Add m-esr52 at 52.6.0

1 files changed, 2560 insertions, 0 deletions

diff --git a/ipc/glue/MessageChannel.cpp b/ipc/glue/MessageChannel.cpp
new file mode 100644
index 000000000..70e2387d5
--- /dev/null
+++ b/ipc/glue/MessageChannel.cpp
@@ -0,0 +1,2560 @@
+/* -*- 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/. */
+
+#include "mozilla/ipc/MessageChannel.h"
+#include "mozilla/ipc/ProtocolUtils.h"
+
+#include "mozilla/dom/ScriptSettings.h"
+
+#include "mozilla/Assertions.h"
+#include "mozilla/DebugOnly.h"
+#include "mozilla/Move.h"
+#include "mozilla/SizePrintfMacros.h"
+#include "mozilla/Sprintf.h"
+#include "mozilla/Telemetry.h"
+#include "mozilla/Logging.h"
+#include "nsAutoPtr.h"
+#include "nsDebug.h"
+#include "nsISupportsImpl.h"
+#include "nsContentUtils.h"
+
+using mozilla::Move;
+
+// Undo the damage done by mozzconf.h
+#undef compress
+
+// Logging seems to be somewhat broken on b2g.
+#ifdef MOZ_B2G
+#define IPC_LOG(...)
+#else
+static mozilla::LazyLogModule sLogModule("ipc");
+#define IPC_LOG(...) MOZ_LOG(sLogModule, LogLevel::Debug, (__VA_ARGS__))
+#endif
+
+/*
+ * IPC design:
+ *
+ * There are three kinds of messages: async, sync, and intr. Sync and intr
+ * messages are blocking.
+ *
+ * Terminology: To dispatch a message Foo is to run the RecvFoo code for
+ * it. This is also called "handling" the message.
+ *
+ * Sync and async messages can sometimes "nest" inside other sync messages
+ * (i.e., while waiting for the sync reply, we can dispatch the inner
+ * message). Intr messages cannot nest.  The three possible nesting levels are
+ * NOT_NESTED, NESTED_INSIDE_SYNC, and NESTED_INSIDE_CPOW.  The intended uses
+ * are:
+ *   NOT_NESTED - most messages.
+ *   NESTED_INSIDE_SYNC - CPOW-related messages, which are always sync
+ *                        and can go in either direction.
+ *   NESTED_INSIDE_CPOW - messages where we don't want to dispatch
+ *                        incoming CPOWs while waiting for the response.
+ * These nesting levels are ordered: NOT_NESTED, NESTED_INSIDE_SYNC,
+ * NESTED_INSIDE_CPOW.  Async messages cannot be NESTED_INSIDE_SYNC but they can
+ * be NESTED_INSIDE_CPOW.
+ *
+ * To avoid jank, the parent process is not allowed to send NOT_NESTED sync messages.
+ * When a process is waiting for a response to a sync message
+ * M0, it will dispatch an incoming message M if:
+ *   1. M has a higher nesting level than M0, or
+ *   2. if M has the same nesting level as M0 and we're in the child, or
+ *   3. if M has the same nesting level as M0 and it was sent by the other side
+ *      while dispatching M0.
+ * The idea is that messages with higher nesting should take precendence. The
+ * purpose of rule 2 is to handle a race where both processes send to each other
+ * simultaneously. In this case, we resolve the race in favor of the parent (so
+ * the child dispatches first).
+ *
+ * Messages satisfy the following properties:
+ *   A. When waiting for a response to a sync message, we won't dispatch any
+ *      messages of nesting level.
+ *   B. Messages of the same nesting level will be dispatched roughly in the
+ *      order they were sent. The exception is when the parent and child send
+ *      sync messages to each other simulataneously. In this case, the parent's
+ *      message is dispatched first. While it is dispatched, the child may send
+ *      further nested messages, and these messages may be dispatched before the
+ *      child's original message. We can consider ordering to be preserved here
+ *      because we pretend that the child's original message wasn't sent until
+ *      after the parent's message is finished being dispatched.
+ *
+ * When waiting for a sync message reply, we dispatch an async message only if
+ * it is NESTED_INSIDE_CPOW. Normally NESTED_INSIDE_CPOW async
+ * messages are sent only from the child. However, the parent can send
+ * NESTED_INSIDE_CPOW async messages when it is creating a bridged protocol.
+ *
+ * Intr messages are blocking and can nest, but they don't participate in the
+ * nesting levels. While waiting for an intr response, all incoming messages are
+ * dispatched until a response is received. When two intr messages race with
+ * each other, a similar scheme is used to ensure that one side wins. The
+ * winning side is chosen based on the message type.
+ *
+ * Intr messages differ from sync messages in that, while sending an intr
+ * message, we may dispatch an async message. This causes some additional
+ * complexity. One issue is that replies can be received out of order. It's also
+ * more difficult to determine whether one message is nested inside
+ * another. Consequently, intr handling uses mOutOfTurnReplies and
+ * mRemoteStackDepthGuess, which are not needed for sync messages.
+ */
+
+using namespace mozilla;
+using namespace mozilla::ipc;
+using namespace std;
+
+using mozilla::dom::AutoNoJSAPI;
+using mozilla::dom::ScriptSettingsInitialized;
+using mozilla::MonitorAutoLock;
+using mozilla::MonitorAutoUnlock;
+
+#define IPC_ASSERT(_cond, ...)                                      \
+    do {                                                            \
+        if (!(_cond))                                               \
+            DebugAbort(__FILE__, __LINE__, #_cond,## __VA_ARGS__);  \
+    } while (0)
+
+static MessageChannel* gParentProcessBlocker;
+
+namespace mozilla {
+namespace ipc {
+
+static const uint32_t kMinTelemetryMessageSize = 8192;
+
+const int32_t MessageChannel::kNoTimeout = INT32_MIN;
+
+// static
+bool MessageChannel::sIsPumpingMessages = false;
+
+enum Direction
+{
+    IN_MESSAGE,
+    OUT_MESSAGE
+};
+
+class MessageChannel::InterruptFrame
+{
+private:
+    enum Semantics
+    {
+        INTR_SEMS,
+        SYNC_SEMS,
+        ASYNC_SEMS
+    };
+
+public:
+    InterruptFrame(Direction direction, const Message* msg)
+      : mMessageName(msg->name()),
+        mMessageRoutingId(msg->routing_id()),
+        mMesageSemantics(msg->is_interrupt() ? INTR_SEMS :
+                          msg->is_sync() ? SYNC_SEMS :
+                          ASYNC_SEMS),
+        mDirection(direction),
+        mMoved(false)
+    {
+        MOZ_RELEASE_ASSERT(mMessageName);
+    }
+
+    InterruptFrame(InterruptFrame&& aOther)
+    {
+        MOZ_RELEASE_ASSERT(aOther.mMessageName);
+        mMessageName = aOther.mMessageName;
+        aOther.mMessageName = nullptr;
+        mMoved = aOther.mMoved;
+        aOther.mMoved = true;
+
+        mMessageRoutingId = aOther.mMessageRoutingId;
+        mMesageSemantics = aOther.mMesageSemantics;
+        mDirection = aOther.mDirection;
+    }
+
+    ~InterruptFrame()
+    {
+        MOZ_RELEASE_ASSERT(mMessageName || mMoved);
+    }
+
+    InterruptFrame& operator=(InterruptFrame&& aOther)
+    {
+        MOZ_RELEASE_ASSERT(&aOther != this);
+        this->~InterruptFrame();
+        new (this) InterruptFrame(Move(aOther));
+        return *this;
+    }
+
+    bool IsInterruptIncall() const
+    {
+        return INTR_SEMS == mMesageSemantics && IN_MESSAGE == mDirection;
+    }
+
+    bool IsInterruptOutcall() const
+    {
+        return INTR_SEMS == mMesageSemantics && OUT_MESSAGE == mDirection;
+    }
+
+    bool IsOutgoingSync() const {
+        return (mMesageSemantics == INTR_SEMS || mMesageSemantics == SYNC_SEMS) &&
+               mDirection == OUT_MESSAGE;
+    }
+
+    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;
+    }
+
+    int32_t GetRoutingId() const
+    {
+        return mMessageRoutingId;
+    }
+
+private:
+    const char* mMessageName;
+    int32_t mMessageRoutingId;
+    Semantics mMesageSemantics;
+    Direction mDirection;
+    bool mMoved;
+
+    // Disable harmful methods.
+    InterruptFrame(const InterruptFrame& aOther) = delete;
+    InterruptFrame& operator=(const InterruptFrame&) = delete;
+};
+
+class MOZ_STACK_CLASS MessageChannel::CxxStackFrame
+{
+public:
+    CxxStackFrame(MessageChannel& that, Direction direction, const Message* msg)
+      : mThat(that)
+    {
+        mThat.AssertWorkerThread();
+
+        if (mThat.mCxxStackFrames.empty())
+            mThat.EnteredCxxStack();
+
+        if (!mThat.mCxxStackFrames.append(InterruptFrame(direction, msg)))
+            MOZ_CRASH();
+
+        const InterruptFrame& frame = mThat.mCxxStackFrames.back();
+
+        if (frame.IsInterruptIncall())
+            mThat.EnteredCall();
+
+        if (frame.IsOutgoingSync())
+            mThat.EnteredSyncSend();
+
+        mThat.mSawInterruptOutMsg |= frame.IsInterruptOutcall();
+    }
+
+    ~CxxStackFrame() {
+        mThat.AssertWorkerThread();
+
+        MOZ_RELEASE_ASSERT(!mThat.mCxxStackFrames.empty());
+
+        const InterruptFrame& frame = mThat.mCxxStackFrames.back();
+        bool exitingSync = frame.IsOutgoingSync();
+        bool exitingCall = frame.IsInterruptIncall();
+        mThat.mCxxStackFrames.shrinkBy(1);
+
+        bool exitingStack = mThat.mCxxStackFrames.empty();
+
+        // According how lifetime is declared, mListener on MessageChannel
+        // lives longer than MessageChannel itself.  Hence is expected to
+        // be alive.  There is nothing to even assert here, there is no place
+        // we would be nullifying mListener on MessageChannel.
+
+        if (exitingCall)
+            mThat.ExitedCall();
+
+        if (exitingSync)
+            mThat.ExitedSyncSend();
+
+        if (exitingStack)
+            mThat.ExitedCxxStack();
+    }
+private:
+    MessageChannel& mThat;
+
+    // Disable harmful methods.
