Adding upstream version 124.0.1.upstream/124.0.1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 2491 insertions, 0 deletions

diff --git a/ipc/glue/MessageChannel.cpp b/ipc/glue/MessageChannel.cpp
new file mode 100644
index 0000000000..a7fc135748
--- /dev/null
+++ b/ipc/glue/MessageChannel.cpp
@@ -0,0 +1,2491 @@
+/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: sw=2 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 <math.h>
+
+#include <utility>
+
+#include "CrashAnnotations.h"
+#include "mozilla/Assertions.h"
+#include "mozilla/CycleCollectedJSContext.h"
+#include "mozilla/DebugOnly.h"
+#include "mozilla/Fuzzing.h"
+#include "mozilla/IntentionalCrash.h"
+#include "mozilla/Logging.h"
+#include "mozilla/Monitor.h"
+#include "mozilla/Mutex.h"
+#include "mozilla/ProfilerMarkers.h"
+#include "mozilla/ScopeExit.h"
+#include "mozilla/Sprintf.h"
+#include "mozilla/StaticMutex.h"
+#include "mozilla/Telemetry.h"
+#include "mozilla/TimeStamp.h"
+#include "mozilla/UniquePtrExtensions.h"
+#include "mozilla/dom/ScriptSettings.h"
+#include "mozilla/ipc/NodeController.h"
+#include "mozilla/ipc/ProcessChild.h"
+#include "mozilla/ipc/ProtocolUtils.h"
+#include "nsAppRunner.h"
+#include "nsContentUtils.h"
+#include "nsIDirectTaskDispatcher.h"
+#include "nsTHashMap.h"
+#include "nsDebug.h"
+#include "nsExceptionHandler.h"
+#include "nsIMemoryReporter.h"
+#include "nsISupportsImpl.h"
+#include "nsPrintfCString.h"
+#include "nsThreadUtils.h"
+
+#ifdef XP_WIN
+#  include "mozilla/gfx/Logging.h"
+#endif
+
+#ifdef FUZZING_SNAPSHOT
+#  include "mozilla/fuzzing/IPCFuzzController.h"
+#endif
+
+// Undo the damage done by mozzconf.h
+#undef compress
+
+static mozilla::LazyLogModule sLogModule("ipc");
+#define IPC_LOG(...) MOZ_LOG(sLogModule, LogLevel::Debug, (__VA_ARGS__))
+
+/*
+ * IPC design:
+ *
+ * There are two kinds of messages: async and sync. Sync 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). 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 a lower 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.
+ */
+
+using namespace mozilla;
+using namespace mozilla::ipc;
+
+using mozilla::MonitorAutoLock;
+using mozilla::MonitorAutoUnlock;
+using mozilla::dom::AutoNoJSAPI;
+
+#define IPC_ASSERT(_cond, ...)                                           \
+  do {                                                                   \
+    AssertWorkerThread();                                                \
+    mMonitor->AssertCurrentThreadOwns();                                 \
+    if (!(_cond)) DebugAbort(__FILE__, __LINE__, #_cond, ##__VA_ARGS__); \
+  } while (0)
+
+static MessageChannel* gParentProcessBlocker = nullptr;
+
+namespace mozilla {
+namespace ipc {
+
+static const uint32_t kMinTelemetryMessageSize = 4096;
+
+// Note: we round the time we spend waiting for a response to the nearest
+// millisecond. So a min value of 1 ms actually captures from 500us and above.
+// This is used for both the sending and receiving side telemetry for sync IPC,
+// (IPC_SYNC_MAIN_LATENCY_MS and IPC_SYNC_RECEIVE_MS).
+static const uint32_t kMinTelemetrySyncIPCLatencyMs = 1;
+
+// static
+bool MessageChannel::sIsPumpingMessages = false;
+
+class AutoEnterTransaction {
+ public:
+  explicit AutoEnterTransaction(MessageChannel* aChan, int32_t aMsgSeqno,
+                                int32_t aTransactionID, int aNestedLevel)
+      MOZ_REQUIRES(*aChan->mMonitor)
+      : 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)
+      MOZ_REQUIRES(*aChan->mMonitor)
+      : 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() {
+    mChan->mMonitor->AssertCurrentThreadOwns();
+    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(UniquePtr<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 = std::move(aMessage);
+    MOZ_RELEASE_ASSERT(IsComplete());
+  }
+
+  void HandleReply(UniquePtr<IPC::Message> aMessage) {
+    mChan->mMonitor->AssertCurrentThreadOwns();
+    AutoEnterTransaction* cur = mChan->mTransactionStack;
+    MOZ_RELEASE_ASSERT(cur == this);
+    while (cur) {
+      MOZ_RELEASE_ASSERT(cur->mActive);
+      if (aMessage->seqno() == cur->mSeqno) {
+        cur->ReceivedReply(std::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();
+  }
+
+  UniquePtr<IPC::Message> GetReply() { return std::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.
+  UniquePtr<IPC::Message> mReply;
+};
+
+class PendingResponseReporter final : public nsIMemoryReporter {
+  ~PendingResponseReporter() = default;
+
+ public:
+  NS_DECL_THREADSAFE_ISUPPORTS
+
+  NS_IMETHOD
+  CollectReports(nsIHandleReportCallback* aHandleReport, nsISupports* aData,
+                 bool aAnonymize) override {
+    MOZ_COLLECT_REPORT(
+        "unresolved-ipc-responses", KIND_OTHER, UNITS_COUNT,
+        MessageChannel::gUnresolvedResponses,
+        "Outstanding IPC async message responses that are still not resolved.");
+    return NS_OK;
+  }
+};
+
+NS_IMPL_ISUPPORTS(PendingResponseReporter, nsIMemoryReporter)
+
+class ChannelCountReporter final : public nsIMemoryReporter {
+  ~ChannelCountReporter() = default;
+
+  struct ChannelCounts {
+    size_t mNow;
+    size_t mMax;
+
+    ChannelCounts() : mNow(0), mMax(0) {}
+
+    void Inc() {
+      ++mNow;
+      if (mMax < mNow) {
+        mMax = mNow;
+      }
+    }
+
+    void Dec() {
+      MOZ_ASSERT(mNow > 0);
+      --mNow;
+    }
+  };
+
+  using CountTable = nsTHashMap<nsDepCharHashKey, ChannelCounts>;
+
+  static StaticMutex sChannelCountMutex;
+  static CountTable* sChannelCounts MOZ_GUARDED_BY(sChannelCountMutex);
+
+ public:
+  NS_DECL_THREADSAFE_ISUPPORTS
+
+  NS_IMETHOD
+  CollectReports(nsIHandleReportCallback* aHandleReport, nsISupports* aData,
+                 bool aAnonymize) override {
+    AutoTArray<std::pair<const char*, ChannelCounts>, 16> counts;
+    {
+      StaticMutexAutoLock countLock(sChannelCountMutex);
+      if (!sChannelCounts) {
+        return NS_OK;
+      }
+      counts.SetCapacity(sChannelCounts->Count());
+      for (const auto& entry : *sChannelCounts) {
+        counts.AppendElement(std::pair{entry.GetKey(), entry.GetData()});
+      }
+    }
+
+    for (const auto& entry : counts) {
+      nsPrintfCString pathNow("ipc-channels/%s", entry.first);
+      nsPrintfCString pathMax("ipc-channels-peak/%s", entry.first);
+      nsPrintfCString descNow(
+          "Number of IPC channels for"
+          " top-level actor type %s",
+          entry.first);
+      nsPrintfCString descMax(
+          "Peak number of IPC channels for"
+          " top-level actor type %s",
+          entry.first);
+
+      aHandleReport->Callback(""_ns, pathNow, KIND_OTHER, UNITS_COUNT,
+                              entry.second.mNow, descNow, aData);
+      aHandleReport->Callback(""_ns, pathMax, KIND_OTHER, UNITS_COUNT,
+                              entry.second.mMax, descMax, aData);
+    }
+    return NS_OK;
+  }
+
+  static void Increment(const char* aName) {
+    StaticMutexAutoLock countLock(sChannelCountMutex);
+    if (!sChannelCounts) {
+      sChannelCounts = new CountTable;
+    }
+    sChannelCounts->LookupOrInsert(aName).Inc();
+  }
+
+  static void Decrement(const char* aName) {
+    StaticMutexAutoLock countLock(sChannelCountMutex);
+    MOZ_ASSERT(sChannelCounts);
+    sChannelCounts->LookupOrInsert(aName).Dec();
+  }
+};
+
+StaticMutex ChannelCountReporter::sChannelCountMutex;
+ChannelCountReporter::CountTable* ChannelCountReporter::sChannelCounts;
+
+NS_IMPL_ISUPPORTS(ChannelCountReporter, nsIMemoryReporter)
+
+// In child processes, the first MessageChannel is created before
+// XPCOM is initialized enough to construct the memory reporter
+// manager.  This retries every time a MessageChannel is constructed,
+// which is good enough in practice.
