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Re: State of NewHash work, future directions, and discussion




On Mon, Jun 11, 2018 at 12:01:03PM -0700, Jonathan Nieder wrote:
> Hi,
> 
> brian m. carlson wrote:
> > I plan on introducing an array of hash algorithms into struct repository
> > (and wrapper macros) which stores, in order, the output hash, and if
> > used, the additional input hash.
> 
> Interesting.  In principle the four following are separate things:
> 
>  1. Hash to be used for command output to the terminal
>  2. Hash used in pack files
>  3. Additional hashes (beyond (2)) that we can look up using the
>     translation table
>  4. Additional hashes (beyond (1)) accepted in input from the command
>     line and stdin
> 
> In principle, (1) and (4) would be globals, and (2) and (3) would be
> tied to the repository.  I think this is always what Duy was hinting
> at.
> 
> All that said, as long as there is some notion of (1) and (4), I'm
> excited. :)  Details of how they are laid out in memory are less
> important.

I'm happy to hear suggestions on how this should or shouldn't work.  I'm
seeing these things in my head, but it can be helpful to have feedback
about what people expect out of the code before I spend a bunch of time
writing it.

> [...]
> > The transition plan anticipates a stage 1 where accept only SHA-1 on
> > input and produce only SHA-1 on output, but store in NewHash.  As I've
> > worked with our tests, I've realized such an implementation is not
> > entirely possible.  We have various tools that expect to accept invalid
> > object IDs, and obviously there's no way to have those continue to work.
> 
> Can you give an example?  Do you mean commands like "git mktree"?

I mean situations like git update-index.  We allow the user to insert
any old invalid value (and in fact check that the user can do this).
t0000 does this, for example.

> You can always use something like e.g. "doubled SHA-1" as a proof of
> concept, but I agree that it's nice to be able to avoid some churn by
> using an actual hash function that we're likely to switch to.

I have a hash that I've been using, but redoing the work would be less
enjoyable.  I'd rather write the tests only once if I can help it.
-- 
brian m. carlson: Houston, Texas, US
OpenPGP: https://keybase.io/bk2204

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