gecko/ipc/glue/MessageChannel.h

/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*-
 * vim: sw=4 ts=4 et :
 */
/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

#ifndef ipc_glue_MessageChannel_h
#define ipc_glue_MessageChannel_h 1

#include "base/basictypes.h"
#include "base/message_loop.h"

#include "mozilla/Assertions.h"
#include "mozilla/DebugOnly.h"
#include "mozilla/Monitor.h"
#include "mozilla/Move.h"
#include "mozilla/Vector.h"
#include "mozilla/WeakPtr.h"
#include "mozilla/ipc/Transport.h"
#include "MessageLink.h"
#include "nsAutoPtr.h"

#include <deque>
#include <stack>
#include <math.h>

namespace mozilla {
namespace ipc {

class MessageChannel;

class RefCountedMonitor : public Monitor
{
  public:
    RefCountedMonitor()
        : Monitor("mozilla.ipc.MessageChannel.mMonitor")
    {}

    NS_INLINE_DECL_THREADSAFE_REFCOUNTING(RefCountedMonitor)
};

class MessageChannel : HasResultCodes
{
    friend class ProcessLink;
    friend class ThreadLink;
    friend class AutoEnterRPCTransaction;

    typedef mozilla::Monitor Monitor;

  public:
    static const int32_t kNoTimeout;

    typedef IPC::Message Message;
    typedef mozilla::ipc::Transport Transport;

    MessageChannel(MessageListener *aListener);
    ~MessageChannel();

    // "Open" from the perspective of the transport layer; the underlying
    // socketpair/pipe should already be created.
    //
    // Returns true iff the transport layer was successfully connected,
    // i.e., mChannelState == ChannelConnected.
    bool Open(Transport* aTransport, MessageLoop* aIOLoop=0, Side aSide=UnknownSide);

    // "Open" a connection to another thread in the same process.
    //
    // Returns true iff the transport layer was successfully connected,
    // i.e., mChannelState == ChannelConnected.
    //
    // For more details on the process of opening a channel between
    // threads, see the extended comment on this function
    // in MessageChannel.cpp.
    bool Open(MessageChannel *aTargetChan, MessageLoop *aTargetLoop, Side aSide);

    // Close the underlying transport channel.
    void Close();

    // Force the channel to behave as if a channel error occurred. Valid
    // for process links only, not thread links.
    void CloseWithError();

    // Asynchronously send a message to the other side of the channel
    bool Send(Message* aMsg);

    // Asynchronously deliver a message back to this side of the
    // channel
    bool Echo(Message* aMsg);

    // Synchronously send |msg| (i.e., wait for |reply|)
    bool Send(Message* aMsg, Message* aReply);

    // Make an Interrupt call to the other side of the channel
    bool Call(Message* aMsg, Message* aReply);

    void SetReplyTimeoutMs(int32_t aTimeoutMs);

    bool IsOnCxxStack() const {
        return !mCxxStackFrames.empty();
    }

    void FlushPendingInterruptQueue();

    // Unsound_IsClosed and Unsound_NumQueuedMessages are safe to call from any
    // thread, but they make no guarantees about whether you'll get an
    // up-to-date value; the values are written on one thread and read without
    // locking, on potentially different threads.  Thus you should only use
    // them when you don't particularly care about getting a recent value (e.g.
    // in a memory report).
    bool Unsound_IsClosed() const {
        return mLink ? mLink->Unsound_IsClosed() : true;
    }
    uint32_t Unsound_NumQueuedMessages() const {
        return mLink ? mLink->Unsound_NumQueuedMessages() : 0;
    }

    static bool IsPumpingMessages() {
        return sIsPumpingMessages;
    }
    static void SetIsPumpingMessages(bool aIsPumping) {
        sIsPumpingMessages = aIsPumping;
    }

#ifdef OS_WIN
    struct MOZ_STACK_CLASS SyncStackFrame
    {
        SyncStackFrame(MessageChannel* channel, bool interrupt);
        ~SyncStackFrame();

        bool mInterrupt;
        bool mSpinNestedEvents;
        bool mListenerNotified;
        MessageChannel* mChannel;

        // The previous stack frame for this channel.
        SyncStackFrame* mPrev;

