Re: Are clone/checkout operations deterministic?

[Stefan Beller <sbeller@google.com>]

On Tue, Jul 17, 2018 at 2:48 AM Ævar Arnfjörð Bjarmason
<avarab@gmail.com> wrote:
>
>
> On Tue, Jul 17 2018, J. Paul Reed wrote:
>
> > Hey Git Devs,
> >
> > I have a bit of an odd question: do git clone/checkout operations have a
> > deterministic ordering?
> >
> > That is: are files guaranteed to be laid down onto disk in any specific
> > (and deterministic) order as a clone and/or checkout operations occurs?
> > (And if so, is it alphabetical order? Depth-first? Something else?)
> >
> > In case the answer is different (and I'd guess that it might be?), I'm
> > mostly interested in the initial clone case... but it would be great to
> > know if, indeed, the answer is different for just-checkouts too.
> >
> > I did some cursory googling, but nothing hopped out at me as an answer to
> > this question.
>
> In practice I think clone, checkout, reset etc. always work in the same
> order you see with `git ls-tree -r --name-only HEAD`, but as far as I
> know this has never been guaranteed or documented, and shouldn't be
> relied on.

The transmission of packfiles is non-deterministic, as the packfiles
(which are packed for each clone separately when using core git as
a server) are not packed in a deterministic fashion, but in a threaded
environment which allows different packing orders.

If you clone from a server that gives you exactly the same pack at
all times (assuming the remote repo doesn't change refs), then
checkout is currently deterministic in unpacking files to the working tree.

>
> E.g. there's probably cases where writing files in parallel is going to
> be faster than writing them sequentially. We don't have such a mode just
> because nobody's written a patch for it, but having that patch would
> break any assumptions of our current order.

+cc Ben who is looking into that, but hasn't spoken up on the mailing list yet.