worktree: teach "add" to check out existing branches

Currently 'git worktree add <path>' creates a new branch named after the
basename of the path by default.  If a branch with that name already
exists, the command refuses to do anything, unless the '--force' option
is given.

However we can do a little better than that, and check the branch out if
it is not checked out anywhere else.  This will help users who just want
to check an existing branch out into a new worktree, and save a few
keystrokes.

As the current behaviour is to simply 'die()' when a branch with the name
of the basename of the path already exists, there are no backwards
compatibility worries here.

We will still 'die()' if the branch is checked out in another worktree,
unless the --force flag is passed.

Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Reviewed-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-worktree.txt

@@ -61,8 +61,13 @@ $ git worktree add --track -b <branch> <path> <remote>/<branch>
 ------------
 +
 If `<commit-ish>` is omitted and neither `-b` nor `-B` nor `--detach` used,
-then, as a convenience, a new branch based at HEAD is created automatically,
-as if `-b $(basename <path>)` was specified.
+then, as a convenience, the new worktree is associated with a branch
+(call it `<branch>`) named after `$(basename <path>)`.  If `<branch>`
+doesn't exist, a new branch based on HEAD is automatically created as
+if `-b <branch>` was given.  If `<branch>` does exist, it will be
+checked out in the new worktree, if it's not checked out anywhere
+else, otherwise the command will refuse to create the worktree (unless
+`--force` is used).
 
 list::
 

builtin/worktree.c

@@ -391,8 +391,17 @@ static const char *dwim_branch(const char *path, const char **new_branch)
 {
 	int n;
 	const char *s = worktree_basename(path, &n);
-	*new_branch = xstrndup(s, n);
-	UNLEAK(*new_branch);
+	const char *branchname = xstrndup(s, n);
+	struct strbuf ref = STRBUF_INIT;
+
+	UNLEAK(branchname);
+	if (!strbuf_check_branch_ref(&ref, branchname) &&
+	    ref_exists(ref.buf)) {
+		strbuf_release(&ref);
+		return branchname;
+	}
+
+	*new_branch = branchname;
 	if (guess_remote) {
 		struct object_id oid;
 		const char *remote =

t/t2025-worktree-add.sh

@@ -198,13 +198,25 @@ test_expect_success '"add" with <branch> omitted' '
 	test_cmp_rev HEAD bat
 '
 
-test_expect_success '"add" auto-vivify does not clobber existing branch' '
-	test_commit c1 &&
-	test_commit c2 &&
-	git branch precious HEAD~1 &&
-	test_must_fail git worktree add precious &&
-	test_cmp_rev HEAD~1 precious &&
-	test_path_is_missing precious
+test_expect_success '"add" checks out existing branch of dwimd name' '
+	git branch dwim HEAD~1 &&
+	git worktree add dwim &&
+	test_cmp_rev HEAD~1 dwim &&
+	(
+		cd dwim &&
+		test_cmp_rev HEAD dwim
+	)
+'
+
+test_expect_success '"add <path>" dwim fails with checked out branch' '
+	git checkout -b test-branch &&
+	test_must_fail git worktree add test-branch &&
+	test_path_is_missing test-branch
+'
+
+test_expect_success '"add --force" with existing dwimd name doesnt die' '
+	git checkout test-branch &&
+	git worktree add --force test-branch
 '
 
 test_expect_success '"add" no auto-vivify with --detach and <branch> omitted' '