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Re: Finer timestamps and serialization in git




On 5/15/2019 7:32 PM, Eric S. Raymond wrote:
> Derrick Stolee <stolee@xxxxxxxxx>:
>> 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.
> 
> It's easy to work around that problem. Each git daemon has to single-thread
> its handling of incoming commits at some level, because you need a lock on the
> file system to guarantee consistent updates to it.
> 
> So if a commit comes in that would be the same as the date of the
> previous commit on the current branch, you bump the incoming commit timestamp.

This changes the commit, causing it to have a different object id, and
now the client that pushed that commit disagrees with your machine on
the history.

> That's the simple case. The complicated case is checking for date
> collisions on *other* branches. But there are ways to make that fast,
> too. There's a very obvious one involving a presort that is is O(log2
> n) in the number of commits.
> 
> I wouldn't have brought this up in the first place if I didn't have a
> pretty clear idea how to do it in code!
> 
>> Even when you specify a committer, there are many environments where a set
>> of parallel machines are creating commits with the same identity.
> 
> If those commit sets become the same commit in the final graph, this is
> not a problem for total ordering.
> 
>>> 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.
> 
> Excuse me, but your premise is incorrect.  A git DAG isn't just "any" DAG.
> The presence of timestamps makes a total ordering possible.
> 
> (I was a theoretical mathematician in a former life. This is all very
> familiar ground to me.)

Same. But you seem to have a fundamental misunderstanding about the immutability
of commits, which is core to how Git works. If you change a commit, then you
get a new object id and now distributed copies don't agree on the history.

>>> 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.
> 
> Yes, I'm aware of that.  The uniqueness properties that make a total
> ordering desirable are not actually dependent on timestamp order
> coinciding with topo order.
> 
>> Changing the granularity of timestamps requires changing the commit format,
>> which is probably a non-starter.
> 
> That's why I started by noting that you're going to have to break the
> format anyway to move to an ECDSA hash (or whatever you end up using).
> 
> I'm saying that *since you'll need to do that anyway*, it's a good time
> to think about making timestamps finer-grained and unique.

That change is difficult enough as it is. I don't think your goals justify
making this more complicated. You are also not considering:

 * The in-memory data type now needs to be a floating-point type, or an
   even larger integer type using a different set of units.

 * This data type now affects our priority queues for commit walks, how
   we store the commit date in the commit-graph file, how we compute
   relative dates for 'git log' pretty formats.

-Stolee