Re: [PATCH 2/2] diff-lib: skip paths outside prefix in oneway_diff()

Jeff King <[email protected]> writes:

>> --- a/diff-lib.c
>> +++ b/diff-lib.c
>> @@ -528,6 +528,11 @@ static int oneway_diff(const struct cache_entry * const *src,
>>  	if (tree == o->df_conflict_entry)
>>  		tree = NULL;
>>  
>> +	if (revs->diffopt.prefix &&
>> +	    strncmp((idx ? idx : tree)->name, revs->diffopt.prefix,
>> +		    revs->diffopt.prefix_length))
>> +		return 0;
>> +
>
> BTW, Coverity complains here that "tree" could be NULL (because we set
> it that way in the lines above due to a D/F conflict).
>
> I _think_ it is fine. We only look at "tree" if idx is NULL, and I think
> idx is only NULL when we have a deletion. So that implies either:
>
>   1. unpack_trees() passed us both entries as NULL, which doesn't make
>      sense. There was no entry to delete!
>
>   2. We set tree to NULL due to a D/F conflict. But a conflict with
>      what? There is nothing at the path in the index to conflict.
>
> So AFAICT this is OK and it's just a false positive from Coverity
> (though an understandable one; the semantics of the relationship between
> "idx" and "tree" are not represented in the code).

Yeah, I agree with all of the above.

> Possibly adding:
>
>   if (!idx && !tree)
> 	BUG("oneway diff with no endpoints");
>
> would help static analysis, but I don't know if that makes things more
> or less clear to a human.

We could help humans that the BUG is not expected to fire and only
to help static analysis by a crafted message, perhaps?

   if (!idx && !tree)
 	BUG("Hey, Coverity, this does not happen");

?