gecko-dev/xpcom/threads/AbstractThread.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 "mozilla/AbstractThread.h"

#include "mozilla/ClearOnShutdown.h"
#include "mozilla/Maybe.h"
#include "mozilla/MozPromise.h" // We initialize the MozPromise logging in this file.
#include "mozilla/StaticPtr.h"
#include "mozilla/StateWatching.h" // We initialize the StateWatching logging in this file.
#include "mozilla/TaskQueue.h"
#include "mozilla/TaskDispatcher.h"
#include "mozilla/Unused.h"

#include "nsThreadUtils.h"
#include "nsContentUtils.h"
#include "nsServiceManagerUtils.h"


namespace mozilla {

LazyLogModule gMozPromiseLog("MozPromise");
LazyLogModule gStateWatchingLog("StateWatching");

StaticRefPtr<AbstractThread> sMainThread;
MOZ_THREAD_LOCAL(AbstractThread*) AbstractThread::sCurrentThreadTLS;

class EventTargetWrapper : public AbstractThread
{
public:
  explicit EventTargetWrapper(nsIEventTarget* aTarget, bool aRequireTailDispatch)
    : AbstractThread(aRequireTailDispatch)
    , mTarget(aTarget)
  {
    // Our current mechanism of implementing tail dispatch is appshell-specific.
    // This is because a very similar mechanism already exists on the main
    // thread, and we want to avoid making event dispatch on the main thread
    // more complicated than it already is.
    //
    // If you need to use tail dispatch on other XPCOM threads, you'll need to
    // implement an nsIThreadObserver to fire the tail dispatcher at the
    // appropriate times. You will also need to modify this assertion.
    MOZ_ASSERT_IF(aRequireTailDispatch, NS_IsMainThread() && aTarget->IsOnCurrentThread());
  }

  virtual nsresult Dispatch(already_AddRefed<nsIRunnable> aRunnable,
                            DispatchReason aReason = NormalDispatch) override
  {
    AbstractThread* currentThread;
    if (aReason != TailDispatch && (currentThread = GetCurrent()) && RequiresTailDispatch(currentThread)) {
      return currentThread->TailDispatcher().AddTask(this, std::move(aRunnable));
    }

    RefPtr<nsIRunnable> runner(new Runner(this, std::move(aRunnable), false /* already drained by TaskGroupRunnable  */));
    return mTarget->Dispatch(runner.forget(), NS_DISPATCH_NORMAL);
  }

  // Prevent a GCC warning about the other overload of Dispatch being hidden.
  using AbstractThread::Dispatch;

  virtual bool IsCurrentThreadIn() override
  {
    return mTarget->IsOnCurrentThread();
  }

  void FireTailDispatcher()
  {
    AutoEnter context(this);

    MOZ_DIAGNOSTIC_ASSERT(mTailDispatcher.isSome());
    mTailDispatcher.ref().DrainDirectTasks();
    mTailDispatcher.reset();
  }

  virtual TaskDispatcher& TailDispatcher() override
  {
    MOZ_ASSERT(IsCurrentThreadIn());
    if (!mTailDispatcher.isSome()) {
      mTailDispatcher.emplace(/* aIsTailDispatcher = */ true);

      nsCOMPtr<nsIRunnable> event =
        NewRunnableMethod("EventTargetWrapper::FireTailDispatcher",
                          this,
                          &EventTargetWrapper::FireTailDispatcher);
      nsContentUtils::RunInStableState(event.forget());
    }

    return mTailDispatcher.ref();
  }

  virtual bool MightHaveTailTasks() override
  {
    return mTailDispatcher.isSome();
  }

  virtual nsIEventTarget* AsEventTarget() override { return mTarget; }

private:
  nsCOMPtr<nsIThread> mRunningThread;
  RefPtr<nsIEventTarget> mTarget;
  Maybe<AutoTaskDispatcher> mTailDispatcher;

