gecko-dev/dom/workers/WorkerRunnable.cpp

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=8 sts=2 et sw=2 tw=80: */
/* 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 "WorkerRunnable.h"

#include "nsGlobalWindow.h"
#include "nsIEventTarget.h"
#include "nsIGlobalObject.h"
#include "nsIRunnable.h"
#include "nsThreadUtils.h"

#include "mozilla/DebugOnly.h"
#include "mozilla/ErrorResult.h"
#include "mozilla/dom/ScriptSettings.h"
#include "mozilla/Telemetry.h"

#include "js/RootingAPI.h"
#include "js/Value.h"

#include "WorkerPrivate.h"
#include "WorkerScope.h"

USING_WORKERS_NAMESPACE

namespace {

const nsIID kWorkerRunnableIID = {
  0x320cc0b5, 0xef12, 0x4084, { 0x88, 0x6e, 0xca, 0x6a, 0x81, 0xe4, 0x1d, 0x68 }
};

// This runnable is used to stop a sync loop and it's meant to be used on the
// main-thread only. As sync loops keep the busy count incremented as long as
// they run this runnable does not modify the busy count
// in any way.
class MainThreadStopSyncLoopRunnable : public WorkerSyncRunnable
{
  bool mResult;

public:
  // Passing null for aSyncLoopTarget is not allowed.
  MainThreadStopSyncLoopRunnable(
                               WorkerPrivate* aWorkerPrivate,
                               already_AddRefed<nsIEventTarget>&& aSyncLoopTarget,
                               bool aResult);

  // By default StopSyncLoopRunnables cannot be canceled since they could leave
  // a sync loop spinning forever.
  nsresult
  Cancel() override;

protected:
  virtual ~MainThreadStopSyncLoopRunnable()
  { }

private:
  virtual bool
  PreDispatch(WorkerPrivate* aWorkerPrivate) override final
  {
    AssertIsOnMainThread();
    return true;
  }

  virtual void
  PostDispatch(WorkerPrivate* aWorkerPrivate, bool aDispatchResult) override;

  virtual bool
  WorkerRun(JSContext* aCx, WorkerPrivate* aWorkerPrivate) override;

  virtual bool
  DispatchInternal() override final;
};

} // namespace

#ifdef DEBUG
WorkerRunnable::WorkerRunnable(WorkerPrivate* aWorkerPrivate,
                               TargetAndBusyBehavior aBehavior)
: mWorkerPrivate(aWorkerPrivate), mBehavior(aBehavior), mCanceled(0),
  mCallingCancelWithinRun(false)
{
  MOZ_ASSERT(aWorkerPrivate);
}
#endif

bool
WorkerRunnable::IsDebuggerRunnable() const
{
  return false;
}

nsIGlobalObject*
WorkerRunnable::DefaultGlobalObject() const
{
  if (IsDebuggerRunnable()) {
    return mWorkerPrivate->DebuggerGlobalScope();
  } else {
    return mWorkerPrivate->GlobalScope();
  }
}

bool
WorkerRunnable::PreDispatch(WorkerPrivate* aWorkerPrivate)
{
#ifdef DEBUG
  MOZ_ASSERT(aWorkerPrivate);

  switch (mBehavior) {
    case ParentThreadUnchangedBusyCount:
      aWorkerPrivate->AssertIsOnWorkerThread();
      break;

    case WorkerThreadModifyBusyCount:
    case WorkerThreadUnchangedBusyCount:
      aWorkerPrivate->AssertIsOnParentThread();
      break;

    default:
      MOZ_ASSERT_UNREACHABLE("Unknown behavior!");
  }
#endif

  if (mBehavior == WorkerThreadModifyBusyCount) {
    return aWorkerPrivate->ModifyBusyCount(true);
  }

  return true;
}

bool
WorkerRunnable::Dispatch()
{
  bool ok = PreDispatch(mWorkerPrivate);
  if (ok) {
    ok = DispatchInternal();
  }
  PostDispatch(mWorkerPrivate, ok);
  return ok;
}

bool
WorkerRunnable::DispatchInternal()
{
  RefPtr<WorkerRunnable> runnable(this);

  if (mBehavior == WorkerThreadModifyBusyCount ||
      mBehavior == WorkerThreadUnchangedBusyCount) {
    if (IsDebuggerRunnable()) {
      return NS_SUCCEEDED(mWorkerPrivate->DispatchDebuggerRunnable(runnable.forget()));
    } else {
      return NS_SUCCEEDED(mWorkerPrivate->Dispatch(runnable.forget()));
    }
  }

