Re: [PATCH 02/17] pack-mtimes: support reading .mtimes files

From: brian m. carlson <sandals@crustytoothpaste.net>
Date: Dec 2, 2021, 22:32 UTC
Message-ID: <YalJgGJFoGGgCazx@camp.crustytoothpaste.net>
In-Reply-To: <ef10c824-e2d9-f113-f010-6a1ac307427a@gmail.com>

On 2021-12-02 at 15:06:07, Derrick Stolee wrote:

Show 19 quoted lines

> On 11/29/2021 5:25 PM, Taylor Blau wrote:
> 
> > +== pack-*.mtimes files have the format:
> > +
> > +  - A 4-byte magic number '0x4d544d45' ('MTME').
> > +
> > +  - A 4-byte version identifier (= 1).
> > +
> > +  - A 4-byte hash function identifier (= 1 for SHA-1, 2 for SHA-256).
> 
> I vaguely remember complaints about using a 1-byte identifier in
> the commit-graph and multi-pack-index formats because the "standard"
> way to refer to these hash functions was a magic number that had a
> meaning in ASCII that helped human readers a bit. I cannot find an
> example of such 4-byte identifiers, but perhaps brian (CC'd) could
> remind us.
> 
> You are using a 4-byte identifier, but using the same values as
> those 1-byte identifiers.

The preferred value is the_hash_algo->format_id. For SHA-1, that's "sha1", big-endian (0x73686131) and for SHA-256 it's "s256", big-endian (0x73323536).

There's also hash_algo_by_id to turn the format ID into an index into the hash_algos array, but you need to check for GIT_HASH_UNKNOWN (0) first.

These will be used in index v3, which I haven't sent out patches for yet.

-- 
brian m. carlson (he/him or they/them)
Toronto, Ontario, CA