event: Refactor async event processing

- Improve the implementation of deferred/immediate events.
- Use the new queue module to change how/when events are queued/processed by
  giving a private queue to each emitter.
- Immediate events(which only exist to break uv_run recursion) are now
  represented in the `loop->fast_events` queue.
- Events pushed to child queues are propagated to the event loop main queue and
  processed as K_EVENT keys.

src/nvim/msgpack_rpc/channel.c

@@ -68,6 +68,7 @@ typedef struct {
   uint64_t next_request_id;
   kvec_t(ChannelCallFrame *) call_stack;
   kvec_t(WBuffer *) delayed_notifications;
+  Queue *events;
 } Channel;
 
 typedef struct {
@@ -224,7 +225,7 @@ Object channel_send_call(uint64_t id,
   ChannelCallFrame frame = {request_id, false, false, NIL};
   kv_push(ChannelCallFrame *, channel->call_stack, &frame);
   channel->pending_requests++;
-  LOOP_POLL_EVENTS_UNTIL(&loop, -1, frame.returned);
+  LOOP_PROCESS_EVENTS_UNTIL(&loop, channel->events, -1, frame.returned);
   (void)kv_pop(channel->call_stack);
   channel->pending_requests--;
 
@@ -459,22 +460,22 @@ static void handle_request(Channel *channel, msgpack_object *request)
     handler.async = true;
   }
 
-  bool async = kv_size(channel->call_stack) || handler.async;
   RequestEvent *event_data = xmalloc(sizeof(RequestEvent));
   event_data->channel = channel;
   event_data->handler = handler;
   event_data->args = args;
   event_data->request_id = request_id;
   incref(channel);
-  loop_push_event(&loop, (Event) {
-    .handler = on_request_event,
-    .data = event_data
-  }, !async);
+  if (handler.async) {
+    on_request_event((void **)&event_data);
+  } else {
+    queue_put(channel->events, on_request_event, 1, event_data);
+  }
 }
 
-static void on_request_event(Event event)
+static void on_request_event(void **argv)
 {
-  RequestEvent *e = event.data;
+  RequestEvent *e = argv[0];
   Channel *channel = e->channel;
   MsgpackRpcRequestHandler handler = e->handler;
   Array args = e->args;
@@ -649,9 +650,8 @@ static void close_channel(Channel *channel)
     case kChannelTypeStdio:
       stream_close(&channel->data.std.in, NULL);
       stream_close(&channel->data.std.out, NULL);
-      loop_push_event(&loop,
-          (Event) { .handler = on_stdio_close, .data = channel }, false);
-      break;
+      queue_put(loop.fast_events, exit_event, 1, channel);
+      return;
     default:
       abort();
   }
@@ -659,9 +659,9 @@ static void close_channel(Channel *channel)
   decref(channel);
 }
 
-static void on_stdio_close(Event e)
+static void exit_event(void **argv)
 {
-  decref(e.data);
+  decref(argv[0]);
 
   if (!exiting) {
     mch_exit(0);
@@ -683,6 +683,7 @@ static void free_channel(Channel *channel)
   pmap_free(cstr_t)(channel->subscribed_events);
   kv_destroy(channel->call_stack);
   kv_destroy(channel->delayed_notifications);
+  queue_free(channel->events);
   xfree(channel);
 }
 
@@ -694,6 +695,7 @@ static void close_cb(Stream *stream, void *data)
 static Channel *register_channel(ChannelType type)
 {
   Channel *rv = xmalloc(sizeof(Channel));
+  rv->events = queue_new_child(loop.events);
   rv->type = type;
   rv->refcount = 1;
   rv->closed = false;