  MOZ_ASSERT(mBehavior == ParentThreadUnchangedBusyCount);

  if (WorkerPrivate* parent = mWorkerPrivate->GetParent()) {
    return NS_SUCCEEDED(parent->Dispatch(runnable.forget()));
  }

  nsCOMPtr<nsIThread> mainThread = do_GetMainThread();
  MOZ_ASSERT(mainThread);

  return NS_SUCCEEDED(mainThread->Dispatch(runnable.forget(), NS_DISPATCH_NORMAL));
}

void
WorkerRunnable::PostDispatch(WorkerPrivate* aWorkerPrivate,
                             bool aDispatchResult)
{
  MOZ_ASSERT(aWorkerPrivate);

#ifdef DEBUG
  switch (mBehavior) {
    case ParentThreadUnchangedBusyCount:
      aWorkerPrivate->AssertIsOnWorkerThread();
      break;

    case WorkerThreadModifyBusyCount:
      aWorkerPrivate->AssertIsOnParentThread();
      break;

    case WorkerThreadUnchangedBusyCount:
      aWorkerPrivate->AssertIsOnParentThread();
      break;

    default:
      MOZ_ASSERT_UNREACHABLE("Unknown behavior!");
  }
#endif

  if (!aDispatchResult) {
    if (mBehavior == WorkerThreadModifyBusyCount) {
      aWorkerPrivate->ModifyBusyCount(false);
    }
  }
}

bool
WorkerRunnable::PreRun(WorkerPrivate* aWorkerPrivate)
{
  return true;
}

void
WorkerRunnable::PostRun(JSContext* aCx, WorkerPrivate* aWorkerPrivate,
                        bool aRunResult)
{
  MOZ_ASSERT(aCx);
  MOZ_ASSERT(aWorkerPrivate);

#ifdef DEBUG
  switch (mBehavior) {
    case ParentThreadUnchangedBusyCount:
      aWorkerPrivate->AssertIsOnParentThread();
      break;

    case WorkerThreadModifyBusyCount:
      aWorkerPrivate->AssertIsOnWorkerThread();
      break;

    case WorkerThreadUnchangedBusyCount:
      aWorkerPrivate->AssertIsOnWorkerThread();
      break;

    default:
      MOZ_ASSERT_UNREACHABLE("Unknown behavior!");
  }
#endif

  if (mBehavior == WorkerThreadModifyBusyCount) {
    aWorkerPrivate->ModifyBusyCountFromWorker(false);
  }
}

// static
WorkerRunnable*
WorkerRunnable::FromRunnable(nsIRunnable* aRunnable)
{
  MOZ_ASSERT(aRunnable);

  WorkerRunnable* runnable;
  nsresult rv = aRunnable->QueryInterface(kWorkerRunnableIID,
                                          reinterpret_cast<void**>(&runnable));
  if (NS_FAILED(rv)) {
    return nullptr;
  }

  MOZ_ASSERT(runnable);
  return runnable;
}

NS_IMPL_ADDREF(WorkerRunnable)
NS_IMPL_RELEASE(WorkerRunnable)

NS_INTERFACE_MAP_BEGIN(WorkerRunnable)
  NS_INTERFACE_MAP_ENTRY(nsIRunnable)
  NS_INTERFACE_MAP_ENTRY(nsICancelableRunnable)
  NS_INTERFACE_MAP_ENTRY_AMBIGUOUS(nsISupports, nsIRunnable)
  // kWorkerRunnableIID is special in that it does not AddRef its result.
  if (aIID.Equals(kWorkerRunnableIID)) {
    *aInstancePtr = this;
    return NS_OK;
  }
  else
NS_INTERFACE_MAP_END

NS_IMETHODIMP
WorkerRunnable::Run()
{
  bool targetIsWorkerThread = mBehavior == WorkerThreadModifyBusyCount ||
                              mBehavior == WorkerThreadUnchangedBusyCount;

