Re: [PATCH] diff: stop output garbled message in dry run mode

> 
> "Lidong Yan via GitGitGadget" <gitgitgadget@gmail.com> writes:
> 
>> From: Lidong Yan <yldhome2d2@gmail.com>
>> 
>> In dry run mode, diff_flush_patch() should not produce any output.
>> However, in commit b55e6d36eb (diff: ensure consistent diff behavior
>> with ignore options, 2025-08-08), only the output during the
>> comparison of two file contents was suppressed. For file deletions
>> or mode changes, diff_flush_patch() still produces output. In
>> run_extern_diff(), set quiet to true if in dry run mode. In
>> emit_diff_symbol_from_struct(), directly return if in dry run mode.
> 
> The above makes it sound as if the dry-run mode was an inherent part
> of the diff machinery that existed even before b55e6d36 came, and
> b55e6d36 somehow broke it.  But that is not what you are telling us,
> I think.
> 
> You may know what the "dry-run" mode is, but others don't.  You
> should tell the backstory a bit better to help them.  I am guessing
> that this patch is to fix a breakage introduced when the dry-run
> mode is added in b55e6d36 (diff: ensure consistent diff behavior
> with ignore options, 2025-08-08)?   If so, I would expect an
> explanation like ...
> 
>    Earlier, b55e6d36 (diff: ensure consistent diff behavior with
>    ignore options, 2025-08-08) introduced "dry-run" mode to the
>    diff machinery so that content based diff filtering (like
>    ignoring space changes or those that match -I<regex>) can first
>    try to produce a patch without emitting any output to see if
>    under the given diff filtering condition we would get any output
>    lines, and a new helper function diff_flush_patch_quietly() was
>    introduced to use the mode to see an individual filepair needs
>    to be shown.
> 
>    However, the solution was not complete.  IN SUCH AND SUCH CASES,
>    THIS BAD THING HAPPENED BECAUSE WE OVERLOOKED THIS AND THAT
>    CONDITION, AND AS A RESULT, DRY-RUN MODE WAS NOT QUIET.
> 
>    To fix this, DO THIS AND THAT.  THIS WOULD AFFECT ONLY SUCH AND
>    SUCH CASES WITHOUT AFFECTING OTHER CODE PATHS LIKE DOING X AND Y.

> 
> ... is given to help readers understand what we wanted to do in the
> earlier commit, what we failed to do there and why, and what we can
> do at this point to clean up the mess without making further
> damange.
> 
>> Signed-off-by: Lidong Yan <yldhome2d2@gmail.com>
>> ---
>>    diff: stop output garbled message in dry run mode
>> 
>>    In dry run mode, diff_flush_patch() should not produce any output.
>>    However, in commit b55e6d36eb (diff: ensure consistent diff behavior
>>    with ignore options, 2025-08-08), only the output during the comparison
>>    of two file contents was suppressed. For file deletions or mode changes,
>>    diff_flush_patch() still produces output. In run_extern_diff(), set
>>    quiet to true if in dry run mode. In emit_diff_symbol_from_struct(),
>>    directly return if in dry run mode.
> 
> The "below three-dash" space is a place to explain what does not
> have to be a part of the resulting commit but would help those who
> are reading the mailing list and reviewing.  Repeating the same
> thing as the proposed log message does not help readers.

> 
>> diff --git a/t/t4013-diff-various.sh b/t/t4013-diff-various.sh
>> index 55a06eadb3..25fa452656 100755
>> --- a/t/t4013-diff-various.sh
>> +++ b/t/t4013-diff-various.sh
>> @@ -661,6 +661,27 @@ test_expect_success 'diff -I<regex>: ignore matching file' '
>> test_grep ! "file1" actual
>> '
>> 
>> +test_expect_success 'diff -I<regex>: ignore all content changes' '
>> + test_when_finished "git rm -f file1 file2" &&
>> + : >file1 &&
>> + git add file1 &&
>> + : >file2 &&
>> + git add file2 &&
>> +
>> + rm -f file1 file2 &&
>> + mkdir file2 &&
>> + test_diff_no_content_changes () {
>> + git diff $1 --ignore-blank-lines -I".*" >actual &&
>> + test_line_count = 2 actual &&
>> + test_grep "file1" actual &&
>> + test_grep "file2" actual &&
>> + test_grep ! "diff --git" actual
>> + } &&
>> + test_diff_no_content_changes "--raw" &&
>> + test_diff_no_content_changes "--name-only" &&
>> + test_diff_no_content_changes "--name-status"
>> +'
> 
> Test that exercises "git diff -I<regex>" is in line with what the
> original b55e6d36eb wanted to address, but given that we saw a
> recent regression report like [*], I would have liked to see "git
> diff --quiet" in the test as well.