Irmin graphql subscriptions (take 2)

[andreas]

The PR description of andreas/ocaml-graphql-server#141 unintendedly closed #598 with no option of reopening, so this PR is a copy of #598 😒

This PR adds the ability to watch the entire store or a specific key with notifications delivered via the GraphQL subscription mechanism (currently only websockets, possibly Server-Sent Events or chunked HTTP responses in the future).

Adding Irmin_unix.set_listen_dir_hook () to cli.ml may be required...

[andreas]

I'm not quite sure why Appveyor cannot find graphql-cohttp.0.10.0, but it works in Travis. Maybe the OPAM repo is cached somehow?

Otherwise, this PR should be ready for review.

[samoht]

Any idea why the irmin-git tests are failing ?

[zshipko]

I think the irmin-git test failure is actually unrelated to this PR. I'm getting the same failure as the one on travis-ci when running tests for the code in mirage/irmin#master without any modifications. However, the failure seems to be kind of inconsistent, so I haven't really been able to debug it.

[zshipko]

This looks great!

One question since I'm not all that familiar with how subscriptions work: it looks like unwatch will be called automatically when the client unsubscribes, but how does the client send the actual unsubscribe request?

[andreas]

One question since I'm not all that familiar with how subscriptions work: it looks like unwatch will be called automatically when the client unsubscribes, but how does the client send the actual unsubscribe request?

The client sends a GQL_STOP message (doc) over the websocket, which will terminate the subscription -- this triggers a call to unwatch.

[andreas]

Any idea why the irmin-git tests are failing ?

I'm not sure either. It's green now without any relevant changes on my part. I did rebase on master though.

[zshipko]

Any idea why the irmin-git tests are failing ?

I'm not sure either. It's green now without any relevant changes on my part. I did rebase on master though.

Yeah, I'm not sure what's going on with the tests 😕