        // The previous stack frame on any channel.
        SyncStackFrame* mStaticPrev;
    };
    friend struct MessageChannel::SyncStackFrame;

    static bool IsSpinLoopActive() {
        for (SyncStackFrame* frame = sStaticTopFrame; frame; frame = frame->mPrev) {
            if (frame->mSpinNestedEvents)
                return true;
        }
        return false;
    }

  protected:
    // The deepest sync stack frame for this channel.
    SyncStackFrame* mTopFrame;

    // The deepest sync stack frame on any channel.
    static SyncStackFrame* sStaticTopFrame;

  public:
    void ProcessNativeEventsInInterruptCall();
    static void NotifyGeckoEventDispatch();

  private:
    void SpinInternalEventLoop();
#endif

  private:
    void CommonThreadOpenInit(MessageChannel *aTargetChan, Side aSide);
    void OnOpenAsSlave(MessageChannel *aTargetChan, Side aSide);

    void PostErrorNotifyTask();
    void OnNotifyMaybeChannelError();
    void ReportConnectionError(const char* aChannelName) const;
    void ReportMessageRouteError(const char* channelName) const;
    bool MaybeHandleError(Result code, const char* channelName);

    void Clear();

    // Send OnChannelConnected notification to listeners.
    void DispatchOnChannelConnected(int32_t peer_pid);

    // Any protocol that requires blocking until a reply arrives, will send its
    // outgoing message through this function. Currently, two protocols do this:
    //
    //  sync, which can only initiate messages from child to parent.
    //  urgent, which can only initiate messages from parent to child.
    //
    // SendAndWait() expects that the worker thread owns the monitor, and that
    // the message has been prepared to be sent over the link. It returns as
    // soon as a reply has been received, or an error has occurred.
    //
    // Note that while the child is blocked waiting for a sync reply, it can wake
    // up to process urgent calls from the parent.
    bool SendAndWait(Message* aMsg, Message* aReply);

    bool RPCCall(Message* aMsg, Message* aReply);
    bool InterruptCall(Message* aMsg, Message* aReply);
    bool UrgentCall(Message* aMsg, Message* aReply);

    bool InterruptEventOccurred();

    bool ProcessPendingUrgentRequest();
    bool ProcessPendingRPCCall();

    void MaybeUndeferIncall();
    void EnqueuePendingMessages();

    // Executed on the worker thread. Dequeues one pending message.
    bool OnMaybeDequeueOne();
    bool DequeueOne(Message *recvd);

    // Dispatches an incoming message to its appropriate handler.
    void DispatchMessage(const Message &aMsg);

    // DispatchMessage will route to one of these functions depending on the
    // protocol type of the message.
    void DispatchSyncMessage(const Message &aMsg);
    void DispatchUrgentMessage(const Message &aMsg);
    void DispatchAsyncMessage(const Message &aMsg);
    void DispatchRPCMessage(const Message &aMsg);
    void DispatchInterruptMessage(const Message &aMsg, size_t aStackDepth);

    // Return true if the wait ended because a notification was received.
    //
    // Return false if the time elapsed from when we started the process of
    // waiting until afterwards exceeded the currently allotted timeout.
    // That *DOES NOT* mean false => "no event" (== timeout); there are many
    // circumstances that could cause the measured elapsed time to exceed the
    // timeout EVEN WHEN we were notified.
    //
    // So in sum: true is a meaningful return value; false isn't,
    // necessarily.
    bool WaitForSyncNotify();
    bool WaitForInterruptNotify();

    bool WaitResponse(bool aWaitTimedOut);

    bool ShouldContinueFromTimeout();

    // The "remote view of stack depth" can be different than the
    // actual stack depth when there are out-of-turn replies.  When we
    // receive one, our actual Interrupt stack depth doesn't decrease, but
    // the other side (that sent the reply) thinks it has.  So, the
    // "view" returned here is |stackDepth| minus the number of
    // out-of-turn replies.
    //
    // Only called from the worker thread.
    size_t RemoteViewOfStackDepth(size_t stackDepth) const {
        AssertWorkerThread();
        return stackDepth - mOutOfTurnReplies.size();
    }

    int32_t NextSeqno() {
        AssertWorkerThread();
        return (mSide == ChildSide) ? --mNextSeqno : ++mNextSeqno;
    }

