Mojo: Report bindings validation errors via MojoNotifyBadMessage

mojo/public/cpp/bindings/message.h

Index: mojo/public/cpp/bindings/message.h
diff --git a/mojo/public/cpp/bindings/message.h b/mojo/public/cpp/bindings/message.h
index d8eed2bfc3ec00a96948954a86ade3b99fdc088c..cebb2041a599f50e3602171617e0aeaaf3a7a80a 100644
--- a/mojo/public/cpp/bindings/message.h
+++ b/mojo/public/cpp/bindings/message.h
@@ -19,6 +19,8 @@
 
 namespace mojo {
 
+class Error;
+
 // Message is a holder for the data and handles to be sent over a MessagePipe.
 // Message owns its data and handles, but a consumer of Message is free to
 // mutate the data and handles. The message's data is comprised of a header
@@ -99,6 +101,10 @@ class Message {
   // ownership of its internal storage and any attached handles.
   ScopedMessageHandle TakeMojoMessage();
 
+  // Notifies the system that this message is "bad," in this case meaning it was
+  // rejected by bindings validation code.
+  void NotifyBadMessage(const std::string& error);
+
  private:
   void CloseHandles();
 
@@ -114,8 +120,9 @@ class MessageReceiver {
 
   // The receiver may mutate the given message.  Returns true if the message
   // was accepted and false otherwise, indicating that the message was invalid
-  // or malformed.
-  virtual bool Accept(Message* message) WARN_UNUSED_RESULT = 0;
+  // or malformed. If this returns false, |*error| may be populated with
+  // additional information about the failure reason.
+  virtual bool Accept(Message* message, Error* error) WARN_UNUSED_RESULT = 0;

[yzshen1, 2016/06/14 16:45:32]

Does it make sense to merge the return value with Error? IIUC, we don't need
orthogonal "accepted" and "error".

Error Accept(Message* message);
or void Accept(Message* message, Error* error);

[Ken Rockot(use gerrit already), 2016/06/14 21:20:40]

Done!

 };
 
 class MessageReceiverWithResponder : public MessageReceiver {
@@ -133,7 +140,9 @@ class MessageReceiverWithResponder : public MessageReceiver {
   //
   // TODO(yzshen): consider changing |responder| to
   // std::unique_ptr<MessageReceiver>.
-  virtual bool AcceptWithResponder(Message* message, MessageReceiver* responder)
+  virtual bool AcceptWithResponder(Message* message,
+                                   MessageReceiver* responder,
+                                   Error* error)
       WARN_UNUSED_RESULT = 0;
 };
 
@@ -173,7 +182,8 @@ class MessageReceiverWithResponderStatus : public MessageReceiver {
   // TODO(yzshen): consider changing |responder| to
   // std::unique_ptr<MessageReceiver>.
   virtual bool AcceptWithResponder(Message* message,
-                                   MessageReceiverWithStatus* responder)
+                                   MessageReceiverWithStatus* responder,
+                                   Error* error)
       WARN_UNUSED_RESULT = 0;
 };