Re: [PATCH 5/6] merge-ort: fix incorrect file handling

> From: Elijah Newren <newren@gmail.com>
> 
> We have multiple bugs here -- accidental silent file deletion,
> accidental silent file retention for files that should be deleted,
> and incorrect number of entries left in the index.
> 
> The series merged at commit d3b88be1b450 (Merge branch
> 'en/merge-dir-rename-corner-case-fix', 2021-07-16) introduced testcase
> 12i-12k in t6423 which checked for rename-to-self cases, and fixed bugs
> that merge-ort and merge-recursive had with these testcases.  At the
> time, I noted that merge-ort had one bug for these cases, while
> merge-recursive had two.  It turns out that merge-ort did in fact have
> another bug, but the "relevant renames" optimizations were masking it.
> If we modify testcase 12i from t6423 to modify the file in the commit
> that renames it (but only modify it enough that it can still be detected
> as a rename), then we can trigger silent deletion of the file.
> 
> Tweak testcase 12i slightly to make the file in question have more than
> one line in it, but which doesn't change how it operates.

> Make this
> change to otherwise minimize the changes between this testcase and a new
> one that we want to add.  Then duplicate the testcase as 12i2, changing
> it so that it adds a single line to the file in question when it is
> renamed, as a testcase for this bug.

> 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)

> Signed-off-by: Elijah Newren <newren@gmail.com>
> ---
>  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;