Re: [PATCH v3 02/20] commit-graph: fix GRAPH_MIN_SIZE

[Jakub Narebski <jnareb@gmail.com>]

"brian m. carlson" <sandals@crustytoothpaste.net> writes:

> On Sat, May 26, 2018 at 08:46:09PM +0200, Jakub Narebski wrote:
>> One issue: in the future when Git moves to NewHash, it could encounter
>> then both commit-graph files using SHA-1 and using NewHash.  What about
>> GRPH_OID_LEN then: for one of those it would be incorrect.  Unless it is
>> about minimal length of checksum, that is we assume that NewHash would
>> be longer than SHA-1, but ten why name it GRAPH_OID_LEN?
>
> My proposal is that whatever we're using in the .git directory is
> consistent.  If we're using SHA-1 for objects, then everything is SHA-1.
> If we're using NewHash for objects, then all data is stored in NewHash
> (except translation tables and such).  Any conversions between SHA-1 and
> NewHash require a lookup through the standard techniques.
>
> I agree that here it would be more helpful if it were a reference to
> the_hash_algo, and I've applied a patch to my object-id-part14 series to
> make that conversion.

All right, I can agree that it would make most sense to always use SHA-1
for OID, or always use NewHash for objects.  This would make
commit-graph file with SHA-1 hash invalid for NewHash-using Git version.

It would be nice, however, to avoid having to redo all the hard work,
like calculating generation numbers (from old commit-graph file, or from
server that does not support NewHash yet -- the latter is not
implemented, but IIUC planned feature).  But we can do it with explicit
conversion step, e.g. 'git commit-graph convert' or 'upgrade'.

But all that is in the future.
-- 
Jakub Narębski