Example of how to interpose yourself in a message stream.

ipc/ipc_sync_channel.h

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef IPC_IPC_SYNC_SENDER_H__
 #define IPC_IPC_SYNC_SENDER_H__
 #pragma once
 
 #include <string>
 #include <deque>
@@ skipping 72 matching lines @@
   // is itself blocked on sending a sync message, not when other channels are.
   //
   // Normally, any unblocking message coming from any channel can be dispatched
   // when any (possibly other) channel is blocked on sending a message. This is
   // needed in some cases to unblock certain loops (e.g. necessary when some
   // processes share a window hierarchy), but may cause re-entrancy issues in
   // some cases where such loops are not possible. This flags allows the tagging
   // 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;
 
   // SyncContext holds the per object data for SyncChannel, so that SyncChannel
   // can be deleted while it's being used in a different thread.  See
   // ChannelProxy::Context for more information.
   class SyncContext : public Context,
                       public base::WaitableEventWatcher::Delegate {
    public:
@@ skipping 58 matching lines @@
 
     typedef std::deque<PendingSyncMsg> PendingSyncMessageQueue;
     PendingSyncMessageQueue deserializers_;
     base::Lock deserializers_lock_;
 
     scoped_refptr<ReceivedSyncMsgQueue> received_sync_msgs_;
 
     base::WaitableEvent* shutdown_event_;
     base::WaitableEventWatcher shutdown_watcher_;
     bool restrict_dispatch_;
+

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

nit

   };
 
  private:
   // WaitableEventWatcher::Delegate implementation.
   virtual void OnWaitableEventSignaled(base::WaitableEvent* arg);
 
   SyncContext* sync_context() {
     return reinterpret_cast<SyncContext*>(context());
   }
 
   // Both these functions wait for a reply, timeout or process shutdown.  The
   // latter one also runs a nested message loop in the meantime.
   static void WaitForReply(
       SyncContext* context, base::WaitableEvent* pump_messages_event);
 
   // Runs a nested message loop until a reply arrives, times out, or the process
   // shuts down.
   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_;
 
   DISALLOW_COPY_AND_ASSIGN(SyncChannel);
 };
 
 }  // namespace IPC
 
 #endif  // IPC_IPC_SYNC_SENDER_H__