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 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@xxxxxxxxxxxx> Signed-off-by: John Cai <johncai86@xxxxxxxxx> --- 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 && -- gitgitgadget