#ifdef DEBUG
  MOZ_ASSERT_IF(mCallingCancelWithinRun, targetIsWorkerThread);
  if (targetIsWorkerThread) {
    mWorkerPrivate->AssertIsOnWorkerThread();
  }
  else {
    MOZ_ASSERT(mBehavior == ParentThreadUnchangedBusyCount);
    mWorkerPrivate->AssertIsOnParentThread();
  }
#endif

  if (IsCanceled() && !mCallingCancelWithinRun) {
    return NS_OK;
  }

  if (targetIsWorkerThread &&
      mWorkerPrivate->AllPendingRunnablesShouldBeCanceled() &&
      !IsCanceled() && !mCallingCancelWithinRun) {

    // Prevent recursion.
    mCallingCancelWithinRun = true;

    Cancel();

    MOZ_ASSERT(mCallingCancelWithinRun);
    mCallingCancelWithinRun = false;

    MOZ_ASSERT(IsCanceled(), "Subclass Cancel() didn't set IsCanceled()!");

    return NS_OK;
  }

  bool result = PreRun(mWorkerPrivate);
  if (!result) {
    MOZ_ASSERT(targetIsWorkerThread,
               "The only PreRun implementation that can fail is "
               "ScriptExecutorRunnable");
    mWorkerPrivate->AssertIsOnWorkerThread();
    MOZ_ASSERT(!JS_IsExceptionPending(mWorkerPrivate->GetJSContext()));
    // We can't enter a useful compartment on the JSContext here; just pass it
    // in as-is.
    PostRun(mWorkerPrivate->GetJSContext(), mWorkerPrivate, false);
    return NS_ERROR_FAILURE;
  }

  // Track down the appropriate global, if any, to use for the AutoEntryScript.
  nsCOMPtr<nsIGlobalObject> globalObject;
  bool isMainThread = !targetIsWorkerThread && !mWorkerPrivate->GetParent();
  MOZ_ASSERT(isMainThread == NS_IsMainThread());
  RefPtr<WorkerPrivate> kungFuDeathGrip;
  if (targetIsWorkerThread) {
    JSContext* cx = GetCurrentThreadJSContext();
    if (NS_WARN_IF(!cx)) {
      return NS_ERROR_FAILURE;
    }

    JSObject* global = JS::CurrentGlobalOrNull(cx);
    if (global) {
      globalObject = xpc::NativeGlobal(global);
    } else {
      globalObject = DefaultGlobalObject();
    }

    // We may still not have a globalObject here: in the case of
    // CompileScriptRunnable, we don't actually create the global object until
    // we have the script data, which happens in a syncloop under
    // CompileScriptRunnable::WorkerRun, so we can't assert that it got created
    // in the PreRun call above.
  } else {
    kungFuDeathGrip = mWorkerPrivate;
    if (isMainThread) {
      globalObject = nsGlobalWindow::Cast(mWorkerPrivate->GetWindow());
    } else {
      globalObject = mWorkerPrivate->GetParent()->GlobalScope();
    }
  }

  // We might run script as part of WorkerRun, so we need an AutoEntryScript.
  // This is part of the HTML spec for workers at:
  // http://www.whatwg.org/specs/web-apps/current-work/#run-a-worker
  // If we don't have a globalObject we have to use an AutoJSAPI instead, but
  // this is OK as we won't be running script in these circumstances.
  Maybe<mozilla::dom::AutoJSAPI> maybeJSAPI;
  Maybe<mozilla::dom::AutoEntryScript> aes;
  JSContext* cx;
  AutoJSAPI* jsapi;
  if (globalObject) {
    aes.emplace(globalObject, "Worker runnable",
                isMainThread,
                isMainThread ? nullptr : GetCurrentThreadJSContext());
    jsapi = aes.ptr();
    cx = aes->cx();
  } else {
    maybeJSAPI.emplace();
    maybeJSAPI->Init();
    jsapi = maybeJSAPI.ptr();
    cx = jsapi->cx();
  }

  // Note that we can't assert anything about mWorkerPrivate->GetWrapper()
  // existing, since it may in fact have been GCed (and we may be one of the
  // runnables cleaning up the worker as a result).

