Lazily initialize MessageLoop for faster thread startup

base/threading/thread.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/threading/thread.h"
 
 #include "base/bind.h"
 #include "base/lazy_instance.h"
 #include "base/profiler/scoped_tracker.h"
 #include "base/synchronization/waitable_event.h"
@@ skipping 18 matching lines @@
     LAZY_INSTANCE_INITIALIZER;
 
 }  // namespace
 
 // This is used to trigger the message loop to exit.
 void ThreadQuitHelper() {
   MessageLoop::current()->QuitWhenIdle();
   Thread::SetThreadWasQuitProperly(true);
 }
 
-// Used to pass data to ThreadMain.  This structure is allocated on the stack
-// from within StartWithOptions.
-struct Thread::StartupData {
-  // We get away with a const reference here because of how we are allocated.
-  const Thread::Options& options;
-
-  // Used to synchronize thread startup.
-  WaitableEvent event;
-
-  explicit StartupData(const Options& opt)
-      : options(opt),
-        event(false, false) {}
-};
-
 Thread::Options::Options()
     : message_loop_type(MessageLoop::TYPE_DEFAULT),
       timer_slack(TIMER_SLACK_NONE),
       stack_size(0) {
 }
 
 Thread::Options::Options(MessageLoop::Type type,
                          size_t size)
     : message_loop_type(type),
       timer_slack(TIMER_SLACK_NONE),
       stack_size(size) {
 }
 
 Thread::Options::~Options() {
 }
 
 Thread::Thread(const std::string& name)
     :
 #if defined(OS_WIN)
       com_status_(NONE),
 #endif
-      started_(false),
       stopping_(false),
       running_(false),
-      startup_data_(NULL),
       thread_(0),
-      message_loop_(NULL),
-      thread_id_(kInvalidThreadId),
+      message_loop_(nullptr),
       name_(name) {
 }
 
 Thread::~Thread() {
   Stop();
 }
 
 bool Thread::Start() {
   Options options;
 #if defined(OS_WIN)
   if (com_status_ == STA)
     options.message_loop_type = MessageLoop::TYPE_UI;
 #endif
   return StartWithOptions(options);
 }
 
 bool Thread::StartWithOptions(const Options& options) {
   DCHECK(!message_loop_);
 #if defined(OS_WIN)
   DCHECK((com_status_ != STA) ||
       (options.message_loop_type == MessageLoop::TYPE_UI));
 #endif
 
   SetThreadWasQuitProperly(false);
 
-  StartupData startup_data(options);
-  startup_data_ = &startup_data;
+  MessageLoop::InitOptions message_loop_options;
+  message_loop_options.message_loop_type = options.message_loop_type;
+  message_loop_options.message_pump_factory = options.message_pump_factory;
+  message_loop_options.timer_slack = options.timer_slack;
+  message_loop_ = new MessageLoop(message_loop_options);
+
+  start_event_.reset(new WaitableEvent(false, false));

[Nico, 2015/04/24 21:31:16]

Move this below the next if, then you don't need the reset() call there

[kinuko, 2015/04/27 16:36:05]

I think we need to do this here since it'll be racy otherwise (new thread may
try to touch start_event_ before we create)

 
   if (!PlatformThread::Create(options.stack_size, this, &thread_)) {
     DLOG(ERROR) << "failed to create thread";
-    startup_data_ = NULL;
+    delete message_loop_;
+    message_loop_ = nullptr;
+    start_event_.reset();
     return false;
   }
 
-  // TODO(kinuko): Remove once crbug.com/465458 is solved.
-  tracked_objects::ScopedTracker tracking_profile_wait(
-      FROM_HERE_WITH_EXPLICIT_FUNCTION(
-          "465458 base::Thread::StartWithOptions (Wait)"));
-
-  // Wait for the thread to start and initialize message_loop_
-  base::ThreadRestrictions::ScopedAllowWait allow_wait;
-  startup_data.event.Wait();
-
-  // set it to NULL so we don't keep a pointer to some object on the stack.
-  startup_data_ = NULL;
-  started_ = true;
-
   DCHECK(message_loop_);
   return true;
 }
 
+bool Thread::StartAndWaitForTesting() {
+  bool result = Start();
+  if (!result)
+    return result;

[Nico, 2015/04/24 21:31:16]

nit: `return false;` reads 0.2s faster

[kinuko, 2015/04/27 16:36:05]

Done.

+  WaitUntilThreadStartedForTesting();
+  return result;
+}
+
+bool Thread::WaitUntilThreadStartedForTesting() {
+  if (!start_event_)
+    return false;
+  // TODO(kinuko): Remove once crbug.com/465458 is solved.
+  tracked_objects::ScopedTracker tracking_profile(
+      FROM_HERE_WITH_EXPLICIT_FUNCTION(
+        "465458 base::Thread::WaitUntilThreadStarted()"));
+  base::ThreadRestrictions::ScopedAllowWait allow_wait;
+  start_event_->Wait();
+  return true;
+}
+
 void Thread::Stop() {
-  if (!started_)
+  if (!start_event_)
     return;
 
   StopSoon();
 
   // Wait for the thread to exit.
   //
   // TODO(darin): Unfortunately, we need to keep message_loop_ around until
   // the thread exits.  Some consumers are abusing the API.  Make them stop.
   //
   PlatformThread::Join(thread_);
 
   // The thread should NULL message_loop_ on exit.
   DCHECK(!message_loop_);
 
   // The thread no longer needs to be joined.
-  started_ = false;
+  start_event_.reset();
 
   stopping_ = false;
 }
 
 void Thread::StopSoon() {
   // We should only be called on the same thread that started us.
 
