Subject: Re: http-protocol question

From: Jeff King <[email-masked]>
Date: Tue, 2 Dec 2014 00:33:32 -0500

On Tue, Dec 02, 2014 at 04:04:11PM +1100, Bryan Turner wrote:

> > Can you say more about the context?  For example, was this actually
> > happening, or is this for the sake of understanding the protocol
> > better?
> 
> I ask because it's actually happening. Heavy CI load sometimes has
> builds fail because git clone dies with "not our ref". That's the
> specific context I'm working to try and address right now. Some teams
> use rebase-heavy workflows, which also evades the check_non_tip easing
> and fails with "not our ref", so I can't be 100% certain it's ref
> deletion in specific causing it (but I guess which of those it is is
> probably largely academic; as long as hosting spans multiple requests
> it seems like this type of race condition is unavoidable).

There is a practical reason to care. Ref deletion will also delete the
reflog, leaving no trace of the reachability. Whereas a non-fast-forward
push could be resolved by looking in the reflog.

One problem with hunting for sha1s in the reflog is that upload-pack
does not know which ref the client thinks they are requesting. So a
search would involve looking in _every_ reflog, which could be
expensive. It might not be too painful if you do the search only after
hitting a "not our ref" condition, which should in theory be rare. A
malicious client could convince you to grep your reflogs repeatedly, but
that is hardly the only way to convince upload-pack to chew CPU. Asking
it to make a pack comes to mind. :)

> I'm trying to decide if there is something I can enable or tune to
> prevent it, and the type of twilighting hinted at by the http-protocol
> documentation seemed like it could be just the thing.

For a public repository, it might make sense to provide a config option
to loosen the is_our_ref check completely (i.e., to just has_sha1_file).
But such an option does not yet exist.

-Peff