  // If we are on the parent thread and that thread is not the main thread,
  // then we must be a dedicated worker (because there are no
  // Shared/ServiceWorkers whose parent is itself a worker) and then we
  // definitely have a globalObject.  If it _is_ the main thread, globalObject
  // can be null for workers started from JSMs or other non-window contexts,
  // sadly.
  MOZ_ASSERT_IF(!targetIsWorkerThread && !isMainThread,
                mWorkerPrivate->IsDedicatedWorker() && globalObject);

  // If we're on the parent thread we might be in a null compartment in the
  // situation described above when globalObject is null.  Make sure to enter
  // the compartment of the worker's reflector if there is one.  There might
  // not be one if we're just starting to compile the script for this worker.
  Maybe<JSAutoCompartment> ac;
  if (!targetIsWorkerThread && mWorkerPrivate->GetWrapper()) {
    // If we're on the parent thread and have a reflector and a globalObject,
    // then the compartments of cx, globalObject, and the worker's reflector
    // should all match.
    MOZ_ASSERT_IF(globalObject,
                  js::GetObjectCompartment(mWorkerPrivate->GetWrapper()) ==
                    js::GetContextCompartment(cx));
    MOZ_ASSERT_IF(globalObject,
                  js::GetObjectCompartment(mWorkerPrivate->GetWrapper()) ==
                    js::GetObjectCompartment(globalObject->GetGlobalJSObject()));

    // If we're on the parent thread and have a reflector, then our
    // JSContext had better be either in the null compartment (and hence
    // have no globalObject) or in the compartment of our reflector.
    MOZ_ASSERT(!js::GetContextCompartment(cx) ||
               js::GetObjectCompartment(mWorkerPrivate->GetWrapper()) ==
                 js::GetContextCompartment(cx),
               "Must either be in the null compartment or in our reflector "
               "compartment");

    ac.emplace(cx, mWorkerPrivate->GetWrapper());
  }

  MOZ_ASSERT(!jsapi->HasException());
  result = WorkerRun(cx, mWorkerPrivate);
  MOZ_ASSERT_IF(result, !jsapi->HasException());
  jsapi->ReportException();

  // We can't even assert that this didn't create our global, since in the case
  // of CompileScriptRunnable it _does_.

  // It would be nice to avoid passing a JSContext to PostRun, but in the case
  // of ScriptExecutorRunnable we need to know the current compartment on the
  // JSContext (the one we set up based on the global returned from PreRun) so
  // that we can sanely do exception reporting.  In particular, we want to make
  // sure that we do our JS_SetPendingException while still in that compartment,
  // because otherwise we might end up trying to create a cross-compartment
  // wrapper when we try to move the JS exception from our runnable's
  // ErrorResult to the JSContext, and that's not desirable in this case.
  //
  // We _could_ skip passing a JSContext here and then in
  // ScriptExecutorRunnable::PostRun end up grabbing it from the WorkerPrivate
  // and looking at its current compartment.  But that seems like slightly weird
  // action-at-a-distance...
  //
  // In any case, we do NOT try to change the compartment on the JSContext at
  // this point; in the one case in which we could do that
  // (CompileScriptRunnable) it actually doesn't matter which compartment we're
  // in for PostRun.
  PostRun(cx, mWorkerPrivate, result);
  MOZ_ASSERT(!jsapi->HasException());

  return result ? NS_OK : NS_ERROR_FAILURE;
}

nsresult
WorkerRunnable::Cancel()
{
  uint32_t canceledCount = ++mCanceled;

  MOZ_ASSERT(canceledCount, "Cancel() overflow!");

  // The docs say that Cancel() should not be called more than once and that we
  // should throw NS_ERROR_UNEXPECTED if it is.
  return (canceledCount == 1) ? NS_OK : NS_ERROR_UNEXPECTED;
}

void
WorkerDebuggerRunnable::PostDispatch(WorkerPrivate* aWorkerPrivate,
                                     bool aDispatchResult)
{
}

WorkerSyncRunnable::WorkerSyncRunnable(WorkerPrivate* aWorkerPrivate,
                                       nsIEventTarget* aSyncLoopTarget)
: WorkerRunnable(aWorkerPrivate, WorkerThreadUnchangedBusyCount),
  mSyncLoopTarget(aSyncLoopTarget)
{
#ifdef DEBUG
  if (mSyncLoopTarget) {
    mWorkerPrivate->AssertValidSyncLoop(mSyncLoopTarget);
  }
#endif
}