+template <class Reporter>
+static void TryRegisterStrongMemoryReporter() {
+  static Atomic<bool> registered;
+  if (registered.compareExchange(false, true)) {
+    RefPtr<Reporter> reporter = new Reporter();
+    if (NS_FAILED(RegisterStrongMemoryReporter(reporter))) {
+      registered = false;
+    }
+  }
+}
+
+Atomic<size_t> MessageChannel::gUnresolvedResponses;
+
+MessageChannel::MessageChannel(const char* aName, IToplevelProtocol* aListener)
+    : mName(aName), mListener(aListener), mMonitor(new RefCountedMonitor()) {
+  MOZ_COUNT_CTOR(ipc::MessageChannel);
+
+#ifdef XP_WIN
+  mEvent = CreateEventW(nullptr, TRUE, FALSE, nullptr);
+  MOZ_RELEASE_ASSERT(mEvent, "CreateEvent failed! Nothing is going to work!");
+#endif
+
+  TryRegisterStrongMemoryReporter<PendingResponseReporter>();
+  TryRegisterStrongMemoryReporter<ChannelCountReporter>();
+}
+
+MessageChannel::~MessageChannel() {
+  MOZ_COUNT_DTOR(ipc::MessageChannel);
+  MonitorAutoLock lock(*mMonitor);
+  MOZ_RELEASE_ASSERT(!mOnCxxStack,
+                     "MessageChannel destroyed while code on CxxStack");
+#ifdef XP_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
+
+  // Make sure that the MessageChannel was closed (and therefore cleared) before
+  // it was destroyed. We can't properly close the channel at this point, as it
+  // would be unsafe to invoke our listener's callbacks, and we may be being
+  // destroyed on a thread other than `mWorkerThread`.
+  if (!IsClosedLocked()) {
+    CrashReporter::AnnotateCrashReport(
+        CrashReporter::Annotation::IPCFatalErrorProtocol,
+        nsDependentCString(mName));
+    switch (mChannelState) {
+      case ChannelConnected:
+        MOZ_CRASH(
+            "MessageChannel destroyed without being closed "
+            "(mChannelState == ChannelConnected).");
+        break;
+      case ChannelClosing:
+        MOZ_CRASH(
+            "MessageChannel destroyed without being closed "
+            "(mChannelState == ChannelClosing).");
+        break;
+      case ChannelError:
+        MOZ_CRASH(
+            "MessageChannel destroyed without being closed "
+            "(mChannelState == ChannelError).");
+        break;
+      default:
+        MOZ_CRASH("MessageChannel destroyed without being closed.");
+    }
+  }
+
+  // Double-check other properties for thoroughness.
+  MOZ_RELEASE_ASSERT(!mLink);
+  MOZ_RELEASE_ASSERT(mPendingResponses.empty());
+  MOZ_RELEASE_ASSERT(!mChannelErrorTask);
+  MOZ_RELEASE_ASSERT(mPending.isEmpty());
+  MOZ_RELEASE_ASSERT(!mShutdownTask);
+}
+
+#ifdef DEBUG
+void MessageChannel::AssertMaybeDeferredCountCorrect() {
+  mMonitor->AssertCurrentThreadOwns();
+
+  size_t count = 0;
+  for (MessageTask* task : mPending) {
+    task->AssertMonitorHeld(*mMonitor);
+    if (!IsAlwaysDeferred(*task->Msg())) {
+      count++;
+    }
+  }
+
+  MOZ_ASSERT(count == mMaybeDeferredPendingCount);
+}
+#endif
+
+// 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) {
+  printf_stderr("\n###!!! [%s][%s] Error: %s\n\n", StringFromIPCSide(side),
+                channelName, msg);
+}
+
+bool MessageChannel::Connected() const {
+  mMonitor->AssertCurrentThreadOwns();
+  return ChannelConnected == mChannelState;
+}
+
+bool MessageChannel::ConnectedOrClosing() const {
+  mMonitor->AssertCurrentThreadOwns();
+  return ChannelConnected == mChannelState || ChannelClosing == mChannelState;
+}
+
+bool MessageChannel::CanSend() const {
+  if (!mMonitor) {
+    return false;
+  }
+  MonitorAutoLock lock(*mMonitor);
+  return Connected();
+}
+
+void MessageChannel::Clear() {
+  AssertWorkerThread();
+  mMonitor->AssertCurrentThreadOwns();
+  MOZ_DIAGNOSTIC_ASSERT(IsClosedLocked(), "MessageChannel cleared too early?");
+  MOZ_ASSERT(ChannelClosed == mChannelState || ChannelError == mChannelState);
+
+  // Don't clear mWorkerThread; we use it in 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.
+
+  if (mShutdownTask) {
+    mShutdownTask->Clear();
+    mWorkerThread->UnregisterShutdownTask(mShutdownTask);
+  }
+  mShutdownTask = nullptr;
+
+  if (NS_IsMainThread() && gParentProcessBlocker == this) {
+    gParentProcessBlocker = nullptr;
+  }
+
+  gUnresolvedResponses -= mPendingResponses.size();
+  {
+    CallbackMap map = std::move(mPendingResponses);
+    MonitorAutoUnlock unlock(*mMonitor);
+    for (auto& pair : map) {
+      pair.second->Reject(ResponseRejectReason::ChannelClosed);
+    }
+  }
+  mPendingResponses.clear();
+
+  SetIsCrossProcess(false);
+
+  mLink = nullptr;
+
+  if (mChannelErrorTask) {
+    mChannelErrorTask->Cancel();
+    mChannelErrorTask = nullptr;
+  }
+
+  if (mFlushLazySendTask) {
+    mFlushLazySendTask->Cancel();
+    mFlushLazySendTask = nullptr;
+  }
+
+  // Free up any memory used by pending messages.
+  mPending.clear();
+
+  mMaybeDeferredPendingCount = 0;
+}
+
+bool MessageChannel::Open(ScopedPort aPort, Side aSide,
+                          const nsID& aMessageChannelId,
+                          nsISerialEventTarget* aEventTarget) {
+  nsCOMPtr<nsISerialEventTarget> eventTarget =
+      aEventTarget ? aEventTarget : GetCurrentSerialEventTarget();
+  MOZ_RELEASE_ASSERT(eventTarget,
+                     "Must open MessageChannel on a nsISerialEventTarget");
+  MOZ_RELEASE_ASSERT(eventTarget->IsOnCurrentThread(),
+                     "Must open MessageChannel from worker thread");
+
+  auto shutdownTask = MakeRefPtr<WorkerTargetShutdownTask>(eventTarget, this);
+  nsresult rv = eventTarget->RegisterShutdownTask(shutdownTask);
+  MOZ_ASSERT(rv != NS_ERROR_NOT_IMPLEMENTED,
+             "target for MessageChannel must support shutdown tasks");
+  if (rv == NS_ERROR_UNEXPECTED) {
+    // If shutdown tasks have already started running, dispatch our shutdown
+    // task manually.
+    NS_WARNING("Opening MessageChannel on EventTarget in shutdown");
+    rv = eventTarget->Dispatch(shutdownTask->AsRunnable());
+  }
+  MOZ_RELEASE_ASSERT(NS_SUCCEEDED(rv),
+                     "error registering ShutdownTask for MessageChannel");
+
+  {
+    MonitorAutoLock lock(*mMonitor);
+    MOZ_RELEASE_ASSERT(!mLink, "Open() called > once");
+    MOZ_RELEASE_ASSERT(ChannelClosed == mChannelState, "Not currently closed");
+    MOZ_ASSERT(mSide == UnknownSide);
+
+    mMessageChannelId = aMessageChannelId;
+    mWorkerThread = eventTarget;
+    mShutdownTask = shutdownTask;
+    mLink = MakeUnique<PortLink>(this, std::move(aPort));
+    mChannelState = ChannelConnected;
+    mSide = aSide;
+  }
+
+  // Notify our listener that the underlying IPC channel has been established.
+  // IProtocol will use this callback to create the ActorLifecycleProxy, and
+  // perform an `AddRef` call to keep the actor alive until the channel is
+  // disconnected.
+  //
+  // We unlock our monitor before calling `OnIPCChannelOpened` to ensure that
+  // any calls back into `MessageChannel` do not deadlock. At this point, we may
+  // be receiving messages on the IO thread, however we cannot process them on
+  // the worker thread or have notified our listener until after this function
+  // returns.
