Re: [PATCH v2 4/6] log: fix common "rev.pending" memory leak in "git show"

[Jeff King <peff@peff.net>]

On Fri, Jul 29, 2022 at 02:55:35PM -0400, Jeff King wrote:

> > @@ -744,7 +745,6 @@ int cmd_show(int argc, const char **argv, const char *prefix)
> >  			rev.shown_one = 1;
> >  			break;
> >  		case OBJ_COMMIT:
> > -			memcpy(&rev.pending, &blank, sizeof(rev.pending));
> >  			add_object_array(o, name, &rev.pending);
> >  			ret = cmd_log_walk_no_free(&rev);
> >  			break;
> 
> We now do not do anything to clean up rev.pending. On the first pass,
> we'd see our blank pending array and add to it. But on a subsequent pass
> (i.e., because we are showing two or more commits), what will we see?
> 
> My initial assumption was we'd have the last pass's commit in "pending"
> here, so we'd be leaking it. But I think in practice it's OK. We end up
> in prepare_revision_walk(), which blanks the list again, and then
> processes each element. Non-commits _do_ end up back in the pending
> list, which would be a leak. But we know that this code triggers only
> for commits, which are placed only in the "commits" list (and that's
> cleaned up as we walk it via get_revision()).

Sorry, just one more clarification here.

The "so we'd be leaking it" in the second paragraph is really: so before
this patch, we'd have been leaking it writing over it with blank. After
this patch, we'd be building up an ever-growing list of pending objects,
and showing a quadratic number of entries. ;)

-Peff