WorkerSyncRunnable::WorkerSyncRunnable(
                               WorkerPrivate* aWorkerPrivate,
                               already_AddRefed<nsIEventTarget>&& aSyncLoopTarget)
: WorkerRunnable(aWorkerPrivate, WorkerThreadUnchangedBusyCount),
  mSyncLoopTarget(aSyncLoopTarget)
{
#ifdef DEBUG
  if (mSyncLoopTarget) {
    mWorkerPrivate->AssertValidSyncLoop(mSyncLoopTarget);
  }
#endif
}

WorkerSyncRunnable::~WorkerSyncRunnable()
{
}

bool
WorkerSyncRunnable::DispatchInternal()
{
  if (mSyncLoopTarget) {
    RefPtr<WorkerSyncRunnable> runnable(this);
    return NS_SUCCEEDED(mSyncLoopTarget->Dispatch(runnable.forget(), NS_DISPATCH_NORMAL));
  }

  return WorkerRunnable::DispatchInternal();
}

void
MainThreadWorkerSyncRunnable::PostDispatch(WorkerPrivate* aWorkerPrivate,
                                           bool aDispatchResult)
{
}

MainThreadStopSyncLoopRunnable::MainThreadStopSyncLoopRunnable(
                               WorkerPrivate* aWorkerPrivate,
                               already_AddRefed<nsIEventTarget>&& aSyncLoopTarget,
                               bool aResult)
: WorkerSyncRunnable(aWorkerPrivate, Move(aSyncLoopTarget)), mResult(aResult)
{
  AssertIsOnMainThread();
#ifdef DEBUG
  mWorkerPrivate->AssertValidSyncLoop(mSyncLoopTarget);
#endif
}

nsresult
MainThreadStopSyncLoopRunnable::Cancel()
{
  nsresult rv = Run();
  NS_WARN_IF_FALSE(NS_SUCCEEDED(rv), "Run() failed");

  nsresult rv2 = WorkerSyncRunnable::Cancel();
  NS_WARN_IF_FALSE(NS_SUCCEEDED(rv2), "Cancel() failed");

  return NS_FAILED(rv) ? rv : rv2;
}

bool
MainThreadStopSyncLoopRunnable::WorkerRun(JSContext* aCx,
                                          WorkerPrivate* aWorkerPrivate)
{
  aWorkerPrivate->AssertIsOnWorkerThread();
  MOZ_ASSERT(mSyncLoopTarget);

  nsCOMPtr<nsIEventTarget> syncLoopTarget;
  mSyncLoopTarget.swap(syncLoopTarget);

  aWorkerPrivate->StopSyncLoop(syncLoopTarget, mResult);
  return true;
}

bool
MainThreadStopSyncLoopRunnable::DispatchInternal()
{
  MOZ_ASSERT(mSyncLoopTarget);

  RefPtr<MainThreadStopSyncLoopRunnable> runnable(this);
  return NS_SUCCEEDED(mSyncLoopTarget->Dispatch(runnable.forget(), NS_DISPATCH_NORMAL));
}

void
MainThreadStopSyncLoopRunnable::PostDispatch(WorkerPrivate* aWorkerPrivate,
                                             bool aDispatchResult)
{
}

#ifdef DEBUG
WorkerControlRunnable::WorkerControlRunnable(WorkerPrivate* aWorkerPrivate,
                                             TargetAndBusyBehavior aBehavior)
: WorkerRunnable(aWorkerPrivate, aBehavior)
{
  MOZ_ASSERT(aWorkerPrivate);
  MOZ_ASSERT(aBehavior == ParentThreadUnchangedBusyCount ||
             aBehavior == WorkerThreadUnchangedBusyCount,
             "WorkerControlRunnables should not modify the busy count");
}
#endif

nsresult
WorkerControlRunnable::Cancel()
{
  if (NS_FAILED(Run())) {
    NS_WARNING("WorkerControlRunnable::Run() failed.");
  }

  return WorkerRunnable::Cancel();
}

bool
WorkerControlRunnable::DispatchInternal()
{
  RefPtr<WorkerControlRunnable> runnable(this);

  if (mBehavior == WorkerThreadUnchangedBusyCount) {
    return NS_SUCCEEDED(mWorkerPrivate->DispatchControlRunnable(runnable.forget()));
  }

  if (WorkerPrivate* parent = mWorkerPrivate->GetParent()) {
    return NS_SUCCEEDED(parent->DispatchControlRunnable(runnable.forget()));
  }

  nsCOMPtr<nsIThread> mainThread = do_GetMainThread();
  MOZ_ASSERT(mainThread);

  return NS_SUCCEEDED(mainThread->Dispatch(runnable.forget(), NS_DISPATCH_NORMAL));
}

void
MainThreadWorkerControlRunnable::PostDispatch(WorkerPrivate* aWorkerPrivate,
                                              bool aDispatchResult)
{
  AssertIsOnMainThread();
}

