From: John Cai <johncai86@xxxxxxxxx> 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 $(git rev-parse topic) without dereferencing it. Fix this bug by adding the RESET_HEAD_DETACH flag in checkout_up_to_date if <branch> is not a valid branch. so that once reset_head() calls update_refs(), it calls update_ref() with REF_NO_DEREF which updates HEAD directly intead of deferencing it and updating the branch that HEAD points to. Also add a test to ensure the correct behavior. 1. https://lore.kernel.org/git/xmqqsfrpbepd.fsf@gitster.g/ Signed-off-by: John Cai <johncai86@xxxxxxxxx> --- builtin/rebase.c | 2 ++ t/t3400-rebase.sh | 8 ++++---- 2 files changed, 6 insertions(+), 4 deletions(-) diff --git a/builtin/rebase.c b/builtin/rebase.c index b29ad2b65e7..27fde7bf281 100644 --- a/builtin/rebase.c +++ b/builtin/rebase.c @@ -829,6 +829,8 @@ static int checkout_up_to_date(struct rebase_options *options) ropts.oid = &options->orig_head; ropts.branch = options->head_name; ropts.flags = RESET_HEAD_RUN_POST_CHECKOUT_HOOK; + if (!ropts.branch) + ropts.flags |= RESET_HEAD_DETACH; ropts.head_msg = buf.buf; if (reset_head(the_repository, &ropts) < 0) ret = error(_("could not switch to %s"), options->switch_to); diff --git a/t/t3400-rebase.sh b/t/t3400-rebase.sh index 2fb3fabe60e..cf55b017ffc 100755 --- a/t/t3400-rebase.sh +++ b/t/t3400-rebase.sh @@ -389,13 +389,13 @@ test_expect_success 'switch to branch not checked out' ' git rebase main other ' -test_expect_success 'switch to non-branch changes branch HEAD points to' ' +test_expect_success 'switch to non-branch detaches HEAD' ' git checkout main && old_main=$(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' ' -- gitgitgadget
