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diff --git a/ipc/ipc_sync_channel.h b/ipc/ipc_sync_channel.h
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+++ b/ipc/ipc_sync_channel.h
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+// 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>
+
+#include "base/basictypes.h"
+#include "base/memory/ref_counted.h"
+#include "base/synchronization/lock.h"
+#include "base/synchronization/waitable_event_watcher.h"
+#include "ipc/ipc_channel_handle.h"
+#include "ipc/ipc_channel_proxy.h"
+#include "ipc/ipc_sync_message.h"
+
+namespace base {
+class WaitableEvent;
+};
+
+namespace IPC {
+
+class SyncMessage;
+class MessageReplyDeserializer;
+
+// This is similar to ChannelProxy, with the added feature of supporting sending
+// synchronous messages.
+//
+// Overview of how the sync channel works
+// --------------------------------------
+// When the sending thread sends a synchronous message, we create a bunch
+// of tracking info (created in SendWithTimeout, stored in the PendingSyncMsg
+// structure) associated with the message that we identify by the unique
+// "MessageId" on the SyncMessage. Among the things we save is the
+// "Deserializer" which is provided by the sync message. This object is in
+// charge of reading the parameters from the reply message and putting them in
+// the output variables provided by its caller.
+//
+// The info gets stashed in a queue since we could have a nested stack of sync
+// messages (each side could send sync messages in response to sync messages,
+// so it works like calling a function). The message is sent to the I/O thread
+// for dispatch and the original thread blocks waiting for the reply.
+//
+// SyncContext maintains the queue in a threadsafe way and listens for replies
+// on the I/O thread. When a reply comes in that matches one of the messages
+// it's looking for (using the unique message ID), it will execute the
+// deserializer stashed from before, and unblock the original thread.
+//
+//
+// Significant complexity results from the fact that messages are still coming
+// in while the original thread is blocked. Normal async messages are queued
+// and dispatched after the blocking call is complete. Sync messages must
+// be dispatched in a reentrant manner to avoid deadlock.
+//
+//
+// Note that care must be taken that the lifetime of the ipc_thread argument
+// is more than this object.  If the message loop goes away while this object
+// is running and it's used to send a message, then it will use the invalid
+// message loop pointer to proxy it to the ipc thread.
+class IPC_EXPORT SyncChannel : public ChannelProxy,
+                               public base::WaitableEventWatcher::Delegate {
+ public:
+  SyncChannel(const IPC::ChannelHandle& channel_handle,
+              Channel::Mode mode,
+              Channel::Listener* listener,
+              base::MessageLoopProxy* ipc_message_loop,
+              bool create_pipe_now,
+              base::WaitableEvent* shutdown_event);
+  virtual ~SyncChannel();
+
+  virtual bool Send(Message* message);
+  virtual bool SendWithTimeout(Message* message, int timeout_ms);
+
+  // Whether we allow sending messages with no time-out.
+  void set_sync_messages_with_no_timeout_allowed(bool value) {
+    sync_messages_with_no_timeout_allowed_ = value;
+  }
+
+  // Sets this channel to only dispatch its incoming unblocking messages when it
+  // 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);
+
+ 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:
+    SyncContext(Channel::Listener* listener,
+                base::MessageLoopProxy* ipc_thread,
+                base::WaitableEvent* shutdown_event);
+
+    // Adds information about an outgoing sync message to the context so that
+    // we know how to deserialize the reply.
+    void Push(SyncMessage* sync_msg);
+
+    // Cleanly remove the top deserializer (and throw it away).  Returns the
+    // result of the Send call for that message.
+    bool Pop();
+
+    // Returns an event that's set when the send is complete, timed out or the
+    // process shut down.
+    base::WaitableEvent* GetSendDoneEvent();
+
+    // Returns an event that's set when an incoming message that's not the reply
+    // needs to get dispatched (by calling SyncContext::DispatchMessages).
+    base::WaitableEvent* GetDispatchEvent();
+
+    void DispatchMessages();
+
+    // Checks if the given message is blocking the listener thread because of a
+    // synchronous send.  If it is, the thread is unblocked and true is
+    // returned. Otherwise the function returns false.
+    bool TryToUnblockListener(const Message* msg);
+
+    // Called on the IPC thread when a sync send that runs a nested message loop
+    // times out.
+    void OnSendTimeout(int message_id);
+
+    base::WaitableEvent* shutdown_event() { return shutdown_event_; }
+
+    ReceivedSyncMsgQueue* received_sync_msgs() {
+      return received_sync_msgs_;
+    }
+
+    void set_restrict_dispatch(bool value) { restrict_dispatch_ = value; }
+    bool restrict_dispatch() const { return restrict_dispatch_; }
+
+   private:
+    virtual ~SyncContext();
+    // ChannelProxy methods that we override.
+
+    // Called on the listener thread.
+    virtual void Clear();
+
+    // Called on the IPC thread.
+    virtual bool OnMessageReceived(const Message& msg);
+    virtual void OnChannelError();
+    virtual void OnChannelOpened();
+    virtual void OnChannelClosed();
+
+    // Cancels all pending Send calls.
+    void CancelPendingSends();
+
+    // WaitableEventWatcher::Delegate implementation.
+    virtual void OnWaitableEventSignaled(base::WaitableEvent* arg);
+
+    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_;
+  };
+
+ 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_;
+
+  // 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__