NS_IMPL_ISUPPORTS_INHERITED0(WorkerControlRunnable, WorkerRunnable)

WorkerMainThreadRunnable::WorkerMainThreadRunnable(WorkerPrivate* aWorkerPrivate,
                                                   const nsACString& aTelemetryKey)
: mWorkerPrivate(aWorkerPrivate)
, mTelemetryKey(aTelemetryKey)
{
  mWorkerPrivate->AssertIsOnWorkerThread();
}

void
WorkerMainThreadRunnable::Dispatch(ErrorResult& aRv)
{
  mWorkerPrivate->AssertIsOnWorkerThread();

  TimeStamp startTime = TimeStamp::NowLoRes();

  AutoSyncLoopHolder syncLoop(mWorkerPrivate);

  mSyncLoopTarget = syncLoop.EventTarget();
  RefPtr<WorkerMainThreadRunnable> runnable(this);

  DebugOnly<nsresult> rv =
    NS_DispatchToMainThread(runnable.forget(), NS_DISPATCH_NORMAL);
  MOZ_ASSERT(NS_SUCCEEDED(rv),
             "Should only fail after xpcom-shutdown-threads and we're gone by then");

  if (!syncLoop.Run()) {
    aRv.ThrowUncatchableException();
  }

  // Telemetry is apparently not threadsafe
  // Telemetry::Accumulate(Telemetry::SYNC_WORKER_OPERATION, mTelemetryKey,
  //                       static_cast<uint32_t>((TimeStamp::NowLoRes() - startTime)
  //                                               .ToMilliseconds()));
}

NS_IMETHODIMP
WorkerMainThreadRunnable::Run()
{
  AssertIsOnMainThread();

  bool runResult = MainThreadRun();

  RefPtr<MainThreadStopSyncLoopRunnable> response =
    new MainThreadStopSyncLoopRunnable(mWorkerPrivate,
                                       mSyncLoopTarget.forget(),
                                       runResult);

  MOZ_ALWAYS_TRUE(response->Dispatch());

  return NS_OK;
}

WorkerCheckAPIExposureOnMainThreadRunnable::WorkerCheckAPIExposureOnMainThreadRunnable(WorkerPrivate* aWorkerPrivate):
  WorkerMainThreadRunnable(aWorkerPrivate,
                           NS_LITERAL_CSTRING("WorkerCheckAPIExposureOnMainThread"))
{}

WorkerCheckAPIExposureOnMainThreadRunnable::~WorkerCheckAPIExposureOnMainThreadRunnable()
{}

bool
WorkerCheckAPIExposureOnMainThreadRunnable::Dispatch()
{
  ErrorResult rv;
  WorkerMainThreadRunnable::Dispatch(rv);
  bool ok = !rv.Failed();
  rv.SuppressException();
  return ok;
}

bool
WorkerSameThreadRunnable::PreDispatch(WorkerPrivate* aWorkerPrivate)
{
  // We don't call WorkerRunnable::PreDispatch, because we're using
  // WorkerThreadModifyBusyCount for mBehavior, and WorkerRunnable will assert
  // that PreDispatch is on the parent thread in that case.
  aWorkerPrivate->AssertIsOnWorkerThread();
  return true;
}

void
WorkerSameThreadRunnable::PostDispatch(WorkerPrivate* aWorkerPrivate,
                                       bool aDispatchResult)
{
  // We don't call WorkerRunnable::PostDispatch, because we're using
  // WorkerThreadModifyBusyCount for mBehavior, and WorkerRunnable will assert
  // that PostDispatch is on the parent thread in that case.
  aWorkerPrivate->AssertIsOnWorkerThread();
  if (aDispatchResult) {
    DebugOnly<bool> willIncrement = aWorkerPrivate->ModifyBusyCountFromWorker(true);
    // Should never fail since if this thread is still running, so should the
    // parent and it should be able to process a control runnable.
    MOZ_ASSERT(willIncrement);
  }
}