Re: topological index field for commit objects

[Junio C Hamano <gitster@pobox.com>]

Stefan Beller <sbeller@google.com> writes:

> On Wed, Jun 29, 2016 at 11:59 AM, Junio C Hamano <gitster@pobox.com> wrote:
>> On Wed, Jun 29, 2016 at 11:31 AM, Marc Strapetz
>> <marc.strapetz@syntevo.com> wrote:
>>> This is no RFE but rather recurring thoughts whenever I'm working with
>>> commit graphs: a topological index attribute for commit objects would be
>>> incredible useful. By "topological index" I mean a simple integer for which
>>> following condition holds true:
>>
>> Look for "generation numbers" in the list archive, perhaps?
>
> Thanks for the pointer to the interesting discussions.
>
> In http://www.spinics.net/lists/git/msg161363.html
> Linus wrote in a discussion with Jeff:
>
>> Right now, we do *have* a "generation number". It's just that it's
>> very easy to corrupt even by mistake. It's called "committer date". We
>> could improve on it.
>
> Would it make sense to refuse creating commits that have a commit date
> prior to its parents commit date (except when the user gives a
> `--dammit-I-know-I-break-a-wildy-used-heuristic`)?

I think that has also been discussed in the past.  I do not think it
would help very much in practice, as projects already have up to 10
years (and the ones migrated from CVS, even more) worth of commits
they cannot rewrite that may record incorrect committer dates.
You'd need something like "you can trust committer dates that are
newer that this date" per project to switch between slow path and
fast path, with an updated fsck that knows how to compute that
number after you pulled from somebody who used that overriding
option.

If the use of generation number can somehow be limited narrowly, we
may be able to incrementally introduce it only for new commits, but
I haven't thought things through, so let me do so aloud here ;-)

Suppose we use it only for this purpose:

 * When we have two commits, C1 and C2, with generation numbers G1
   and G2, we can say "C1 cannot possibly be an ancestor of C2" if
   G1 > G2.  We cannot say anything else based on generation
   numbers (or lack thereof).

then I think we could just say "A newly created commit must record
generation number G that is larger than generation numbers of its
parent commits; ignore parents that lack generation number for the
purpose of this sentence".

I am not sure if that limited use is all that useful, though.