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 XPCOMThreadWrapper : public AbstractThread
{
public:
  explicit XPCOMThreadWrapper(nsIThread* 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.
    MOZ_ASSERT_IF(aRequireTailDispatch,
                  NS_IsMainThread() && NS_GetCurrentThread() == aTarget);
  }

  virtual void Dispatch(already_AddRefed<nsIRunnable> aRunnable,
                        DispatchFailureHandling aFailureHandling = AssertDispatchSuccess,
                        DispatchReason aReason = NormalDispatch) override
  {
    nsCOMPtr<nsIRunnable> r = aRunnable;
    AbstractThread* currentThread;
    if (aReason != TailDispatch && (currentThread = GetCurrent()) && RequiresTailDispatch(currentThread)) {
      currentThread->TailDispatcher().AddTask(this, r.forget(), aFailureHandling);
      return;
    }

    nsresult rv = mTarget->Dispatch(r, NS_DISPATCH_NORMAL);
    MOZ_DIAGNOSTIC_ASSERT(aFailureHandling == DontAssertDispatchSuccess || NS_SUCCEEDED(rv));
    Unused << rv;
  }

  virtual bool IsCurrentThreadIn() override
  {
    // Compare NSPR threads so that this works after shutdown when
    // NS_GetCurrentThread starts returning null.
    PRThread* thread = nullptr;
    mTarget->GetPRThread(&thread);
    bool in = PR_GetCurrentThread() == thread;
    MOZ_ASSERT(in == (GetCurrent() == this));
    return in;
  }

  void FireTailDispatcher()
  {
    MOZ_DIAGNOSTIC_ASSERT(mTailDispatcher.isSome());
    mTailDispatcher.ref().DrainDirectTasks();
    mTailDispatcher.reset();
  }

  virtual TaskDispatcher& TailDispatcher() override
  {
    MOZ_ASSERT(this == sMainThread); // See the comment in the constructor.
    MOZ_ASSERT(IsCurrentThreadIn());
    if (!mTailDispatcher.isSome()) {
      mTailDispatcher.emplace(/* aIsTailDispatcher = */ true);

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

    return mTailDispatcher.ref();
  }

  virtual nsIThread* AsXPCOMThread() override { return mTarget; }

private:
  RefPtr<nsIThread> mTarget;
  Maybe<AutoTaskDispatcher> mTailDispatcher;
};

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

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

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

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

/* static */
already_AddRefed<AbstractThread>
AbstractThread::CreateXPCOMThreadWrapper(nsIThread* aThread, bool aRequireTailDispatch)
{
  RefPtr<XPCOMThreadWrapper> wrapper = new XPCOMThreadWrapper(aThread, aRequireTailDispatch);
  // 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([wrapper]() { sCurrentThreadTLS.set(wrapper); });
  aThread->Dispatch(r.forget(), NS_DISPATCH_NORMAL);
  return wrapper.forget();
}

} // namespace mozilla