Re: [PATCH 1/2] merge-ort: fix small memory leak in detect_and_process_renames()

[Elijah Newren <newren@gmail.com>]

On Sat, Feb 19, 2022 at 1:45 PM Ævar Arnfjörð Bjarmason
<avarab@gmail.com> wrote:
>
> On Sat, Feb 19 2022, Elijah Newren via GitGitGadget wrote:
>
> > From: Elijah Newren <newren@gmail.com>
> >
> > detect_and_process_renames() detects renames on both sides of history
> > and then combines these into a single diff_queue_struct.  The combined
> > diff_queue_struct needs to be able to hold the renames found on either
> > side, and since it knows the (maximum) size it needs, it pre-emptively
> > grows the array to the appropriate size:
> >
> >       ALLOC_GROW(combined.queue,
> >                  renames->pairs[1].nr + renames->pairs[2].nr,
> >                  combined.alloc);
> >
> > It then collects the items from each side:
> >
> >       collect_renames(opt, &combined, MERGE_SIDE1, ...)
> >       collect_renames(opt, &combined, MERGE_SIDE2, ...)
> >
> > Note, though, that collect_renames() sometimes determines that some
> > pairs are unnecessary and does not include them in the combined array.
> > When it is done, detect_and_process_renames() frees this memory:
> >
> >       if (combined.nr) {
> >                 ...
> >               free(combined.queue);
> >         }
> >
> > The problem is that sometimes even when there are pairs, none of them are
> > necessary.  Instead of checking combined.nr, we should check
> > combined.alloc.  Doing so fixes the following memory leak, as reported
> > by valgrind:
> >
> > ==PID== 192 bytes in 1 blocks are definitely lost in loss record 107 of 134
> > ==PID==    at 0xADDRESS: malloc
> > ==PID==    by 0xADDRESS: realloc
> > ==PID==    by 0xADDRESS: xrealloc (wrapper.c:126)
> > ==PID==    by 0xADDRESS: detect_and_process_renames (merge-ort.c:3134)
> > ==PID==    by 0xADDRESS: merge_ort_nonrecursive_internal (merge-ort.c:4610)
> > ==PID==    by 0xADDRESS: merge_ort_internal (merge-ort.c:4709)
> > ==PID==    by 0xADDRESS: merge_incore_recursive (merge-ort.c:4760)
> > ==PID==    by 0xADDRESS: merge_ort_recursive (merge-ort-wrappers.c:57)
> > ==PID==    by 0xADDRESS: try_merge_strategy (merge.c:753)
> > ==PID==    by 0xADDRESS: cmd_merge (merge.c:1676)
> > ==PID==    by 0xADDRESS: run_builtin (git.c:461)
> > ==PID==    by 0xADDRESS: handle_builtin (git.c:713)
> > ==PID==    by 0xADDRESS: run_argv (git.c:780)
> > ==PID==    by 0xADDRESS: cmd_main (git.c:911)
> > ==PID==    by 0xADDRESS: main (common-main.c:52)
> >
> > Reported-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
> > Signed-off-by: Elijah Newren <newren@gmail.com>
> > ---
> >  merge-ort.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/merge-ort.c b/merge-ort.c
> > index d85b1cd99e9..4f5abc558c5 100644
> > --- a/merge-ort.c
> > +++ b/merge-ort.c
> > @@ -3175,7 +3175,7 @@ simple_cleanup:
> >               free(renames->pairs[s].queue);
> >               DIFF_QUEUE_CLEAR(&renames->pairs[s]);
> >       }
> > -     if (combined.nr) {
> > +     if (combined.alloc) {
> >               int i;
> >               for (i = 0; i < combined.nr; i++)
> >                       pool_diff_free_filepair(&opt->priv->pool,
>
> This looks correct in that it'll work, but I still don't get why the
> pre-image or post-image is going through this indirection of checking
> "alloc" at all. Wouldn't this be correct & more straightforward (the
> memset part is optional, just did it for good measure)?:
>
> diff --git a/merge-ort.c b/merge-ort.c
> index 40ae4dc4e92..a01f28586a1 100644
> --- a/merge-ort.c
> +++ b/merge-ort.c
> @@ -3092,12 +3092,12 @@ static int detect_and_process_renames(struct merge_options *opt,
>                                       struct tree *side1,
>                                       struct tree *side2)
>  {
> -       struct diff_queue_struct combined;
> +       struct diff_queue_struct combined = { 0 };
>         struct rename_info *renames = &opt->priv->renames;
>         int need_dir_renames, s, clean = 1;
>         unsigned detection_run = 0;
> +       int i;
>
> -       memset(&combined, 0, sizeof(combined));
>         if (!possible_renames(renames))
>                 goto cleanup;
>
> @@ -3181,13 +3181,10 @@ static int detect_and_process_renames(struct merge_options *opt,
>                 free(renames->pairs[s].queue);
>                 DIFF_QUEUE_CLEAR(&renames->pairs[s]);
>         }
> -       if (combined.alloc) {
> -               int i;
> -               for (i = 0; i < combined.nr; i++)
> -                       pool_diff_free_filepair(&opt->priv->pool,
> -                                               combined.queue[i]);
> -               free(combined.queue);
> -       }
> +
> +       for (i = 0; i < combined.nr; i++)
> +               pool_diff_free_filepair(&opt->priv->pool, combined.queue[i]);
> +       free(combined.queue);
>
>         return clean;
>  }

Hmm, good point; I like your version better.  I'll change it, thanks.