From: Phillip Wood Date: Tue, 20 Sep 2022 08:26:27 GMT Subject: Re: [PATCH v2] sequencer: avoid dropping fixup commit that targets self via commit-ish Message-ID: <8909c02d-3fd0-a0bb-ebc2-0a640febce53@dunelm.org.uk> In-Reply-To: <20220920031140.1220220-1-aclopte@gmail.com> Hi Johannes On 20/09/2022 04:11, Johannes Altmanninger wrote: > Commit 68d5d03bc4 (rebase: teach --autosquash to match on sha1 in > addition to message, 2010-11-04) taught autosquash to recognize > subjects like "fixup! 7a235b" where 7a235b is an OID-prefix. It > actually did more than advertised: 7a235b can be an arbitrary > commit-ish (as long as it's not trailed by spaces). > > Accidental(?) use of this secret feature revealed a bug where we > would silently drop a fixup commit. The bug can also be triggered > when using an OID-prefix but that's unlikely in practice. > > Given a commit with subject "fixup! main" that is the tip of the > branch "main". When computing the fixup target for this commit, we > find the commit itself. This is wrong because, by definition, a fixup > target must be an earlier commit in the todo list. We wrongly find > the current commit because we added it to the todo list prematurely. > Avoid these fixup-cycles by only adding the current commit after we > have finished finding its target. Thanks for working on this, the fix for the fixup self reference looks good. It's unfortunate that the implementation is not stricter when parsing "fixup! " but it is more or less consistent with the shell version which used "git rev-parse $subject"[1]. We should think about being stricter but this fix avoids on of the worst pitfalls of our lax parsing. Best Wishes Phillip [1] With regard to the oid vs subject prefix issue, I think the shell version chose to fixup the first commit that matched either the oid or the subject. At least the C version is consistent in preferring an oid match over a subject prefix match even if I wish it was the other way round. > Reported-by: Erik Cervin Edin > Signed-off-by: Johannes Altmanninger > --- > sequencer.c | 4 ++-- > t/t3415-rebase-autosquash.sh | 26 ++++++++++++++++++++++++++ > 2 files changed, 28 insertions(+), 2 deletions(-) > > Changes to v1. > - Remove wrong assumptions from commit message. The commit message should > be clearer now (though I didn't spend too much time on it). > - Drop one test because it's not related to the fix (and doesn't test anything > I care about) and modify the other test so it requires the fix to pass. > > 1: cb2ee0e003 ! 1: 410ca51936 sequencer: avoid dropping fixup commit that targets self via commit-ish > @@ Commit message > sequencer: avoid dropping fixup commit that targets self via commit-ish > > Commit 68d5d03bc4 (rebase: teach --autosquash to match on sha1 in > - addition to message, 2010-11-04) made --autosquash apply a commit > - with subject "fixup! 012345" to the first commit in the todo list > - whose OID starts with 012345. So far so good. > + addition to message, 2010-11-04) taught autosquash to recognize > + subjects like "fixup! 7a235b" where 7a235b is an OID-prefix. It > + actually did more than advertised: 7a235b can be an arbitrary > + commit-ish (as long as it's not trailed by spaces). > > - More recently, c44a4c650c (rebase -i: rearrange fixup/squash lines > - using the rebase--helper, 2017-07-14) reimplemented this logic in C > - and introduced two behavior changes. > - First, OID matches are given precedence over subject prefix > - matches. Second, instead of prefix-matching OIDs, we use > - lookup_commit_reference_by_name(). This means that if 012345 is a > - branch name, we will apply the fixup commit to the tip of that branch > - (if that is present in the todo list). > + Accidental(?) use of this secret feature revealed a bug where we > + would silently drop a fixup commit. The bug can also be triggered > + when using an OID-prefix but that's unlikely in practice. > > - Both behavior changes might be motivated by performance concerns > - (since the commit message mentions performance). Looking through > - the todo list to find a commit that matches the given prefix can be > - more expensive than looking up an OID. The runtime of the former is > - of O(n*m) where n is the size of the todo list and m is the length > - of a commit subject. However, if this is really a problem, we could > - easily make it O(m) by constructing a trie (prefix tree). > - > - Demonstrate both behavior changes by adding two test cases for > - "fixup! foo" where foo is a commit-ish that is not an OID-prefix. > - Arguably, this feature is very weird. If no one uses it we should > - consider removing it. > - > - Regardless, there is one bad edge case to fix. Let refspec "foo" point > - to a commit with the subject "fixup! foo". Since rebase --autosquash > - finds the fixup target via lookup_commit_reference_by_name(), the > - fixup target is the fixup commit itself. Obviously this can't work. > - We proceed with the broken invariant and drop the fixup commit > - entirely. > - > - The self-fixup was only