Re: [PATCH 18/41] index-pack: abstract away hash function constant

>> I agree that pack v2 is not going to have anything but SHA-1.  However,
>> writing all the code such that it's algorithm agnostic means that we can
>> do testing of new algorithms by wholesale replacing the algorithm with a
>> new one, which simplifies things considerably.
>
> Ok. I do sort of wonder if a "successful" test run after globally
> substituting Hash-Foo for SHA-1 (regardless of whether the size changes
> or not) hints at a problem. That is, nowhere do we test that this code
> uses 20-byte SHA-1s, regardless of what other hash functions are
> available and configured. Of course, until soon, that did not really
> have to be tested since there was only one hash function available to
> choose from. As for identifying all the places that matter ... no idea.
>
> Of course I can see how this helps get things to a point where Git does
> not crash and burn because the hash has a different size, and where the
> test suite doesn't spew failures because the initial chaining value of
> "SHA-1" is changed.
>
> Once that is accomplished, I sort of suspect that this code will want to
> be updated to not always blindly use the_hash_algo, but to always work
> with SHA-1 sizes. Or rather, this would turn into more generic code to
> handle both "v2 with SHA-1" and "v3 with some hash function(s)". This
> commit might be a good first step in that direction.