Re: [PATCH] diff: restore redirection to /dev/null for diff_from_contents
- From
Jeff King <peff@peff.net>
- Date
- Oct 21, 2025, 07:52 UTC
- Message-ID
- <20251021075226.GC259661@coredump.intra.peff.net>
- In-Reply-To
- <d5895f9c-5b3c-7a69-46e0-cf16cda5bf3a@gmx.de>
On Sun, Oct 19, 2025 at 11:09:28PM +0200, Johannes Schindelin wrote:
Show 21 quoted lines
> > @@ -6890,6 +6890,15 @@ void diff_flush(struct diff_options *options)
> > if (output_format & DIFF_FORMAT_NO_OUTPUT &&
> > options->flags.exit_with_status &&
> > options->flags.diff_from_contents) {
> > + /*
> > + * run diff_flush_patch for the exit status. setting
> > + * options->file to /dev/null should be safe, because we
> > + * aren't supposed to produce any output anyway.
> > + */
> > + diff_free_file(options);
> > + options->file = xfopen("/dev/null", "w");
> > + options->close_file = 1;
> > + options->color_moved = 0;
>
> I do not see any discussion about the `color_moved` line in
> https://lore.kernel.org/git/20250808033019.78817-1-yldhome2d2@gmail.com/#r,
> nor here.
>
> Since you re-add it, I consider at least a little bit of reasonsing in
> order, e.g. why this is necessary, and if it is necessary, why isn't
> `options->use_color` forced to 0 also?My patch is a revert of the hunk from b55e6d36eb which caused the "--quiet" regression, and hence includes that line. Perhaps I could have made that more clear in the commit message.
I don't think use_color is related here. There's no clue in the commit message which added that color_moved line (it was just the commit which added the color_moved feature in the first place). But knowing the code, I'd guess that it is not about trying to avoid producing color (which is, after all, just going to go to /dev/null anyway) but rather avoiding the computation to detect moved lines, since nobody will see them.
So probably (but I did not do any experimenting) the code produces the correct output with or without color_moved. But it is also probably wasting some extra CPU since b55e6d36eb. In a world with a dry_run flag, it probably would make sense to skip the color_moved feature when dry_run is set.
> Taking a step back to see the 100ft view, I can understand why you want > that "extra level of protection" here. An even more important thing, that > is missing, is a plan to avoid the need for this protection.
Sure. The goal of my patch was not to fix the dry-run feature. It was to do the release engineering to undo the "--quiet" regression in the simplest and least error-prone way possible. One way to do that is to just revert b55e6d36eb entirely, add a new test covering the regression, and then try again on top (perhaps on master this time). But I did the more selective revert to reduce the back-and-forth noise of dropping the dry_run code and then adding it back, which I thought gave the original author a better base to work from.
Whether that /dev/null redirection survives once we are confident that dry_run is hitting all of the code paths is up for debate.
I take it that you would prefer to try to fix dry_run in place on 'maint'. I think that can work, too. It's just not how I would do it (not because I think this particular case is so hard, but because as a general release engineering principle I prefer to fix regressions by backing out changes rather than piling more changes on top). I am OK if you want to go the other way, though.
Show 5 quoted lines
> Given that you're still on GitHub's payroll if the hallway rumors are > correct, I am quite a bit puzzled that you did not immediately reach for > CodeQL (which is a GitHub-sponsored technology, after all) to get clarity > on the code paths that would make this exra "layer of protection" still > necessary, and thereby provide said plan.
There is no need to be puzzled. I have never actually used CodeQL at all, beyond analyzing some of the false positives I've seen it report. And the fact that GitHub sponsors my work on git.git is not really relevant to how I go about that work.
Show 5 quoted lines
> I started an AI-assisted brainstorm session and ended up with this query > (which is neither as concise nor as comprehensible as I would have liked, > but at least it does the job of finding the `run_diff_cmd()` code path > that I also find, and no other code path, and in v4 of Lidong Yan's patch, > it finds no remaining code path):
Neat, though it is very hard for me to quickly assess whether that CodeQL block is doing the right thing. Your idea of manually tracing the paths that touch opts->file seemed much simpler to me (and I think came up with similar results).
-Peff