+  mListener->OnIPCChannelOpened();
+  return true;
+}
+
+static Side GetOppSide(Side aSide) {
+  switch (aSide) {
+    case ChildSide:
+      return ParentSide;
+    case ParentSide:
+      return ChildSide;
+    default:
+      return UnknownSide;
+  }
+}
+
+bool MessageChannel::Open(MessageChannel* aTargetChan,
+                          nsISerialEventTarget* aEventTarget, Side aSide) {
+  // Opens a connection to another thread in the same process.
+
+  MOZ_ASSERT(aTargetChan, "Need a target channel");
+
+  nsID channelId = nsID::GenerateUUID();
+
+  std::pair<ScopedPort, ScopedPort> ports =
+      NodeController::GetSingleton()->CreatePortPair();
+
+  // NOTE: This dispatch must be sync as it captures locals by non-owning
+  // reference, however we can't use `NS_DispatchAndSpinEventLoopUntilComplete`
+  // as that will spin a nested event loop, and doesn't work with certain types
+  // of calling event targets.
+  base::WaitableEvent event(/* manual_reset */ true,
+                            /* initially_signaled */ false);
+  MOZ_ALWAYS_SUCCEEDS(aEventTarget->Dispatch(NS_NewCancelableRunnableFunction(
+      "ipc::MessageChannel::OpenAsOtherThread", [&]() {
+        aTargetChan->Open(std::move(ports.second), GetOppSide(aSide), channelId,
+                          aEventTarget);
+        event.Signal();
+      })));
+  bool ok = event.Wait();
+  MOZ_RELEASE_ASSERT(ok);
+
+  // Now that the other side has connected, open the port on our side.
+  return Open(std::move(ports.first), aSide, channelId);
+}
+
+bool MessageChannel::OpenOnSameThread(MessageChannel* aTargetChan,
+                                      mozilla::ipc::Side aSide) {
+  auto [porta, portb] = NodeController::GetSingleton()->CreatePortPair();
+
+  nsID channelId = nsID::GenerateUUID();
+
+  aTargetChan->mIsSameThreadChannel = true;
+  mIsSameThreadChannel = true;
+
+  auto* currentThread = GetCurrentSerialEventTarget();
+  return aTargetChan->Open(std::move(portb), GetOppSide(aSide), channelId,
+                           currentThread) &&
+         Open(std::move(porta), aSide, channelId, currentThread);
+}
+
+bool MessageChannel::Send(UniquePtr<Message> aMsg) {
+  if (aMsg->size() >= kMinTelemetryMessageSize) {
+    Telemetry::Accumulate(Telemetry::IPC_MESSAGE_SIZE2, aMsg->size());
+  }
+
+  MOZ_RELEASE_ASSERT(!aMsg->is_sync());
+  MOZ_RELEASE_ASSERT(aMsg->nested_level() != IPC::Message::NESTED_INSIDE_SYNC);
+
+  AutoSetValue<bool> setOnCxxStack(mOnCxxStack, true);
+
+  AssertWorkerThread();
+  mMonitor->AssertNotCurrentThreadOwns();
+  if (MSG_ROUTING_NONE == aMsg->routing_id()) {
+    ReportMessageRouteError("MessageChannel::Send");
+    return false;
+  }
+
+  if (aMsg->seqno() == 0) {
+    aMsg->set_seqno(NextSeqno());
+  }
+
+  MonitorAutoLock lock(*mMonitor);
+  if (!Connected()) {
+    ReportConnectionError("Send", aMsg->type());
+    return false;
+  }
+
+  AddProfilerMarker(*aMsg, MessageDirection::eSending);
+  SendMessageToLink(std::move(aMsg));
+  return true;
+}
+
+void MessageChannel::SendMessageToLink(UniquePtr<Message> aMsg) {
+  AssertWorkerThread();
+  mMonitor->AssertCurrentThreadOwns();
+
+  // If the channel is not cross-process, there's no reason to be lazy, so we
+  // ignore the flag in that case.
+  if (aMsg->is_lazy_send() && mIsCrossProcess) {
+    // If this is the first lazy message in the queue and our worker thread
+    // supports direct task dispatch, dispatch a task to flush messages,
+    // ensuring we don't leave them pending forever.
+    if (!mFlushLazySendTask) {
+      if (nsCOMPtr<nsIDirectTaskDispatcher> dispatcher =
+              do_QueryInterface(mWorkerThread)) {
+        mFlushLazySendTask = new FlushLazySendMessagesRunnable(this);
+        MOZ_ALWAYS_SUCCEEDS(
+            dispatcher->DispatchDirectTask(do_AddRef(mFlushLazySendTask)));
+      }
+    }
+    if (mFlushLazySendTask) {
+      mFlushLazySendTask->PushMessage(std::move(aMsg));
+      return;
+    }
+  }
+
+  if (mFlushLazySendTask) {
+    FlushLazySendMessages();
+  }
+  mLink->SendMessage(std::move(aMsg));
+}
+
+void MessageChannel::FlushLazySendMessages() {
+  AssertWorkerThread();
+  mMonitor->AssertCurrentThreadOwns();
+
+  // Clean up any SendLazyTask which might be pending.
+  auto messages = mFlushLazySendTask->TakeMessages();
+  mFlushLazySendTask = nullptr;
+
+  // Send all lazy messages, then clear the queue.
+  for (auto& msg : messages) {
+    mLink->SendMessage(std::move(msg));
+  }
+}
+
+UniquePtr<MessageChannel::UntypedCallbackHolder> MessageChannel::PopCallback(
+    const Message& aMsg, int32_t aActorId) {
+  auto iter = mPendingResponses.find(aMsg.seqno());
+  if (iter != mPendingResponses.end() && iter->second->mActorId == aActorId &&
+      iter->second->mReplyMsgId == aMsg.type()) {
+    UniquePtr<MessageChannel::UntypedCallbackHolder> ret =
+        std::move(iter->second);
+    mPendingResponses.erase(iter);
+    gUnresolvedResponses--;
+    return ret;
+  }
+  return nullptr;
+}
+
+void MessageChannel::RejectPendingResponsesForActor(int32_t aActorId) {
+  auto itr = mPendingResponses.begin();
+  while (itr != mPendingResponses.end()) {
+    if (itr->second.get()->mActorId != aActorId) {
+      ++itr;
+      continue;
+    }
+    itr->second.get()->Reject(ResponseRejectReason::ActorDestroyed);
+    // Take special care of advancing the iterator since we are
+    // removing it while iterating.
+    itr = mPendingResponses.erase(itr);
+    gUnresolvedResponses--;
+  }
+}
+
+class BuildIDsMatchMessage : public IPC::Message {
+ public:
+  BuildIDsMatchMessage()
+      : IPC::Message(MSG_ROUTING_NONE, BUILD_IDS_MATCH_MESSAGE_TYPE) {}
+  void Log(const std::string& aPrefix, FILE* aOutf) const {
+    fputs("(special `Build IDs match' message)", aOutf);
+  }
+};
+
+// Send the parent a special async message to confirm when the parent and child
+// are of the same buildID. Skips sending the message and returns false if the
+// buildIDs don't match. This is a minor variation on
+// MessageChannel::Send(Message* aMsg).
+bool MessageChannel::SendBuildIDsMatchMessage(const char* aParentBuildID) {
+  MOZ_ASSERT(!XRE_IsParentProcess());
+
+  nsCString parentBuildID(aParentBuildID);
+  nsCString childBuildID(mozilla::PlatformBuildID());
+
+  if (parentBuildID != childBuildID) {
+    // The build IDs didn't match, usually because an update occurred in the
+    // background.
+    return false;
+  }
+
+  auto msg = MakeUnique<BuildIDsMatchMessage>();
+
+  MOZ_RELEASE_ASSERT(!msg->is_sync());
+  MOZ_RELEASE_ASSERT(msg->nested_level() != IPC::Message::NESTED_INSIDE_SYNC);
+
+  AssertWorkerThread();
+  mMonitor->AssertNotCurrentThreadOwns();
+  // Don't check for MSG_ROUTING_NONE.
+
+  MonitorAutoLock lock(*mMonitor);
+  if (!Connected()) {
+    ReportConnectionError("SendBuildIDsMatchMessage", msg->type());
+    return false;
+  }
+
+#if defined(MOZ_DEBUG) && defined(ENABLE_TESTS)
+  // Technically, the behavior is interesting for any kind of process
+  // but when exercising tests, we want to crash only a content process and
+  // avoid making noise with other kind of processes crashing
+  if (const char* dontSend = PR_GetEnv("MOZ_BUILDID_MATCH_DONTSEND")) {
+    if (dontSend[0] == '1') {
+      // Bug 1732999: We are going to crash, so we need to advise leak check
+      // tooling to avoid intermittent missing leakcheck
+      NoteIntentionalCrash(XRE_GetProcessTypeString());
+      if (XRE_IsContentProcess()) {
+        return false;
+      }
+    }
+  }
+#endif
+
+  SendMessageToLink(std::move(msg));
+  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) {
+  mMonitor->AssertCurrentThreadOwns();
+
+  if (MSG_ROUTING_NONE == aMsg.routing_id()) {
+    if (GOODBYE_MESSAGE_TYPE == aMsg.type()) {
+      // We've received a GOODBYE message, close the connection and mark
+      // ourselves as "Closing".
