
+cat >expect <<\EOF
+## b5...brokenbase [broken]
+EOF
+
+test_expect_success 'status -s -b (broken upstream)' '
+	(
+		cd test &&
+		git checkout b5 >/dev/null &&
+		git status -s -b | head -1
+	) >actual &&
+	test_i18ncmp expect actual
+'
+
 test_expect_success 'fail to track lightweight tags' '
 	git checkout master &&
 	git tag light &&

@@ -1363,6 +1363,7 @@ static void wt_shortstatus_print_tracking(struct wt_status *s)
 	const char *base;
 	const char *branch_name;
 	int num_ours, num_theirs;
+	int broken_upstream = 0;
 
 	color_fprintf(s->fp, color(WT_STATUS_HEADER, s), "## ");
 

@@ -1383,17 +1384,21 @@ static void wt_shortstatus_print_tracking(struct wt_status *s)
 
 	color_fprintf(s->fp, branch_color_local, "%s", branch_name);
 
-	/*
-	 * Not report tracking info if no tracking branch found
-	 * or no difference found.
-	 */
-	if (!stat_tracking_info(branch, &num_ours, &num_theirs)) {
-		fputc(s->null_termination ? '\0' : '\n', s->fp);
-		return;
-	}
-	if (!num_ours && !num_theirs) {
+	switch (stat_tracking_info(branch, &num_ours, &num_theirs)) {
+	case 0:
+		/* Not report tracking info if no tracking branch found. */
 		fputc(s->null_termination ? '\0' : '\n', s->fp);
 		return;
+	case -1:
+		/* Upstream is missing or invalid. */
+		broken_upstream = 1;
+		break;
+	default:
+		if (!num_ours && !num_theirs) {
+			fputc(s->null_termination ? '\0' : '\n', s->fp);
+			return;
+		}
+		break;
 	}
 
 	base = branch->merge[0]->dst;

@@ -1402,7 +1407,9 @@ static void wt_shortstatus_print_tracking(struct wt_status *s)
 	color_fprintf(s->fp, branch_color_remote, "%s", base);
 
 	color_fprintf(s->fp, header_color, " [");
-	if (!num_ours) {
+	if (broken_upstream) {
+		color_fprintf(s->fp, header_color, _("broken"));
+	} else if (!num_ours) {
 		color_fprintf(s->fp, header_color, _("behind "));
 		color_fprintf(s->fp, branch_color_remote, "%d", num_theirs);
 	} else if (!num_theirs) {