Re: [RFC] Generation Number v2

From: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Date: Oct 31, 2018, 12:54 UTC
Message-ID: <875zxil1if.fsf@evledraar.gmail.com>
In-Reply-To: <xmqqy3ag13us.fsf@gitster-ct.c.googlers.com>

On Tue, Oct 30 2018, Junio C Hamano wrote:

Show 19 quoted lines

> Derrick Stolee <stolee@gmail.com> writes:
>> In contrast, maximum generation numbers and corrected commit
>> dates both performed quite well. They are frequently the top
>> two performing indexes, and rarely significantly different.
>>
>> The trade-off here now seems to be: which _property_ is more important,
>> locally-computable or backwards-compatible?
>
> Nice summary.
>
> As I already said, I personally do not think being compatible with
> currently deployed clients is important at all (primarily because I
> still consider the whole thing experimental), and there is a clear
> way forward once we correct the mistake of not having a version
> number in the file format that tells the updated clients to ignore
> the generation numbers.  For longer term viability, we should pick
> something that is immutable, reproducible, computable with minimum
> input---all of which would lead to being incrementally computable, I
> would think.

I think it depends on what we mean by backwards compatibility. None of our docs are saying this is experimental right now, just that it's opt-in like so many other git-config(1) options.

So if we mean breaking backwards compatibility in that we'll write a new file or clobber the existing one with a version older clients can't use as an optimization, fine.

But it would be bad to produce a hard error on older clients, but avoiding that seems as easy as just creating a "commit-graph2" file in .git/objects/info/.