+    CxxStackFrame() = delete;
+    CxxStackFrame(const CxxStackFrame&) = delete;
+    CxxStackFrame& operator=(const CxxStackFrame&) = delete;
+};
+
+class AutoEnterTransaction
+{
+public:
+    explicit AutoEnterTransaction(MessageChannel *aChan,
+                                  int32_t aMsgSeqno,
+                                  int32_t aTransactionID,
+                                  int aNestedLevel)
+      : mChan(aChan),
+        mActive(true),
+        mOutgoing(true),
+        mNestedLevel(aNestedLevel),
+        mSeqno(aMsgSeqno),
+        mTransaction(aTransactionID),
+        mNext(mChan->mTransactionStack)
+    {
+        mChan->mMonitor->AssertCurrentThreadOwns();
+        mChan->mTransactionStack = this;
+    }
+
+    explicit AutoEnterTransaction(MessageChannel *aChan, const IPC::Message &aMessage)
+      : mChan(aChan),
+        mActive(true),
+        mOutgoing(false),
+        mNestedLevel(aMessage.nested_level()),
+        mSeqno(aMessage.seqno()),
+        mTransaction(aMessage.transaction_id()),
+        mNext(mChan->mTransactionStack)
+    {
+        mChan->mMonitor->AssertCurrentThreadOwns();
+
+        if (!aMessage.is_sync()) {
+            mActive = false;
+            return;
+        }
+
+        mChan->mTransactionStack = this;
+    }
+
+    ~AutoEnterTransaction() {
+        mChan->mMonitor->AssertCurrentThreadOwns();
+        if (mActive) {
+            mChan->mTransactionStack = mNext;
+        }
+    }
+
+    void Cancel() {
+        AutoEnterTransaction *cur = mChan->mTransactionStack;
+        MOZ_RELEASE_ASSERT(cur == this);
+        while (cur && cur->mNestedLevel != IPC::Message::NOT_NESTED) {
+            // Note that, in the following situation, we will cancel multiple
+            // transactions:
+            // 1. Parent sends NESTED_INSIDE_SYNC message P1 to child.
+            // 2. Child sends NESTED_INSIDE_SYNC message C1 to child.
+            // 3. Child dispatches P1, parent blocks.
+            // 4. Child cancels.
+            // In this case, both P1 and C1 are cancelled. The parent will
+            // remove C1 from its queue when it gets the cancellation message.
+            MOZ_RELEASE_ASSERT(cur->mActive);
+            cur->mActive = false;
+            cur = cur->mNext;
+        }
+
+        mChan->mTransactionStack = cur;
+
+        MOZ_RELEASE_ASSERT(IsComplete());
+    }
+
+    bool AwaitingSyncReply() const {
+        MOZ_RELEASE_ASSERT(mActive);
+        if (mOutgoing) {
+            return true;
+        }
+        return mNext ? mNext->AwaitingSyncReply() : false;
+    }
+
+    int AwaitingSyncReplyNestedLevel() const {
+        MOZ_RELEASE_ASSERT(mActive);
+        if (mOutgoing) {
+            return mNestedLevel;
+        }
+        return mNext ? mNext->AwaitingSyncReplyNestedLevel() : 0;
+    }
+
+    bool DispatchingSyncMessage() const {
+        MOZ_RELEASE_ASSERT(mActive);
+        if (!mOutgoing) {
+            return true;
+        }
+        return mNext ? mNext->DispatchingSyncMessage() : false;
+    }
+
+    int DispatchingSyncMessageNestedLevel() const {
+        MOZ_RELEASE_ASSERT(mActive);
+        if (!mOutgoing) {
+            return mNestedLevel;
+        }
+        return mNext ? mNext->DispatchingSyncMessageNestedLevel() : 0;
+    }
+
+    int NestedLevel() const {
+        MOZ_RELEASE_ASSERT(mActive);
+        return mNestedLevel;
+    }
+
+    int32_t SequenceNumber() const {
+        MOZ_RELEASE_ASSERT(mActive);
+        return mSeqno;
+    }
+
+    int32_t TransactionID() const {
+        MOZ_RELEASE_ASSERT(mActive);
+        return mTransaction;
+    }
+
+    void ReceivedReply(IPC::Message&& aMessage) {
+        MOZ_RELEASE_ASSERT(aMessage.seqno() == mSeqno);
+        MOZ_RELEASE_ASSERT(aMessage.transaction_id() == mTransaction);
+        MOZ_RELEASE_ASSERT(!mReply);
+        IPC_LOG("Reply received on worker thread: seqno=%d", mSeqno);
+        mReply = new IPC::Message(Move(aMessage));
+        MOZ_RELEASE_ASSERT(IsComplete());
+    }
+
+    void HandleReply(IPC::Message&& aMessage) {
+        AutoEnterTransaction *cur = mChan->mTransactionStack;
+        MOZ_RELEASE_ASSERT(cur == this);
+        while (cur) {
+            MOZ_RELEASE_ASSERT(cur->mActive);
+            if (aMessage.seqno() == cur->mSeqno) {
+                cur->ReceivedReply(Move(aMessage));
+                break;
+            }
+            cur = cur->mNext;
+            MOZ_RELEASE_ASSERT(cur);
+        }
+    }
+
+    bool IsComplete() {
+        return !mActive || mReply;
+    }
+
+    bool IsOutgoing() {
+        return mOutgoing;
+    }
+
+    bool IsCanceled() {
+        return !mActive;
+    }
+
+    bool IsBottom() const {
+        return !mNext;
+    }
+
+    bool IsError() {
+        MOZ_RELEASE_ASSERT(mReply);
+        return mReply->is_reply_error();
+    }
+
+    nsAutoPtr<IPC::Message> GetReply() {
+        return Move(mReply);
+    }
+
+private:
+    MessageChannel *mChan;
+
+    // Active is true if this transaction is on the mChan->mTransactionStack
+    // stack. Generally we're not on the stack if the transaction was canceled
+    // or if it was for a message that doesn't require transactions (an async
+    // message).
+    bool mActive;
+
+    // Is this stack frame for an outgoing message?
+    bool mOutgoing;
+
+    // Properties of the message being sent/received.
+    int mNestedLevel;
+    int32_t mSeqno;
+    int32_t mTransaction;
+
+    // Next item in mChan->mTransactionStack.
+    AutoEnterTransaction *mNext;
+
+    // Pointer the a reply received for this message, if one was received.
+    nsAutoPtr<IPC::Message> mReply;
+};
+
+MessageChannel::MessageChannel(IToplevelProtocol *aListener)
+  : mListener(aListener),
+    mChannelState(ChannelClosed),
+    mSide(UnknownSide),
+    mLink(nullptr),
+    mWorkerLoop(nullptr),
+    mChannelErrorTask(nullptr),
+    mWorkerLoopID(-1),
+    mTimeoutMs(kNoTimeout),
+    mInTimeoutSecondHalf(false),
+    mNextSeqno(0),
+    mLastSendError(SyncSendError::SendSuccess),
+    mDispatchingAsyncMessage(false),
+    mDispatchingAsyncMessageNestedLevel(0),
+    mTransactionStack(nullptr),
+    mTimedOutMessageSeqno(0),
+    mTimedOutMessageNestedLevel(0),
+    mRemoteStackDepthGuess(0),
+    mSawInterruptOutMsg(false),
+    mIsWaitingForIncoming(false),
+    mAbortOnError(false),
+    mNotifiedChannelDone(false),
+    mFlags(REQUIRE_DEFAULT),
+    mPeerPidSet(false),
+    mPeerPid(-1)
+{
+    MOZ_COUNT_CTOR(ipc::MessageChannel);
+
+#ifdef OS_WIN
+    mTopFrame = nullptr;
+    mIsSyncWaitingOnNonMainThread = false;
+#endif
+
+    mOnChannelConnectedTask =
+        NewNonOwningCancelableRunnableMethod(this, &MessageChannel::DispatchOnChannelConnected);
+
+#ifdef OS_WIN
+    mEvent = CreateEventW(nullptr, TRUE, FALSE, nullptr);
+    MOZ_RELEASE_ASSERT(mEvent, "CreateEvent failed! Nothing is going to work!");
+#endif
+}
+
+MessageChannel::~MessageChannel()
+{
+    MOZ_COUNT_DTOR(ipc::MessageChannel);
+    IPC_ASSERT(mCxxStackFrames.empty(), "mismatched CxxStackFrame ctor/dtors");
+#ifdef OS_WIN
+    if (mEvent) {
+        BOOL ok = CloseHandle(mEvent);
+        mEvent = nullptr;
+
+        if (!ok) {
+            gfxDevCrash(mozilla::gfx::LogReason::MessageChannelCloseFailure) <<
+                "MessageChannel failed to close. GetLastError: " <<
+                GetLastError();
+        }
+        MOZ_RELEASE_ASSERT(ok);
+    } else {
+        gfxDevCrash(mozilla::gfx::LogReason::MessageChannelCloseFailure) <<
+                "MessageChannel destructor ran without an mEvent Handle";
+    }
+#endif
+    Clear();
+}
+
+// This function returns the current transaction ID. Since the notion of a
+// "current transaction" can be hard to define when messages race with each
+// other and one gets canceled and the other doesn't, we require that this
+// function is only called when the current transaction is known to be for a
+// NESTED_INSIDE_SYNC message. In that case, we know for sure what the caller is
+// looking for.
+int32_t
+MessageChannel::CurrentNestedInsideSyncTransaction() const
+{
+    mMonitor->AssertCurrentThreadOwns();
+    if (!mTransactionStack) {
+        return 0;
+    }
+    MOZ_RELEASE_ASSERT(mTransactionStack->NestedLevel() == IPC::Message::NESTED_INSIDE_SYNC);
+    return mTransactionStack->TransactionID();
+}
+
+bool
+MessageChannel::AwaitingSyncReply() const
+{
+    mMonitor->AssertCurrentThreadOwns();
+    return mTransactionStack ? mTransactionStack->AwaitingSyncReply() : false;
+}
+
+int
+MessageChannel::AwaitingSyncReplyNestedLevel() const
+{
+    mMonitor->AssertCurrentThreadOwns();
+    return mTransactionStack ? mTransactionStack->AwaitingSyncReplyNestedLevel() : 0;
+}
+
+bool
+MessageChannel::DispatchingSyncMessage() const
+{
+    mMonitor->AssertCurrentThreadOwns();
+    return mTransactionStack ? mTransactionStack->DispatchingSyncMessage() : false;
+}
+
+int
+MessageChannel::DispatchingSyncMessageNestedLevel() const
+{
+    mMonitor->AssertCurrentThreadOwns();
+    return mTransactionStack ? mTransactionStack->DispatchingSyncMessageNestedLevel() : 0;
+}
+
+static void
+PrintErrorMessage(Side side, const char* channelName, const char* msg)
+{
+    const char *from = (side == ChildSide)
+                       ? "Child"
+                       : ((side == ParentSide) ? "Parent" : "Unknown");
+    printf_stderr("\n###!!! [%s][%s] Error: %s\n\n", from, channelName, msg);
+}
+
+bool
+MessageChannel::Connected() const
+{
+    mMonitor->AssertCurrentThreadOwns();
+
+    // The transport layer allows us to send messages before
+    // receiving the "connected" ack from the remote side.
+    return (ChannelOpening == mChannelState || ChannelConnected == mChannelState);
+}
+
+bool
+MessageChannel::CanSend() const
+{
+    if (!mMonitor) {
+        return false;
+    }
+    MonitorAutoLock lock(*mMonitor);
+    return Connected();
+}
+
+void
+MessageChannel::Clear()
+{
+    // Don't clear mWorkerLoopID; we use it in AssertLinkThread() and
+    // AssertWorkerThread().
+    //
+    // Also don't clear mListener.  If we clear it, then sending a message
+    // through this channel after it's Clear()'ed can cause this process to
+    // crash.
+    //
+    // In practice, mListener owns the channel, so the channel gets deleted
+    // before mListener.  But just to be safe, mListener is a weak pointer.
+
+    if (gParentProcessBlocker == this) {
+        gParentProcessBlocker = nullptr;
+    }
+
+    mWorkerLoop = nullptr;
+    delete mLink;
+    mLink = nullptr;
+
+    mOnChannelConnectedTask->Cancel();
+
+    if (mChannelErrorTask) {
+        mChannelErrorTask->Cancel();
+        mChannelErrorTask = nullptr;
+    }
+
+    // Free up any memory used by pending messages.