  virtual already_AddRefed<nsIRunnable>
  CreateDirectTaskDrainer(already_AddRefed<nsIRunnable> aRunnable) override
  {
    RefPtr<Runner> runner =
      new Runner(this, std::move(aRunnable), /* aDrainDirectTasks */ true);
    return runner.forget();
  }

  class Runner : public CancelableRunnable {
    class MOZ_STACK_CLASS AutoTaskGuard final {
    public:
      explicit AutoTaskGuard(EventTargetWrapper* aThread)
        : mLastCurrentThread(nullptr)
      {
        MOZ_ASSERT(aThread);
        mLastCurrentThread = sCurrentThreadTLS.get();
        sCurrentThreadTLS.set(aThread);
      }

      ~AutoTaskGuard()
      {
        sCurrentThreadTLS.set(mLastCurrentThread);
      }
    private:
      AbstractThread* mLastCurrentThread;
    };

  public:
    explicit Runner(EventTargetWrapper* aThread,
                    already_AddRefed<nsIRunnable> aRunnable,
                    bool aDrainDirectTasks)
      : CancelableRunnable("EventTargetWrapper::Runner")
      , mThread(aThread)
      , mRunnable(aRunnable)
      , mDrainDirectTasks(aDrainDirectTasks)
    {
    }

    NS_IMETHOD Run() override
    {
      AutoTaskGuard taskGuard(mThread);

      MOZ_ASSERT(mThread == AbstractThread::GetCurrent());
      MOZ_ASSERT(mThread->IsCurrentThreadIn());
      nsresult rv = mRunnable->Run();

      if (mDrainDirectTasks) {
        mThread->TailDispatcher().DrainDirectTasks();
      }

      return rv;
    }

    nsresult Cancel() override
    {
      // Set the TLS during Cancel() just in case it calls Run().
      AutoTaskGuard taskGuard(mThread);

      nsresult rv = NS_OK;

      // Try to cancel the runnable if it implements the right interface.
      // Otherwise just skip the runnable.
      nsCOMPtr<nsICancelableRunnable> cr = do_QueryInterface(mRunnable);
      if (cr) {
        rv = cr->Cancel();
      }

      return rv;
    }

#ifdef MOZ_COLLECTING_RUNNABLE_TELEMETRY
    NS_IMETHOD GetName(nsACString& aName) override
    {
      aName.AssignLiteral("AbstractThread::Runner");
      if (nsCOMPtr<nsINamed> named = do_QueryInterface(mRunnable)) {
        nsAutoCString name;
        named->GetName(name);
        if (!name.IsEmpty()) {
          aName.AppendLiteral(" for ");
          aName.Append(name);
        }
      }
      return NS_OK;
    }
#endif

  private:
    RefPtr<EventTargetWrapper> mThread;
    RefPtr<nsIRunnable> mRunnable;
    bool mDrainDirectTasks;
  };
};

NS_IMPL_ISUPPORTS(AbstractThread, nsIEventTarget, nsISerialEventTarget)

NS_IMETHODIMP_(bool)
AbstractThread::IsOnCurrentThreadInfallible()
{
  return IsCurrentThreadIn();
}

NS_IMETHODIMP
AbstractThread::IsOnCurrentThread(bool* aResult)
{
  *aResult = IsCurrentThreadIn();
  return NS_OK;
}

NS_IMETHODIMP
AbstractThread::DispatchFromScript(nsIRunnable* aEvent, uint32_t aFlags)
{
  nsCOMPtr<nsIRunnable> event(aEvent);
  return Dispatch(event.forget(), aFlags);
}

NS_IMETHODIMP
AbstractThread::Dispatch(already_AddRefed<nsIRunnable> aEvent, uint32_t aFlags)
{
  return Dispatch(std::move(aEvent), NormalDispatch);
}

