`Invalid capability index!` when trying to return a capability reference from the server to the client.

[RyanGibb]

I've followed the tutorial in the README, and am trying to adapt it to define a schema that has a Connection.create method which returns a Connection.Stream:

@0xdcaa43aa8c57d4a8;

interface Connection {
  create @0 () -> (stream :Stream);
  interface Stream {
    read @0 () -> (data :Data);
    write @1 (data :Data) -> ();
    close @2 () -> ();
  }
}

My create implementation instantiates a Stream and returns it as a result:

      method create_impl _params release_param_caps =
        let open Connection.Create in
        release_param_caps ();
        let stream = Stream.local in
        let response, results = Service.Response.create Results.init_pointer in
        Results.stream_set results (Some stream);
        Service.return response

The client calls this with:

  let create t =
    let open Api.Client.Connection.Create in
    let request, _params = Capability.Request.create Params.init_pointer in
    Capability.call_for_value_exn t method_id request |> Results.stream_get

However when I try to use the resulting Capability.t stream I get the exception: Invalid capability index!.

I've checked the reference counting, and it appears to be 2 after Results.stream_set results (Some stream);.

The full example can be found here, with the relevant files being:

• main.ml
• pipe.ml
• pipe_api.capnp

The output of which is:

Connection(rc=1)
Stream(rc=1)
Stream(rc=2)
Failed: Invalid capability index!
Failed: Invalid capability index!

Does the capability need to be a sturdy ref in order to be passed to the client?

I'm using the EIO port.

[avsm]

Not had a chance to look at your code yet, but here's an older repository that has a similar setup (client/server+callbacks) that did work a few years ago https://github.com/ocurrent/ocluster/tree/prototype

[RyanGibb]

Thanks Anil, it looks like Capability.call_for_caps was what I needed!