    // This helper class manages mCxxStackDepth on behalf of MessageChannel.
    // When the stack depth is incremented from zero to non-zero, it invokes
    // a callback, and similarly for when the depth goes from non-zero to zero.
    void EnteredCxxStack() {
       mListener->OnEnteredCxxStack();
    }

    void ExitedCxxStack();

    void EnteredCall() {
        mListener->OnEnteredCall();
    }

    void ExitedCall() {
        mListener->OnExitedCall();
    }

    MessageListener *Listener() const {
        return mListener.get();
    }

    enum Direction { IN_MESSAGE, OUT_MESSAGE };
    struct InterruptFrame {
        enum Semantics { INTR_SEMS, SYNC_SEMS, ASYNC_SEMS };

        InterruptFrame(Direction direction, const Message* msg)
          : mMessageName(strdup(msg->name())),
            mMessageRoutingId(msg->routing_id()),
            mMesageSemantics(msg->is_interrupt() ? INTR_SEMS :
                             msg->is_sync() ? SYNC_SEMS :
                             ASYNC_SEMS),
            mDirection(direction),
            mMoved(false)
        {
            MOZ_ASSERT(mMessageName);
        }

        InterruptFrame(InterruptFrame&& aOther)
        {
            MOZ_ASSERT(aOther.mMessageName);
            mMessageName = aOther.mMessageName;
            aOther.mMessageName = nullptr;
            aOther.mMoved = true;

            mMessageRoutingId = aOther.mMessageRoutingId;
            mMesageSemantics = aOther.mMesageSemantics;
            mDirection = aOther.mDirection;
        }

        ~InterruptFrame()
        {
            MOZ_ASSERT_IF(!mMessageName, mMoved);

            if (mMessageName)
                free(const_cast<char*>(mMessageName));
        }

        InterruptFrame& operator=(InterruptFrame&& aOther)
        {
            MOZ_ASSERT(&aOther != this);
            this->~InterruptFrame();
            new (this) InterruptFrame(mozilla::Move(aOther));
            return *this;
        }

        bool IsInterruptIncall() const
        {
            return INTR_SEMS == mMesageSemantics && IN_MESSAGE == mDirection;
        }

        bool IsInterruptOutcall() const
        {
            return INTR_SEMS == mMesageSemantics && OUT_MESSAGE == mDirection;
        }

        void Describe(int32_t* id, const char** dir, const char** sems,
                      const char** name) const
        {
            *id = mMessageRoutingId;
            *dir = (IN_MESSAGE == mDirection) ? "in" : "out";
            *sems = (INTR_SEMS == mMesageSemantics) ? "intr" :
                    (SYNC_SEMS == mMesageSemantics) ? "sync" :
                    "async";
            *name = mMessageName;
        }

        const char* mMessageName;
        int32_t mMessageRoutingId;
        Semantics mMesageSemantics;
        Direction mDirection;
        DebugOnly<bool> mMoved;

    private:
        InterruptFrame(const InterruptFrame& aOther) MOZ_DELETE;
        InterruptFrame& operator=(const InterruptFrame&) MOZ_DELETE;
    };

    class MOZ_STACK_CLASS CxxStackFrame
    {
      public:
        CxxStackFrame(MessageChannel& that, Direction direction, const Message* msg)
          : mThat(that)
        {
            mThat.AssertWorkerThread();

            if (mThat.mCxxStackFrames.empty())
                mThat.EnteredCxxStack();

            mThat.mCxxStackFrames.append(InterruptFrame(direction, msg));

            const InterruptFrame& frame = mThat.mCxxStackFrames.back();

            if (frame.IsInterruptIncall())
                mThat.EnteredCall();

            mThat.mSawInterruptOutMsg |= frame.IsInterruptOutcall();
        }

        ~CxxStackFrame() {
            mThat.AssertWorkerThread();

            MOZ_ASSERT(!mThat.mCxxStackFrames.empty());

            bool exitingCall = mThat.mCxxStackFrames.back().IsInterruptIncall();
            mThat.mCxxStackFrames.shrinkBy(1);

            bool exitingStack = mThat.mCxxStackFrames.empty();

            // mListener could have gone away if Close() was called while
            // MessageChannel code was still on the stack
            if (!mThat.mListener)
                return;

            if (exitingCall)
                mThat.ExitedCall();

            if (exitingStack)
                mThat.ExitedCxxStack();
        }
      private:
        MessageChannel& mThat;