NS_IMETHODIMP
AbstractThread::DelayedDispatch(already_AddRefed<nsIRunnable> aEvent,
                                 uint32_t aDelayMs)
{
  return NS_ERROR_NOT_IMPLEMENTED;
}

nsresult
AbstractThread::TailDispatchTasksFor(AbstractThread* aThread)
{
  if (MightHaveTailTasks()) {
    return TailDispatcher().DispatchTasksFor(aThread);
  }

  return NS_OK;
}

bool
AbstractThread::HasTailTasksFor(AbstractThread* aThread)
{
  if (!MightHaveTailTasks()) {
    return false;
  }
  return TailDispatcher().HasTasksFor(aThread);
}

bool
AbstractThread::RequiresTailDispatch(AbstractThread* aThread) const
{
  MOZ_ASSERT(aThread);
  // We require tail dispatch if both the source and destination
  // threads support it.
  return SupportsTailDispatch() && aThread->SupportsTailDispatch();
}

bool
AbstractThread::RequiresTailDispatchFromCurrentThread() const
{
  AbstractThread* current = GetCurrent();
  return current && RequiresTailDispatch(current);
}

AbstractThread*
AbstractThread::MainThread()
{
  MOZ_ASSERT(sMainThread);
  return sMainThread;
}

void
AbstractThread::InitTLS()
{
  if (!sCurrentThreadTLS.init()) {
    MOZ_CRASH();
  }
}

void
AbstractThread::InitMainThread()
{
  MOZ_ASSERT(NS_IsMainThread());
  MOZ_ASSERT(!sMainThread);
  nsCOMPtr<nsIThread> mainThread;
  NS_GetMainThread(getter_AddRefs(mainThread));
  MOZ_DIAGNOSTIC_ASSERT(mainThread);
  sMainThread = new EventTargetWrapper(mainThread.get(), /* aRequireTailDispatch = */ true);
  ClearOnShutdown(&sMainThread);

  if (!sCurrentThreadTLS.init()) {
    MOZ_CRASH();
  }
}

void
AbstractThread::DispatchStateChange(already_AddRefed<nsIRunnable> aRunnable)
{
  GetCurrent()->TailDispatcher().AddStateChangeTask(this, std::move(aRunnable));
}

/* static */ void
AbstractThread::DispatchDirectTask(already_AddRefed<nsIRunnable> aRunnable)
{
  GetCurrent()->TailDispatcher().AddDirectTask(std::move(aRunnable));
}

/* static */
already_AddRefed<AbstractThread>
AbstractThread::CreateXPCOMThreadWrapper(nsIThread* aThread, bool aRequireTailDispatch)
{
  RefPtr<EventTargetWrapper> wrapper = new EventTargetWrapper(aThread, aRequireTailDispatch);

  bool onCurrentThread = false;
  Unused << aThread->IsOnCurrentThread(&onCurrentThread);

  if (onCurrentThread) {
    sCurrentThreadTLS.set(wrapper);
    return wrapper.forget();
  }

  // Set the thread-local sCurrentThreadTLS to point to the wrapper on the
  // target thread. This ensures that sCurrentThreadTLS is as expected by
  // AbstractThread::GetCurrent() on the target thread.
  nsCOMPtr<nsIRunnable> r =
    NS_NewRunnableFunction("AbstractThread::CreateXPCOMThreadWrapper",
                           [wrapper]() { sCurrentThreadTLS.set(wrapper); });
  aThread->Dispatch(r.forget(), NS_DISPATCH_NORMAL);
  return wrapper.forget();
}

/* static  */
already_AddRefed<AbstractThread>
AbstractThread::CreateEventTargetWrapper(nsIEventTarget* aEventTarget,
                                         bool aRequireTailDispatch)
{
  MOZ_ASSERT(aEventTarget);
  nsCOMPtr<nsIThread> thread(do_QueryInterface(aEventTarget));
  Unused << thread; // simpler than DebugOnly<nsCOMPtr<nsIThread>>
  MOZ_ASSERT(!thread, "nsIThread should be wrapped by CreateXPCOMThreadWrapper!");

  RefPtr<EventTargetWrapper> wrapper =
    new EventTargetWrapper(aEventTarget, aRequireTailDispatch);

  return wrapper.forget();
}

} // namespace mozilla