On Mon, Aug 4, 2025 at 9:39 PM Patrick Steinhardt <ps@xxxxxx> wrote: > > On Mon, Aug 04, 2025 at 03:08:50PM -0700, Elijah Newren wrote: > > On Fri, Aug 1, 2025 at 1:31 AM Patrick Steinhardt <ps@xxxxxx> wrote: > > > On Tue, Jul 22, 2025 at 03:23:10PM +0000, Elijah Newren via GitGitGadget wrote: > > > > Further, commit 98a1a00d5301 (t6423: add a testcase causing a failed > > > > assertion in process_renames, 2025-03-06), fixed an issue with > > > > rename-to-self but added a new testcase, 12n, that only checked for > > > > whether the merge ran to completion. A few commits ago, we modified > > > > this test to check for the number of entries in the index -- but noted > > > > that the number was wrong. And we also noted a > > > > silently-keep-instead-of-delete bug at the same time in the new testcase > > > > 12n2. > > > > > > > > Fix to merge-ort to prevent multiple bugs with rename-to-self cases: > > > > * silent deletion of file expected to be kept (t6423 testcase 12i2) > > > > * silent retention of file expected to be removed (t6423 testcase 12n2) > > > > * wrong number of extries left in the index (t6423 testcase 12n) > > > > > > I think it would have been nice to also go a bit more in depth for what > > > the bug actually was and how it's fixed. You do add a comment, but that > > > only adds a single sentence of context. > > > > Would something like this help: > > > > ...all of these issues come up because in a rename-to-self case, when > > we have a rename A->B, both A and B name the same file. The code in > > process_renames() assumes A & B are different, and tries to move the > > higher order stages and file contents so that they are associated just > > with the new path, but the assumptions of A & B being different can > > cause A to be deleted when it's not supposed to be or mark B as > > resolved and kept in place when it's supposed to be deleted. Since A > > & B are already the same path in the rename-to-self case, we can > > simply skip the steps in process_renames() for such files. > > Yes, it would! Great, I'll include it in the next round, which I'll send out soon. > > > > Signed-off-by: Elijah Newren <newren@xxxxxxxxx> > > > > --- > > > > merge-ort.c | 11 +++++ > > > > t/t6423-merge-rename-directories.sh | 69 +++++++++++++++++++++++++++-- > > > > 2 files changed, 77 insertions(+), 3 deletions(-) > > > > > > > > diff --git a/merge-ort.c b/merge-ort.c > > > > index 9b9d82ed10f7..feb06720c7e1 100644 > > > > --- a/merge-ort.c > > > > +++ b/merge-ort.c > > > > @@ -2873,6 +2873,17 @@ static int process_renames(struct merge_options *opt, > > > > newinfo = new_ent->value; > > > > } > > > > > > > > + /* > > > > + * Directory renames can result in rename-to-self, which we > > > > + * want to skip so we don't mark oldpath for deletion. > > > > + * > > > > + * Note that we can avoid strcmp here because of prior > > > > + * diligence in apply_directory_rename_modifications() to > > > > + * ensure we reused existing paths from opt->priv->paths. > > > > + */ > > > > + if (oldpath == newpath) > > > > + continue; > > > > > > Makes me wonder whether the additional brittleness is worth the saved > > > `strcmp()` comparison. But on the other hand we do have tests now that > > > would break if the memory allocation patterns ever changed, so that's > > > reassuring. > > > > There's no brittleness here; one of the many optimizations in > > merge-ort.c was to intern *all* pathnames in struct > > merge_options_internal's "paths" member; any code that needs to > > generate/compute a filename that may be part of the merge must check > > if that path already exists in opt->priv->paths, and if so use that > > pointer instead so that all filename comparisons can be done with > > cheap pointer comparisons. See the big comment near the top of > > merge_options_internal. Nearly all such > > string-equality-via-pointer-equality checks were introduced about the > > same time, and in other functions, which makes this one kind of an > > outlier. I figured whoever reviewed this patch or ran across this in > > the code might get surprised by the pointer comparison, so I tried to > > add a comment to address any questions. Looks like I wasn't thorough > > enough (and the first paragraph of the comment pre-dated my finding > > other bugs this patch fixed, which makes it slightly confusing), so > > I'll try to see if I can improve it for v2. > > I think the current version is good enough -- it feels brittle to me, > but I don't see a strong reason to change it. Okay. Note that if there was a strong reason to change it, the logic to do so would mean that the pointer-comparison-instead-of-strcmp optimization used on pathnames in several other places of merge-ort would also need to change, because they are all based on the same interning-of-pathname-strings to allow that optimization to be safe. I did notice that the merge_options_internal comment on the paths member could perhaps be improved slightly; I'll include that too.