+    for (RefPtr<MessageTask> task : mPending) {
+        task->Clear();
+    }
+    mPending.clear();
+
+    mOutOfTurnReplies.clear();
+    while (!mDeferred.empty()) {
+        mDeferred.pop();
+    }
+}
+
+bool
+MessageChannel::Open(Transport* aTransport, MessageLoop* aIOLoop, Side aSide)
+{
+    NS_PRECONDITION(!mLink, "Open() called > once");
+
+    mMonitor = new RefCountedMonitor();
+    mWorkerLoop = MessageLoop::current();
+    mWorkerLoopID = mWorkerLoop->id();
+
+    ProcessLink *link = new ProcessLink(this);
+    link->Open(aTransport, aIOLoop, aSide); // :TODO: n.b.: sets mChild
+    mLink = link;
+    return true;
+}
+
+bool
+MessageChannel::Open(MessageChannel *aTargetChan, MessageLoop *aTargetLoop, Side aSide)
+{
+    // Opens a connection to another thread in the same process.
+
+    //  This handshake proceeds as follows:
+    //  - Let A be the thread initiating the process (either child or parent)
+    //    and B be the other thread.
+    //  - A spawns thread for B, obtaining B's message loop
+    //  - A creates ProtocolChild and ProtocolParent instances.
+    //    Let PA be the one appropriate to A and PB the side for B.
+    //  - A invokes PA->Open(PB, ...):
+    //    - set state to mChannelOpening
+    //    - this will place a work item in B's worker loop (see next bullet)
+    //      and then spins until PB->mChannelState becomes mChannelConnected
+    //    - meanwhile, on PB's worker loop, the work item is removed and:
+    //      - invokes PB->SlaveOpen(PA, ...):
+    //        - sets its state and that of PA to Connected
+    NS_PRECONDITION(aTargetChan, "Need a target channel");
+    NS_PRECONDITION(ChannelClosed == mChannelState, "Not currently closed");
+
+    CommonThreadOpenInit(aTargetChan, aSide);
+
+    Side oppSide = UnknownSide;
+    switch(aSide) {
+      case ChildSide: oppSide = ParentSide; break;
+      case ParentSide: oppSide = ChildSide; break;
+      case UnknownSide: break;
+    }
+
+    mMonitor = new RefCountedMonitor();
+
+    MonitorAutoLock lock(*mMonitor);
+    mChannelState = ChannelOpening;
+    aTargetLoop->PostTask(NewNonOwningRunnableMethod
+                          <MessageChannel*, Side>(aTargetChan,
+                                                  &MessageChannel::OnOpenAsSlave,
+                                                  this, oppSide));
+
+    while (ChannelOpening == mChannelState)
+        mMonitor->Wait();
+    MOZ_RELEASE_ASSERT(ChannelConnected == mChannelState, "not connected when awoken");
+    return (ChannelConnected == mChannelState);
+}
+
+void
+MessageChannel::OnOpenAsSlave(MessageChannel *aTargetChan, Side aSide)
+{
+    // Invoked when the other side has begun the open.
+    NS_PRECONDITION(ChannelClosed == mChannelState,
+                    "Not currently closed");
+    NS_PRECONDITION(ChannelOpening == aTargetChan->mChannelState,
+                    "Target channel not in the process of opening");
+
+    CommonThreadOpenInit(aTargetChan, aSide);
+    mMonitor = aTargetChan->mMonitor;
+
+    MonitorAutoLock lock(*mMonitor);
+    MOZ_RELEASE_ASSERT(ChannelOpening == aTargetChan->mChannelState,
+                          "Target channel not in the process of opening");
+    mChannelState = ChannelConnected;
+    aTargetChan->mChannelState = ChannelConnected;
+    aTargetChan->mMonitor->Notify();
+}
+
+void
+MessageChannel::CommonThreadOpenInit(MessageChannel *aTargetChan, Side aSide)
+{
+    mWorkerLoop = MessageLoop::current();
+    mWorkerLoopID = mWorkerLoop->id();
+    mLink = new ThreadLink(this, aTargetChan);
+    mSide = aSide;
+}
+
+bool
+MessageChannel::Echo(Message* aMsg)
+{
+    nsAutoPtr<Message> msg(aMsg);
+    AssertWorkerThread();
+    mMonitor->AssertNotCurrentThreadOwns();
+    if (MSG_ROUTING_NONE == msg->routing_id()) {
+        ReportMessageRouteError("MessageChannel::Echo");
+        return false;
+    }
+
+    MonitorAutoLock lock(*mMonitor);
+
+    if (!Connected()) {
+        ReportConnectionError("MessageChannel", msg);
+        return false;
+    }
+
+    mLink->EchoMessage(msg.forget());
+    return true;
+}
+
+bool
+MessageChannel::Send(Message* aMsg)
+{
+    if (aMsg->size() >= kMinTelemetryMessageSize) {
+        Telemetry::Accumulate(Telemetry::IPC_MESSAGE_SIZE,
+                              nsDependentCString(aMsg->name()), aMsg->size());
+    }
+
+    MOZ_RELEASE_ASSERT(!aMsg->is_sync());
+    MOZ_RELEASE_ASSERT(aMsg->nested_level() != IPC::Message::NESTED_INSIDE_SYNC);
+
+    CxxStackFrame frame(*this, OUT_MESSAGE, aMsg);
+
+    nsAutoPtr<Message> msg(aMsg);
+    AssertWorkerThread();
+    mMonitor->AssertNotCurrentThreadOwns();
+    if (MSG_ROUTING_NONE == msg->routing_id()) {
+        ReportMessageRouteError("MessageChannel::Send");
+        return false;
+    }
+
+    MonitorAutoLock lock(*mMonitor);
+    if (!Connected()) {
+        ReportConnectionError("MessageChannel", msg);
+        return false;
+    }
+    mLink->SendMessage(msg.forget());
+    return true;
+}
+
+class CancelMessage : public IPC::Message
+{
+public:
+    explicit CancelMessage(int transaction) :
+        IPC::Message(MSG_ROUTING_NONE, CANCEL_MESSAGE_TYPE)
+    {
+        set_transaction_id(transaction);
+    }
+    static bool Read(const Message* msg) {
+        return true;
+    }
+    void Log(const std::string& aPrefix, FILE* aOutf) const {
+        fputs("(special `Cancel' message)", aOutf);
+    }
+};
+
+bool
+MessageChannel::MaybeInterceptSpecialIOMessage(const Message& aMsg)
+{
+    AssertLinkThread();
+    mMonitor->AssertCurrentThreadOwns();
+
+    if (MSG_ROUTING_NONE == aMsg.routing_id()) {
+        if (GOODBYE_MESSAGE_TYPE == aMsg.type()) {
+            // :TODO: Sort out Close() on this side racing with Close() on the
+            // other side
+            mChannelState = ChannelClosing;
+            if (LoggingEnabled()) {
+                printf("NOTE: %s process received `Goodbye', closing down\n",
+                       (mSide == ChildSide) ? "child" : "parent");
+            }
+            return true;
+        } else if (CANCEL_MESSAGE_TYPE == aMsg.type()) {
+            IPC_LOG("Cancel from message");
+            CancelTransaction(aMsg.transaction_id());
+            NotifyWorkerThread();
+            return true;
+        }
+    }
+    return false;
+}
+
+bool
+MessageChannel::ShouldDeferMessage(const Message& aMsg)
+{
+    // Never defer messages that have the highest nested level, even async
+    // ones. This is safe because only the child can send these messages, so
+    // they can never nest.
+    if (aMsg.nested_level() == IPC::Message::NESTED_INSIDE_CPOW)
+        return false;
+
+    // Unless they're NESTED_INSIDE_CPOW, we always defer async messages.
+    // Note that we never send an async NESTED_INSIDE_SYNC message.
+    if (!aMsg.is_sync()) {
+        MOZ_RELEASE_ASSERT(aMsg.nested_level() == IPC::Message::NOT_NESTED);
+        return true;
+    }
+
+    int msgNestedLevel = aMsg.nested_level();
+    int waitingNestedLevel = AwaitingSyncReplyNestedLevel();
+
+    // Always defer if the nested level of the incoming message is less than the
+    // nested level of the message we're awaiting.
+    if (msgNestedLevel < waitingNestedLevel)
+        return true;
+
+    // Never defer if the message has strictly greater nested level.
+    if (msgNestedLevel > waitingNestedLevel)
+        return false;
+
+    // When both sides send sync messages of the same nested level, we resolve the
+    // race by dispatching in the child and deferring the incoming message in
+    // the parent. However, the parent still needs to dispatch nested sync
+    // messages.
+    //
+    // Deferring in the parent only sort of breaks message ordering. When the
+    // child's message comes in, we can pretend the child hasn't quite
+    // finished sending it yet. Since the message is sync, we know that the
+    // child hasn't moved on yet.
+    return mSide == ParentSide && aMsg.transaction_id() != CurrentNestedInsideSyncTransaction();
+}
+
+void
+MessageChannel::OnMessageReceivedFromLink(Message&& aMsg)
+{
+    AssertLinkThread();
+    mMonitor->AssertCurrentThreadOwns();
+
+    if (MaybeInterceptSpecialIOMessage(aMsg))
+        return;
+
+    // Regardless of the Interrupt stack, if we're awaiting a sync reply,
+    // we know that it needs to be immediately handled to unblock us.
+    if (aMsg.is_sync() && aMsg.is_reply()) {
+        IPC_LOG("Received reply seqno=%d xid=%d", aMsg.seqno(), aMsg.transaction_id());
+
+        if (aMsg.seqno() == mTimedOutMessageSeqno) {
+            // Drop the message, but allow future sync messages to be sent.
+            IPC_LOG("Received reply to timedout message; igoring; xid=%d", mTimedOutMessageSeqno);
+            EndTimeout();
+            return;
+        }
+
+        MOZ_RELEASE_ASSERT(AwaitingSyncReply());
+        MOZ_RELEASE_ASSERT(!mTimedOutMessageSeqno);
+
+        mTransactionStack->HandleReply(Move(aMsg));
+        NotifyWorkerThread();
+        return;
+    }
+
+    // Nested messages cannot be compressed.
+    MOZ_RELEASE_ASSERT(aMsg.compress_type() == IPC::Message::COMPRESSION_NONE ||
+                       aMsg.nested_level() == IPC::Message::NOT_NESTED);
+
+    bool reuseTask = false;
+    if (aMsg.compress_type() == IPC::Message::COMPRESSION_ENABLED) {
+        bool compress = (!mPending.isEmpty() &&
+                         mPending.getLast()->Msg().type() == aMsg.type() &&
+                         mPending.getLast()->Msg().routing_id() == aMsg.routing_id());
+        if (compress) {
+            // This message type has compression enabled, and the back of the
+            // queue was the same message type and routed to the same destination.
+            // Replace it with the newer message.
+            MOZ_RELEASE_ASSERT(mPending.getLast()->Msg().compress_type() ==
+                               IPC::Message::COMPRESSION_ENABLED);
+            mPending.getLast()->Msg() = Move(aMsg);
+
+            reuseTask = true;
+        }
+    } else if (aMsg.compress_type() == IPC::Message::COMPRESSION_ALL && !mPending.isEmpty()) {
+        for (RefPtr<MessageTask> p = mPending.getLast(); p; p = p->getPrevious()) {
+            if (p->Msg().type() == aMsg.type() &&
+                p->Msg().routing_id() == aMsg.routing_id())
+            {
+                // This message type has compression enabled, and the queue
+                // holds a message with the same message type and routed to the
+                // same destination. Erase it. Note that, since we always
+                // compress these redundancies, There Can Be Only One.
