[mojo-edk] Add MojoWatch and MojoCancelWatch APIs

mojo/edk/system/node_controller.cc

Index: mojo/edk/system/node_controller.cc
diff --git a/mojo/edk/system/node_controller.cc b/mojo/edk/system/node_controller.cc
index e2b1b08f5f307c6846f1e84a8af7c92b6de355b5..337bcac8b132c59a197c6143a9084d21d2e2b57d 100644
--- a/mojo/edk/system/node_controller.cc
+++ b/mojo/edk/system/node_controller.cc
@@ -21,6 +21,7 @@
 #include "mojo/edk/system/broker_host.h"
 #include "mojo/edk/system/core.h"
 #include "mojo/edk/system/ports_message.h"
+#include "mojo/edk/system/request_context.h"
 
 namespace mojo {
 namespace edk {
@@ -716,6 +717,12 @@ void NodeController::OnAcceptBrokerClient(const ports::NodeName& from_node,
 void NodeController::OnPortsMessage(Channel::MessagePtr channel_message) {
   DCHECK(io_task_runner_->RunsTasksOnCurrentThread());
 
+  // This is a singular chokepoint through which dispatcher port statuses may
+  // be changed without an explicit call to some Mojo system API. We need to
+  // ensure that a RequestContext exists in case a dispatcher has any watchers
+  // to notify.
+  RequestContext request_context;
+
   void* data;
   size_t num_data_bytes;
   NodeChannel::GetPortsMessageData(