Re: Git commit generation numbers

From: Phil Hord <hordp@cisco.com>
Date: Jul 21, 2011, 00:47 UTC
Message-ID: <4E27772B.60306@cisco.com>
In-Reply-To: <CAJo=hJuS_iYSS8iVWoJ1BiUANsGtYJoYm-WRa863isVNsq=5vw@mail.gmail.com>

On 07/20/2011 08:37 PM, Shawn Pearce wrote:

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> On Wed, Jul 20, 2011 at 17:18,<david@lang.hm>  wrote:
>> if it's just locally generated, then I could easily see generation numbers
>> being different on different people's ssstems, dependin on the order that
>> they see commits (either locally generated or pulled from others)
> But this should only happen if the user fudges with their Git sources
> and makes Git produce a different generation number.
>
> If the algorithm is always "gen(A) = max(gen(P) for each parent_of(A))
> + 1" then it doesn't matter who merged what commits, the same commit
> appears at the same part of the graph relative to all of its
> ancestors, and therefore always has the same generation number. This
> is true whether or not the commit contains the generation number.

Interesting. I was going to disagree with the latter part of your statement, but then I realized you're right.

And that your algorithm allows duplicate generation numbers.

And that there's nothing wrong with that.

Because it meets the one quasi-essential need, "for each A in ancestors_of(B), gen(A) < gen(B)".

Show 10 quoted lines

>> If it's part of the commit, then as that commit gets propogated the
>> generation number gets propogated as well, and every repository will agree
>> on what the generation number is for any commit that's shared.
> This isn't really as beneficial as you are making it out to be. We
> already can agree on what the generation number should be for any
> given commit, if you topo-sort the commit DAG, you get the same
> result.
>
>> I agree that this consistancy guarantee seems to be valuable.
> Its valuable, but its consistent either with a cache, or not.

I still fail to see the value.

Phil