+                MOZ_RELEASE_ASSERT(p->Msg().compress_type() == IPC::Message::COMPRESSION_ALL);
+                p->remove();
+                break;
+            }
+        }
+    }
+
+    bool wakeUpSyncSend = AwaitingSyncReply() && !ShouldDeferMessage(aMsg);
+
+    bool shouldWakeUp = AwaitingInterruptReply() ||
+                        wakeUpSyncSend ||
+                        AwaitingIncomingMessage();
+
+    // Although we usually don't need to post a message task if
+    // shouldWakeUp is true, it's easier to post anyway than to have to
+    // guarantee that every Send call processes everything it's supposed to
+    // before returning.
+    bool shouldPostTask = !shouldWakeUp || wakeUpSyncSend;
+
+    IPC_LOG("Receive on link thread; seqno=%d, xid=%d, shouldWakeUp=%d",
+            aMsg.seqno(), aMsg.transaction_id(), shouldWakeUp);
+
+    if (reuseTask) {
+        return;
+    }
+
+    // There are three cases we're concerned about, relating to the state of the
+    // main thread:
+    //
+    // (1) We are waiting on a sync reply - main thread is blocked on the
+    //     IPC monitor.
+    //   - If the message is NESTED_INSIDE_SYNC, we wake up the main thread to
+    //     deliver the message depending on ShouldDeferMessage. Otherwise, we
+    //     leave it in the mPending queue, posting a task to the main event
+    //     loop, where it will be processed once the synchronous reply has been
+    //     received.
+    //
+    // (2) We are waiting on an Interrupt reply - main thread is blocked on the
+    //     IPC monitor.
+    //   - Always notify and wake up the main thread.
+    //
+    // (3) We are not waiting on a reply.
+    //   - We post a task to the main event loop.
+    //
+    // Note that, we may notify the main thread even though the monitor is not
+    // blocked. This is okay, since we always check for pending events before
+    // blocking again.
+
+    RefPtr<MessageTask> task = new MessageTask(this, Move(aMsg));
+    mPending.insertBack(task);
+
+    if (shouldWakeUp) {
+        NotifyWorkerThread();
+    }
+
+    if (shouldPostTask) {
+        task->Post();
+    }
+}
+
+void
+MessageChannel::PeekMessages(mozilla::function<bool(const Message& aMsg)> aInvoke)
+{
+    // FIXME: We shouldn't be holding the lock for aInvoke!
+    MonitorAutoLock lock(*mMonitor);
+
+    for (RefPtr<MessageTask> it : mPending) {
+        const Message &msg = it->Msg();
+        if (!aInvoke(msg)) {
+            break;
+        }
+    }
+}
+
+void
+MessageChannel::ProcessPendingRequests(AutoEnterTransaction& aTransaction)
+{
+    mMonitor->AssertCurrentThreadOwns();
+
+    IPC_LOG("ProcessPendingRequests for seqno=%d, xid=%d",
+            aTransaction.SequenceNumber(), aTransaction.TransactionID());
+
+    // Loop until there aren't any more nested messages to process.
+    for (;;) {
+        // If we canceled during ProcessPendingRequest, then we need to leave
+        // immediately because the results of ShouldDeferMessage will be
+        // operating with weird state (as if no Send is in progress). That could
+        // cause even NOT_NESTED sync messages to be processed (but not
+        // NOT_NESTED async messages), which would break message ordering.
+        if (aTransaction.IsCanceled()) {
+            return;
+        }
+
+        mozilla::Vector<Message> toProcess;
+
+        for (RefPtr<MessageTask> p = mPending.getFirst(); p; ) {
+            Message &msg = p->Msg();
+
+            MOZ_RELEASE_ASSERT(!aTransaction.IsCanceled(),
+                               "Calling ShouldDeferMessage when cancelled");
+            bool defer = ShouldDeferMessage(msg);
+
+            // Only log the interesting messages.
+            if (msg.is_sync() || msg.nested_level() == IPC::Message::NESTED_INSIDE_CPOW) {
+                IPC_LOG("ShouldDeferMessage(seqno=%d) = %d", msg.seqno(), defer);
+            }
+
+            if (!defer) {
+                if (!toProcess.append(Move(msg)))
+                    MOZ_CRASH();
+
+                p = p->removeAndGetNext();
+                continue;
+            }
+            p = p->getNext();
+        }
+
+        if (toProcess.empty()) {
+            break;
+        }
+
+        // Processing these messages could result in more messages, so we
+        // loop around to check for more afterwards.
+
+        for (auto it = toProcess.begin(); it != toProcess.end(); it++) {
+            ProcessPendingRequest(Move(*it));
+        }
+    }
+}
+
+bool
+MessageChannel::Send(Message* aMsg, Message* aReply)
+{
+    if (aMsg->size() >= kMinTelemetryMessageSize) {
+        Telemetry::Accumulate(Telemetry::IPC_MESSAGE_SIZE,
+                              nsDependentCString(aMsg->name()), aMsg->size());
+    }
+
+    nsAutoPtr<Message> msg(aMsg);
+
+    // Sanity checks.
+    AssertWorkerThread();
+    mMonitor->AssertNotCurrentThreadOwns();
+
+#ifdef OS_WIN
+    SyncStackFrame frame(this, false);
+    NeuteredWindowRegion neuteredRgn(mFlags & REQUIRE_DEFERRED_MESSAGE_PROTECTION);
+#endif
+
+    CxxStackFrame f(*this, OUT_MESSAGE, msg);
+
+    MonitorAutoLock lock(*mMonitor);
+
+    if (mTimedOutMessageSeqno) {
+        // Don't bother sending another sync message if a previous one timed out
+        // and we haven't received a reply for it. Once the original timed-out
+        // message receives a reply, we'll be able to send more sync messages
+        // again.
+        IPC_LOG("Send() failed due to previous timeout");
+        mLastSendError = SyncSendError::PreviousTimeout;
+        return false;
+    }
+
+    if (DispatchingSyncMessageNestedLevel() == IPC::Message::NOT_NESTED &&
+        msg->nested_level() > IPC::Message::NOT_NESTED)
+    {
+        // Don't allow sending CPOWs while we're dispatching a sync message.
+        // If you want to do that, use sendRpcMessage instead.
+        IPC_LOG("Nested level forbids send");
+        mLastSendError = SyncSendError::SendingCPOWWhileDispatchingSync;
+        return false;
+    }
+
+    if (DispatchingSyncMessageNestedLevel() == IPC::Message::NESTED_INSIDE_CPOW ||
+        DispatchingAsyncMessageNestedLevel() == IPC::Message::NESTED_INSIDE_CPOW)
+    {
+        // Generally only the parent dispatches urgent messages. And the only
+        // sync messages it can send are NESTED_INSIDE_SYNC. Mainly we want to ensure
+        // here that we don't return false for non-CPOW messages.
+        MOZ_RELEASE_ASSERT(msg->nested_level() == IPC::Message::NESTED_INSIDE_SYNC);
+        IPC_LOG("Sending while dispatching urgent message");
+        mLastSendError = SyncSendError::SendingCPOWWhileDispatchingUrgent;
+        return false;
+    }
+
+    if (msg->nested_level() < DispatchingSyncMessageNestedLevel() ||
+        msg->nested_level() < AwaitingSyncReplyNestedLevel())
+    {
+        MOZ_RELEASE_ASSERT(DispatchingSyncMessage() || DispatchingAsyncMessage());
+        IPC_LOG("Cancel from Send");
+        CancelMessage *cancel = new CancelMessage(CurrentNestedInsideSyncTransaction());
+        CancelTransaction(CurrentNestedInsideSyncTransaction());
+        mLink->SendMessage(cancel);
+    }
+
+    IPC_ASSERT(msg->is_sync(), "can only Send() sync messages here");
+
+    IPC_ASSERT(msg->nested_level() >= DispatchingSyncMessageNestedLevel(),
+               "can't send sync message of a lesser nested level than what's being dispatched");
+    IPC_ASSERT(AwaitingSyncReplyNestedLevel() <= msg->nested_level(),
+               "nested sync message sends must be of increasing nested level");
+    IPC_ASSERT(DispatchingSyncMessageNestedLevel() != IPC::Message::NESTED_INSIDE_CPOW,
+               "not allowed to send messages while dispatching urgent messages");
+
+    IPC_ASSERT(DispatchingAsyncMessageNestedLevel() != IPC::Message::NESTED_INSIDE_CPOW,
+               "not allowed to send messages while dispatching urgent messages");
+
+    if (!Connected()) {
+        ReportConnectionError("MessageChannel::SendAndWait", msg);
+        mLastSendError = SyncSendError::NotConnectedBeforeSend;
+        return false;
+    }
+
+    msg->set_seqno(NextSeqno());
+
+    int32_t seqno = msg->seqno();
+    int nestedLevel = msg->nested_level();
+    msgid_t replyType = msg->type() + 1;
+
+    AutoEnterTransaction *stackTop = mTransactionStack;
+
+    // If the most recent message on the stack is NESTED_INSIDE_SYNC, then our
+    // message should nest inside that and we use the same transaction
+    // ID. Otherwise we need a new transaction ID (so we use the seqno of the
+    // message we're sending).
+    bool nest = stackTop && stackTop->NestedLevel() == IPC::Message::NESTED_INSIDE_SYNC;
+    int32_t transaction = nest ? stackTop->TransactionID() : seqno;
+    msg->set_transaction_id(transaction);
+
+    bool handleWindowsMessages = mListener->HandleWindowsMessages(*aMsg);
+    AutoEnterTransaction transact(this, seqno, transaction, nestedLevel);
+
+    IPC_LOG("Send seqno=%d, xid=%d", seqno, transaction);
+
+    // msg will be destroyed soon, but name() is not owned by msg.
+    const char* msgName = msg->name();
+
+    mLink->SendMessage(msg.forget());
+
+    while (true) {
+        MOZ_RELEASE_ASSERT(!transact.IsCanceled());
+        ProcessPendingRequests(transact);
+        if (transact.IsComplete()) {
+            break;
+        }
+        if (!Connected()) {
+            ReportConnectionError("MessageChannel::Send");
+            mLastSendError = SyncSendError::DisconnectedDuringSend;
+            return false;
+        }
+
+        MOZ_RELEASE_ASSERT(!mTimedOutMessageSeqno);
+        MOZ_RELEASE_ASSERT(!transact.IsComplete());
+        MOZ_RELEASE_ASSERT(mTransactionStack == &transact);
+
+        bool maybeTimedOut = !WaitForSyncNotify(handleWindowsMessages);
+
+        if (mListener->NeedArtificialSleep()) {
+            MonitorAutoUnlock unlock(*mMonitor);
+            mListener->ArtificialSleep();
+        }
+
+        if (!Connected()) {
+            ReportConnectionError("MessageChannel::SendAndWait");
+            mLastSendError = SyncSendError::DisconnectedDuringSend;
+            return false;
+        }
+
+        if (transact.IsCanceled()) {
+            break;
+        }
+
+        MOZ_RELEASE_ASSERT(mTransactionStack == &transact);
+
+        // We only time out a message if it initiated a new transaction (i.e.,
+        // if neither side has any other message Sends on the stack).