+      mLink->Close();
+      mChannelState = ChannelClosing;
+      if (LoggingEnabledFor(mListener->GetProtocolName(), mSide)) {
+        printf(
+            "[%s %u] NOTE: %s%s actor received `Goodbye' message.  Closing "
+            "channel.\n",
+            XRE_GeckoProcessTypeToString(XRE_GetProcessType()),
+            static_cast<uint32_t>(base::GetCurrentProcId()),
+            mListener->GetProtocolName(), StringFromIPCSide(mSide));
+      }
+
+      // Notify the worker thread that the connection has been closed, as we
+      // will not receive an `OnChannelErrorFromLink` after calling
+      // `mLink->Close()`.
+      if (AwaitingSyncReply()) {
+        NotifyWorkerThread();
+      }
+      PostErrorNotifyTask();
+      return true;
+    } else if (CANCEL_MESSAGE_TYPE == aMsg.type()) {
+      IPC_LOG("Cancel from message");
+      CancelTransaction(aMsg.transaction_id());
+      NotifyWorkerThread();
+      return true;
+    } else if (BUILD_IDS_MATCH_MESSAGE_TYPE == aMsg.type()) {
+      IPC_LOG("Build IDs match message");
+      mBuildIDsConfirmedMatch = true;
+      return true;
+    } else if (IMPENDING_SHUTDOWN_MESSAGE_TYPE == aMsg.type()) {
+      IPC_LOG("Impending Shutdown received");
+      ProcessChild::NotifiedImpendingShutdown();
+      return true;
+    }
+  }
+  return false;
+}
+
+/* static */
+bool MessageChannel::IsAlwaysDeferred(const Message& aMsg) {
+  // If a message is not NESTED_INSIDE_CPOW and not sync, then we always defer
+  // it.
+  return aMsg.nested_level() != IPC::Message::NESTED_INSIDE_CPOW &&
+         !aMsg.is_sync();
+}
+
+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) {
+    MOZ_ASSERT(!IsAlwaysDeferred(aMsg));
+    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);
+    MOZ_ASSERT(IsAlwaysDeferred(aMsg));
+    return true;
+  }
+
+  MOZ_ASSERT(!IsAlwaysDeferred(aMsg));
+
+  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(UniquePtr<Message> aMsg) {
+  mMonitor->AssertCurrentThreadOwns();
+  MOZ_ASSERT(mChannelState == ChannelConnected);
+
+  if (MaybeInterceptSpecialIOMessage(*aMsg)) {
+    return;
+  }
+
+  mListener->OnChannelReceivedMessage(*aMsg);
+
+  // 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(std::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);
+
+  if (aMsg->compress_type() == IPC::Message::COMPRESSION_ENABLED &&
+      !mPending.isEmpty()) {
+    auto* last = mPending.getLast();
+    last->AssertMonitorHeld(*mMonitor);
+    bool compress = last->Msg()->type() == aMsg->type() &&
+                    last->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(last->Msg()->compress_type() ==
+                         IPC::Message::COMPRESSION_ENABLED);
+      last->Msg() = std::move(aMsg);
+      return;
+    }
+  } else if (aMsg->compress_type() == IPC::Message::COMPRESSION_ALL &&
+             !mPending.isEmpty()) {
+    for (MessageTask* p = mPending.getLast(); p; p = p->getPrevious()) {
+      p->AssertMonitorHeld(*mMonitor);
+      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);
+        MOZ_RELEASE_ASSERT(IsAlwaysDeferred(*p->Msg()));
+        p->remove();
+        break;
+      }
+    }
+  }
+
+  bool alwaysDeferred = IsAlwaysDeferred(*aMsg);
+
+  bool shouldWakeUp = AwaitingSyncReply() && !ShouldDeferMessage(*aMsg);
+
+  IPC_LOG("Receive from link; seqno=%d, xid=%d, shouldWakeUp=%d", aMsg->seqno(),
+          aMsg->transaction_id(), shouldWakeUp);
+
+  // There are two cases we're concerned about, relating to the state of the
+  // worker thread:
+  //
+  // (1) We are waiting on a sync reply - worker thread is blocked on the
+  //     IPC monitor.
+  //   - If the message is NESTED_INSIDE_SYNC, we wake up the worker thread to
+  //     deliver the message depending on ShouldDeferMessage. Otherwise, we
+  //     leave it in the mPending queue, posting a task to the worker event
+  //     loop, where it will be processed once the synchronous reply has been
+  //     received.
+  //
+  // (2) We are not waiting on a reply.
+  //   - We post a task to the worker event loop.
+  //
+  // Note that, we may notify the worker 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, std::move(aMsg));
+  mPending.insertBack(task);
+
+  if (!alwaysDeferred) {
+    mMaybeDeferredPendingCount++;
+  }
+
+  if (shouldWakeUp) {
+    NotifyWorkerThread();
+  }
+
+  // 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.
+  task->AssertMonitorHeld(*mMonitor);
+  task->Post();
+}
+
+void MessageChannel::PeekMessages(
+    const std::function<bool(const Message& aMsg)>& aInvoke) {
+  // FIXME: We shouldn't be holding the lock for aInvoke!
+  MonitorAutoLock lock(*mMonitor);
+
+  for (MessageTask* it : mPending) {
+    it->AssertMonitorHeld(*mMonitor);
+    const Message& msg = *it->Msg();
+    if (!aInvoke(msg)) {
+      break;
+    }
+  }
+}
+
+void MessageChannel::ProcessPendingRequests(
+    ActorLifecycleProxy* aProxy, AutoEnterTransaction& aTransaction) {
+  mMonitor->AssertCurrentThreadOwns();
+
+  AssertMaybeDeferredCountCorrect();
+  if (mMaybeDeferredPendingCount == 0) {
+    return;
+  }
+
+  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;
+    }
+
+    Vector<UniquePtr<Message>> toProcess;
+
+    for (MessageTask* p = mPending.getFirst(); p;) {
+      p->AssertMonitorHeld(*mMonitor);
+      UniquePtr<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) {
+        MOZ_ASSERT(!IsAlwaysDeferred(*msg));
+
+        if (!toProcess.append(std::move(msg))) MOZ_CRASH();
+
+        mMaybeDeferredPendingCount--;
+
+        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& msg : toProcess) {
+      ProcessPendingRequest(aProxy, std::move(msg));
+    }
+  }
+
+  AssertMaybeDeferredCountCorrect();
+}
+
+bool MessageChannel::Send(UniquePtr<Message> aMsg, UniquePtr<Message>* aReply) {
+  mozilla::TimeStamp start = TimeStamp::Now();
+  if (aMsg->size() >= kMinTelemetryMessageSize) {
+    Telemetry::Accumulate(Telemetry::IPC_MESSAGE_SIZE2, aMsg->size());
+  }
+
+  // Sanity checks.
+  AssertWorkerThread();
+  mMonitor->AssertNotCurrentThreadOwns();
+  MOZ_RELEASE_ASSERT(!mIsSameThreadChannel,
+                     "sync send over same-thread channel will deadlock!");
+
+  RefPtr<ActorLifecycleProxy> proxy = Listener()->GetLifecycleProxy();
+
+#ifdef XP_WIN
+  SyncStackFrame frame(this);
+  NeuteredWindowRegion neuteredRgn(mFlags &
+                                   REQUIRE_DEFERRED_MESSAGE_PROTECTION);
+#endif
+
+  AutoSetValue<bool> setOnCxxStack(mOnCxxStack, true);
+
+  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 &&
+      aMsg->nested_level() > IPC::Message::NOT_NESTED) {
+    // Don't allow sending CPOWs while we're dispatching a sync message.