allowed because the fixup commit was already > - added to the preliminary todo list, which it shouldn't be. Rather, > - we should first compute the fixup target and only then add the fixup > - commit to the todo list. Make it so, avoiding this error by design, > - and add a third test for this case. > + Given a commit with subject "fixup! main" that is the tip of the > + branch "main". When computing the fixup target for this commit, we > + find the commit itself. This is wrong because, by definition, a fixup > + target must be an earlier commit in the todo list. We wrongly find > + the current commit because we added it to the todo list prematurely. > + Avoid these fixup-cycles by only adding the current commit after we > + have finished finding its target. > > Reported-by: Erik Cervin Edin > - Signed-off-by: Johannes Altmanninger > - Signed-off-by: Junio C Hamano > > ## sequencer.c ## > @@ sequencer.c: int todo_list_rearrange_squash(struct todo_list *todo_list) > @@ t/t3415-rebase-autosquash.sh: test_expect_success 'auto squash that matches long > +test_expect_success 'auto squash that matches regex' ' > + git reset --hard base && > + git commit --allow-empty -m "hay needle hay" && > -+ git commit --allow-empty -m "fixup! :/[n]eedle" && > ++ git commit --allow-empty -m "fixup! :/needle" && > + GIT_SEQUENCE_EDITOR="cat >tmp" git rebase --autosquash -i HEAD^^ && > -+ sed -ne "/^[^#]/{s/[0-9a-f]\{7,\}/HASH/g;p}" tmp >actual && > ++ sed -ne "/^[^#]/{s/[0-9a-f]\{7,\}/HASH/g;p;}" tmp >actual && > + cat <<-EOF >expect && > + pick HASH hay needle hay # empty > -+ fixup HASH fixup! :/[n]eedle # empty > -+ EOF > -+ test_cmp expect actual > -+' > -+ > -+test_expect_success 'auto squash of fixup commit that matches branch name' ' > -+ git reset --hard base && > -+ git commit --allow-empty -m "wip commit (just a prefix match so overshadowed by branch)" && > -+ git commit --allow-empty -m "tip of wip" && > -+ git branch wip && > -+ git commit --allow-empty -m "unrelated commit" && > -+ git commit --allow-empty -m "fixup! wip" && > -+ GIT_SEQUENCE_EDITOR="cat >tmp" git rebase --autosquash -i HEAD^^^^ && > -+ sed -ne "/^[^#]/{s/[0-9a-f]\{7,\}/HASH/g;p}" tmp >actual && > -+ cat <<-EOF >expect && > -+ pick HASH wip commit (just a prefix match so overshadowed by branch) # empty > -+ pick HASH tip of wip # empty > -+ fixup HASH fixup! wip # empty > -+ pick HASH unrelated commit # empty > ++ fixup HASH fixup! :/needle # empty > + EOF > + test_cmp expect actual > +' > @@ t/t3415-rebase-autosquash.sh: test_expect_success 'auto squash that matches long > + git commit --allow-empty -m "fixup! self-cycle" && > + git branch self-cycle && > + GIT_SEQUENCE_EDITOR="cat >tmp" git rebase --autosquash -i HEAD^^ && > -+ sed -ne "/^[^#]/{s/[0-9a-f]\{7,\}/HASH/g;p}" tmp >actual && > ++ sed -ne "/^[^#]/{s/[0-9a-f]\{7,\}/HASH/g;p;}" tmp >actual && > + cat <<-EOF >expect && > + pick HASH second commit > + pick HASH fixup! self-cycle # empty > > > diff --git a/sequencer.c b/sequencer.c > index 484ca9aa50..777200a6dc 100644 > --- a/sequencer.c > +++ b/sequencer.c > @@ -6287,8 +6287,6 @@ int todo_list_rearrange_squash(struct todo_list *todo_list) > return error(_("the script was already rearranged.")); > } > > - *commit_todo_item_at(&commit_todo, item->commit) = item; > - > parse_commit(item->commit); > commit_buffer = logmsg_reencode(item->commit, NULL, "UTF-8"); > find_commit_subject(commit_buffer, &subject); > @@ -6355,6 +6353,8 @@ int todo_list_rearrange_squash(struct todo_list *todo_list) > strhash(entry->subject)); > hashmap_put(&subject2item, &entry->entry); > } > + > + *commit_todo_item_at(&commit_todo, item->commit) = item; > } > > if (rearranged) { > diff --git a/t/t3415-rebase-autosquash.sh b/t/t3415-rebase-autosquash.sh > index 78c27496d6..98af865268 100755 > --- a/t/t3415-rebase-autosquash.sh > +++ b/t/t3415-rebase-autosquash.sh > @@ -232,6 +232,32 @@ test_expect_success 'auto squash that matches longer sha1' ' > test_line_count = 1 actual > ' > > +test_expect_success 'auto squash that matches regex' ' > + git reset --hard base && > + git commit --allow-empty -m "hay needle hay" && > + git commit --allow-empty -m "fixup! :/needle" && > + GIT_SEQUENCE_EDITOR="cat >tmp" git rebase --autosquash -i HEAD^^ && > + sed -ne "/^[^#]/{s/[0-9a-f]\{7,\}/HASH/g;p;}" tmp >actual && > + cat <<-EOF >expect && > + pick HASH hay needle hay # empty > + fixup HASH fixup! :/needle # empty > + EOF > + test_cmp expect actual > +' > + > +test_expect_success 'auto squash of fixup commit that matches branch name which points back to fixup commit' ' > + git reset --hard base && > + git commit --allow-empty -m "fixup! self-cycle" && > + git branch self-cycle && > + GIT_SEQUENCE_EDITOR="cat >tmp" git rebase --autosquash -i HEAD^^ && > + sed -ne "/^[^#]/{s/[0-9a-f]\{7,\}/HASH/g;p;}" tmp >actual && > + cat <<-EOF >expect && > + pick HASH second commit > + pick HASH fixup! self-cycle # empty > + EOF > + test_cmp expect actual > +' > + > test_auto_commit_flags () { > git reset --hard base && > echo 1 >file1 &&