+        bool canTimeOut = transact.IsBottom();
+        if (maybeTimedOut && canTimeOut && !ShouldContinueFromTimeout()) {
+            // Since ShouldContinueFromTimeout drops the lock, we need to
+            // re-check all our conditions here. We shouldn't time out if any of
+            // these things happen because there won't be a reply to the timed
+            // out message in these cases.
+            if (transact.IsComplete()) {
+                break;
+            }
+
+            IPC_LOG("Timing out Send: xid=%d", transaction);
+
+            mTimedOutMessageSeqno = seqno;
+            mTimedOutMessageNestedLevel = nestedLevel;
+            mLastSendError = SyncSendError::TimedOut;
+            return false;
+        }
+
+        if (transact.IsCanceled()) {
+            break;
+        }
+    }
+
+    if (transact.IsCanceled()) {
+        IPC_LOG("Other side canceled seqno=%d, xid=%d", seqno, transaction);
+        mLastSendError = SyncSendError::CancelledAfterSend;
+        return false;
+    }
+
+    if (transact.IsError()) {
+        IPC_LOG("Error: seqno=%d, xid=%d", seqno, transaction);
+        mLastSendError = SyncSendError::ReplyError;
+        return false;
+    }
+
+    IPC_LOG("Got reply: seqno=%d, xid=%d", seqno, transaction);
+
+    nsAutoPtr<Message> reply = transact.GetReply();
+
+    MOZ_RELEASE_ASSERT(reply);
+    MOZ_RELEASE_ASSERT(reply->is_reply(), "expected reply");
+    MOZ_RELEASE_ASSERT(!reply->is_reply_error());
+    MOZ_RELEASE_ASSERT(reply->seqno() == seqno);
+    MOZ_RELEASE_ASSERT(reply->type() == replyType, "wrong reply type");
+    MOZ_RELEASE_ASSERT(reply->is_sync());
+
+    *aReply = Move(*reply);
+    if (aReply->size() >= kMinTelemetryMessageSize) {
+        Telemetry::Accumulate(Telemetry::IPC_REPLY_SIZE,
+                              nsDependentCString(msgName), aReply->size());
+    }
+    return true;
+}
+
+bool
+MessageChannel::Call(Message* aMsg, Message* aReply)
+{
+    nsAutoPtr<Message> msg(aMsg);
+    AssertWorkerThread();
+    mMonitor->AssertNotCurrentThreadOwns();
+
+#ifdef OS_WIN
+    SyncStackFrame frame(this, true);
+#endif
+
+    // This must come before MonitorAutoLock, as its destructor acquires the
+    // monitor lock.
+    CxxStackFrame cxxframe(*this, OUT_MESSAGE, msg);
+
+    MonitorAutoLock lock(*mMonitor);
+    if (!Connected()) {
+        ReportConnectionError("MessageChannel::Call", msg);
+        return false;
+    }
+
+    // Sanity checks.
+    IPC_ASSERT(!AwaitingSyncReply(),
+               "cannot issue Interrupt call while blocked on sync request");
+    IPC_ASSERT(!DispatchingSyncMessage(),
+               "violation of sync handler invariant");
+    IPC_ASSERT(msg->is_interrupt(), "can only Call() Interrupt messages here");
+
+    msg->set_seqno(NextSeqno());
+    msg->set_interrupt_remote_stack_depth_guess(mRemoteStackDepthGuess);
+    msg->set_interrupt_local_stack_depth(1 + InterruptStackDepth());
+    mInterruptStack.push(MessageInfo(*msg));
+    mLink->SendMessage(msg.forget());
+
+    while (true) {
+        // if a handler invoked by *Dispatch*() spun a nested event
+        // loop, and the connection was broken during that loop, we
+        // might have already processed the OnError event. if so,
+        // trying another loop iteration will be futile because
+        // channel state will have been cleared
+        if (!Connected()) {
+            ReportConnectionError("MessageChannel::Call");
+            return false;
+        }
+
+#ifdef OS_WIN
+        // We need to limit the scoped of neuteredRgn to this spot in the code.
+        // Window neutering can't be enabled during some plugin calls because
+        // we then risk the neutered window procedure being subclassed by a
+        // plugin.
+        {
+            NeuteredWindowRegion neuteredRgn(mFlags & REQUIRE_DEFERRED_MESSAGE_PROTECTION);
+            /* We should pump messages at this point to ensure that the IPC peer
+               does not become deadlocked on a pending inter-thread SendMessage() */
+            neuteredRgn.PumpOnce();
+        }
+#endif
+
+        // Now might be the time to process a message deferred because of race
+        // resolution.
+        MaybeUndeferIncall();
+
+        // Wait for an event to occur.
+        while (!InterruptEventOccurred()) {
+            bool maybeTimedOut = !WaitForInterruptNotify();
+
+            // We might have received a "subtly deferred" message in a nested
+            // loop that it's now time to process.
+            if (InterruptEventOccurred() ||
+                (!maybeTimedOut && (!mDeferred.empty() || !mOutOfTurnReplies.empty())))
+            {
+                break;
+            }
+
+            if (maybeTimedOut && !ShouldContinueFromTimeout())
+                return false;
+        }
+
+        Message recvd;
+        MessageMap::iterator it;
+
+        if ((it = mOutOfTurnReplies.find(mInterruptStack.top().seqno()))
+            != mOutOfTurnReplies.end())
+        {
+            recvd = Move(it->second);
+            mOutOfTurnReplies.erase(it);
+        } else if (!mPending.isEmpty()) {
+            RefPtr<MessageTask> task = mPending.popFirst();
+            recvd = Move(task->Msg());
+        } else {
+            // because of subtleties with nested event loops, it's possible
+            // that we got here and nothing happened.  or, we might have a
+            // deferred in-call that needs to be processed.  either way, we
+            // won't break the inner while loop again until something new
+            // happens.
+            continue;
+        }
+
+        // If the message is not Interrupt, we can dispatch it as normal.
+        if (!recvd.is_interrupt()) {
+            DispatchMessage(Move(recvd));
+            if (!Connected()) {
+                ReportConnectionError("MessageChannel::DispatchMessage");
+                return false;
+            }
+            continue;
+        }
+
+        // If the message is an Interrupt reply, either process it as a reply to our
+        // call, or add it to the list of out-of-turn replies we've received.
+        if (recvd.is_reply()) {
+            IPC_ASSERT(!mInterruptStack.empty(), "invalid Interrupt stack");
+
+            // If this is not a reply the call we've initiated, add it to our
+            // out-of-turn replies and keep polling for events.
+            {
+                const MessageInfo &outcall = mInterruptStack.top();
+
+                // Note, In the parent, sequence numbers increase from 0, and
+                // in the child, they decrease from 0.
+                if ((mSide == ChildSide && recvd.seqno() > outcall.seqno()) ||
+                    (mSide != ChildSide && recvd.seqno() < outcall.seqno()))
+                {
+                    mOutOfTurnReplies[recvd.seqno()] = Move(recvd);
+                    continue;
+                }
+
+                IPC_ASSERT(recvd.is_reply_error() ||
+                           (recvd.type() == (outcall.type() + 1) &&
+                            recvd.seqno() == outcall.seqno()),
+                           "somebody's misbehavin'", true);
+            }
+
+            // We received a reply to our most recent outstanding call. Pop
+            // this frame and return the reply.
+            mInterruptStack.pop();
+
+            bool is_reply_error = recvd.is_reply_error();
+            if (!is_reply_error) {
+                *aReply = Move(recvd);
+            }
+
+            // If we have no more pending out calls waiting on replies, then
+            // the reply queue should be empty.
+            IPC_ASSERT(!mInterruptStack.empty() || mOutOfTurnReplies.empty(),
+                       "still have pending replies with no pending out-calls",
+                       true);
+
+            return !is_reply_error;
+        }
+
+        // Dispatch an Interrupt in-call. Snapshot the current stack depth while we
+        // own the monitor.
+        size_t stackDepth = InterruptStackDepth();
+        {
+            MonitorAutoUnlock unlock(*mMonitor);
+
+            CxxStackFrame frame(*this, IN_MESSAGE, &recvd);
+            DispatchInterruptMessage(Move(recvd), stackDepth);
+        }
+        if (!Connected()) {
+            ReportConnectionError("MessageChannel::DispatchInterruptMessage");
+            return false;
+        }
+    }
+
+    return true;
+}
+
+bool
+MessageChannel::WaitForIncomingMessage()
+{
+#ifdef OS_WIN
+    SyncStackFrame frame(this, true);
+    NeuteredWindowRegion neuteredRgn(mFlags & REQUIRE_DEFERRED_MESSAGE_PROTECTION);
+#endif
+
+    MonitorAutoLock lock(*mMonitor);
+    AutoEnterWaitForIncoming waitingForIncoming(*this);
+    if (mChannelState != ChannelConnected) {
+        return false;
+    }
+    if (!HasPendingEvents()) {
+        return WaitForInterruptNotify();
+    }
+
+    MOZ_RELEASE_ASSERT(!mPending.isEmpty());
+    RefPtr<MessageTask> task = mPending.getFirst();
+    RunMessage(*task);
+    return true;
+}
+
+bool
+MessageChannel::HasPendingEvents()
+{
+    AssertWorkerThread();
+    mMonitor->AssertCurrentThreadOwns();
+    return Connected() && !mPending.isEmpty();
+}
+
+bool
+MessageChannel::InterruptEventOccurred()
+{
+    AssertWorkerThread();
+    mMonitor->AssertCurrentThreadOwns();
+    IPC_ASSERT(InterruptStackDepth() > 0, "not in wait loop");
+
+    return (!Connected() ||
+            !mPending.isEmpty() ||
+            (!mOutOfTurnReplies.empty() &&
+             mOutOfTurnReplies.find(mInterruptStack.top().seqno()) !=
+             mOutOfTurnReplies.end()));
+}
+
+bool
+MessageChannel::ProcessPendingRequest(Message &&aUrgent)
+{
+    AssertWorkerThread();
+    mMonitor->AssertCurrentThreadOwns();
+
+    IPC_LOG("Process pending: seqno=%d, xid=%d", aUrgent.seqno(), aUrgent.transaction_id());
+
+    DispatchMessage(Move(aUrgent));
+    if (!Connected()) {
+        ReportConnectionError("MessageChannel::ProcessPendingRequest");
+        return false;
+    }
+
+    return true;
+}
+
+bool
+MessageChannel::ShouldRunMessage(const Message& aMsg)
+{
+    if (!mTimedOutMessageSeqno) {
+        return true;
+    }
+
+    // If we've timed out a message and we're awaiting the reply to the timed
+    // out message, we have to be careful what messages we process. Here's what
+    // can go wrong:
+    // 1. child sends a NOT_NESTED sync message S
+    // 2. parent sends a NESTED_INSIDE_SYNC sync message H at the same time
+    // 3. parent times out H
+    // 4. child starts processing H and sends a NESTED_INSIDE_SYNC message H' nested
+    //    within the same transaction
+    // 5. parent dispatches S and sends reply
+    // 6. child asserts because it instead expected a reply to H'.
+    //
+    // To solve this, we refuse to process S in the parent until we get a reply
+    // to H. More generally, let the timed out message be M. We don't process a
+    // message unless the child would need the response to that message in order
+    // to process M. Those messages are the ones that have a higher nested level
+    // than M or that are part of the same transaction as M.
