Re: [PATCH v2 3/4] Integrate hash algorithm support with repo setup

> Is the plan to allow running with multiple hash algorithms in
> parallel?  I thought what we want to see in the future codebase was
> to have the default hash algorithm used for everything except for a
> select few codepaths, and assumed that the way we achieve it is to
> 
>  - allow very low level helper functions (e.g. read_sha1_file(),
>    write_sha1_file_prepare(), etc.) to take a pointer to the hash
>    algorithm structure;
> 
>  - have higher level helper functions to call these low level
>    helpers with a fixed singleton instance of hash algorithm
>    structure that represents that default one (SHA2-something?); and
> 
>  - a few selected codepaths (e.g. index-pack that reads SHA-1 stream
>    and converts it into NewHash pack/index while building the object
>    name mapping) use an additional singleton instance of hash
>    algorithm structure that represents the SHA-1 hash, and the way
>    they use it is *NOT* by replacing "the current" one with SHA-1,
>    but by explicitly passing the instance to the low level helpers
>    as parameter.

> So, "current" does indeed sound quite wrong, as it makes it sound as
> if you can swap it anytime and ask "which one is in effect now?".
> If we do not want to call the default instance "SHA-2" because we
> want to prepare for migrating again by not having the name of a
> concrete hash algorithm sprinkled in the codebase all over like we
> currently do, why not call the instance "the_hash_algo"?