Subject: Re: [PATCH v3] diffcore-break: use a goto instead of a redundant if statement

From: Junio C Hamano <[email-masked]>
Date: Mon, 30 Sep 2019 10:14:31 +0900

CB Bailey <[email-masked]> writes:

> For easier discussion, I've snipped the original patch and replaced with
> one with enough context to show the entire function.
>
> I was reviewing this patch and it appeared to introduce a change in
> behaviour.
>
>> diff --git a/diffcore-break.c b/diffcore-break.c
>> index 875aefd3fe..f6ab74141b 100644
>> --- a/diffcore-break.c
>> +++ b/diffcore-break.c
>> @@ -262,44 +262,43 @@ static void merge_broken(struct diff_filepair *p,
>> 
>>  void diffcore_merge_broken(void)
>>  {
>>  	struct diff_queue_struct *q = &diff_queued_diff;
>>  	struct diff_queue_struct outq;
>>  	int i, j;
>> 
>>  	DIFF_QUEUE_CLEAR(&outq);
>> 
>>  	for (i = 0; i < q->nr; i++) {
>>  		struct diff_filepair *p = q->queue[i];
>>  		if (!p)
>>  			/* we already merged this with its peer */
>>  			continue;
>>  		else if (p->broken_pair &&
>>  			 !strcmp(p->one->path, p->two->path)) {
>>  			/* If the peer also survived rename/copy, then
>>  			 * we merge them back together.
>>  			 */
>>  			for (j = i + 1; j < q->nr; j++) {
>>  				struct diff_filepair *pp = q->queue[j];
>>  				if (pp->broken_pair &&
>>  				    !strcmp(pp->one->path, pp->two->path) &&
>>  				    !strcmp(p->one->path, pp->two->path)) {
>>  					/* Peer survived.  Merge them */
>>  					merge_broken(p, pp, &outq);
>>  					q->queue[j] = NULL;
>> -					break;
>> +					goto done;
>
> Previously, if the condition matched in the inner loop, the function
> would null out the entry in the queue that that inner loop had reached
> (q->queue[j] = NULL) and then break out of the inner loop. This meant
> that the outer loop would skip over this entry (if (!p)).
>
> The change introduced seems to break out of both loops as soon as we
> reach one match, whereas before other subsequent matches would be
> considered and merged. Not only this, but the outer 'else' case for all
> subsequent entries is skipped so the rest of the entries the original
> queue are missing from 'outq'.

Thanks.

Sometimes judicious use of 'goto' makes the resulting code easier to
follow, but quite honestly, I do not see it happening with this
change.  The original makes it much more clear that there are three
cases to worry about:

    A. an earlier round handled this one already;

    B. we have a broken pair and need to find the other one,
       B-1. if there is, we process it;
       B-2. otherwise we keep it in the outq.

    C. a normal one that does not need the complication of B is
       sent to the outq.

and I find it much easier to follow without any goto.

>
>>  				}
>>  			}
>> -			if (q->nr <= j)
>> -				/* The peer did not survive, so we keep
>> -				 * it in the output.
>> -				 */
>> -				diff_q(&outq, p);
>> +			/* The peer did not survive, so we keep
>> +			 * it in the output.
>> +			 */
>>  		}
>> -		else
>> -			diff_q(&outq, p);
>> +		diff_q(&outq, p);
>>  	}
>> +
>> +done:
>>  	free(q->queue);
>>  	*q = outq;
>> 
>>  	return;
>>  }
>
> I spent a bit of time trying to see if this change was user visible
> which turned out to be unneeded as t4008-diff-break-rewrite.sh already
> fails with this change for me in my environment, initially with this
> test but also 3 other tests in this file.
>
>> expecting success of 4008.6 'run diff with -B (#3)':
>> 	git diff-index -B reference >current &&
>> 	cat >expect <<-EOF &&
>> 	:100644 100644 $blob0_id $blob1_id M100	file0
>> 	:100644 100644 $blob1_id $blob0_id M100	file1
>> 	EOF
>> 	compare_diff_raw expect current
>> 
>> --- .tmp-1	2019-09-29 09:21:07.089070076 +0000
>> +++ .tmp-2	2019-09-29 09:21:07.093070086 +0000
>> @@ -1,2 +1 @@
>>  :100644 100644 548142c327a6790ff8821d67c2ee1eff7a656b52 6ff87c4664981e4397625791c8ea3bbb5f2279a3 M#	file0
>> -:100644 100644 6ff87c4664981e4397625791c8ea3bbb5f2279a3 548142c327a6790ff8821d67c2ee1eff7a656b52 M#	file1
>> not ok 6 - run diff with -B (#3)