Subject: Re: [PATCH v4 3/3] rebase: set REF_HEAD_DETACH in checkout_up_to_date()

From: Junio C Hamano <[email-masked]>
Date: Thu, 17 Mar 2022 14:36:16 -0700

"John Cai via GitGitGadget" <[email-masked]> writes:

> From: John Cai <[email-masked]>
>
> Fixes a bug whereby rebase updates the deferenced reference HEAD points
> to instead of HEAD directly.

Perhaps

    "git rebase A B", where B is not a commit, should behave as if
    the HEAD got detached at B and then the detached HEAD got
    rebased on top of A.  A bug however overwrites the current
    branch to point at B, when B is a descendant of A (i.e. the
    rebase ends up being a fast-forward).

> ... See [1] for
> the original bug report.

OK (URL is wrong; see below).

The explanation of how the bug occurs (elided) in the patch looked
all reasonable.  It read well.

> ...
> Also add a test to ensure the correct behavior.

Yup.  _Add_ a test to ensure that.  Not replace a misleading test
that expected to see a wrong behaviour.

> 1. https://lore.kernel.org/git/[email-masked]/

This is not the original bug report.  It was an early hint for
diagnosis.

[1] https://lore.kernel.org/git/YiokTm3GxIZQQUow@newk/

would be a more appropriate pointer.

>  	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;

OK.  If head_name is not set, we do not want to touch the branch
the HEAD happens to be pointing at, so we want to detach.

> +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

As we want (1) HEAD (detached) is pointing at Second, (2) 'main'
stayed at $old_main, and (3) HEAD is detched, these three conditions
look sane.

Thanks.

For reference, I discarded [1/3], queued [2/3] and replaced this
[3/3] with the following for now.

---- >8 ---- ---- >8 ---- ---- >8 ---- ---- >8 ---- ---- >8 ----
From: John Cai <[email-masked]>
Subject: [PATCH] rebase: set REF_HEAD_DETACH in checkout_up_to_date()

"git rebase A B" where B is not a commit should behave as if the
HEAD got detached at B and then the detached HEAD got rebased on top
of A.  A bug however overwrites the current branch to point at B,
when B is a descendant of A (i.e. the rebase ends up being a
fast-forward).  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 B 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. We want the HEAD detached at B instead.

Fix this bug by adding the RESET_HEAD_DETACH flag in
checkout_up_to_date if B 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/YiokTm3GxIZQQUow@newk/

Reported-by: Michael McClimon <[email-masked]>
Signed-off-by: John Cai <[email-masked]>
Signed-off-by: Junio C Hamano <[email-masked]>
---
 builtin/rebase.c  | 2 ++
 t/t3400-rebase.sh | 9 +++++++++
 2 files changed, 11 insertions(+)

diff --git a/builtin/rebase.c b/builtin/rebase.c
index b29ad2b65e..27fde7bf28 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 6dc8df8be7..cf55b017ff 100755
--- a/t/t3400-rebase.sh
+++ b/t/t3400-rebase.sh
@@ -389,6 +389,15 @@ 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 &&
-- 
2.35.1-757-g4266a5c05c