msgpack: Replace FUNC_ATTR_DEFERRED by FUNC_ATTR_ASYNC

API functions exposed via msgpack-rpc now fall into two categories:

- async functions, which are executed as soon as the request is parsed
- sync functions, which are invoked in nvim main loop when processing the
  `K_EVENT special key

Only a few functions which can be safely executed in any context are marked as
async.

src/nvim/msgpack_rpc/channel.c

@@ -450,16 +450,16 @@ static void handle_request(Channel *channel, msgpack_object *request)
                                           method->via.bin.size);
   } else {
     handler.fn = msgpack_rpc_handle_missing_method;
-    handler.defer = false;
+    handler.async = true;
   }
 
   Array args = ARRAY_DICT_INIT;
   if (!msgpack_rpc_to_array(msgpack_rpc_args(request), &args)) {
     handler.fn = msgpack_rpc_handle_invalid_arguments;
-    handler.defer = false;
+    handler.async = true;
   }
 
-  bool defer = (!kv_size(channel->call_stack) && handler.defer);
+  bool async = kv_size(channel->call_stack) || handler.async;
   RequestEvent *event_data = xmalloc(sizeof(RequestEvent));
   event_data->channel = channel;
   event_data->handler = handler;
@@ -469,7 +469,7 @@ static void handle_request(Channel *channel, msgpack_object *request)
   loop_push_event(&loop, (Event) {
     .handler = on_request_event,
     .data = event_data
-  }, defer);
+  }, !async);
 }
 
 static void on_request_event(Event event)

src/nvim/msgpack_rpc/defs.h

@@ -11,9 +11,8 @@ typedef struct {
                uint64_t request_id,
                Array args,
                Error *error);
-  bool defer;  // Should the call be deferred to the main loop? This should
-               // be true if the function mutates editor data structures such
-               // as buffers, windows, tabs, or if it executes vimscript code.
+  bool async;  // function is always safe to run immediately instead of being
+               // put in a request queue for handling when nvim waits for input.
 } MsgpackRpcRequestHandler;
 
 /// Initializes the msgpack-rpc method table

src/nvim/msgpack_rpc/remote_ui.c

@@ -28,7 +28,7 @@ void remote_ui_init(void)
   connected_uis = pmap_new(uint64_t)();
   // Add handler for "attach_ui"
   String method = cstr_as_string("ui_attach");
-  MsgpackRpcRequestHandler handler = {.fn = remote_ui_attach, .defer = false};
+  MsgpackRpcRequestHandler handler = {.fn = remote_ui_attach, .async = true};
   msgpack_rpc_add_method_handler(method, handler);
   method = cstr_as_string("ui_detach");
   handler.fn = remote_ui_detach;