Fix "method too large" error

[cb372]

Refactors the macro-generated code to avoid generating giant method bodies.

Before the fix, the @service macro would generate the following code for the bindService method:

def bindService(...) =
  higherkindness.mu.rpc.internal.service.GRPCServiceDefBuilder.build[F](
    "fully.qualified.ServiceName",
    (FooMethodDescriptor, io.grpc.stub.ServerCalls.asyncUnaryCall(higherkindness.mu.rpc.internal.server.unaryCalls.unaryMethod(algebra.foo, None))),
    (BarMethodDescriptor, io.grpc.stub.ServerCalls.asyncUnaryCall(higherkindness.mu.rpc.internal.server.unaryCalls.unaryMethod(algebra.bar, None))),
    ...
  )

One tuple is added for each endpoint Foo, Bar, ... in the service.

The problem is that FooMethodDescriptor is a call to a (macro-generated) method which looks like:

def FooMethodDescriptor(implicit
  ReqM: io.grpc.MethodDescriptor.Marshaller[FooRequest],
  RespM: io.grpc.MethodDescriptor.Marshaller[FooResponse]
): ...

When the macro-generated code is compiled, the implicits are expanded. Because these marshallers are auto-derived using avro4s or PBDirect, the resulting reified code can be huge, and that huge chunk of bytecode ends up inside the body of bindService. This can lead to Method too large errors when emitting the bytecode.

Fixed by refactoring the generated code. It now looks like:

object MyService {

  object FooMethodDescriptor {
    def methodDescriptor(implicit reqM: ..., respM: ...): ...
    val _methodDescriptor = methodDescriptor
  }

  object BarMethodDescriptor {
    def methodDescriptor(implicit reqM: ..., respM: ...): ...
    val _methodDescriptor = methodDescriptor
  }

  ...

  def bindService(...) =
    higherkindness.mu.rpc.internal.service.GRPCServiceDefBuilder.build[F](
      "fully.qualified.ServiceName",
      (FooMethodDescriptor._methodDescriptor, ...),
      (BarMethodDescriptor._methodDescriptor, ...),
      ...
    )

}

This way the huge chunks of reified code end up inside the static initializers of the MyService$FooMethodDescriptor$ and MyService$BarMethodDescriptor$ classes instead of inside the body of bindService.

It's still possible to run into the Method too large error if the models are really big and deeply nested, but this gives us a bit of breathing space because the code has been split up by endpoint.

[BenFradet]

👍