        // disable harmful methods
        CxxStackFrame() MOZ_DELETE;
        CxxStackFrame(const CxxStackFrame&) MOZ_DELETE;
        CxxStackFrame& operator=(const CxxStackFrame&) MOZ_DELETE;
    };

    void DebugAbort(const char* file, int line, const char* cond,
                    const char* why,
                    bool reply=false) const;

    // This method is only safe to call on the worker thread, or in a
    // debugger with all threads paused.
    void DumpInterruptStack(const char* const pfx="") const;

  private:
    // Called from both threads
    size_t InterruptStackDepth() const {
        mMonitor->AssertCurrentThreadOwns();
        return mInterruptStack.size();
    }

    // Returns true if we're blocking waiting for a reply.
    bool AwaitingSyncReply() const {
        mMonitor->AssertCurrentThreadOwns();
        return mPendingSyncReplies > 0;
    }
    bool AwaitingUrgentReply() const {
        mMonitor->AssertCurrentThreadOwns();
        return mPendingUrgentReplies > 0;
    }
    bool AwaitingRPCReply() const {
        mMonitor->AssertCurrentThreadOwns();
        return mPendingRPCReplies > 0;
    }
    bool AwaitingInterruptReply() const {
        mMonitor->AssertCurrentThreadOwns();
        return !mInterruptStack.empty();
    }

    // Returns true if we're dispatching a sync message's callback.
    bool DispatchingSyncMessage() const {
        return mDispatchingSyncMessage;
    }

    // Returns true if we're dispatching an urgent message's callback.
    bool DispatchingUrgentMessage() const {
        return mDispatchingUrgentMessageCount > 0;
    }

    bool Connected() const;

  private:
    // Executed on the IO thread.
    void NotifyWorkerThread();

    // Return true if |aMsg| is a special message targeted at the IO
    // thread, in which case it shouldn't be delivered to the worker.
    bool MaybeInterceptSpecialIOMessage(const Message& aMsg);

    void OnChannelConnected(int32_t peer_id);

    // Tell the IO thread to close the channel and wait for it to ACK.
    void SynchronouslyClose();

    void OnMessageReceivedFromLink(const Message& aMsg);
    void OnChannelErrorFromLink();

  private:
    // Run on the not current thread.
    void NotifyChannelClosed();
    void NotifyMaybeChannelError();

  private:
    // Can be run on either thread
    void AssertWorkerThread() const
    {
        NS_ABORT_IF_FALSE(mWorkerLoopID == MessageLoop::current()->id(),
                          "not on worker thread!");
    }

    // The "link" thread is either the I/O thread (ProcessLink) or the
    // other actor's work thread (ThreadLink).  In either case, it is
    // NOT our worker thread.
    void AssertLinkThread() const
    {
        NS_ABORT_IF_FALSE(mWorkerLoopID != MessageLoop::current()->id(),
                          "on worker thread but should not be!");
    }

  private:
    typedef IPC::Message::msgid_t msgid_t;
    typedef std::deque<Message> MessageQueue;
    typedef std::map<size_t, Message> MessageMap;

    // All dequeuing tasks require a single point of cancellation,
    // which is handled via a reference-counted task.
    class RefCountedTask
    {
      public:
        RefCountedTask(CancelableTask* aTask)
          : mTask(aTask)
        { }
        ~RefCountedTask() { delete mTask; }
        void Run() { mTask->Run(); }
        void Cancel() { mTask->Cancel(); }

        NS_INLINE_DECL_THREADSAFE_REFCOUNTING(RefCountedTask)

      private:
        CancelableTask* mTask;
    };

    // Wrap an existing task which can be cancelled at any time
    // without the wrapper's knowledge.
    class DequeueTask : public Task
    {
      public:
        DequeueTask(RefCountedTask* aTask)
          : mTask(aTask)
        { }
        void Run() { mTask->Run(); }

      private:
        nsRefPtr<RefCountedTask> mTask;
    };

  private:
    mozilla::WeakPtr<MessageListener> mListener;
    ChannelState mChannelState;
    nsRefPtr<RefCountedMonitor> mMonitor;
    Side mSide;
    MessageLink* mLink;
    MessageLoop* mWorkerLoop;           // thread where work is done
    CancelableTask* mChannelErrorTask;  // NotifyMaybeChannelError runnable

