Re: [RFC][GSoC] Implement Generation Number v2

[Abhishek Kumar <abhishekkumar8222@gmail.com>]

On Sun, Mar 22, 2020 at 10:32:32PM -0700, Junio C Hamano wrote:
> Abhishek Kumar <abhishekkumar8222@gmail.com> writes:
> 
> >> Unfortunately for the time being we cannot use commit-graph format
> >> version; the idea that was proposed on the mailing list (when we found
> >> about the bug in handling commit-graph versioning, during incremental
> >> commit-graph implementation), was to create and use metadata chunk or
> >> versioning chunk (the final version of incremental format do not use
> >> this mechanism).  This could be used by gen2 compatibile Git to
> >> distinguish between situation where old commit-graph file to be updated
> >> uses generation number v1, and when it uses v2.
> >> 
> >> If you have a better idea, please say so.
> >
> > We could also use a flag file. Here's how it works:
> >
> > If the file `.git/info/generation-number-v2` exists, use gen2.
> > Otherwise use gen1.
> 
> If the file is lost then we will try to read the other file that has
> the commit-graph data as if it were in old format?  And if such a
> file was created (say, with "touch .git/info/generation-number-v2"),
> a file in the original format will be read as if it is in new
> format?  If that is the case, it is likely that we'd see a segfault;
> sounds too brittle to me.

Agreed. Flag file has too many issues to be pursued further.

> 
> It appears that the format of "CDAT", and the fact that generation
> is represented as higher 30-bit of a be32 integer, is very much
> hardcoded in the design and is hard to change, but your new version
> of graph file can be designed not to use "CDAT" chunk at all, and
> instead have the commit data with new version of generation numbers
> stored in a different chunk (say "CDA2") to force older version of
> Git not to use the new graph file---would that work?

A commit-graph without "CDAT" chunk will hard fail on older versions of
Git. verify_commit_graph_lite() errors out if chunk_commit_data is null.

Metadata chunk seems the way to go.

Regards
Abhishek