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