+    if (aMsg.nested_level() < mTimedOutMessageNestedLevel ||
+        (aMsg.nested_level() == mTimedOutMessageNestedLevel
+         && aMsg.transaction_id() != mTimedOutMessageSeqno))
+    {
+        return false;
+    }
+
+    return true;
+}
+
+void
+MessageChannel::RunMessage(MessageTask& aTask)
+{
+    AssertWorkerThread();
+    mMonitor->AssertCurrentThreadOwns();
+
+    Message& msg = aTask.Msg();
+
+    if (!Connected()) {
+        ReportConnectionError("RunMessage");
+        return;
+    }
+
+    // Check that we're going to run the first message that's valid to run.
+#ifdef DEBUG
+    for (RefPtr<MessageTask> task : mPending) {
+        if (task == &aTask) {
+            break;
+        }
+
+        MOZ_ASSERT(!ShouldRunMessage(task->Msg()) ||
+                   aTask.Msg().priority() != task->Msg().priority());
+
+    }
+#endif
+
+    if (!mDeferred.empty()) {
+        MaybeUndeferIncall();
+    }
+
+    if (!ShouldRunMessage(msg)) {
+        return;
+    }
+
+    MOZ_RELEASE_ASSERT(aTask.isInList());
+    aTask.remove();
+
+    if (IsOnCxxStack() && msg.is_interrupt() && msg.is_reply()) {
+        // We probably just received a reply in a nested loop for an
+        // Interrupt call sent before entering that loop.
+        mOutOfTurnReplies[msg.seqno()] = Move(msg);
+        return;
+    }
+
+    DispatchMessage(Move(msg));
+}
+
+NS_IMPL_ISUPPORTS_INHERITED(MessageChannel::MessageTask, CancelableRunnable, nsIRunnablePriority)
+
+nsresult
+MessageChannel::MessageTask::Run()
+{
+    if (!mChannel) {
+        return NS_OK;
+    }
+
+    mChannel->AssertWorkerThread();
+    mChannel->mMonitor->AssertNotCurrentThreadOwns();
+
+    MonitorAutoLock lock(*mChannel->mMonitor);
+
+    // In case we choose not to run this message, we may need to be able to Post
+    // it again.
+    mScheduled = false;
+
+    if (!isInList()) {
+        return NS_OK;
+    }
+
+    mChannel->RunMessage(*this);
+    return NS_OK;
+}
+
+// Warning: This method removes the receiver from whatever list it might be in.
+nsresult
+MessageChannel::MessageTask::Cancel()
+{
+    if (!mChannel) {
+        return NS_OK;
+    }
+
+    mChannel->AssertWorkerThread();
+    mChannel->mMonitor->AssertNotCurrentThreadOwns();
+
+    MonitorAutoLock lock(*mChannel->mMonitor);
+
+    if (!isInList()) {
+        return NS_OK;
+    }
+    remove();
+
+    return NS_OK;
+}
+
+void
+MessageChannel::MessageTask::Post()
+{
+    MOZ_RELEASE_ASSERT(!mScheduled);
+    MOZ_RELEASE_ASSERT(isInList());
+
+    mScheduled = true;
+
+    RefPtr<MessageTask> self = this;
+    mChannel->mWorkerLoop->PostTask(self.forget());
+}
+
+void
+MessageChannel::MessageTask::Clear()
+{
+    mChannel->AssertWorkerThread();
+
+    mChannel = nullptr;
+}
+
+NS_IMETHODIMP
+MessageChannel::MessageTask::GetPriority(uint32_t* aPriority)
+{
+  *aPriority = mMessage.priority() == Message::HIGH_PRIORITY ?
+               PRIORITY_HIGH : PRIORITY_NORMAL;
+  return NS_OK;
+}
+
+void
+MessageChannel::DispatchMessage(Message &&aMsg)
+{
+    AssertWorkerThread();
+    mMonitor->AssertCurrentThreadOwns();
+
+    Maybe<AutoNoJSAPI> nojsapi;
+    if (ScriptSettingsInitialized() && NS_IsMainThread())
+        nojsapi.emplace();
+
+    nsAutoPtr<Message> reply;
+
+    IPC_LOG("DispatchMessage: seqno=%d, xid=%d", aMsg.seqno(), aMsg.transaction_id());
+
+    {
+        AutoEnterTransaction transaction(this, aMsg);
+
+        int id = aMsg.transaction_id();
+        MOZ_RELEASE_ASSERT(!aMsg.is_sync() || id == transaction.TransactionID());
+
+        {
+            MonitorAutoUnlock unlock(*mMonitor);
+            CxxStackFrame frame(*this, IN_MESSAGE, &aMsg);
+
+            mListener->ArtificialSleep();
+
+            if (aMsg.is_sync())
+                DispatchSyncMessage(aMsg, *getter_Transfers(reply));
+            else if (aMsg.is_interrupt())
+                DispatchInterruptMessage(Move(aMsg), 0);
+            else
+                DispatchAsyncMessage(aMsg);
+
+            mListener->ArtificialSleep();
+        }
+
+        if (reply && transaction.IsCanceled()) {
+            // The transaction has been canceled. Don't send a reply.
+            IPC_LOG("Nulling out reply due to cancellation, seqno=%d, xid=%d", aMsg.seqno(), id);
+            reply = nullptr;
+        }
+    }
+
+    if (reply && ChannelConnected == mChannelState) {
+        IPC_LOG("Sending reply seqno=%d, xid=%d", aMsg.seqno(), aMsg.transaction_id());
+        mLink->SendMessage(reply.forget());
+    }
+}
+
+void
+MessageChannel::DispatchSyncMessage(const Message& aMsg, Message*& aReply)
+{
+    AssertWorkerThread();
+
+    int nestedLevel = aMsg.nested_level();
+
+    MOZ_RELEASE_ASSERT(nestedLevel == IPC::Message::NOT_NESTED || NS_IsMainThread());
+
+    MessageChannel* dummy;
+    MessageChannel*& blockingVar = mSide == ChildSide && NS_IsMainThread() ? gParentProcessBlocker : dummy;
+
+    Result rv;
+    {
+        AutoSetValue<MessageChannel*> blocked(blockingVar, this);
+        rv = mListener->OnMessageReceived(aMsg, aReply);
+    }
+
+    if (!MaybeHandleError(rv, aMsg, "DispatchSyncMessage")) {
+        aReply = new Message();
+        aReply->set_sync();
+        aReply->set_nested_level(aMsg.nested_level());
+        aReply->set_reply();
+        aReply->set_reply_error();
+    }
+    aReply->set_seqno(aMsg.seqno());
+    aReply->set_transaction_id(aMsg.transaction_id());
+}
+
+void
+MessageChannel::DispatchAsyncMessage(const Message& aMsg)
+{
+    AssertWorkerThread();
+    MOZ_RELEASE_ASSERT(!aMsg.is_interrupt() && !aMsg.is_sync());
+
+    if (aMsg.routing_id() == MSG_ROUTING_NONE) {
+        NS_RUNTIMEABORT("unhandled special message!");
+    }
+
+    Result rv;
+    {
+        int nestedLevel = aMsg.nested_level();
+        AutoSetValue<bool> async(mDispatchingAsyncMessage, true);
+        AutoSetValue<int> nestedLevelSet(mDispatchingAsyncMessageNestedLevel, nestedLevel);
+        rv = mListener->OnMessageReceived(aMsg);
+    }
+    MaybeHandleError(rv, aMsg, "DispatchAsyncMessage");
+}
+
+bool
+MessageChannel::ShouldDeferInterruptMessage(const Message& aMsg, size_t aStackDepth)
+{
+    AssertWorkerThread();
+
+    // We may or may not own the lock in this function, so don't access any
+    // channel state.
+
+    IPC_ASSERT(aMsg.is_interrupt() && !aMsg.is_reply(), "wrong message type");
+
+    // Race detection: see the long comment near mRemoteStackDepthGuess in
+    // MessageChannel.h. "Remote" stack depth means our side, and "local" means
+    // the other side.
+    if (aMsg.interrupt_remote_stack_depth_guess() == RemoteViewOfStackDepth(aStackDepth)) {
+        return false;
+    }
+
+    // Interrupt in-calls have raced. The winner, if there is one, gets to defer
+    // processing of the other side's in-call.
+    bool defer;
+    const MessageInfo parentMsgInfo =
+        (mSide == ChildSide) ? MessageInfo(aMsg) : mInterruptStack.top();
+    const MessageInfo childMsgInfo =
+        (mSide == ChildSide) ? mInterruptStack.top() : MessageInfo(aMsg);
+    switch (mListener->MediateInterruptRace(parentMsgInfo, childMsgInfo))
+    {
+        case RIPChildWins:
+            defer = (mSide == ChildSide);
+            break;
+        case RIPParentWins:
+            defer = (mSide != ChildSide);
+            break;
+        case RIPError:
+            MOZ_CRASH("NYI: 'Error' Interrupt race policy");
+        default:
+            MOZ_CRASH("not reached");
+    }
+
+    return defer;
+}
+
+void
+MessageChannel::DispatchInterruptMessage(Message&& aMsg, size_t stackDepth)
+{
+    AssertWorkerThread();
+    mMonitor->AssertNotCurrentThreadOwns();
+
+    IPC_ASSERT(aMsg.is_interrupt() && !aMsg.is_reply(), "wrong message type");
+
+    if (ShouldDeferInterruptMessage(aMsg, stackDepth)) {
+        // We now know the other side's stack has one more frame
+        // than we thought.
+        ++mRemoteStackDepthGuess; // decremented in MaybeProcessDeferred()
+        mDeferred.push(Move(aMsg));
+        return;
+    }
+
+    // If we "lost" a race and need to process the other side's in-call, we
+    // don't need to fix up the mRemoteStackDepthGuess here, because we're just
+    // about to increment it, which will make it correct again.