+    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(aMsg->nested_level() ==
+                       IPC::Message::NESTED_INSIDE_SYNC);
+    IPC_LOG("Sending while dispatching urgent message");
+    mLastSendError = SyncSendError::SendingCPOWWhileDispatchingUrgent;
+    return false;
+  }
+
+  if (aMsg->nested_level() < DispatchingSyncMessageNestedLevel() ||
+      aMsg->nested_level() < AwaitingSyncReplyNestedLevel()) {
+    MOZ_RELEASE_ASSERT(DispatchingSyncMessage() || DispatchingAsyncMessage());
+    IPC_LOG("Cancel from Send");
+    auto cancel =
+        MakeUnique<CancelMessage>(CurrentNestedInsideSyncTransaction());
+    CancelTransaction(CurrentNestedInsideSyncTransaction());
+    SendMessageToLink(std::move(cancel));
+  }
+
+  IPC_ASSERT(aMsg->is_sync(), "can only Send() sync messages here");
+
+  IPC_ASSERT(aMsg->nested_level() >= DispatchingSyncMessageNestedLevel(),
+             "can't send sync message of a lesser nested level than what's "
+             "being dispatched");
+  IPC_ASSERT(AwaitingSyncReplyNestedLevel() <= aMsg->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("SendAndWait", aMsg->type());
+    mLastSendError = SyncSendError::NotConnectedBeforeSend;
+    return false;
+  }
+
+  aMsg->set_seqno(NextSeqno());
+
+  int32_t seqno = aMsg->seqno();
+  int nestedLevel = aMsg->nested_level();
+  msgid_t replyType = aMsg->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;
+  aMsg->set_transaction_id(transaction);
+
+  AutoEnterTransaction transact(this, seqno, transaction, nestedLevel);
+
+  IPC_LOG("Send seqno=%d, xid=%d", seqno, transaction);
+
+  // aMsg will be destroyed soon, let's keep its type.
+  const char* msgName = aMsg->name();
+  const msgid_t msgType = aMsg->type();
+
+  AddProfilerMarker(*aMsg, MessageDirection::eSending);
+  SendMessageToLink(std::move(aMsg));
+
+  while (true) {
+    MOZ_RELEASE_ASSERT(!transact.IsCanceled());
+    ProcessPendingRequests(proxy, transact);
+    if (transact.IsComplete()) {
+      break;
+    }
+    if (!Connected()) {
+      ReportConnectionError("Send", msgType);
+      mLastSendError = SyncSendError::DisconnectedDuringSend;
+      return false;
+    }
+
+    MOZ_RELEASE_ASSERT(!mTimedOutMessageSeqno);
+    MOZ_RELEASE_ASSERT(!transact.IsComplete());
+    MOZ_RELEASE_ASSERT(mTransactionStack == &transact);
+
+    bool maybeTimedOut = !WaitForSyncNotify();
+
+    if (mListener->NeedArtificialSleep()) {
+      MonitorAutoUnlock unlock(*mMonitor);
+      mListener->ArtificialSleep();
+    }
+
+    if (!Connected()) {
+      ReportConnectionError("SendAndWait", msgType);
+      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;
+  }
+
+  uint32_t latencyMs = round((TimeStamp::Now() - start).ToMilliseconds());
+  IPC_LOG("Got reply: seqno=%d, xid=%d, msgName=%s, latency=%ums", seqno,
+          transaction, msgName, latencyMs);
+
+  UniquePtr<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());
+
+  AddProfilerMarker(*reply, MessageDirection::eReceiving);
+
+  if (reply->size() >= kMinTelemetryMessageSize) {
+    Telemetry::Accumulate(Telemetry::IPC_REPLY_SIZE,
+                          nsDependentCString(msgName), reply->size());
+  }
+
+  *aReply = std::move(reply);
+
+  // NOTE: Only collect IPC_SYNC_MAIN_LATENCY_MS on the main thread (bug
+  // 1343729)
+  if (NS_IsMainThread() && latencyMs >= kMinTelemetrySyncIPCLatencyMs) {
+    Telemetry::Accumulate(Telemetry::IPC_SYNC_MAIN_LATENCY_MS,
+                          nsDependentCString(msgName), latencyMs);
+  }
+  return true;
+}
+
+bool MessageChannel::HasPendingEvents() {
+  AssertWorkerThread();
+  mMonitor->AssertCurrentThreadOwns();
+  return ConnectedOrClosing() && !mPending.isEmpty();
+}
+
+bool MessageChannel::ProcessPendingRequest(ActorLifecycleProxy* aProxy,
+                                           UniquePtr<Message> aUrgent) {
+  AssertWorkerThread();
+  mMonitor->AssertCurrentThreadOwns();
+
+  IPC_LOG("Process pending: seqno=%d, xid=%d", aUrgent->seqno(),
+          aUrgent->transaction_id());
+
+  // keep the error relevant information
+  msgid_t msgType = aUrgent->type();
+
+  DispatchMessage(aProxy, std::move(aUrgent));
+  if (!ConnectedOrClosing()) {
+    ReportConnectionError("ProcessPendingRequest", msgType);
+    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(ActorLifecycleProxy* aProxy,
+                                MessageTask& aTask) {
+  AssertWorkerThread();
+  mMonitor->AssertCurrentThreadOwns();
+  aTask.AssertMonitorHeld(*mMonitor);
+
+  UniquePtr<Message>& msg = aTask.Msg();
+
+  if (!ConnectedOrClosing()) {
+    ReportConnectionError("RunMessage", msg->type());
+    return;
+  }
+
+  // Check that we're going to run the first message that's valid to run.
+#if 0
+#  ifdef DEBUG
+    for (MessageTask* task : mPending) {
+        if (task == &aTask) {
+            break;
+        }
+
+        MOZ_ASSERT(!ShouldRunMessage(*task->Msg()) ||
+                   aTask.Msg()->priority() != task->Msg()->priority());
+
+    }
+#  endif
+#endif
+
+  if (!ShouldRunMessage(*msg)) {
+    return;
+  }
+
+  MOZ_RELEASE_ASSERT(aTask.isInList());
+  aTask.remove();
+
+  if (!IsAlwaysDeferred(*msg)) {
+    mMaybeDeferredPendingCount--;
+  }
+
+  DispatchMessage(aProxy, std::move(msg));
+}
+
+NS_IMPL_ISUPPORTS_INHERITED(MessageChannel::MessageTask, CancelableRunnable,
+                            nsIRunnablePriority, nsIRunnableIPCMessageType)
+
+static uint32_t ToRunnablePriority(IPC::Message::PriorityValue aPriority) {
+  switch (aPriority) {
+    case IPC::Message::NORMAL_PRIORITY:
+      return nsIRunnablePriority::PRIORITY_NORMAL;
+    case IPC::Message::INPUT_PRIORITY:
+      return nsIRunnablePriority::PRIORITY_INPUT_HIGH;
+    case IPC::Message::VSYNC_PRIORITY:
+      return nsIRunnablePriority::PRIORITY_VSYNC;
+    case IPC::Message::MEDIUMHIGH_PRIORITY:
+      return nsIRunnablePriority::PRIORITY_MEDIUMHIGH;
+    case IPC::Message::CONTROL_PRIORITY:
+      return nsIRunnablePriority::PRIORITY_CONTROL;
+    default:
+      MOZ_ASSERT_UNREACHABLE();
+      return nsIRunnablePriority::PRIORITY_NORMAL;
+  }
+}
+
+MessageChannel::MessageTask::MessageTask(MessageChannel* aChannel,
+                                         UniquePtr<Message> aMessage)
+    : CancelableRunnable(aMessage->name()),
+      mMonitor(aChannel->mMonitor),
+      mChannel(aChannel),
+      mMessage(std::move(aMessage)),
+      mPriority(ToRunnablePriority(mMessage->priority())),
+      mScheduled(false)
+#ifdef FUZZING_SNAPSHOT
+      ,
+      mIsFuzzMsg(mMessage->IsFuzzMsg()),
+      mFuzzStopped(false)
+#endif
+{
+  MOZ_DIAGNOSTIC_ASSERT(mMessage, "message may not be null");
+#ifdef FUZZING_SNAPSHOT
+  if (mIsFuzzMsg) {
+    MOZ_FUZZING_IPC_MT_CTOR();
+  }
+#endif
+}
+
+MessageChannel::MessageTask::~MessageTask() {
+#ifdef FUZZING_SNAPSHOT
+  // We track fuzzing messages until their run is complete. To make sure
+  // that we don't miss messages that are for some reason destroyed without
+  // being run (e.g. canceled), we catch this condition in the destructor.
+  if (mIsFuzzMsg && !mFuzzStopped) {
+    MOZ_FUZZING_IPC_MT_STOP();
+  } else if (!mIsFuzzMsg && !fuzzing::Nyx::instance().started()) {
+    MOZ_FUZZING_IPC_PRE_FUZZ_MT_STOP();
+  }
+#endif
+}
+
+nsresult MessageChannel::MessageTask::Run() {
+  mMonitor->AssertNotCurrentThreadOwns();
+
+  // Drop the toplevel actor's lifecycle proxy outside of our monitor if we take
+  // it, as destroying our ActorLifecycleProxy reference can acquire the
+  // monitor.
