Re: [PATCH 7/9] commit-graph: drop count_distinct_commits() function

> On Fri, Dec 04, 2020 at 03:06:24PM -0500, Derrick Stolee wrote:
> 
>> On 12/4/2020 1:56 PM, Jeff King wrote:
>>> That turns out not to be a problem, though. The only things we do with
>>> the count are:
>>>
>>>   - check if our count will overflow our data structures. But the limit
>>>     there is 2^31 commits, so it's not likely to happen in practice.
>>
>> You do point out that you are removing this logic, done in this
>> if statement:
>>
>>> -	if (count_distinct >= GRAPH_EDGE_LAST_MASK) {
>>> -		error(_("the commit graph format cannot write %d commits"), count_distinct);
>>> -		res = -1;
>>> -		goto cleanup;
>>> -	}
>>
>> What is the harm of keeping this? I know it is very unlikely, but
>> I'd rather fail here than write a commit-graph that gets parsed as
>> garbage.
> 
> I agree it's important to have. But this one is redundant. Look a few
> lines below this hunk, and we have:
> 
> 	copy_oids_to_commits(ctx);
> 
> 	if (ctx->commits.nr >= GRAPH_EDGE_LAST_MASK) {
> 		error(_("too many commits to write graph"));
> 		res = -1;
> 		goto cleanup;
> 	}

> So before we were counting distinct commits, checking that our count
> fits, and then _ignoring_ that count in order to create the actual list
> of commits, and then checking that the actual list's count fits. We only
> need one of those checks, and the important one is the one from the
> actual list (they _should_ match, but due to the bug, they sometimes
> didn't).
> 
> My "not likely to happen in practice" is not about the quality of the
> check, but rather that being off by one would never matter in practice.
> 
> Does that make more sense?