[PATCH] 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). This bug was
reported by Michael McClimon. See [1].

This is happening because on a fast foward with an oid as a <branch>,
update_refs() will only call update_ref() with REF_NO_DEREF if
RESET_HEAD_DETACH is set. This change was made in 176f5d96 (built-in rebase
--autostash: leave the current branch alone if possible,
2018-11-07). In rebase, we are not setting the RESET_HEAD_DETACH flag,
which means that the update_ref() call ends up dereferencing
HEAD and updating it to the oid used as <branch>.

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
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/

Signed-off-by: John Cai <johncai86@gmail.com>
---
    rebase: update HEAD when is an oid
    
    Fixes a bug [1] reported by Michael McClimon where rebase with oids will
    erroneously update the branch HEAD points to.
    
     1. https://lore.kernel.org/git/xmqqsfrpbepd.fsf@gitster.g/

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-1226%2Fjohn-cai%2Fjc%2Ffix-rebase-oids-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-1226/john-cai/jc/fix-rebase-oids-v1
Pull-Request: https://github.com/git/git/pull/1226

 builtin/rebase.c  |  2 +-
 t/t3400-rebase.sh | 21 +++++++++++++++++++++
 2 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/builtin/rebase.c b/builtin/rebase.c
index b29ad2b65e7..52afeffcc2e 100644
--- a/builtin/rebase.c
+++ b/builtin/rebase.c
@@ -828,7 +828,7 @@ static int checkout_up_to_date(struct rebase_options *options)
 		    options->switch_to);
 	ropts.oid = &options->orig_head;
 	ropts.branch = options->head_name;
-	ropts.flags = RESET_HEAD_RUN_POST_CHECKOUT_HOOK;
+	ropts.flags = RESET_HEAD_RUN_POST_CHECKOUT_HOOK | 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 71b1735e1dd..0b92e78976c 100755
--- a/t/t3400-rebase.sh
+++ b/t/t3400-rebase.sh
@@ -437,4 +437,25 @@ test_expect_success 'rebase when inside worktree subdirectory' '
 	)
 '
 
+test_expect_success 'rebase with oids' '
+	git init main-wt &&
+	(
+		cd main-wt &&
+		>file &&
+		git add file &&
+		git commit -m initial &&
+		git checkout -b side &&
+		echo >>file &&
+		git commit -a -m side &&
+		git checkout main &&
+		git tag hold &&
+		git checkout -B main hold &&
+		git rev-parse main >pre &&
+		git rebase $(git rev-parse main) $(git rev-parse side) &&
+		git rev-parse main >post &&
+		test "$(git rev-parse side)" = "$(cat .git/HEAD)" &&
+		test_cmp pre post
+	)
+'
+
 test_done

base-commit: 1a4874565fa3b6668042216189551b98b4dc0b1b
-- 
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