    // id() of mWorkerLoop.  This persists even after mWorkerLoop is cleared
    // during channel shutdown.
    int mWorkerLoopID;

    // A task encapsulating dequeuing one pending message.
    nsRefPtr<RefCountedTask> mDequeueOneTask;

    // Timeout periods are broken up in two to prevent system suspension from
    // triggering an abort. This method (called by WaitForEvent with a 'did
    // timeout' flag) decides if we should wait again for half of mTimeoutMs
    // or give up.
    int32_t mTimeoutMs;
    bool mInTimeoutSecondHalf;

    // Worker-thread only; sequence numbers for messages that require
    // synchronous replies.
    int32_t mNextSeqno;

    static bool sIsPumpingMessages;

    class AutoEnterPendingReply {
      public:
        AutoEnterPendingReply(size_t &replyVar)
          : mReplyVar(replyVar)
        {
            mReplyVar++;
        }
        ~AutoEnterPendingReply() {
            mReplyVar--;
        }
      private:
        size_t& mReplyVar;
    };

    // Worker-thread only; type we're expecting for the reply to a sync
    // out-message. This will never be greater than 1.
    size_t mPendingSyncReplies;

    // Worker-thread only; Number of urgent and rpc replies we're waiting on.
    // These are mutually exclusive since one channel cannot have outcalls of
    // both kinds.
    size_t mPendingUrgentReplies;
    size_t mPendingRPCReplies;

    // When we send an urgent request from the parent process, we could race
    // with an RPC message that was issued by the child beforehand. In this
    // case, if the parent were to wake up while waiting for the urgent reply,
    // and process the RPC, it could send an additional urgent message. The
    // child would wake up to process the urgent message (as it always will),
    // then send a reply, which could be received by the parent out-of-order
    // with respect to the first urgent reply.
    //
    // To address this problem, urgent or RPC requests are associated with a
    // "transaction". Whenever one side of the channel wishes to start a
    // chain of RPC/urgent messages, it allocates a new transaction ID. Any
    // messages the parent receives, not apart of this transaction, are
    // deferred. When issuing RPC/urgent requests on top of a started
    // transaction, the initiating transaction ID is used.
    // 
    // To ensure IDs are unique, we use sequence numbers for transaction IDs,
    // which grow in opposite directions from child to parent.

    // The current transaction ID.
    int32_t mCurrentRPCTransaction;

    class AutoEnterRPCTransaction
    {
      public:
       AutoEnterRPCTransaction(MessageChannel *aChan)
        : mChan(aChan),
          mOldTransaction(mChan->mCurrentRPCTransaction)
       {
           mChan->mMonitor->AssertCurrentThreadOwns();
           if (mChan->mCurrentRPCTransaction == 0)
               mChan->mCurrentRPCTransaction = mChan->NextSeqno();
       }
       AutoEnterRPCTransaction(MessageChannel *aChan, Message *message)
        : mChan(aChan),
          mOldTransaction(mChan->mCurrentRPCTransaction)
       {
           mChan->mMonitor->AssertCurrentThreadOwns();

           if (!message->is_rpc() && !message->is_urgent())
               return;

           MOZ_ASSERT_IF(mChan->mSide == ParentSide,
                         !mOldTransaction || mOldTransaction == message->transaction_id());
           mChan->mCurrentRPCTransaction = message->transaction_id();
       }
       ~AutoEnterRPCTransaction() {
           mChan->mMonitor->AssertCurrentThreadOwns();
           mChan->mCurrentRPCTransaction = mOldTransaction;
       }

      private:
       MessageChannel *mChan;
       int32_t mOldTransaction;
    };

    // If waiting for the reply to a sync out-message, it will be saved here
    // on the I/O thread and then read and cleared by the worker thread.
    nsAutoPtr<Message> mRecvd;

    // Set while we are dispatching a synchronous message.
    bool mDispatchingSyncMessage;

    // Count of the recursion depth of dispatching urgent messages.
    size_t mDispatchingUrgentMessageCount;

