Subject: Re* [rebase] `fatal: cannot force update the branch ... checkout out at ...` is confusing when it isn't active

From: Junio C Hamano <[email-masked]>
Date: Wed, 12 Jul 2023 15:25:27 -0700

Junio C Hamano <[email-masked]> writes:

> It shouldn't be a rocket surgery to teach the function to leave that
> information, but I am not offhand sure how valuable it would be to
> do so.  After all, once you learn which worktree of yours is using
> the branch you wanted to touch, you'll either know already or it
> would be easy for you to find out what you have been doing in that
> other worktree anyway, no?

So, before we forget, let's make a minimum patch and let the list
archive it.

----- >8 --------- >8 --------- >8 --------- >8 --------- >8 -----

Subject: [PATCH] branch: update the message to refuse touching a branch in-use

The "git branch -f" command can refuse to force-update a branch that
is in use in another worktree.  The original rationale for this
behaviour was that updating a branch that is checked out in another
worktree, without making a matching change to the index and the
working tree files in that worktree, will lead to a very confused
user.  "git diff HEAD" will no longer give a useful patch, because
HEAD is a commit unrelated to what the index and the working tree in
the worktree were based on, for example.

These days, however, the same mechanism also protects branches that
are being rebased or bisected, and the same machanism is expected to
be the right place to add more checks, when we decide to protect
branches undergoing other kinds of operations.  We forgot to rethink
the messaging, which originally said that we are refusing to touch
the branch because it is "checked out" elsewhere, when d2ba271a
(branch: check for bisects and rebases, 2022-06-14) started to
protect branches that are being rebased or bisected.

The spirit of the check is that we do not want to disrupt the use of
the same branch in other worktrees.  Let's reword the message slightly
to say that the branch is "in use", instead of "checked out".

We could teach the branch.c:prepare_checked_out_branches() function
to remember why it decided that a particular branch needs protecting
(i.e. was it because it was checked out?  being bisected?  something
else?) in addition to which worktree the branch was in use, and use
that in the error message to say "you cannot force update this
branch because it is being bisected in the worktree X", etc., but it
is dubious that such extra complexity is worth it.  The message
already tells which directory the worktree in question is, and it
should be just a "chdir" away for the user to find out what state it
is in, if the user felt curious enough.  So let's not go there yet.

Signed-off-by: Junio C Hamano <[email-masked]>
---
 branch.c                  |  2 +-
 t/t2407-worktree-heads.sh | 10 +++++-----
 2 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/branch.c b/branch.c
index cdf70b0ef0..6cb3b4c56d 100644
--- a/branch.c
+++ b/branch.c
@@ -471,7 +471,7 @@ int validate_new_branchname(const char *name, struct strbuf *ref, int force)
 
 	if ((path = branch_checked_out(ref->buf)))
 		die(_("cannot force update the branch '%s' "
-		      "checked out at '%s'"),
+		      "in use at '%s'"),
 		    ref->buf + strlen("refs/heads/"), path);
 
 	return 1;
diff --git a/t/t2407-worktree-heads.sh b/t/t2407-worktree-heads.sh
index 019a40df2c..d005ce2f20 100755
--- a/t/t2407-worktree-heads.sh
+++ b/t/t2407-worktree-heads.sh
@@ -58,7 +58,7 @@ test_expect_success !SANITIZE_LEAK 'refuse to overwrite: worktree in bisect' '
 	git -C wt-4 bisect good wt-1 &&
 
 	test_must_fail git branch -f wt-4 HEAD 2>err &&
-	grep "cannot force update the branch '\''wt-4'\'' checked out at.*wt-4" err
+	grep "cannot force update the branch '\''wt-4'\'' in use at.*wt-4" err
 '
 
 test_expect_success !SANITIZE_LEAK 'refuse to overwrite: worktree in rebase (apply)' '
@@ -68,7 +68,7 @@ test_expect_success !SANITIZE_LEAK 'refuse to overwrite: worktree in rebase (app
 	test_must_fail git -C wt-2 rebase --apply conflict-2 &&
 
 	test_must_fail git branch -f wt-2 HEAD 2>err &&
-	grep "cannot force update the branch '\''wt-2'\'' checked out at.*wt-2" err
+	grep "cannot force update the branch '\''wt-2'\'' in use at.*wt-2" err
 '
 
 test_expect_success !SANITIZE_LEAK 'refuse to overwrite: worktree in rebase (merge)' '
@@ -78,7 +78,7 @@ test_expect_success !SANITIZE_LEAK 'refuse to overwrite: worktree in rebase (mer
 	test_must_fail git -C wt-2 rebase conflict-2 &&
 
 	test_must_fail git branch -f wt-2 HEAD 2>err &&
-	grep "cannot force update the branch '\''wt-2'\'' checked out at.*wt-2" err
+	grep "cannot force update the branch '\''wt-2'\'' in use at.*wt-2" err
 '
 
 test_expect_success !SANITIZE_LEAK 'refuse to overwrite: worktree in rebase with --update-refs' '
@@ -90,7 +90,7 @@ test_expect_success !SANITIZE_LEAK 'refuse to overwrite: worktree in rebase with
 	for i in 3 4
 	do
 		test_must_fail git branch -f can-be-updated HEAD 2>err &&
-		grep "cannot force update the branch '\''can-be-updated'\'' checked out at.*wt-3" err ||
+		grep "cannot force update the branch '\''can-be-updated'\'' in use at.*wt-3" err ||
 			return 1
 	done
 '
@@ -150,7 +150,7 @@ test_expect_success 'refuse to overwrite when in error states' '
 	for i in 1 2
 	do
 		test_must_fail git branch -f fake-$i HEAD 2>err &&
-		grep "cannot force update the branch '\''fake-$i'\'' checked out at" err ||
+		grep "cannot force update the branch '\''fake-$i'\'' in use at" err ||
 			return 1
 	done
 '
-- 
2.41.0-327-gaa9166bcc0