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

From: Junio C Hamano <gitster@pobox.com>
Date: Mar 23, 2020, 16:04 UTC
Message-ID: <xmqqtv2f5a6x.fsf@gitster.c.googlers.com>
In-Reply-To: <86mu87qj92.fsf@gmail.com>

Jakub Narebski <jnareb@gmail.com> writes:

Show 5 quoted lines

> About moving commit data with generation number v2 to "CDA2" chunk: if
> "CDAT" chunk is missing then (I think) old Git would simply not use
> commit-graph file at all; it may crash, but I don't think so.  If "CDAT"
> chunk has zero length... I don't know what would happen then, possibly
> also old Git would simply not use commit-graph data at all.

Yeah, if it makes it crash, then we cannot use that "missing CDAT" approach.

Show 8 quoted lines

> Putting generation number v2 into separate chunk (which might be called
> "GEN2" or "OFFS"/"DOFF") has the disadvantage of increasing the on disk
> size of the commit graph, and possibly also increasing memory
> consumption (the latter depends on how it would be handled), but has the
> advantage of being fullly backward compatibile.  Old Git would simply
> use generation numbers v1 in "CDAT", new Git would use generation
> numbers v2 in "OFFS" -- combining commit creation date from "CDAT" and
> offset from "OFFS"),

Do we have an option *not* to record meaningful generation numbers in CDAT and have the current Git binaries understand and still use the rest of the graph file, while not using the optimizations that rely on having generation numbers? If not, then the new version of Git that tries to be compatible with old one needs to compute both generation numbers, and we would need to keep the topological number for quite some time.

> and there should be no problems with updating
> commit-graph file (either rewriting, or adding new commit-graph to the
> chain).

Would merging by the current Git also work well (meaning, would "GEN2" or whatever it does not understand be omitted)?

Thanks.