[PATCH v4 3/3] 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 an oid, 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().

Then update_refs() checks ropts.branch to decide whether or not to switch
branches. 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 the correct behavior.

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

Signed-off-by: John Cai <johncai86@gmail.com>
---
 builtin/rebase.c  | 2 ++
 t/t3400-rebase.sh | 8 ++++----
 2 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/builtin/rebase.c b/builtin/rebase.c
index b29ad2b65e7..27fde7bf281 100644
--- a/builtin/rebase.c
+++ b/builtin/rebase.c
@@ -829,6 +829,8 @@ static int checkout_up_to_date(struct rebase_options *options)
 	ropts.oid = &options->orig_head;
 	ropts.branch = options->head_name;
 	ropts.flags = RESET_HEAD_RUN_POST_CHECKOUT_HOOK;
+	if (!ropts.branch)
+		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/t/t3400-rebase.sh b/t/t3400-rebase.sh
index 2fb3fabe60e..cf55b017ffc 100755
--- a/t/t3400-rebase.sh
+++ b/t/t3400-rebase.sh
@@ -389,13 +389,13 @@ test_expect_success 'switch to branch not checked out' '
 	git rebase main other
 '
 
-test_expect_success 'switch to non-branch changes branch HEAD points to' '
+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 main &&
-	test_cmp_rev main $(git rev-parse Second) &&
-	git symbolic-ref HEAD
+	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' '
-- 
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