Re: Git commit generation numbers

On 21/07/2011 5:24 PM, Phil Hord wrote:

Maybe the confusion comes from the different storage mechanisms being
discussed. If the generation numbers are in a local cache and used by a
single client, the determinism of the specific numbers doesn't much
matter. If they are part of the commit, it still doesn't need to be
completely deterministic. However, interoperability requires standards,
and standards favor determinism, so dogmatic determinism may triumph in
that case.

1. gen(06) might make sense if you mean to implement --date-order using
gen-numbers, for example. But I don't think it's practical in any case.

2. gen(06)+1 might make sense if you mean to require that gen-numbers
are unique per repo. But this is both unsupportable and unnecessary, so
it's a non-starter.

3. gen(B)+1 is what you'd get from the the algorithm I saw proposed.

All three of these are provably correct by my definition of "correct":
"for each A in ancestors_of(B), gen(A) < gen(B)".

However, [1] and [2] have some extra features of dubious value. Simpler
is better for interoperability, so I like [3] for this purpose.

Even [3] has an extra feature I think is unnecessary: determinism. If
that "requirement" is dropped, I think all three of these algorithms are
(functionally) roughly equivalent.

quoted

I don't think everybody MEANT to be saying such
different things--that's just how they appeared on this end.

Now, did you mean something different by "commit number?"

I remain unconvinced that there is value in gen-number distribution, so
to my mind, the specific algorithm and whether or not it is
deterministic are unimportant.

The beauty of Git is that no two copies of a Git repository as a whole are the same:  some people make shallow copies;  others prune away all branches except for the one they are interested in;  yet others graft together multiple original repositories.  The upshot is that two copies of the same repository may end up having different commits as their root commits, and so the generation numbers computed for their repositories would be different.  Indeed, the shallow repository copy could later be filled out with additional underlying commits, and so on.

Given this context, I can't see the value in fixing generation numbers within commits.  In my mind generation numbers are extremely useful transient helper objects in every Git repository but they have no meaning outside that repository, sort of like GIT_WORK_TREE.

Peter