[PATCH v2 2/2] rebase: set REF_HEAD_DETACH in checkout_up_to_date()

["John Cai via GitGitGadget" <gitgitgadget@gmail.com>]

From: John Cai <johncai86@gmail.com>

Fixes a bug whereby rebase updates the deferenced reference HEAD points
to instead of HEAD directly.

If HEAD is on main and if the following is a fast-forward operation,

git rebase $(git rev-parse main) $(git rev-parse topic)

Instead of HEAD being set to $(git rev-parse topic), rebase erroneously
dereferences HEAD and sets main to $(git rev-parse topic). See [1] for
the original bug report.

The callstack from checkout_up_to_date() is the following:

cmd_rebase() -> checkout_up_to_date() -> reset_head() -> update_refs()
 -> update_ref()

When <branch> is not a valid branch but a sha, rebase sets the head_name
of rebase_options to NULL. This value gets passed down this call chain
through the branch member of reset_head_opts also getting set to NULL
all the way to update_refs(). update_refs() checks ropts.branch to
decide whether or not to switch brancheds. If ropts.branch is NULL, it
calls update_ref() to update HEAD. At this point however, from rebase's
point of view, we want a detached HEAD. But, since checkout_up_to_date()
does not set the RESET_HEAD_DETACH flag, the update_ref() call will
deference HEAD and update the branch its pointing to, which in the above
example is main.

The correct behavior is that git rebase should update HEAD to $(git
rev-parse topic) without dereferencing it.

Fix this bug by adding the RESET_HEAD_DETACH flag in checkout_up_to_date
if <branch> is not a valid branch. so that once reset_head() calls
update_refs(), it calls update_ref() with REF_NO_DEREF which updates
HEAD directly intead of deferencing it and updating the branch that HEAD
points to.

Also add a test to ensure this behavior.

1. https://lore.kernel.org/git/xmqqsfrpbepd.fsf@gitster.g/

Reported-by: Michael McClimon <michael@mcclimon.org>
Signed-off-by: John Cai <johncai86@gmail.com>
---
 builtin/rebase.c  | 5 ++++-
 reset.c           | 3 +++
 t/t3400-rebase.sh | 9 +++++++++
 3 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/builtin/rebase.c b/builtin/rebase.c
index b29ad2b65e7..5ae7fa2a169 100644
--- a/builtin/rebase.c
+++ b/builtin/rebase.c
@@ -827,8 +827,11 @@ static int checkout_up_to_date(struct rebase_options *options)
 		    getenv(GIT_REFLOG_ACTION_ENVIRONMENT),
 		    options->switch_to);
 	ropts.oid = &options->orig_head;
-	ropts.branch = options->head_name;
 	ropts.flags = RESET_HEAD_RUN_POST_CHECKOUT_HOOK;
+	if (options->head_name)
+		ropts.branch = options->head_name;
+	else
+		ropts.flags |=  RESET_HEAD_DETACH;
 	ropts.head_msg = buf.buf;
 	if (reset_head(the_repository, &ropts) < 0)
 		ret = error(_("could not switch to %s"), options->switch_to);
diff --git a/reset.c b/reset.c
index e3383a93343..f8e32fcc240 100644
--- a/reset.c
+++ b/reset.c
@@ -101,6 +101,9 @@ int reset_head(struct repository *r, const struct reset_head_opts *opts)
 	if (opts->branch_msg && !opts->branch)
 		BUG("branch reflog message given without a branch");
 
+	if (switch_to_branch && opts->flags & RESET_HEAD_DETACH)
+		BUG("attempting to detach HEAD when branch is given");
+
 	if (!refs_only && repo_hold_locked_index(r, &lock, LOCK_REPORT_ON_ERROR) < 0) {
 		ret = -1;
 		goto leave_reset_head;
diff --git a/t/t3400-rebase.sh b/t/t3400-rebase.sh
index 0643d015255..d5a8ee39fc4 100755
--- a/t/t3400-rebase.sh
+++ b/t/t3400-rebase.sh
@@ -394,6 +394,15 @@ test_expect_success 'switch to branch not checked out' '
 	git rebase main other
 '
 
+test_expect_success 'switch to non-branch detaches HEAD' '
+	git checkout main &&
+	old_main=$(git rev-parse HEAD) &&
+	git rebase First Second^0 &&
+	test_cmp_rev HEAD Second &&
+	test_cmp_rev main $old_main &&
+	test_must_fail git symbolic-ref HEAD
+'
+
 test_expect_success 'refuse to switch to branch checked out elsewhere' '
 	git checkout main &&
 	git worktree add wt &&
-- 
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