Subject: Re: [PATCH v2 3/3] Revert "pack-objects: lazily set up "struct rev_info", don't leak"

From: Ævar Arnfjörð Bjarmason <[email-masked]>
Date: Mon, 28 Nov 2022 19:32:29 +0100

On Mon, Nov 28 2022, René Scharfe wrote:

> Am 28.11.2022 um 16:56 schrieb René Scharfe:>
>> The problem is "How to use struct rev_info without leaks?".  No matter
>> where you move it, the leak will be present until the TODO in
>> release_revisions() is done.
>
> Wrong.  The following sequence leaks:
>
> 	struct rev_info revs;
> 	repo_init_revisions(the_repository, &revs, NULL);
> 	release_revisions(&revs);
>
> ... and this here doesn't:
>
> 	struct rev_info revs;
> 	repo_init_revisions(the_repository, &revs, NULL);
> 	setup_revisions(0, NULL, &revs, NULL);  // leak plugger
> 	release_revisions(&revs);
>
> That's because setup_revisions() calls diff_setup_done(), which frees
> revs->diffopt.parseopts, and release_revisions() doesn't.
>
> And since builtin/pack-objects.c::get_object_list() calls
> setup_revisions(), it really frees that memory, as you claimed from the
> start.  Sorry, I was somehow assuming that a setup function wouldn't
> clean up.  D'oh!
>
> The first sequence is used in some other places. e.g. builtin/prune.c.
> But there LeakSanitizer doesn't complain for some reason; Valgrind
> reports the parseopts allocation as "possibly lost".

Yes, some of the interactions are tricky. It's really useful to run the
tests with GIT_TEST_PASSING_SANITIZE_LEAK=[true|check] (see t/README) to
check these sorts of assumptions for sanity.

> I still think the assumption that "init_x(x); release_x(x);" doesn't
> leak is reasonable.  Let's make it true.  How about this?  It's safe
> in the sense that we don't risk double frees and it's close to the
> TODO comment so we probably won't forget removing it once diff_free()
> becomes used.
>
> ---
>  revision.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/revision.c b/revision.c
> index 439e34a7c5..6a51ef9418 100644
> --- a/revision.c
> +++ b/revision.c
> @@ -3055,6 +3055,7 @@ void release_revisions(struct rev_info *revs)
>  	release_revisions_mailmap(revs->mailmap);
>  	free_grep_patterns(&revs->grep_filter);
>  	/* TODO (need to handle "no_free"): diff_free(&revs->diffopt) */
> +	FREE_AND_NULL(revs->diffopt.parseopts);
>  	diff_free(&revs->pruning);
>  	reflog_walk_info_release(revs->reflog_info);
>  	release_revisions_topo_walk_info(revs->topo_walk_info);

At this point I'm unclear on what & why this is needed? I.e. once we
narrowly fix the >1 "--filter" options what still fails?

But in general: I don't really think this sort of thing is worth
it. Here we're reaching into a member of "revs->diffopt" behind its back
rather than calling diff_free(). I think we should just focus on being
able to do do that safely.

WIP patches I have in that direction, partially based on your previous
"is_dead" suggestion:

	https://github.com/avar/git/commit/e02a15f6206
	https://github.com/avar/git/commit/c718f36566a

I haven't poked at that in a while, I think the only outstanding issue
with it is that fclose() interaction.

I think for this particular thing there aren't going to be any bad
side-effects in practice, but I also think convincing oneself of that
basically means putting the same amount of work in as just fixing some
of these properly.