On Mon, Aug 4, 2025 at 9:39 PM Patrick Steinhardt [off-list ref] wrote:

On Mon, Aug 04, 2025 at 03:08:50PM -0700, Elijah Newren wrote:

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On Fri, Aug 1, 2025 at 1:31 AM Patrick Steinhardt [off-list ref] wrote:

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On Tue, Jul 22, 2025 at 03:23:10PM +0000, Elijah Newren via GitGitGadget wrote:

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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.

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Signed-off-by: Elijah Newren <redacted>
---
 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.