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

From: Ævar Arnfjörð Bjarmason <[email-masked]>
Date: Sat, 19 Feb 2022 22:44:19 +0100

On Sat, Feb 19 2022, Elijah Newren via GitGitGadget wrote:

> From: Elijah Newren <[email-masked]>
>
> 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 <[email-masked]>
> Signed-off-by: Elijah Newren <[email-masked]>
> ---
>  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;
 }