Re: bug in "git fsck"?

["René Scharfe" <l.s.r@web.de>]

Am 02.07.21 um 20:15 schrieb Junio C Hamano:
> "Ulrich Windl" <Ulrich.Windl@rz.uni-regensburg.de> writes:
>
>> I was wondering whether git fsck should be able to cleanup
>> orphaned branches ("HEAD points to an unborn branch") as described
>> in https://stackoverflow.com/q/68226081/6607497 It seems I can fix
>> it be editing files in the repository, but I feed that's not the
>> way it should be.
>
> HEAD pointing at an unborn branch is not even a corruption, isn't
> it?
>
>    $ rm -rf trash && git init trash
>
> would point HEAD at an unborn one, ready to be used.

True, but the scenario described on StackOverflow is a bit different.
Commits were filtered out, and branches still pointing to them cannot
be deleted with "git branch -d" or "git branch -D".  Git fsck only
reports them.

You *can* overwrite them using "git branch --force foo" and then
"git branch -d foo" works.

I think it makes sense to let "git branch -D foo" work directly in such
a case.  That would be a user-visible change that may cause data loss,
so we better be careful.  I can't imagine a practical data-loss
scenario, but that might be just me.  Under which circumstances do we
want to keep a branch that does not point to a commit?

Anyway, here's what the change would look like:

--- >8 ---
Subject: [PATCH] branch: allow deleting dangling branches with --force

git branch only allows deleting branches that point to valid commits.
Skip that check if --force is given, as the caller is indicating with
it that they know what they are doing and accept the consequences.
This allows deleting dangling branches, which previously had to be
reset to a valid start-point using --force first.

Signed-off-by: René Scharfe <l.s.r@web.de>
---
 Documentation/git-branch.txt | 3 ++-
 builtin/branch.c             | 2 +-
 t/t3200-branch.sh            | 7 +++++++
 3 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/Documentation/git-branch.txt b/Documentation/git-branch.txt
index 94dc9a54f2..5449767121 100644
--- a/Documentation/git-branch.txt
+++ b/Documentation/git-branch.txt
@@ -118,7 +118,8 @@ OPTIONS
 	Reset <branchname> to <startpoint>, even if <branchname> exists
 	already. Without `-f`, 'git branch' refuses to change an existing branch.
 	In combination with `-d` (or `--delete`), allow deleting the
-	branch irrespective of its merged status. In combination with
+	branch irrespective of its merged status, or whether it even
+	points to a valid commit. In combination with
 	`-m` (or `--move`), allow renaming the branch even if the new
 	branch name already exists, the same applies for `-c` (or `--copy`).

diff --git a/builtin/branch.c b/builtin/branch.c
index b23b1d1752..03c7b7253a 100644
--- a/builtin/branch.c
+++ b/builtin/branch.c
@@ -168,7 +168,7 @@ static int check_branch_commit(const char *branchname, const char *refname,
 			       int kinds, int force)
 {
 	struct commit *rev = lookup_commit_reference(the_repository, oid);
-	if (!rev) {
+	if (!force && !rev) {
 		error(_("Couldn't look up commit object for '%s'"), refname);
 		return -1;
 	}
diff --git a/t/t3200-branch.sh b/t/t3200-branch.sh
index cc4b10236e..ec61a10c29 100755
--- a/t/t3200-branch.sh
+++ b/t/t3200-branch.sh
@@ -1272,6 +1272,13 @@ test_expect_success 'attempt to delete a branch merged to its base' '
 	test_must_fail git branch -d my10
 '

+test_expect_success 'branch --delete --force removes dangling branch' '
+	test_when_finished "rm -f .git/refs/heads/dangling" &&
+	echo $ZERO_OID >.git/refs/heads/dangling &&
+	git branch --delete --force dangling &&
+	test_path_is_missing .git/refs/heads/dangling
+'
+
 test_expect_success 'use --edit-description' '
 	write_script editor <<-\EOF &&
 		echo "New contents" >"$1"
--
2.32.0