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Re: Finer timestamps and serialization in git




On 5/15/2019 3:16 PM, Eric S. Raymond wrote:
> The deeper problem is that I want something from Git that I cannot
> have with 1-second granularity. That is: a unique timestamp on each
> commit in a repository.

This is impossible in a distributed version control system like Git
(where the commits are immutable). No matter your precision, there is
a chance that two machiens commit at the exact same moment on two different
machines and then those commits are merged into the same branch. Even
when you specify a committer, there are many environments where a set
of parallel machines are creating commits with the same identity.

> Why do I want this? There are number of reasons, all related to a
> mathematical concept called "total ordering".  At present, commits in
> a Git repository only have partial ordering. 

This is true of any directed acyclic graph. If you want a total ordering
that is completely unambiguous, then you should think about maintaining
a linear commit history by requiring rebasing instead of merging.

> One consequence is that
> action stamps - the committer/date pairs I use as VCS-independent commit
> identifications in reposurgeon - are not unique.  When a patch sequence
> is applied, it can easily happen fast enough to give several successive
> commits the same committer-ID and timestamp.

Sorting by committer/date pairs sounds like an unhelpful idea, as that
does not take any graph topology into account. It happens that commits
can actually have an _earlier_ commit date than its parent.

> More deeply, the lack of total ordering means that repository graphs
> don't have a single canonical serialized form.  This sounds abstract
> but it means there are surgical operations I can't regression-test
> properly.  My colleague Edward Cree has found cases where git fast-export
> can issue a stream dump for which git fast-import won't necessarily
> re-color certain interior nodes the same way when it's read back in
> and I'm pretty sure the absence of total ordering on the branch tips
> is at the bottom of that.

If you use `git rev-list --topo-order` with a fixed set of refs to start,
then the total ordering given is well-defined (and it is a linear
extension of the partial order given by the commit graph). However, this
ordering is not stable: adding another merge commit may swap the order between
two commits lower in the order.

> I'm willing to write patches if this direction is accepted.  I've figured
> out how to make fast-import streams upward-compatible with finer-grained
> timestamps.

Changing the granularity of timestamps requires changing the commit format,
which is probably a non-starter. More universally-useful suggestions have
been blocked due to keeping the file format consistent.

Thanks,
-Stolee