Re: Revision resolution for remote-helpers?

[Mike Hommey <mh@glandium.org>]

On Fri, Aug 18, 2017 at 08:15:09AM -0400, Jeff King wrote:
> On Fri, Aug 18, 2017 at 03:42:08PM +0900, Mike Hommey wrote:
> 
> > I was thinking it could be useful to have a special syntax for revisions
> > that would query a helper program. The helper program could use a
> > similar protocol to that of the remote helpers.
> 
> That sounds like a reasonable thing to want.
> 
> > My thought is that a string like <helper>::<revision> could be used
> > wherever a committish is expected. That would call some helper
> > and request to resolve revision, and the helper would provide a git
> > commit as a response.
> 
> So I'm guessing this would look something like:
> 
>   git log svn::12345
> 
> I think even without Git support, you could do something like:
> 
>   git log $(git svn map 12345)

That's what I do, but subshells and all is extra cumbersome.

> which is similarly complex in terms of concepts, and not too many more
> characters. That would be a little more awkward outside of a shell,
> though.
> 
> But it did get me wondering if we could do _better_ with something built
> into Git. For example, could we have an external resolution helper that
> resolves names to object ids as a fallback after internal resolution has
> failed. And then you could do:
> 
>  git log 12345
> 
> and it would just work. Efficiency shouldn't be a big problem, because
> we'd hit the helper only in the error case.
> 
> I'd be more concerned about awkward ambiguities, though. If mercurial is
> also using sha1s, then there's nothing syntactic to differentiate the
> two. For that matter, 12345 has the same problem, since it could be a
> partial sha1.

For something as short, the likelihood of hitting an actual existing
abbreviated sha1 is quite high, too.

> It might work to actually check if we have the object and then bail
> to the remote resolver only if we don't. But that's actually conflating
> name resolution with object lookup, which our internals typically keep
> separate.
> 
> So maybe this is a bad direction to go in. I'm mostly just thinking out
> loud here.
> 
> > Which leads me to think some "virtual" ref namespace could be a solution
> > to the problem. So instead of <helper>::, the prefix would be <helper>/.
> > For e.g. svn, svn/$remote/$rev would be a natural way to specify the
> > revision for a given remote. For mercurial, hg/$sha1.
> 
> Interesting. I do like the general idea of having external helpers to
> fill in bits of the virtual namespace. But it may also open many cans of
> worms. :)
> 
> > Potentially, this could be a sort of pluggable ref stores, which could
> > be used for extensions such as the currently discussed reftable.
> 
> The current pluggable code is definitely geared for build-time
> pluggability, not runtime. But I think you could have a builtin
> pluggable store that does the overlay, and then chains to another
> backend. I.e., configure something like:
> 
>   [extensions]
>   refBackend = externalOverlay
> 
>   [externalOverlay "svn"]
>   namespace = refs/svn
>   command = my-svn-mapper
> 
>   [externalOverlay]
>   chain = reftable
> 
> That would allow the externalOverlay thing to develop independent of the
> core of Git's refs code.

That's a lot of configuration, but it's definitely an interesting
proposition.

> > On the opposite end of the problem, I'm also thinking about git log
> > --decorate=<helper> displaying the mercurial revisions where branch
> > decorations would normally go.
> 
> Interesting thought. I'm not sure if that would be a good thing or a bad
> thing. But one of the virtual methods for pluggable backends is
> "enumerate all refs". If you're mapping every mercurial revision, that's
> going to be a lot of refs (and potentially a lot of overhead for certain
> operations).
> 
> I think the decorate code just looks at a few parts of the refs
> namespace right now (so a "refs/svn" would probably get ignored by
> default).

I think decorate would need its own special entry to the "ref query" API
to answer the question "what ref points to <sha1>" instead of scanning
the whole namespace.

Mike