checkout: teach check_linked_checkout() about symbolic link HEAD

check_linked_checkout() only understands symref-style HEAD (i.e. "ref:
refs/heads/master"), however, HEAD may also be a an actual symbolic link
(on platforms which support it). To accurately detect if a branch is
checked out elsewhere, it needs to handle symbolic link HEAD, as well.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/checkout.c

@@ -889,7 +889,11 @@ static void check_linked_checkout(const char *branch, const char *id)
 	else
 		strbuf_addf(&path, "%s/HEAD", get_git_common_dir());
 
-	if (strbuf_read_file(&sb, path.buf, 0) >= 0 &&
+	if (!strbuf_readlink(&sb, path.buf, 0)) {
+		if (!starts_with(sb.buf, "refs/") ||
+		    check_refname_format(sb.buf, 0))
+			goto done;
+	} else if (strbuf_read_file(&sb, path.buf, 0) >= 0 &&
 	    starts_with(sb.buf, "ref:")) {
 		strbuf_remove(&sb, 0, strlen("ref:"));
 		strbuf_trim(&sb);

t/t2025-worktree-add.sh

@@ -83,6 +83,14 @@ test_expect_success 'die the same branch is already checked out' '
 	)
 '
 
+test_expect_success SYMLINKS 'die the same branch is already checked out (symlink)' '
+	head=$(git -C there rev-parse --git-path HEAD) &&
+	ref=$(git -C there symbolic-ref HEAD) &&
+	rm "$head" &&
+	ln -s "$ref" "$head" &&
+	test_must_fail git -C here checkout newmaster
+'
+
 test_expect_success 'not die the same branch is already checked out' '
 	(
 		cd here &&