-  // Reading thread_id_ without a lock can lead to a benign data race
-  // with ThreadMain, so we annotate it to stay silent under ThreadSanitizer.
-  DCHECK_NE(ANNOTATE_UNPROTECTED_READ(thread_id_), PlatformThread::CurrentId());
+  DCHECK_NE(thread_id(), PlatformThread::CurrentId());
 
   if (stopping_ || !message_loop_)
     return;
 
   stopping_ = true;
   message_loop_->PostTask(FROM_HERE, base::Bind(&ThreadQuitHelper));
 }
 
 bool Thread::IsRunning() const {
+  if (message_loop_ && !stopping_)
+    return true;
+  AutoLock lock(lock_);

[Nico, 2015/04/24 21:31:16]

Why is this lock needed now when it wasn't before? Why isn't it needed to read
stopping_

[kinuko, 2015/04/27 16:36:05]

Actually I think we should have had this lock even without this change and I'm
not fully sure why we didn't have.

stopping_ seems to be touched only on the same thread that started the thread,
hence no locking.

[Nico, 2015/04/27 22:49:34]

Maybe add a comment that says why accessing message_loop_ and stopping_ outside
of the lock is safe.

[kinuko, 2015/04/28 15:43:25]

Done.

   return running_;
 }
 
 void Thread::SetPriority(ThreadPriority priority) {
   // The thread must be started (and id known) for this to be
   // compatible with all platforms.
-  DCHECK_NE(thread_id_, kInvalidThreadId);
+  DCHECK(message_loop_ != nullptr);
   PlatformThread::SetThreadPriority(thread_, priority);
 }
 
 void Thread::Run(MessageLoop* message_loop) {
   message_loop->Run();
 }
 
 void Thread::SetThreadWasQuitProperly(bool flag) {
   lazy_tls_bool.Pointer()->Set(flag);
 }
 
 bool Thread::GetThreadWasQuitProperly() {
   bool quit_properly = true;
 #ifndef NDEBUG
   quit_properly = lazy_tls_bool.Pointer()->Get();
 #endif
   return quit_properly;
 }
 
 void Thread::ThreadMain() {
-  {
-    // The message loop for this thread.
-    // Allocated on the heap to centralize any leak reports at this line.
-    scoped_ptr<MessageLoop> message_loop;
-    if (!startup_data_->options.message_pump_factory.is_null()) {
-      message_loop.reset(
-          new MessageLoop(startup_data_->options.message_pump_factory.Run()));
-    } else {
-      message_loop.reset(
-          new MessageLoop(startup_data_->options.message_loop_type));
-    }
+  // Complete the initialization of our Thread object.
+  DCHECK_EQ(thread_id(), PlatformThread::CurrentId());
+  PlatformThread::SetName(name_.c_str());
+  ANNOTATE_THREAD_NAME(name_.c_str());  // Tell the name to race detector.
 
-    // Complete the initialization of our Thread object.
-    thread_id_ = PlatformThread::CurrentId();
-    PlatformThread::SetName(name_.c_str());
-    ANNOTATE_THREAD_NAME(name_.c_str());  // Tell the name to race detector.
-    message_loop->set_thread_name(name_);
-    message_loop->SetTimerSlack(startup_data_->options.timer_slack);
-    message_loop_ = message_loop.get();
+  // Lazily initialize the message_loop so that it can run on this thread.
+  DCHECK(message_loop_);
+  scoped_ptr<MessageLoop> message_loop(message_loop_);
+  message_loop_->set_thread_name(name_);
+  message_loop_->Init();
 
 #if defined(OS_WIN)
-    scoped_ptr<win::ScopedCOMInitializer> com_initializer;
-    if (com_status_ != NONE) {
-      com_initializer.reset((com_status_ == STA) ?
-          new win::ScopedCOMInitializer() :
-          new win::ScopedCOMInitializer(win::ScopedCOMInitializer::kMTA));
-    }
+  scoped_ptr<win::ScopedCOMInitializer> com_initializer;
+  if (com_status_ != NONE) {
+    com_initializer.reset((com_status_ == STA) ?
+        new win::ScopedCOMInitializer() :
+        new win::ScopedCOMInitializer(win::ScopedCOMInitializer::kMTA));
+  }
 #endif
 
-    // Let the thread do extra initialization.
-    // Let's do this before signaling we are started.
-    Init();
+  // Let the thread do extra initialization.
+  Init();
 
+  {
+    AutoLock lock(lock_);
     running_ = true;
-    startup_data_->event.Signal();
-    // startup_data_ can't be touched anymore since the starting thread is now
-    // unlocked.
+  }
 
-    Run(message_loop_);
+  start_event_->Signal();
+
+  Run(message_loop_);
+
+  {
+    AutoLock lock(lock_);
     running_ = false;
+  }
 
-    // Let the thread do extra cleanup.
-    CleanUp();
+  // Let the thread do extra cleanup.
+  CleanUp();
 
 #if defined(OS_WIN)
-    com_initializer.reset();
+  com_initializer.reset();
 #endif
 
-    // Assert that MessageLoop::Quit was called by ThreadQuitHelper.
-    DCHECK(GetThreadWasQuitProperly());
+  // Assert that MessageLoop::Quit was called by ThreadQuitHelper.
+  DCHECK(GetThreadWasQuitProperly());
 
-    // We can't receive messages anymore.
-    message_loop_ = NULL;
-  }
+  // We can't receive messages anymore.
+  // (The message loop is destructed at the end of this block)
+  message_loop_ = NULL;
 }
 
 }  // namespace base