Re: Git commit generation numbers

From: Nicolas Pitre <nico@fluxnic.net>
Date: Jul 20, 2011, 20:51 UTC
Message-ID: <alpine.LFD.2.00.1107201538590.21187@xanadu.home>
In-Reply-To: <20110718114834.12406.qmail@science.horizon.com>

On Mon, 18 Jul 2011, George Spelvin wrote:

Show 7 quoted lines

> Storing the generation number inside the commit means that a commit
> with a generation number has a different hash than a commit without one.
> This means that people won't want to break the hashes of existing commits
> by adding them.  In many cases, ever.
> 
> Which means that git will have to be able to work without the generation
> numbers forever.

I've been diverting myself from $day_job by reading through this thread. Still, I couldn't make my mind between having the generation number stored in the commit object or in a separate cache by reading all the arguments for each until now. Admittedly I'm not as involved in the design of Git as I once was, so my comments can be considered with the same proportions.

Obviously, with a perfect design, we would have had gen numbers from the beginning. But we did mistakes, and now have to regret and live with them (and yes I have my own share of responsibility for some of those regrets which are now embodied in the Git data format).

Show 5 quoted lines

> If the generation numbers are stored in a separate data structure that
> can be added to an existing repository, then a new version of git can
> do that when needed.  Which lets git depend on always having the the
> generation numbers to do all history walking and stop using commit date
> based heuristics completely.

To me this is the killer argument. Being able to forget about the broken date heuristics entirely and simplify the code is what makes the external cache so fundamentally better as it can be applied to any existing repositories. And it has no backward compatibility issues as old Git version won't work any worse if they can't make any usage of that cache.

The alternative of having to sometimes use the generation number, sometimes use the possibly broken commit date, makes for much more complicated code that has to be maintained forever. Having a solution that starts working only after a certain point in history doesn't look eleguant to me at all. It is not like having different pack formats where back and forth conversions can be made for the _entire_ history.

And if you don't care about graft/replace then the cached data is immutable just like the in-commit version would, so there is no consistency issues. If you do care about graft/replace (or who knows what other dag alteration scheme might be created in 5 years from now) then a separate cache will be required _anyway_, regardless of any in-commit gen number.

So to say that if a generation number is _really_ needed, then it should go in a separate cache. Saying that if we would have done it initially then it would have been inside the commit object is not a good enough justification to do it today if it can't be applied to the whole of already existing repositories and avoid special cases.

I however have not formed any opinion on that fundamental question i.e. whether or not gen numbers are worth it in today's conditions. Neither did I think about the actual cache format (I don't think that adding it to the pack index is a good idea if grafts are to be honored) which certainly has bearing on that fundamental question too.

But I don't see the point of starting to add them now to commit objects, even if we regret not doing it initially, simply because having them appear randomly based on the Git version/implementation being used is still much uglier than some ad hoc cache or even not having them at all.

Nicolas