+
+#ifdef OS_WIN
+    SyncStackFrame frame(this, true);
+#endif
+
+    nsAutoPtr<Message> reply;
+
+    ++mRemoteStackDepthGuess;
+    Result rv = mListener->OnCallReceived(aMsg, *getter_Transfers(reply));
+    --mRemoteStackDepthGuess;
+
+    if (!MaybeHandleError(rv, aMsg, "DispatchInterruptMessage")) {
+        reply = new Message();
+        reply->set_interrupt();
+        reply->set_reply();
+        reply->set_reply_error();
+    }
+    reply->set_seqno(aMsg.seqno());
+
+    MonitorAutoLock lock(*mMonitor);
+    if (ChannelConnected == mChannelState) {
+        mLink->SendMessage(reply.forget());
+    }
+}
+
+void
+MessageChannel::MaybeUndeferIncall()
+{
+    AssertWorkerThread();
+    mMonitor->AssertCurrentThreadOwns();
+
+    if (mDeferred.empty())
+        return;
+
+    size_t stackDepth = InterruptStackDepth();
+
+    Message& deferred = mDeferred.top();
+
+    // the other side can only *under*-estimate our actual stack depth
+    IPC_ASSERT(deferred.interrupt_remote_stack_depth_guess() <= stackDepth,
+               "fatal logic error");
+
+    if (ShouldDeferInterruptMessage(deferred, stackDepth)) {
+        return;
+    }
+
+    // maybe time to process this message
+    Message call(Move(deferred));
+    mDeferred.pop();
+
+    // fix up fudge factor we added to account for race
+    IPC_ASSERT(0 < mRemoteStackDepthGuess, "fatal logic error");
+    --mRemoteStackDepthGuess;
+
+    MOZ_RELEASE_ASSERT(call.nested_level() == IPC::Message::NOT_NESTED);
+    RefPtr<MessageTask> task = new MessageTask(this, Move(call));
+    mPending.insertBack(task);
+    task->Post();
+}
+
+void
+MessageChannel::EnteredCxxStack()
+{
+    mListener->EnteredCxxStack();
+}
+
+void
+MessageChannel::ExitedCxxStack()
+{
+    mListener->ExitedCxxStack();
+    if (mSawInterruptOutMsg) {
+        MonitorAutoLock lock(*mMonitor);
+        // see long comment in OnMaybeDequeueOne()
+        EnqueuePendingMessages();
+        mSawInterruptOutMsg = false;
+    }
+}
+
+void
+MessageChannel::EnteredCall()
+{
+    mListener->EnteredCall();
+}
+
+void
+MessageChannel::ExitedCall()
+{
+    mListener->ExitedCall();
+}
+
+void
+MessageChannel::EnteredSyncSend()
+{
+    mListener->OnEnteredSyncSend();
+}
+
+void
+MessageChannel::ExitedSyncSend()
+{
+    mListener->OnExitedSyncSend();
+}
+
+void
+MessageChannel::EnqueuePendingMessages()
+{
+    AssertWorkerThread();
+    mMonitor->AssertCurrentThreadOwns();
+
+    MaybeUndeferIncall();
+
+    // XXX performance tuning knob: could process all or k pending
+    // messages here, rather than enqueuing for later processing
+
+    RepostAllMessages();
+}
+
+static inline bool
+IsTimeoutExpired(PRIntervalTime aStart, PRIntervalTime aTimeout)
+{
+    return (aTimeout != PR_INTERVAL_NO_TIMEOUT) &&
+           (aTimeout <= (PR_IntervalNow() - aStart));
+}
+
+bool
+MessageChannel::WaitResponse(bool aWaitTimedOut)
+{
+    if (aWaitTimedOut) {
+        if (mInTimeoutSecondHalf) {
+            // We've really timed out this time.
+            return false;
+        }
+        // Try a second time.
+        mInTimeoutSecondHalf = true;
+    } else {
+        mInTimeoutSecondHalf = false;
+    }
+    return true;
+}
+
+#ifndef OS_WIN
+bool
+MessageChannel::WaitForSyncNotify(bool /* aHandleWindowsMessages */)
+{
+#ifdef DEBUG
+    // WARNING: We don't release the lock here. We can't because the link thread
+    // could signal at this time and we would miss it. Instead we require
+    // ArtificialTimeout() to be extremely simple.
+    if (mListener->ArtificialTimeout()) {
+        return false;
+    }
+#endif
+
+    PRIntervalTime timeout = (kNoTimeout == mTimeoutMs) ?
+                             PR_INTERVAL_NO_TIMEOUT :
+                             PR_MillisecondsToInterval(mTimeoutMs);
+    // XXX could optimize away this syscall for "no timeout" case if desired
+    PRIntervalTime waitStart = PR_IntervalNow();
+
+    mMonitor->Wait(timeout);
+
+    // If the timeout didn't expire, we know we received an event. The
+    // converse is not true.
+    return WaitResponse(IsTimeoutExpired(waitStart, timeout));
+}
+
+bool
+MessageChannel::WaitForInterruptNotify()
+{
+    return WaitForSyncNotify(true);
+}
+
+void
+MessageChannel::NotifyWorkerThread()
+{
+    mMonitor->Notify();
+}
+#endif
+
+bool
+MessageChannel::ShouldContinueFromTimeout()
+{
+    AssertWorkerThread();
+    mMonitor->AssertCurrentThreadOwns();
+
+    bool cont;
+    {
+        MonitorAutoUnlock unlock(*mMonitor);
+        cont = mListener->ShouldContinueFromReplyTimeout();
+        mListener->ArtificialSleep();
+    }
+
+    static enum { UNKNOWN, NOT_DEBUGGING, DEBUGGING } sDebuggingChildren = UNKNOWN;
+
+    if (sDebuggingChildren == UNKNOWN) {
+        sDebuggingChildren = getenv("MOZ_DEBUG_CHILD_PROCESS") ? DEBUGGING : NOT_DEBUGGING;
+    }
+    if (sDebuggingChildren == DEBUGGING) {
+        return true;
+    }
+
+    return cont;
+}
+
+void
+MessageChannel::SetReplyTimeoutMs(int32_t aTimeoutMs)
+{
+    // Set channel timeout value. Since this is broken up into
+    // two period, the minimum timeout value is 2ms.
+    AssertWorkerThread();
+    mTimeoutMs = (aTimeoutMs <= 0)
+                 ? kNoTimeout
+                 : (int32_t)ceil((double)aTimeoutMs / 2.0);
+}
+
+void
+MessageChannel::OnChannelConnected(int32_t peer_id)
+{
+    MOZ_RELEASE_ASSERT(!mPeerPidSet);
+    mPeerPidSet = true;
+    mPeerPid = peer_id;
+    RefPtr<CancelableRunnable> task = mOnChannelConnectedTask;
+    mWorkerLoop->PostTask(task.forget());
+}
+
+void
+MessageChannel::DispatchOnChannelConnected()
+{
+    AssertWorkerThread();
+    MOZ_RELEASE_ASSERT(mPeerPidSet);
+    mListener->OnChannelConnected(mPeerPid);
+}
+
+void
+MessageChannel::ReportMessageRouteError(const char* channelName) const
+{
+    PrintErrorMessage(mSide, channelName, "Need a route");
+    mListener->ProcessingError(MsgRouteError, "MsgRouteError");
+}
+
+void
+MessageChannel::ReportConnectionError(const char* aChannelName, Message* aMsg) const
+{
+    AssertWorkerThread();
+    mMonitor->AssertCurrentThreadOwns();
+
+    const char* errorMsg = nullptr;
+    switch (mChannelState) {
+      case ChannelClosed:
+        errorMsg = "Closed channel: cannot send/recv";
+        break;
+      case ChannelOpening:
+        errorMsg = "Opening channel: not yet ready for send/recv";
+        break;
+      case ChannelTimeout:
+        errorMsg = "Channel timeout: cannot send/recv";
+        break;
+      case ChannelClosing:
+        errorMsg = "Channel closing: too late to send/recv, messages will be lost";
+        break;
+      case ChannelError:
+        errorMsg = "Channel error: cannot send/recv";
+        break;
+
+      default:
+        NS_RUNTIMEABORT("unreached");
+    }
+
+    if (aMsg) {
+        char reason[512];
+        SprintfLiteral(reason,"(msgtype=0x%X,name=%s) %s",
+                       aMsg->type(), aMsg->name(), errorMsg);
+
+        PrintErrorMessage(mSide, aChannelName, reason);
+    } else {
+        PrintErrorMessage(mSide, aChannelName, errorMsg);
+    }
+
+    MonitorAutoUnlock unlock(*mMonitor);
+    mListener->ProcessingError(MsgDropped, errorMsg);
+}
+
+bool
+MessageChannel::MaybeHandleError(Result code, const Message& aMsg, const char* channelName)
+{
+    if (MsgProcessed == code)
+        return true;
+
+    const char* errorMsg = nullptr;
+    switch (code) {
+      case MsgNotKnown:
+        errorMsg = "Unknown message: not processed";
+        break;
+      case MsgNotAllowed:
+        errorMsg = "Message not allowed: cannot be sent/recvd in this state";
+        break;
+      case MsgPayloadError:
+        errorMsg = "Payload error: message could not be deserialized";
+        break;
+      case MsgProcessingError:
+        errorMsg = "Processing error: message was deserialized, but the handler returned false (indicating failure)";
+        break;
+      case MsgRouteError:
+        errorMsg = "Route error: message sent to unknown actor ID";
+        break;
+      case MsgValueError:
+        errorMsg = "Value error: message was deserialized, but contained an illegal value";
+        break;
+
+    default:
+        NS_RUNTIMEABORT("unknown Result code");
+        return false;
+    }
+
+    char reason[512];
+    const char* msgname = StringFromIPCMessageType(aMsg.type());
+    if (msgname[0] == '?') {
+        SprintfLiteral(reason,"(msgtype=0x%X) %s", aMsg.type(), errorMsg);
+    } else {
+        SprintfLiteral(reason,"%s %s", msgname, errorMsg);
+    }
+
+    PrintErrorMessage(mSide, channelName, reason);
+
+    mListener->ProcessingError(code, reason);
+
+    return false;
+}
+
+void
+MessageChannel::OnChannelErrorFromLink()
+{
+    AssertLinkThread();
+    mMonitor->AssertCurrentThreadOwns();
+
+    IPC_LOG("OnChannelErrorFromLink");
+
+    if (InterruptStackDepth() > 0)
+        NotifyWorkerThread();
+
+    if (AwaitingSyncReply() || AwaitingIncomingMessage())
+        NotifyWorkerThread();
+
+    if (ChannelClosing != mChannelState) {
+        if (mAbortOnError) {
+            NS_RUNTIMEABORT("Aborting on channel error.");
+        }
+        mChannelState = ChannelError;
+        mMonitor->Notify();
+    }
+
+    PostErrorNotifyTask();
+}
+
+void
+MessageChannel::NotifyMaybeChannelError()
+{
+    mMonitor->AssertNotCurrentThreadOwns();
+
+    // TODO sort out Close() on this side racing with Close() on the other side
+    if (ChannelClosing == mChannelState) {
+        // the channel closed, but we received a "Goodbye" message warning us
+        // about it. no worries
+        mChannelState = ChannelClosed;
+        NotifyChannelClosed();
+        return;
+    }
+
+    Clear();
+
+    // Oops, error!  Let the listener know about it.
+    mChannelState = ChannelError;
+
+    // IPDL assumes these notifications do not fire twice, so we do not let
+    // that happen.
+    if (mNotifiedChannelDone) {
+      return;
+    }
+    mNotifiedChannelDone = true;
+
+    // After this, the channel may be deleted.  Based on the premise that
+    // mListener owns this channel, any calls back to this class that may
+    // work with mListener should still work on living objects.
+    mListener->OnChannelError();
+}
+
+void
+MessageChannel::OnNotifyMaybeChannelError()
+{
+    AssertWorkerThread();
+    mMonitor->AssertNotCurrentThreadOwns();
+
+    mChannelErrorTask = nullptr;
+
+    // OnChannelError holds mMonitor when it posts this task and this
+    // task cannot be allowed to run until OnChannelError has
+    // exited. We enforce that order by grabbing the mutex here which
+    // should only continue once OnChannelError has completed.
+    {
+        MonitorAutoLock lock(*mMonitor);
+        // nothing to do here
+    }
+
+    if (IsOnCxxStack()) {
+        mChannelErrorTask =
+            NewNonOwningCancelableRunnableMethod(this, &MessageChannel::OnNotifyMaybeChannelError);
+        RefPtr<Runnable> task = mChannelErrorTask;
+        // 10 ms delay is completely arbitrary
+        mWorkerLoop->PostDelayedTask(task.forget(), 10);
+        return;
+    }
+
+    NotifyMaybeChannelError();
+}
+
+void
+MessageChannel::PostErrorNotifyTask()
+{
+    mMonitor->AssertCurrentThreadOwns();
+
+    if (mChannelErrorTask)
+        return;
+
+    // This must be the last code that runs on this thread!
+    mChannelErrorTask =
+        NewNonOwningCancelableRunnableMethod(this, &MessageChannel::OnNotifyMaybeChannelError);
+    RefPtr<Runnable> task = mChannelErrorTask;
+    mWorkerLoop->PostTask(task.forget());
+}
+
+// Special async message.