+  RefPtr<ActorLifecycleProxy> proxy;
+
+  MonitorAutoLock lock(*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;
+  }
+
+#ifdef FUZZING_SNAPSHOT
+  if (!mIsFuzzMsg) {
+    if (fuzzing::Nyx::instance().started()) {
+      // Once we started fuzzing, prevent non-fuzzing tasks from being
+      // run and potentially blocking worker threads.
+      //
+      // TODO: This currently blocks all MessageTasks from running, not
+      // just those belonging to the target process pair. We currently
+      // do this for performance reasons, but it should be re-evaluated
+      // at a later stage when we found a better snapshot point.
+      return NS_OK;
+    }
+    // Record all running tasks prior to fuzzing, so we can wait for
+    // them to settle before snapshotting.
+    MOZ_FUZZING_IPC_PRE_FUZZ_MT_RUN();
+  }
+#endif
+
+  Channel()->AssertWorkerThread();
+  mMonitor->AssertSameMonitor(*Channel()->mMonitor);
+  proxy = Channel()->Listener()->GetLifecycleProxy();
+  Channel()->RunMessage(proxy, *this);
+
+#ifdef FUZZING_SNAPSHOT
+  if (mIsFuzzMsg && !mFuzzStopped) {
+    MOZ_FUZZING_IPC_MT_STOP();
+    mFuzzStopped = true;
+  }
+#endif
+  return NS_OK;
+}
+
+// Warning: This method removes the receiver from whatever list it might be in.
+nsresult MessageChannel::MessageTask::Cancel() {
+  mMonitor->AssertNotCurrentThreadOwns();
+
+  MonitorAutoLock lock(*mMonitor);
+
+  if (!isInList()) {
+    return NS_OK;
+  }
+
+  Channel()->AssertWorkerThread();
+  mMonitor->AssertSameMonitor(*Channel()->mMonitor);
+  if (!IsAlwaysDeferred(*Msg())) {
+    Channel()->mMaybeDeferredPendingCount--;
+  }
+
+  remove();
+
+#ifdef FUZZING_SNAPSHOT
+  if (mIsFuzzMsg && !mFuzzStopped) {
+    MOZ_FUZZING_IPC_MT_STOP();
+    mFuzzStopped = true;
+  }
+#endif
+
+  return NS_OK;
+}
+
+void MessageChannel::MessageTask::Post() {
+  mMonitor->AssertCurrentThreadOwns();
+  mMonitor->AssertSameMonitor(*Channel()->mMonitor);
+  MOZ_RELEASE_ASSERT(!mScheduled);
+  MOZ_RELEASE_ASSERT(isInList());
+
+  mScheduled = true;
+
+  Channel()->mWorkerThread->Dispatch(do_AddRef(this));
+}
+
+NS_IMETHODIMP
+MessageChannel::MessageTask::GetPriority(uint32_t* aPriority) {
+  *aPriority = mPriority;
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+MessageChannel::MessageTask::GetType(uint32_t* aType) {
+  mMonitor->AssertNotCurrentThreadOwns();
+
+  MonitorAutoLock lock(*mMonitor);
+  if (!mMessage) {
+    // If mMessage has been moved already elsewhere, we can't know what the type
+    // has been.
+    return NS_ERROR_FAILURE;
+  }
+
+  *aType = mMessage->type();
+  return NS_OK;
+}
+
+void MessageChannel::DispatchMessage(ActorLifecycleProxy* aProxy,
+                                     UniquePtr<Message> aMsg) {
+  AssertWorkerThread();
+  mMonitor->AssertCurrentThreadOwns();
+
+  Maybe<AutoNoJSAPI> nojsapi;
+  if (NS_IsMainThread() && CycleCollectedJSContext::Get()) {
+    nojsapi.emplace();
+  }
+
+  UniquePtr<Message> reply;
+
+#ifdef FUZZING_SNAPSHOT
+  if (IsCrossProcess()) {
+    aMsg = mozilla::fuzzing::IPCFuzzController::instance().replaceIPCMessage(
+        std::move(aMsg));
+  }
+#endif
+
+  IPC_LOG("DispatchMessage: seqno=%d, xid=%d", aMsg->seqno(),
+          aMsg->transaction_id());
+  AddProfilerMarker(*aMsg, MessageDirection::eReceiving);
+
+  {
+    AutoEnterTransaction transaction(this, *aMsg);
+
+    int id = aMsg->transaction_id();
+    MOZ_RELEASE_ASSERT(!aMsg->is_sync() || id == transaction.TransactionID());
+
+    {
+      MonitorAutoUnlock unlock(*mMonitor);
+      AutoSetValue<bool> setOnCxxStack(mOnCxxStack, true);
+
+      mListener->ArtificialSleep();
+
+      if (aMsg->is_sync()) {
+        DispatchSyncMessage(aProxy, *aMsg, reply);
+      } else {
+        DispatchAsyncMessage(aProxy, *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;
+    }
+  }
+
+#ifdef FUZZING_SNAPSHOT
+  if (aMsg->IsFuzzMsg()) {
+    mozilla::fuzzing::IPCFuzzController::instance().syncAfterReplace();
+  }
+#endif
+
+  if (reply && ChannelConnected == mChannelState) {
+    IPC_LOG("Sending reply seqno=%d, xid=%d", aMsg->seqno(),
+            aMsg->transaction_id());
+    AddProfilerMarker(*reply, MessageDirection::eSending);
+
+    SendMessageToLink(std::move(reply));
+  }
+}
+
+void MessageChannel::DispatchSyncMessage(ActorLifecycleProxy* aProxy,
+                                         const Message& aMsg,
+                                         UniquePtr<Message>& aReply) {
+  AssertWorkerThread();
+
+  mozilla::TimeStamp start = TimeStamp::Now();
+
+  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 = aProxy->Get()->OnMessageReceived(aMsg, aReply);
+  }
+
+  uint32_t latencyMs = round((TimeStamp::Now() - start).ToMilliseconds());
+  if (latencyMs >= kMinTelemetrySyncIPCLatencyMs) {
+    Telemetry::Accumulate(Telemetry::IPC_SYNC_RECEIVE_MS,
+                          nsDependentCString(aMsg.name()), latencyMs);
+  }
+
+  if (!MaybeHandleError(rv, aMsg, "DispatchSyncMessage")) {
+    aReply = Message::ForSyncDispatchError(aMsg.nested_level());
+  }
+  aReply->set_seqno(aMsg.seqno());
+  aReply->set_transaction_id(aMsg.transaction_id());
+}
+
+void MessageChannel::DispatchAsyncMessage(ActorLifecycleProxy* aProxy,
+                                          const Message& aMsg) {
+  AssertWorkerThread();
+  MOZ_RELEASE_ASSERT(!aMsg.is_sync());
+
+  if (aMsg.routing_id() == MSG_ROUTING_NONE) {
+    NS_WARNING("unhandled special message!");
+    MaybeHandleError(MsgNotKnown, aMsg, "DispatchAsyncMessage");
+    return;
+  }
+
+  Result rv;
+  {
+    int nestedLevel = aMsg.nested_level();
+    AutoSetValue<bool> async(mDispatchingAsyncMessage, true);
+    AutoSetValue<int> nestedLevelSet(mDispatchingAsyncMessageNestedLevel,
+                                     nestedLevel);
+    rv = aProxy->Get()->OnMessageReceived(aMsg);
+  }
+  MaybeHandleError(rv, aMsg, "DispatchAsyncMessage");
+}
+
+void MessageChannel::EnqueuePendingMessages() {
+  AssertWorkerThread();
+  mMonitor->AssertCurrentThreadOwns();
+
+  // XXX performance tuning knob: could process all or k pending
+  // messages here, rather than enqueuing for later processing
+
+  RepostAllMessages();
+}
+
+bool MessageChannel::WaitResponse(bool aWaitTimedOut) {
+  AssertWorkerThread();
+  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 XP_WIN
+bool MessageChannel::WaitForSyncNotify() {
+  AssertWorkerThread();
+#  ifdef DEBUG
+  // WARNING: We don't release the lock here. We can't because the link
+  // could signal at this time and we would miss it. Instead we require
+  // ArtificialTimeout() to be extremely simple.
+  if (mListener->ArtificialTimeout()) {
+    return false;
+  }
+#  endif
+
+  MOZ_RELEASE_ASSERT(!mIsSameThreadChannel,
+                     "Wait on same-thread channel will deadlock!");
+
+  TimeDuration timeout = (kNoTimeout == mTimeoutMs)
+                             ? TimeDuration::Forever()
+                             : TimeDuration::FromMilliseconds(mTimeoutMs);
+  CVStatus status = mMonitor->Wait(timeout);
+
+  // If the timeout didn't expire, we know we received an event. The
+  // converse is not true.
