Hi Junio, On 17 Mar 2022, at 17:36, Junio C Hamano wrote: > "John Cai via GitGitGadget" <gitgitgadget@xxxxxxxxx> writes: > >> From: John Cai <johncai86@xxxxxxxxx> >> >> 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/xmqqsfrpbepd.fsf@gitster.g/ > > 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. Sounds good--this is what I was about to do anyways! > > ---- >8 ---- ---- >8 ---- ---- >8 ---- ---- >8 ---- ---- >8 ---- > From: John Cai <johncai86@xxxxxxxxx> > 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 <michael@xxxxxxxxxxxx> > Signed-off-by: John Cai <johncai86@xxxxxxxxx> > Signed-off-by: Junio C Hamano <gitster@xxxxxxxxx> > --- > 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 Thanks John