+class GoodbyeMessage : public IPC::Message
+{
+public:
+    GoodbyeMessage() :
+        IPC::Message(MSG_ROUTING_NONE, GOODBYE_MESSAGE_TYPE)
+    {
+    }
+    static bool Read(const Message* msg) {
+        return true;
+    }
+    void Log(const std::string& aPrefix, FILE* aOutf) const {
+        fputs("(special `Goodbye' message)", aOutf);
+    }
+};
+
+void
+MessageChannel::SynchronouslyClose()
+{
+    AssertWorkerThread();
+    mMonitor->AssertCurrentThreadOwns();
+    mLink->SendClose();
+    while (ChannelClosed != mChannelState)
+        mMonitor->Wait();
+}
+
+void
+MessageChannel::CloseWithError()
+{
+    AssertWorkerThread();
+
+    MonitorAutoLock lock(*mMonitor);
+    if (ChannelConnected != mChannelState) {
+        return;
+    }
+    SynchronouslyClose();
+    mChannelState = ChannelError;
+    PostErrorNotifyTask();
+}
+
+void
+MessageChannel::CloseWithTimeout()
+{
+    AssertWorkerThread();
+
+    MonitorAutoLock lock(*mMonitor);
+    if (ChannelConnected != mChannelState) {
+        return;
+    }
+    SynchronouslyClose();
+    mChannelState = ChannelTimeout;
+}
+
+void
+MessageChannel::Close()
+{
+    AssertWorkerThread();
+
+    {
+        MonitorAutoLock lock(*mMonitor);
+
+        if (ChannelError == mChannelState || ChannelTimeout == mChannelState) {
+            // See bug 538586: if the listener gets deleted while the
+            // IO thread's NotifyChannelError event is still enqueued
+            // and subsequently deletes us, then the error event will
+            // also be deleted and the listener will never be notified
+            // of the channel error.
+            if (mListener) {
+                MonitorAutoUnlock unlock(*mMonitor);
+                NotifyMaybeChannelError();
+            }
+            return;
+        }
+
+        if (ChannelOpening == mChannelState) {
+            // SynchronouslyClose() waits for an ack from the other side, so
+            // the opening sequence should complete before this returns.
+            SynchronouslyClose();
+            mChannelState = ChannelError;
+            NotifyMaybeChannelError();
+            return;
+        }
+
+        if (ChannelClosed == mChannelState) {
+            // XXX be strict about this until there's a compelling reason
+            // to relax
+            NS_RUNTIMEABORT("Close() called on closed channel!");
+        }
+
+        // Notify the other side that we're about to close our socket. If we've
+        // already received a Goodbye from the other side (and our state is
+        // ChannelClosing), there's no reason to send one.
+        if (ChannelConnected == mChannelState) {
+          mLink->SendMessage(new GoodbyeMessage());
+        }
+        SynchronouslyClose();
+    }
+
+    NotifyChannelClosed();
+}
+
+void
+MessageChannel::NotifyChannelClosed()
+{
+    mMonitor->AssertNotCurrentThreadOwns();
+
+    if (ChannelClosed != mChannelState)
+        NS_RUNTIMEABORT("channel should have been closed!");
+
+    Clear();
+
+    // IPDL assumes these notifications do not fire twice, so we do not let
+    // that happen.
+    if (mNotifiedChannelDone) {
+      return;
+    }
+    mNotifiedChannelDone = true;
+
+    // OK, the IO thread just closed the channel normally.  Let the
+    // listener know about it. After this point the channel may be
+    // deleted.
+    mListener->OnChannelClose();
+}
+
+void
+MessageChannel::DebugAbort(const char* file, int line, const char* cond,
+                           const char* why,
+                           bool reply)
+{
+    printf_stderr("###!!! [MessageChannel][%s][%s:%d] "
+                  "Assertion (%s) failed.  %s %s\n",
+                  mSide == ChildSide ? "Child" : "Parent",
+                  file, line, cond,
+                  why,
+                  reply ? "(reply)" : "");
+    // technically we need the mutex for this, but we're dying anyway
+    DumpInterruptStack("  ");
+    printf_stderr("  remote Interrupt stack guess: %" PRIuSIZE "\n",
+                  mRemoteStackDepthGuess);
+    printf_stderr("  deferred stack size: %" PRIuSIZE "\n",
+                  mDeferred.size());
+    printf_stderr("  out-of-turn Interrupt replies stack size: %" PRIuSIZE "\n",
+                  mOutOfTurnReplies.size());
+
+    MessageQueue pending = Move(mPending);
+    while (!pending.isEmpty()) {
+        printf_stderr("    [ %s%s ]\n",
+                      pending.getFirst()->Msg().is_interrupt() ? "intr" :
+                      (pending.getFirst()->Msg().is_sync() ? "sync" : "async"),
+                      pending.getFirst()->Msg().is_reply() ? "reply" : "");
+        pending.popFirst();
+    }
+
+    NS_RUNTIMEABORT(why);
+}
+
+void
+MessageChannel::DumpInterruptStack(const char* const pfx) const
+{
+    NS_WARNING_ASSERTION(
+      MessageLoop::current() != mWorkerLoop,
+      "The worker thread had better be paused in a debugger!");
+
+    printf_stderr("%sMessageChannel 'backtrace':\n", pfx);
+
+    // print a python-style backtrace, first frame to last
+    for (uint32_t i = 0; i < mCxxStackFrames.length(); ++i) {
+        int32_t id;
+        const char* dir;
+        const char* sems;
+        const char* name;
+        mCxxStackFrames[i].Describe(&id, &dir, &sems, &name);
+
+        printf_stderr("%s[(%u) %s %s %s(actor=%d) ]\n", pfx,
+                      i, dir, sems, name, id);
+    }
+}
+
+int32_t
+MessageChannel::GetTopmostMessageRoutingId() const
+{
+    MOZ_RELEASE_ASSERT(MessageLoop::current() == mWorkerLoop);
+    if (mCxxStackFrames.empty()) {
+        return MSG_ROUTING_NONE;
+    }
+    const InterruptFrame& frame = mCxxStackFrames.back();
+    return frame.GetRoutingId();
+}
+
+void
+MessageChannel::EndTimeout()
+{
+    mMonitor->AssertCurrentThreadOwns();
+
+    IPC_LOG("Ending timeout of seqno=%d", mTimedOutMessageSeqno);
+    mTimedOutMessageSeqno = 0;
+    mTimedOutMessageNestedLevel = 0;
+
+    RepostAllMessages();
+}
+
+void
+MessageChannel::RepostAllMessages()
+{
+    bool needRepost = false;
+    for (RefPtr<MessageTask> task : mPending) {
+        if (!task->IsScheduled()) {
+            needRepost = true;
+        }
+    }
+    if (!needRepost) {
+        // If everything is already scheduled to run, do nothing.
+        return;
+    }
+
+    // In some cases we may have deferred dispatch of some messages in the
+    // queue. Now we want to run them again. However, we can't just re-post
+    // those messages since the messages after them in mPending would then be
+    // before them in the event queue. So instead we cancel everything and
+    // re-post all messages in the correct order.
+    MessageQueue queue = Move(mPending);
+    while (RefPtr<MessageTask> task = queue.popFirst()) {
+        RefPtr<MessageTask> newTask = new MessageTask(this, Move(task->Msg()));
+        mPending.insertBack(newTask);
+        newTask->Post();
+    }
+}
+
+void
+MessageChannel::CancelTransaction(int transaction)
+{
+    mMonitor->AssertCurrentThreadOwns();
+
+    // When we cancel a transaction, we need to behave as if there's no longer
+    // any IPC on the stack. Anything we were dispatching or sending will get
+    // canceled. Consequently, we have to update the state variables below.
+    //
+    // We also need to ensure that when any IPC functions on the stack return,
+    // they don't reset these values using an RAII class like AutoSetValue. To
+    // avoid that, these RAII classes check if the variable they set has been
+    // tampered with (by us). If so, they don't reset the variable to the old
+    // value.
+
+    IPC_LOG("CancelTransaction: xid=%d", transaction);
+
+    // An unusual case: We timed out a transaction which the other side then
+    // cancelled. In this case we just leave the timedout state and try to
+    // forget this ever happened.
+    if (transaction == mTimedOutMessageSeqno) {
+        IPC_LOG("Cancelled timed out message %d", mTimedOutMessageSeqno);
+        EndTimeout();
+
+        // Normally mCurrentTransaction == 0 here. But it can be non-zero if:
+        // 1. Parent sends NESTED_INSIDE_SYNC message H.
+        // 2. Parent times out H.
+        // 3. Child dispatches H and sends nested message H' (same transaction).
+        // 4. Parent dispatches H' and cancels.
+        MOZ_RELEASE_ASSERT(!mTransactionStack || mTransactionStack->TransactionID() == transaction);
+        if (mTransactionStack) {
+            mTransactionStack->Cancel();
+        }
+    } else {
+        MOZ_RELEASE_ASSERT(mTransactionStack->TransactionID() == transaction);
+        mTransactionStack->Cancel();
+    }
+
+    bool foundSync = false;
+    for (RefPtr<MessageTask> p = mPending.getFirst(); p; ) {
+        Message &msg = p->Msg();
+
+        // If there was a race between the parent and the child, then we may
+        // have a queued sync message. We want to drop this message from the
+        // queue since if will get cancelled along with the transaction being
+        // cancelled. This happens if the message in the queue is NESTED_INSIDE_SYNC.
+        if (msg.is_sync() && msg.nested_level() != IPC::Message::NOT_NESTED) {
+            MOZ_RELEASE_ASSERT(!foundSync);
+            MOZ_RELEASE_ASSERT(msg.transaction_id() != transaction);
+            IPC_LOG("Removing msg from queue seqno=%d xid=%d", msg.seqno(), msg.transaction_id());
+            foundSync = true;
+            p = p->removeAndGetNext();
+            continue;
+        }
+
+        p = p->getNext();
+    }
+}
+
+bool
+MessageChannel::IsInTransaction() const
+{
+    MonitorAutoLock lock(*mMonitor);
+    return !!mTransactionStack;
+}
+
+void
+MessageChannel::CancelCurrentTransaction()
+{
+    MonitorAutoLock lock(*mMonitor);
+    if (DispatchingSyncMessageNestedLevel() >= IPC::Message::NESTED_INSIDE_SYNC) {
+        if (DispatchingSyncMessageNestedLevel() == IPC::Message::NESTED_INSIDE_CPOW ||
+            DispatchingAsyncMessageNestedLevel() == IPC::Message::NESTED_INSIDE_CPOW)
+        {
+            mListener->IntentionalCrash();
+        }
+
+        IPC_LOG("Cancel requested: current xid=%d", CurrentNestedInsideSyncTransaction());
+        MOZ_RELEASE_ASSERT(DispatchingSyncMessage());
+        CancelMessage *cancel = new CancelMessage(CurrentNestedInsideSyncTransaction());
+        CancelTransaction(CurrentNestedInsideSyncTransaction());
+        mLink->SendMessage(cancel);
+    }
+}
+
+void
+CancelCPOWs()
+{
+    if (gParentProcessBlocker) {
+        mozilla::Telemetry::Accumulate(mozilla::Telemetry::IPC_TRANSACTION_CANCEL, true);
+        gParentProcessBlocker->CancelCurrentTransaction();
+    }
+}
+
+} // namespace ipc
+} // namespace mozilla