    // Queue of all incoming messages, except for replies to sync and urgent
    // messages, which are delivered directly to mRecvd, and any pending urgent
    // incall, which is stored in mPendingUrgentRequest.
    //
    // If both this side and the other side are functioning correctly, the queue
    // can only be in certain configurations.  Let
    //
    //   |A<| be an async in-message,
    //   |S<| be a sync in-message,
    //   |C<| be an Interrupt in-call,
    //   |R<| be an Interrupt reply.
    //
    // The queue can only match this configuration
    //
    //  A<* (S< | C< | R< (?{mStack.size() == 1} A<* (S< | C<)))
    //
    // The other side can send as many async messages |A<*| as it wants before
    // sending us a blocking message.
    //
    // The first case is |S<|, a sync in-msg.  The other side must be blocked,
    // and thus can't send us any more messages until we process the sync
    // in-msg.
    //
    // The second case is |C<|, an Interrupt in-call; the other side must be blocked.
    // (There's a subtlety here: this in-call might have raced with an
    // out-call, but we detect that with the mechanism below,
    // |mRemoteStackDepth|, and races don't matter to the queue.)
    //
    // Final case, the other side replied to our most recent out-call |R<|.
    // If that was the *only* out-call on our stack, |?{mStack.size() == 1}|,
    // then other side "finished with us," and went back to its own business.
    // That business might have included sending any number of async message
    // |A<*| until sending a blocking message |(S< | C<)|.  If we had more than
    // one Interrupt call on our stack, the other side *better* not have sent us
    // another blocking message, because it's blocked on a reply from us.
    //
    MessageQueue mPending;

    // Note that these two pointers are mutually exclusive. One channel cannot
    // send both urgent requests (parent -> child) and RPC calls (child->parent).
    // Also note that since initiating either requires blocking, they cannot
    // queue up on the other side. One message slot is enough.
    //
    // Normally, all other message types are deferred into into mPending, and
    // only these two types have special treatment (since they wake up blocked
    // requests). However, when an RPC in-call races with an urgent out-call,
    // the RPC message will be put into mPending instead of its slot below.
    nsAutoPtr<Message> mPendingUrgentRequest;
    nsAutoPtr<Message> mPendingRPCCall;

    // Stack of all the out-calls on which this channel is awaiting responses.
    // Each stack refers to a different protocol and the stacks are mutually
    // exclusive: multiple outcalls of the same kind cannot be initiated while
    // another is active.
    std::stack<Message> mInterruptStack;

    // This is what we think the Interrupt stack depth is on the "other side" of this
    // Interrupt channel.  We maintain this variable so that we can detect racy Interrupt
    // calls.  With each Interrupt out-call sent, we send along what *we* think the
    // stack depth of the remote side is *before* it will receive the Interrupt call.
    //
    // After sending the out-call, our stack depth is "incremented" by pushing
    // that pending message onto mPending.
    //
    // Then when processing an in-call |c|, it must be true that
    //
    //   mStack.size() == c.remoteDepth
    //
    // I.e., my depth is actually the same as what the other side thought it
    // was when it sent in-call |c|.  If this fails to hold, we have detected
    // racy Interrupt calls.
    //
    // We then increment mRemoteStackDepth *just before* processing the
    // in-call, since we know the other side is waiting on it, and decrement
    // it *just after* finishing processing that in-call, since our response
    // will pop the top of the other side's |mPending|.
    //
    // One nice aspect of this race detection is that it is symmetric; if one
    // side detects a race, then the other side must also detect the same race.
    size_t mRemoteStackDepthGuess;

    // Approximation of code frames on the C++ stack. It can only be
    // interpreted as the implication:
    //
    //  !mCxxStackFrames.empty() => MessageChannel code on C++ stack
    //
    // This member is only accessed on the worker thread, and so is not
    // protected by mMonitor.  It is managed exclusively by the helper
    // |class CxxStackFrame|.
    mozilla::Vector<InterruptFrame> mCxxStackFrames;

    // Did we process an Interrupt out-call during this stack?  Only meaningful in
    // ExitedCxxStack(), from which this variable is reset.
    bool mSawInterruptOutMsg;

    // Map of replies received "out of turn", because of Interrupt
    // in-calls racing with replies to outstanding in-calls.  See
    // https://bugzilla.mozilla.org/show_bug.cgi?id=521929.
    MessageMap mOutOfTurnReplies;

    // Stack of Interrupt in-calls that were deferred because of race
    // conditions.
    std::stack<Message> mDeferred;

#ifdef OS_WIN
    HANDLE mEvent;
#endif
};

} // namespace ipc
} // namespace mozilla

#endif  // ifndef ipc_glue_MessageChannel_h