Re: Lockless Refs?

[Martin Fick <mfick@codeaurora.org>]

Jeff King <peff@peff.net> wrote:

>On Fri, Dec 28, 2012 at 09:15:52AM -0800, Junio C Hamano wrote:
>
>> Martin Fick <mfick@codeaurora.org> writes:
>> 
>> > Hmm, actually I believe that with a small modification to the 
>> > semantics described here it would be possible to make multi 
>> > repo/branch commits work....
>> >
>> > Shawn talked about adding multi repo/branch transaction 
>> > semantics to jgit, this might be something that git wants to 
>> > support also at some point?
>> 
>> Shawn may have talked about it and you may have listened to it, but
>> others wouldn't have any idea what kind of "multi repo/branch
>> transaction" you are talking about.  Is it about "I want to push
>> this ref to that repo and push this other ref to that other repo",
>> in what situation will it be used/useful, what are the failure
>> modes, what are failure tolerances by the expected use cases, ...?
>> 
>> Care to explain?
>
>I cannot speak for Martin, but I am assuming the point is to atomically
>update 2 (or more) refs on the same repo. That is, if I have a branch
>"refs/heads/foo" and a ref pointing to meta-information (say, notes
>about commits in foo, in "refs/notes/meta/foo"), I would want to "git
>push" them, and only update them if _both_ will succeed, and otherwise
>fail and update nothing.

My use case was cross repo/branch dependencies in Gerrit (which do not yet exist). Users want to be able to define several changes (destined for different project/branches) which can only be merged together.  If one change cannot be merged, the others should fail too.  The solutions we can think of generally need to hold ref locks while acquiring more ref locks, this drastically increases the opportunities for stale locks over the simple "lock, check, update, unlock" mode which git locks are currently used for.

I was perhaps making too big of a leap to assume that there would be other non Gerrit uses cases for this?  I assumed that other git projects which are spread across several git repos would need this? But maybe this simply wouldn't be practical with other git server solutions?

-Martin

Employee of Qualcomm Innovation Center,Inc. which is a member of Code Aurora Forum