checkout: reject if the branch is already checked out elsewhere

One branch obviously can't be checked out at two places (but detached
heads are ok). Give the user a choice in this case: --detach, -b
new-branch, switch branch in the other checkout first or simply 'cd'
and continue to work there.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/checkout.c

@@ -430,6 +430,11 @@ struct branch_info {
 	const char *name; /* The short name used */
 	const char *path; /* The full name of a real branch */
 	struct commit *commit; /* The named commit */
+	/*
+	 * if not null the branch is detached because it's already
+	 * checked out in this checkout
+	 */
+	char *checkout;
 };
 
 static void setup_branch_path(struct branch_info *branch)
@@ -958,12 +963,78 @@ static const char *unique_tracking_name(const char *name, unsigned char *sha1)
 	return NULL;
 }
 
+static void check_linked_checkout(struct branch_info *new, const char *id)
+{
+	struct strbuf sb = STRBUF_INIT;
+	struct strbuf path = STRBUF_INIT;
+	struct strbuf gitdir = STRBUF_INIT;
+	const char *start, *end;
+
+	if (id)
+		strbuf_addf(&path, "%s/worktrees/%s/HEAD", get_git_common_dir(), id);
+	else
+		strbuf_addf(&path, "%s/HEAD", get_git_common_dir());
+
+	if (strbuf_read_file(&sb, path.buf, 0) < 0 ||
+	    !skip_prefix(sb.buf, "ref:", &start))
+		goto done;
+	while (isspace(*start))
+		start++;
+	end = start;
+	while (*end && !isspace(*end))
+		end++;
+	if (strncmp(start, new->path, end - start) || new->path[end - start] != '\0')
+		goto done;
+	if (id) {
+		strbuf_reset(&path);
+		strbuf_addf(&path, "%s/worktrees/%s/gitdir", get_git_common_dir(), id);
+		if (strbuf_read_file(&gitdir, path.buf, 0) <= 0)
+			goto done;
+		strbuf_rtrim(&gitdir);
+	} else
+		strbuf_addstr(&gitdir, get_git_common_dir());
+	die(_("'%s' is already checked out at '%s'"), new->name, gitdir.buf);
+done:
+	strbuf_release(&path);
+	strbuf_release(&sb);
+	strbuf_release(&gitdir);
+}
+
+static void check_linked_checkouts(struct branch_info *new)
+{
+	struct strbuf path = STRBUF_INIT;
+	DIR *dir;
+	struct dirent *d;
+
+	strbuf_addf(&path, "%s/worktrees", get_git_common_dir());
+	if ((dir = opendir(path.buf)) == NULL) {
+		strbuf_release(&path);
+		return;
+	}
+
+	/*
+	 * $GIT_COMMON_DIR/HEAD is practically outside
+	 * $GIT_DIR so resolve_ref_unsafe() won't work (it
+	 * uses git_path). Parse the ref ourselves.
+	 */
+	check_linked_checkout(new, NULL);
+
+	while ((d = readdir(dir)) != NULL) {
+		if (!strcmp(d->d_name, ".") || !strcmp(d->d_name, ".."))
+			continue;
+		check_linked_checkout(new, d->d_name);
+	}
+	strbuf_release(&path);
+	closedir(dir);
+}
+
 static int parse_branchname_arg(int argc, const char **argv,
 				int dwim_new_local_branch_ok,
 				struct branch_info *new,
 				struct tree **source_tree,
 				unsigned char rev[20],
-				const char **new_branch)
+				const char **new_branch,
+				int force_detach)
 {
 	int argcount = 0;
 	unsigned char branch_rev[20];
@@ -1085,6 +1156,16 @@ static int parse_branchname_arg(int argc, const char **argv,
 	else
 		new->path = NULL; /* not an existing branch */
 
+	if (new->path && !force_detach && !*new_branch) {
+		unsigned char sha1[20];
+		int flag;
+		char *head_ref = resolve_refdup("HEAD", 0, sha1, &flag);
+		if (head_ref &&
+		    (!(flag & REF_ISSYMREF) || strcmp(head_ref, new->path)))
+			check_linked_checkouts(new);
+		free(head_ref);
+	}
+
 	new->commit = lookup_commit_reference_gently(rev, 1);
 	if (!new->commit) {
 		/* not a commit */
@@ -1289,7 +1370,8 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 			!opts.new_branch;
 		int n = parse_branchname_arg(argc, argv, dwim_ok,
 					     &new, &opts.source_tree,
-					     rev, &opts.new_branch);
+					     rev, &opts.new_branch,
+					     opts.force_detach);
 		argv += n;
 		argc -= n;
 	}

t/t2025-checkout-to.sh

@@ -18,13 +18,14 @@ test_expect_success 'checkout --to an existing worktree' '
 '
 
 test_expect_success 'checkout --to a new worktree' '
-	git checkout --to here master &&
+	git rev-parse HEAD >expect &&
+	git checkout --detach --to here master &&
 	(
 		cd here &&
 		test_cmp ../init.t init.t &&
-		git symbolic-ref HEAD >actual &&
-		echo refs/heads/master >expect &&
-		test_cmp expect actual &&
+		test_must_fail git symbolic-ref HEAD &&
+		git rev-parse HEAD >actual &&
+		test_cmp ../expect actual &&
 		git fsck
 	)
 '
@@ -42,7 +43,7 @@ test_expect_success 'checkout --to a new worktree from a subdir' '
 test_expect_success 'checkout --to from a linked checkout' '
 	(
 		cd here &&
-		git checkout --to nested-here master &&
+		git checkout --detach --to nested-here master &&
 		cd nested-here &&
 		git fsck
 	)
@@ -60,4 +61,18 @@ test_expect_success 'checkout --to a new worktree creating new branch' '
 	)
 '
 
+test_expect_success 'die the same branch is already checked out' '
+	(
+		cd here &&
+		test_must_fail git checkout newmaster
+	)
+'
+
+test_expect_success 'not die on re-checking out current branch' '
+	(
+		cd there &&
+		git checkout newmaster
+	)
+'
+
 test_done