+  return WaitResponse(status == CVStatus::Timeout);
+}
+
+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") || getenv("MOZ_DEBUG_CHILD_PAUSE")
+            ? 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::ReportConnectionError(const char* aFunctionName,
+                                           const uint32_t aMsgType) const {
+  AssertWorkerThread();
+  mMonitor->AssertCurrentThreadOwns();
+
+  const char* errorMsg = nullptr;
+  switch (mChannelState) {
+    case ChannelClosed:
+      errorMsg = "Closed channel: cannot send/recv";
+      break;
+    case ChannelClosing:
+      errorMsg = "Channel closing: too late to send, messages will be lost";
+      break;
+    case ChannelError:
+      errorMsg = "Channel error: cannot send/recv";
+      break;
+
+    default:
+      MOZ_CRASH("unreached");
+  }
+
+  // IPC connection errors are fairly common, especially "Channel closing: too
+  // late to send/recv, messages will be lost", so shouldn't be being reported
+  // on release builds, as that's misleading as to their severity.
+  NS_WARNING(nsPrintfCString("IPC Connection Error: [%s][%s] %s(msgname=%s) %s",
+                             StringFromIPCSide(mSide), mName, aFunctionName,
+                             IPC::StringFromIPCMessageType(aMsgType), errorMsg)
+                 .get());
+
+  MonitorAutoUnlock unlock(*mMonitor);
+  mListener->ProcessingError(MsgDropped, errorMsg);
+}
+
+void MessageChannel::ReportMessageRouteError(const char* channelName) const {
+  PrintErrorMessage(mSide, channelName, "Need a route");
+  mListener->ProcessingError(MsgRouteError, "MsgRouteError");
+}
+
+bool MessageChannel::MaybeHandleError(Result code, const Message& aMsg,
+                                      const char* channelName) {
+  if (MsgProcessed == code) return true;
+
+#ifdef FUZZING_SNAPSHOT
+  mozilla::fuzzing::IPCFuzzController::instance().OnMessageError(code, aMsg);
+#endif
+
+  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:
+      MOZ_CRASH("unknown Result code");
+      return false;
+  }
+
+  char reason[512];
+  const char* msgname = aMsg.name();
+  if (msgname[0] == '?') {
+    SprintfLiteral(reason, "(msgtype=0x%X) %s", aMsg.type(), errorMsg);
+  } else {
+    SprintfLiteral(reason, "%s %s", msgname, errorMsg);
+  }
+
+  PrintErrorMessage(mSide, channelName, reason);
+
+  // Error handled in mozilla::ipc::IPCResult.
+  if (code == MsgProcessingError) {
+    return false;
+  }
+
+  mListener->ProcessingError(code, reason);
+
+  return false;
+}
+
+void MessageChannel::OnChannelErrorFromLink() {
+  mMonitor->AssertCurrentThreadOwns();
+  MOZ_ASSERT(mChannelState == ChannelConnected);
+
+  IPC_LOG("OnChannelErrorFromLink");
+
+  if (AwaitingSyncReply()) {
+    NotifyWorkerThread();
+  }
+
+  if (mAbortOnError) {
+    // mAbortOnError is set by main actors (e.g., ContentChild) to ensure
+    // that the process terminates even if normal shutdown is prevented.
+    // A MOZ_CRASH() here is not helpful because crash reporting relies
+    // on the parent process which we know is dead or otherwise unusable.
+    //
+    // Additionally, the parent process can (and often is) killed on Android
+    // when apps are backgrounded. We don't need to report a crash for
+    // normal behavior in that case.
+    printf_stderr("Exiting due to channel error.\n");
+    ProcessChild::QuickExit();
+  }
+  mChannelState = ChannelError;
+  mMonitor->Notify();
+
+  PostErrorNotifyTask();
+}
+
+void MessageChannel::NotifyMaybeChannelError(ReleasableMonitorAutoLock& aLock) {
+  AssertWorkerThread();
+  mMonitor->AssertCurrentThreadOwns();
+  aLock.AssertCurrentThreadOwns();
+  MOZ_ASSERT(mChannelState != ChannelConnected);
+
+  if (ChannelClosing == mChannelState || ChannelClosed == mChannelState) {
+    // the channel closed, but we received a "Goodbye" message warning us
+    // about it. no worries
+    mChannelState = ChannelClosed;
+    NotifyChannelClosed(aLock);
+    return;
+  }
+
+  MOZ_ASSERT(ChannelError == mChannelState);
+
+  Clear();
+
+  // IPDL assumes these notifications do not fire twice, so we do not let
+  // that happen.
+  if (mNotifiedChannelDone) {
+    return;
+  }
+  mNotifiedChannelDone = true;
+
+  // Let our listener know that the channel errored. This may cause the
+  // channel to be deleted. Release our caller's `MonitorAutoLock` before
+  // invoking the listener, as this may call back into MessageChannel, and/or
+  // cause the channel to be destroyed.
+  aLock.Unlock();
+  mListener->OnChannelError();
+}
+
+void MessageChannel::OnNotifyMaybeChannelError() {
+  AssertWorkerThread();
+  mMonitor->AssertNotCurrentThreadOwns();
+
+  // This lock guard may be reset by `NotifyMaybeChannelError` before invoking
+  // listener callbacks which may destroy this `MessageChannel`.
+  //
+  // Acquiring the lock here also allows us to ensure that
+  // `OnChannelErrorFromLink` has finished running before this task is allowed
+  // to continue.
+  ReleasableMonitorAutoLock lock(*mMonitor);
+
+  mChannelErrorTask = nullptr;
+
+  if (IsOnCxxStack()) {
+    // This used to post a 10ms delayed task; however not all
+    // nsISerialEventTarget implementations support delayed dispatch.
+    // The delay being completely arbitrary, we may not as well have any.
+    PostErrorNotifyTask();
+    return;
+  }
+
+  // This call may destroy `this`.
+  NotifyMaybeChannelError(lock);
+}
+
+void MessageChannel::PostErrorNotifyTask() {
+  mMonitor->AssertCurrentThreadOwns();
+
+  if (mChannelErrorTask) {
+    return;
+  }
+
+  // This must be the last code that runs on this thread!
+  mChannelErrorTask = NewNonOwningCancelableRunnableMethod(
+      "ipc::MessageChannel::OnNotifyMaybeChannelError", this,
+      &MessageChannel::OnNotifyMaybeChannelError);
+  mWorkerThread->Dispatch(do_AddRef(mChannelErrorTask));
+}
+
+// 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::InduceConnectionError() {
+  MonitorAutoLock lock(*mMonitor);
+
+  // Either connected or closing, immediately convert to an error and notify.
+  switch (mChannelState) {
+    case ChannelConnected:
+      // The channel is still actively connected. Immediately shut down the
+      // connection with our peer and simulate it invoking
+      // OnChannelErrorFromLink on us.
+      //
+      // This will update the state to ChannelError, preventing new messages
+      // from being processed, leading to an error being reported asynchronously
+      // to our listener.
+      mLink->Close();
+      OnChannelErrorFromLink();
+      return;
+
+    case ChannelClosing:
+      // An notify task has already been posted. Update mChannelState to stop
+      // processing new messages and treat the notification as an error.
+      mChannelState = ChannelError;
+      return;
+
+    default:
+      // Either already closed or errored. Nothing to do.
+      MOZ_ASSERT(mChannelState == ChannelClosed ||
+                 mChannelState == ChannelError);
+      return;
+  }
+}
+
+void MessageChannel::NotifyImpendingShutdown() {
+  UniquePtr<Message> msg =
+      MakeUnique<Message>(MSG_ROUTING_NONE, IMPENDING_SHUTDOWN_MESSAGE_TYPE);
+  MonitorAutoLock lock(*mMonitor);
+  if (Connected()) {
+    SendMessageToLink(std::move(msg));
+  }
+}
+
+void MessageChannel::Close() {
+  AssertWorkerThread();
+  mMonitor->AssertNotCurrentThreadOwns();
+
+  // This lock guard may be reset by `Notify{ChannelClosed,MaybeChannelError}`
+  // before invoking listener callbacks which may destroy this `MessageChannel`.
+  ReleasableMonitorAutoLock lock(*mMonitor);
+
+  switch (mChannelState) {
+    case ChannelError:
+      // 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.
+      NotifyMaybeChannelError(lock);
+      return;
+    case ChannelClosed:
+      // Slightly unexpected but harmless; ignore.  See bug 1554244.
+      return;
+
+    default:
+      // 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) {
+        SendMessageToLink(MakeUnique<GoodbyeMessage>());
+      }
+      mLink->Close();
+      mChannelState = ChannelClosed;
+      NotifyChannelClosed(lock);
+      return;
+  }
+}
+
+void MessageChannel::NotifyChannelClosed(ReleasableMonitorAutoLock& aLock) {
+  AssertWorkerThread();
+  mMonitor->AssertCurrentThreadOwns();
+  aLock.AssertCurrentThreadOwns();
+
+  if (ChannelClosed != mChannelState) {
+    MOZ_CRASH("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;
+
+  // Let our listener know that the channel was closed. This may cause the
+  // channel to be deleted. Release our caller's `MonitorAutoLock` before
+  // invoking the listener, as this may call back into MessageChannel, and/or
+  // cause the channel to be destroyed.
