Lazily initialize MessageLoop for faster thread startup

base/threading/thread.h

 // 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.
 
 #ifndef BASE_THREADING_THREAD_H_
 #define BASE_THREADING_THREAD_H_
 
 #include <string>
 
 #include "base/base_export.h"
 #include "base/callback.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/message_loop/message_loop.h"
 #include "base/message_loop/message_loop_proxy.h"
 #include "base/message_loop/timer_slack.h"
+#include "base/synchronization/lock.h"
 #include "base/threading/platform_thread.h"
 
 namespace base {
 
 class MessagePump;
+class WaitableEvent;
 
 // A simple thread abstraction that establishes a MessageLoop on a new thread.
 // The consumer uses the MessageLoop of the thread to cause code to execute on
 // the thread.  When this object is destroyed the thread is terminated.  All
 // pending tasks queued on the thread's message loop will run to completion
 // before the thread is terminated.
 //
 // WARNING! SUBCLASSES MUST CALL Stop() IN THEIR DESTRUCTORS!  See ~Thread().
 //
 // After the thread is stopped, the destruction sequence is:
 //
 //  (1) Thread::CleanUp()
 //  (2) MessageLoop::~MessageLoop
 //  (3.b)    MessageLoop::DestructionObserver::WillDestroyCurrentMessageLoop
 class BASE_EXPORT Thread : PlatformThread::Delegate {
  public:
   struct BASE_EXPORT Options {
     typedef Callback<scoped_ptr<MessagePump>()> MessagePumpFactory;
 
     Options();
     Options(MessageLoop::Type type, size_t size);
     ~Options();
 
     // Specifies the type of message loop that will be allocated on the thread.
     // This is ignored if message_pump_factory.is_null() is false.
     MessageLoop::Type message_loop_type;
 
-    // Specify timer slack for thread message loop.
+    // Specifies timer slack for thread message loop.
     TimerSlack timer_slack;
 
     // Used to create the MessagePump for the MessageLoop. The callback is Run()
     // on the thread. If message_pump_factory.is_null(), then a MessagePump
     // appropriate for |message_loop_type| is created. Setting this forces the
     // MessageLoop::Type to TYPE_CUSTOM.
     MessagePumpFactory message_pump_factory;
 
     // Specifies the maximum stack size that the thread is allowed to use.
     // This does not necessarily correspond to the thread's initial stack size.
@@ skipping 15 matching lines @@
   // before it is destructed.
   ~Thread() override;
 
 #if defined(OS_WIN)
   // Causes the thread to initialize COM.  This must be called before calling
   // Start() or StartWithOptions().  If |use_mta| is false, the thread is also
   // started with a TYPE_UI message loop.  It is an error to call
   // init_com_with_mta(false) and then StartWithOptions() with any message loop
   // type other than TYPE_UI.
   void init_com_with_mta(bool use_mta) {
-    DCHECK(!started_);
+    DCHECK(!start_event_);
     com_status_ = use_mta ? MTA : STA;
   }
 #endif
 
   // Starts the thread.  Returns true if the thread was successfully started;
   // otherwise, returns false.  Upon successful return, the message_loop()
   // getter will return non-null.
   //
   // Note: This function can't be called on Windows with the loader lock held;
   // i.e. during a DllMain, global object construction or destruction, atexit()
   // callback.
   bool Start();
 
   // Starts the thread. Behaves exactly like Start in addition to allow to
   // override the default options.
   //
   // Note: This function can't be called on Windows with the loader lock held;
   // i.e. during a DllMain, global object construction or destruction, atexit()
   // callback.
   bool StartWithOptions(const Options& options);
 
+  // Starts the thread and wait for the thread to start and run initialization
+  // before returning. It's same as calling Start() and then
+  // WaitUntilThreadStarted().
+  // Note that using this (instead of Start() or StartWithOptions() causes
+  // jank on the calling thread, and must be used carefully for production code.
+  bool StartAndWait();
+
+  // Blocks until the thread starts running. Supposed to be used for testing.
+  bool WaitUntilThreadStarted();

[rvargas (doing something else), 2015/03/26 02:19:28]

nit: Add ForTest(/ing) to the name?

[kinuko, 2015/04/13 02:03:00]

It's also internally called in StartAndWait().. added a bit more comment.

+
   // Signals the thread to exit and returns once the thread has exited.  After
   // this method returns, the Thread object is completely reset and may be used
   // as if it were newly constructed (i.e., Start may be called again).
   //
   // Stop may be called multiple times and is simply ignored if the thread is
   // already stopped.
   //
   // NOTE: If you are a consumer of Thread, it is not necessary to call this
   // before deleting your Thread objects, as the destructor will do it.
   // IF YOU ARE A SUBCLASS OF Thread, YOU MUST CALL THIS IN YOUR DESTRUCTOR.
@@ skipping 43 matching lines @@
     return message_loop_proxy();
   }
 
   // Returns the name of this thread (for display in debugger too).
   const std::string& thread_name() const { return name_; }
 
   // The native thread handle.
   PlatformThreadHandle thread_handle() { return thread_; }
 
   // The thread ID.
-  PlatformThreadId thread_id() const { return thread_id_; }
+  PlatformThreadId thread_id() const { return thread_.id(); }
 
   // Returns true if the thread has been started, and not yet stopped.
   bool IsRunning() const;
 
   // Sets the thread priority. The thread must already be started.
   void SetPriority(ThreadPriority priority);
 
  protected:
   // Called just prior to starting the message loop
   virtual void Init() {}
@@ skipping 16 matching lines @@
   enum ComStatus {
     NONE,
     STA,
     MTA,
   };
 #endif
 
   // PlatformThread::Delegate methods:
   void ThreadMain() override;
 
+  // Returns true while inside of Run().
+  bool IsInRunLoop() const;

[rvargas (doing something else), 2015/03/26 02:19:28]

Why is this not a replacement for IsRunning()? And if it is, why change the
name? ...as in, IsInRunLoop tells me more about what to expect, but I don't
think the implementation fulfills my new expectation :)

+
 #if defined(OS_WIN)
   // Whether this thread needs to initialize COM, and if so, in what mode.
   ComStatus com_status_;
 #endif
 
-  // Whether we successfully started the thread.
-  bool started_;
-
   // If true, we're in the middle of stopping, and shouldn't access
   // |message_loop_|. It may non-NULL and invalid.
   bool stopping_;
 
   // True while inside of Run().
+  mutable base::Lock lock_;
   bool running_;
 
-  // Used to pass data to ThreadMain.
-  struct StartupData;
-  StartupData* startup_data_;
-
   // The thread's handle.
   PlatformThreadHandle thread_;
 
   // The thread's message loop.  Valid only while the thread is alive.  Set
   // by the created thread.
   MessageLoop* message_loop_;
 
-  // Our thread's ID.
-  PlatformThreadId thread_id_;
-
   // The name of the thread.  Used for debugging purposes.
   std::string name_;
 
+  // Non-null if the thread has successfully started.
+  scoped_ptr<WaitableEvent> start_event_;
+
   friend void ThreadQuitHelper();
 
   DISALLOW_COPY_AND_ASSIGN(Thread);
 };
 
 }  // namespace base
 
 #endif  // BASE_THREADING_THREAD_H_