Example of how to interpose yourself in a message stream.

ipc/ipc_sync_channel.h

Index: ipc/ipc_sync_channel.h
===================================================================
--- ipc/ipc_sync_channel.h	(revision 83074)
+++ ipc/ipc_sync_channel.h	(working copy)
@@ -90,6 +90,21 @@
   // of some particular channels to not re-enter in such cases.
   void SetRestrictDispatchToSameChannel(bool value);
 
+  // Interface for a filter to be imposed on outgoing messages which can
+  // re-write the message.  Used mainly for testing.
+  class OutgoingMessageFilter {
+   public:
+    virtual ~OutgoingMessageFilter() = 0;
+
+    // Returns a re-written message, freeing the original, or simply the
+    // original unchanged if no rewrite indicated.
+    virtual Message *Rewrite(Message *message) = 0;
+  };
+
+  void set_outgoing_message_filter(OutgoingMessageFilter *filter) {
+    outgoing_message_filter_ = filter;
+  }
+
  protected:
   class ReceivedSyncMsgQueue;
   friend class ReceivedSyncMsgQueue;
@@ -168,6 +183,7 @@
     base::WaitableEvent* shutdown_event_;
     base::WaitableEventWatcher shutdown_watcher_;
     bool restrict_dispatch_;
+

[jam, 2011/04/27 21:58:07]

nit

   };
 
  private:
@@ -188,6 +204,7 @@
   static void WaitForReplyWithNestedMessageLoop(SyncContext* context);
 
   bool sync_messages_with_no_timeout_allowed_;
+  OutgoingMessageFilter *outgoing_message_filter_;
 
   // Used to signal events between the IPC and listener threads.
   base::WaitableEventWatcher dispatch_watcher_;