+  aLock.Unlock();
+  mListener->OnChannelClose();
+}
+
+void MessageChannel::DebugAbort(const char* file, int line, const char* cond,
+                                const char* why, bool reply) {
+  AssertWorkerThread();
+  mMonitor->AssertCurrentThreadOwns();
+
+  printf_stderr(
+      "###!!! [MessageChannel][%s][%s:%d] "
+      "Assertion (%s) failed.  %s %s\n",
+      StringFromIPCSide(mSide), file, line, cond, why, reply ? "(reply)" : "");
+
+  MessageQueue pending = std::move(mPending);
+  while (!pending.isEmpty()) {
+    pending.getFirst()->AssertMonitorHeld(*mMonitor);
+    printf_stderr("    [ %s%s ]\n",
+                  pending.getFirst()->Msg()->is_sync() ? "sync" : "async",
+                  pending.getFirst()->Msg()->is_reply() ? "reply" : "");
+    pending.popFirst();
+  }
+
+  MOZ_CRASH_UNSAFE(why);
+}
+
+void MessageChannel::AddProfilerMarker(const IPC::Message& aMessage,
+                                       MessageDirection aDirection) {
+  mMonitor->AssertCurrentThreadOwns();
+
+  if (profiler_feature_active(ProfilerFeature::IPCMessages)) {
+    base::ProcessId pid = mListener->OtherPidMaybeInvalid();
+    // Only record markers for IPCs with a valid pid.
+    // And if one of the profiler mutexes is locked on this thread, don't record
+    // markers, because we don't want to expose profiler IPCs due to the
+    // profiler itself, and also to avoid possible re-entrancy issues.
+    if (pid != base::kInvalidProcessId &&
+        !profiler_is_locked_on_current_thread()) {
+      // The current timestamp must be given to the `IPCMarker` payload.
+      [[maybe_unused]] const TimeStamp now = TimeStamp::Now();
+      bool isThreadBeingProfiled =
+          profiler_thread_is_being_profiled_for_markers();
+      PROFILER_MARKER(
+          "IPC", IPC,
+          mozilla::MarkerOptions(
+              mozilla::MarkerTiming::InstantAt(now),
+              // If the thread is being profiled, add the marker to
+              // the current thread. If the thread is not being
+              // profiled, add the marker to the main thread. It
+              // will appear in the main thread's IPC track. Profiler analysis
+              // UI correlates all the IPC markers from different threads and
+              // generates processed markers.
+              isThreadBeingProfiled ? mozilla::MarkerThreadId::CurrentThread()
+                                    : mozilla::MarkerThreadId::MainThread()),
+          IPCMarker, now, now, pid, aMessage.seqno(), aMessage.type(), mSide,
+          aDirection, MessagePhase::Endpoint, aMessage.is_sync(),
+          // aOriginThreadId: If the thread is being profiled, do not include a
+          // thread ID, as it's the same as the markers. Only include this field
+          // when the marker is being sent from another thread.
+          isThreadBeingProfiled ? mozilla::MarkerThreadId{}
+                                : mozilla::MarkerThreadId::CurrentThread());
+    }
+  }
+}
+
+void MessageChannel::EndTimeout() {
+  mMonitor->AssertCurrentThreadOwns();
+
+  IPC_LOG("Ending timeout of seqno=%d", mTimedOutMessageSeqno);
+  mTimedOutMessageSeqno = 0;
+  mTimedOutMessageNestedLevel = 0;
+
+  RepostAllMessages();
+}
+
+void MessageChannel::RepostAllMessages() {
+  mMonitor->AssertCurrentThreadOwns();
+
+  bool needRepost = false;
+  for (MessageTask* task : mPending) {
+    task->AssertMonitorHeld(*mMonitor);
+    if (!task->IsScheduled()) {
+      needRepost = true;
+      break;
+    }
+  }
+  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 = std::move(mPending);
+  while (RefPtr<MessageTask> task = queue.popFirst()) {
+    task->AssertMonitorHeld(*mMonitor);
+    RefPtr<MessageTask> newTask = new MessageTask(this, std::move(task->Msg()));
+    newTask->AssertMonitorHeld(*mMonitor);
+    mPending.insertBack(newTask);
+    newTask->Post();
+  }
+
+  AssertMaybeDeferredCountCorrect();
+}
+
+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 (MessageTask* p = mPending.getFirst(); p;) {
+    p->AssertMonitorHeld(*mMonitor);
+    UniquePtr<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;
+      if (!IsAlwaysDeferred(*msg)) {
+        mMaybeDeferredPendingCount--;
+      }
+      p = p->removeAndGetNext();
+      continue;
+    }
+
+    p = p->getNext();
+  }
+
+  AssertMaybeDeferredCountCorrect();
+}
+
+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());
+    auto cancel =
+        MakeUnique<CancelMessage>(CurrentNestedInsideSyncTransaction());
+    CancelTransaction(CurrentNestedInsideSyncTransaction());
+    SendMessageToLink(std::move(cancel));
+  }
+}
+
+void CancelCPOWs() {
+  MOZ_ASSERT(NS_IsMainThread());
+
+  if (gParentProcessBlocker) {
+    mozilla::Telemetry::Accumulate(mozilla::Telemetry::IPC_TRANSACTION_CANCEL,
+                                   true);
+    gParentProcessBlocker->CancelCurrentTransaction();
+  }
+}
+
+bool MessageChannel::IsCrossProcess() const {
+  mMonitor->AssertCurrentThreadOwns();
+  return mIsCrossProcess;
+}
+
+void MessageChannel::SetIsCrossProcess(bool aIsCrossProcess) {
+  mMonitor->AssertCurrentThreadOwns();
+  if (aIsCrossProcess == mIsCrossProcess) {
+    return;
+  }
+  mIsCrossProcess = aIsCrossProcess;
+  if (mIsCrossProcess) {
+    ChannelCountReporter::Increment(mName);
+  } else {
+    ChannelCountReporter::Decrement(mName);
+  }
+}
+
+NS_IMPL_ISUPPORTS(MessageChannel::WorkerTargetShutdownTask,
+                  nsITargetShutdownTask)
+
+MessageChannel::WorkerTargetShutdownTask::WorkerTargetShutdownTask(
+    nsISerialEventTarget* aTarget, MessageChannel* aChannel)
+    : mTarget(aTarget), mChannel(aChannel) {}
+
+void MessageChannel::WorkerTargetShutdownTask::TargetShutdown() {
+  MOZ_RELEASE_ASSERT(mTarget->IsOnCurrentThread());
+  IPC_LOG("Closing channel due to event target shutdown");
+  if (MessageChannel* channel = std::exchange(mChannel, nullptr)) {
+    channel->Close();
+  }
+}
+
+void MessageChannel::WorkerTargetShutdownTask::Clear() {
+  MOZ_RELEASE_ASSERT(mTarget->IsOnCurrentThread());
+  mChannel = nullptr;
+}
+
+NS_IMPL_ISUPPORTS_INHERITED0(MessageChannel::FlushLazySendMessagesRunnable,
+                             CancelableRunnable)
+
+MessageChannel::FlushLazySendMessagesRunnable::FlushLazySendMessagesRunnable(
+    MessageChannel* aChannel)
+    : CancelableRunnable("MessageChannel::FlushLazyMessagesRunnable"),
+      mChannel(aChannel) {}
+
+NS_IMETHODIMP MessageChannel::FlushLazySendMessagesRunnable::Run() {
+  if (mChannel) {
+    MonitorAutoLock lock(*mChannel->mMonitor);
+    MOZ_ASSERT(mChannel->mFlushLazySendTask == this);
+    mChannel->FlushLazySendMessages();
+  }
+  return NS_OK;
+}
+
+nsresult MessageChannel::FlushLazySendMessagesRunnable::Cancel() {
+  mQueue.Clear();
+  mChannel = nullptr;
+  return NS_OK;
+}
+
+void MessageChannel::FlushLazySendMessagesRunnable::PushMessage(
+    UniquePtr<Message> aMsg) {
+  MOZ_ASSERT(mChannel);
+  mQueue.AppendElement(std::move(aMsg));
+}
+
+nsTArray<UniquePtr<IPC::Message>>
+MessageChannel::FlushLazySendMessagesRunnable::TakeMessages() {
+  mChannel = nullptr;
+  return std::move(mQueue);
+}
+
+}  // namespace ipc
+}  // namespace mozilla