refactor(grpc): drop empty messages for google.protobuf.Empty

Remove the empty StopVmResponse and ListVmsRequest messages; StopVm now
returns google.protobuf.Empty and ListVms takes it. Codec/server updated to
use Google.Protobuf.Empty. protoc resolves the well-known import with no extra
include path.

Author: markovejnovic

docs/grpc.md

@@ -8,6 +8,13 @@ create, stop, locate, and list microVMs.
 > **v0 -- unstable.** The contract may change without notice during early
 > development. Pin to a commit if you depend on it.
 
+## VMs
+
+With `Hyper` running, you can create a new `gRPC` client:
+
+```python
+```
+
 ## The contract
 
 The service is `hyper.grpc.v0.Hyper`, defined in

lib/hyper/grpc/codec.ex

@@ -8,12 +8,13 @@ defmodule Hyper.Grpc.Codec do
       or a `GRPC.RPCError` for the server to raise.
   """
 
+  alias Google.Protobuf.Empty
+
   alias Hyper.Grpc.V0.{
     CreateVmRequest,
     CreateVmResponse,
     GetVmResponse,
     ListVmsResponse,
-    StopVmResponse,
     Vm
   }
 
@@ -84,8 +85,8 @@ defmodule Hyper.Grpc.Codec do
   def to_grpc({:vms, vms}),
     do: %ListVmsResponse{vms: Enum.map(vms, &vm/1)}
 
-  @spec to_grpc(:stopped) :: StopVmResponse.t()
-  def to_grpc(:stopped), do: %StopVmResponse{}
+  @spec to_grpc(:stopped) :: Empty.t()
+  def to_grpc(:stopped), do: %Empty{}
 
   @spec to_grpc({:error, term()}) :: GRPC.RPCError.t()
   def to_grpc({:error, reason}), do: rpc_error(reason)

lib/hyper/grpc/server.ex

@@ -12,17 +12,16 @@ defmodule Hyper.Grpc.Server do
 
   use GRPC.Server, service: Hyper.Grpc.V0.Hyper.Service
 
+  alias Google.Protobuf.Empty
   alias Hyper.Grpc.Codec
 
   alias Hyper.Grpc.V0.{
     CreateVmRequest,
     CreateVmResponse,
     GetVmRequest,
     GetVmResponse,
-    ListVmsRequest,
     ListVmsResponse,
-    StopVmRequest,
-    StopVmResponse
+    StopVmRequest
   }
 
   @spec create_vm(CreateVmRequest.t(), GRPC.Server.Stream.t()) :: CreateVmResponse.t()
@@ -39,7 +38,7 @@ defmodule Hyper.Grpc.Server do
     end
   end
 
-  @spec stop_vm(StopVmRequest.t(), GRPC.Server.Stream.t()) :: StopVmResponse.t()
+  @spec stop_vm(StopVmRequest.t(), GRPC.Server.Stream.t()) :: Empty.t()
   def stop_vm(%StopVmRequest{vm_id: vm_id}, _stream) do
     Hyper.Node.FireVMM.State.stop(vm_id)
     Codec.to_grpc(:stopped)
@@ -55,8 +54,8 @@ defmodule Hyper.Grpc.Server do
     end
   end
 
-  @spec list_vms(ListVmsRequest.t(), GRPC.Server.Stream.t()) :: ListVmsResponse.t()
-  def list_vms(%ListVmsRequest{}, _stream) do
+  @spec list_vms(Empty.t(), GRPC.Server.Stream.t()) :: ListVmsResponse.t()
+  def list_vms(%Empty{}, _stream) do
     Codec.to_grpc({:vms, Hyper.Cluster.Routing.all()})
   end
 end

proto/hyper/grpc/v0/hyper.proto

@@ -14,6 +14,8 @@ syntax = "proto3";
 
 package hyper.grpc.v0;
 
+import "google/protobuf/empty.proto";
+
 // The VM lifecycle service.
 //
 // Errors are returned as standard gRPC statuses (see each RPC). Beyond the ones
@@ -40,7 +42,7 @@ service Hyper {
   // Errors:
   //   NOT_FOUND   -- no VM with this `vm_id` is running in the cluster.
   //   UNAVAILABLE -- the VM's host node is unreachable.
-  rpc StopVm(StopVmRequest) returns (StopVmResponse);
+  rpc StopVm(StopVmRequest) returns (google.protobuf.Empty);
 
   // Locate a microVM: report the cluster node it currently runs on.
   //
@@ -49,7 +51,7 @@ service Hyper {
   rpc GetVm(GetVmRequest) returns (GetVmResponse);
 
   // List every microVM currently known to the cluster, across all nodes.
-  rpc ListVms(ListVmsRequest) returns (ListVmsResponse);
+  rpc ListVms(google.protobuf.Empty) returns (ListVmsResponse);
 }
 
 // A fixed (vCPU, memory, disk) size, like a cloud instance class. Each step up
@@ -111,9 +113,6 @@ message StopVmRequest {
   string vm_id = 1;
 }
 
-// Result of a successful StopVm. Intentionally empty.
-message StopVmResponse {}
-
 // Request to locate a VM.
 message GetVmRequest {
   // The id of the VM to look up.
@@ -129,9 +128,6 @@ message GetVmResponse {
   string node = 2;
 }
 
-// Request to list all VMs. Intentionally empty.
-message ListVmsRequest {}
-
 // Result of ListVms.
 message ListVmsResponse {
   // Every VM known to the cluster, in no particular order.