ipc: Add a lock to guard modifications of state of the AttachmentBroker.

ipc/attachment_broker.h

 // Copyright 2015 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_ATTACHMENT_BROKER_H_
 #define IPC_ATTACHMENT_BROKER_H_
 
 #include "base/gtest_prod_util.h"
 #include "base/macros.h"
 #include "base/process/process_handle.h"
+#include "base/synchronization/lock.h"
 #include "ipc/brokerable_attachment.h"
 #include "ipc/ipc_export.h"
 #include "ipc/ipc_listener.h"
 
 // If the platform has no attachments that need brokering, then it shouldn't
 // compile any code that calls member functions of AttachmentBroker. This
 // prevents symbols only used by AttachmentBroker and its subclasses from
 // making it into the binary.
 #if defined(OS_WIN) || (defined(OS_MACOSX) && !defined(OS_IOS))
 #define USE_ATTACHMENT_BROKER 1
@@ skipping 71 matching lines @@
   // Adds |attachment| to |attachments_|, and notifies the observers.
   void HandleReceivedAttachment(
       const scoped_refptr<BrokerableAttachment>& attachment);
 
   // Informs the observers that a new BrokerableAttachment has been received.
   void NotifyObservers(const BrokerableAttachment::AttachmentId& id);
 
   // This method is exposed for testing only.
   AttachmentVector* get_attachments() { return &attachments_; }
 
+  base::Lock* get_lock() { return &lock_; }
+
  private:
 #if defined(OS_WIN)
   FRIEND_TEST_ALL_PREFIXES(AttachmentBrokerUnprivilegedWinTest,
                            ReceiveValidMessage);
   FRIEND_TEST_ALL_PREFIXES(AttachmentBrokerUnprivilegedWinTest,
                            ReceiveInvalidMessage);
 #endif  // defined(OS_WIN)
 
   // A vector of BrokerableAttachments that have been received, but not yet
   // consumed.
   // A std::vector is used instead of a std::map because this container is
   // expected to have few elements, for which a std::vector is expected to have
   // better performance.
   AttachmentVector attachments_;
 
   std::vector<Observer*> observers_;
+
+  // The AttachmentBroker can be accessed from any thread, so modifications to
+  // internal state must be guarded by a lock.
+  base::Lock lock_;
   DISALLOW_COPY_AND_ASSIGN(AttachmentBroker);
 };
 
 }  // namespace IPC
 
 #endif  // IPC_ATTACHMENT_BROKER_H_