Allow embedder service request handlers to return JSON-RPC errors.

runtime/vm/service.cc

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/service.h"
 
 #include "include/dart_api.h"
 #include "include/dart_native_api.h"
 #include "platform/globals.h"
 
@@ skipping 841 matching lines @@
       return;
     }
 
     EmbedderServiceHandler* handler = FindIsolateEmbedderHandler(c_method_name);
     if (handler == NULL) {
       handler = FindRootEmbedderHandler(c_method_name);
     }
 
     if (handler != NULL) {
       EmbedderHandleMessage(handler, &js);
-      js.PostReply();
       return;
     }
 
     const Instance& extension_handler = Instance::Handle(Z,
         I->LookupServiceExtensionHandler(method_name));
     if (!extension_handler.IsNull()) {
       ScheduleExtensionHandler(extension_handler,
                                method_name,
                                param_keys,
                                param_values,
@@ skipping 246 matching lines @@
   void* user_data_;
   EmbedderServiceHandler* next_;
 };
 
 
 void Service::EmbedderHandleMessage(EmbedderServiceHandler* handler,
                                     JSONStream* js) {
   ASSERT(handler != NULL);
   Dart_ServiceRequestCallback callback = handler->callback();
   ASSERT(callback != NULL);
-  const char* r = NULL;
-  const char* method = js->method();
-  const char** keys = js->param_keys();
-  const char** values = js->param_values();
-  r = callback(method, keys, values, js->num_params(), handler->user_data());
-  ASSERT(r != NULL);
-  // TODO(johnmccutchan): Allow for NULL returns?
-  TextBuffer* buffer = js->buffer();
-  buffer->AddString(r);
-  free(const_cast<char*>(r));
+  const char* response = NULL;
+  bool success = callback(js->method(), js->param_keys(), js->param_values(),
+                          js->num_params(), handler->user_data(), &response);
+  ASSERT(response != NULL);
+  if (!success) {
+    js->SetupError();
+  }
+  js->buffer()->AddString(response);
+  js->PostReply();
+  free(const_cast<char*>(response));
 }
 
 
 void Service::RegisterIsolateEmbedderCallback(
     const char* name,
     Dart_ServiceRequestCallback callback,
     void* user_data) {
   if (name == NULL) {
     return;
   }
@@ skipping 2907 matching lines @@
     if (strcmp(method_name, method.name) == 0) {
       return &method;
     }
   }
   return NULL;
 }
 
 #endif  // !PRODUCT
 
 }  // namespace dart