Lazily initialize MessageLoop for faster thread startup

base/threading/thread.h

Index: base/threading/thread.h
diff --git a/base/threading/thread.h b/base/threading/thread.h
index 5010f0ec428bdcc69e54a9fdfe723862cacd75aa..4a74d017107b7b453e5acd827c7d51dae6e0a059 100644
--- a/base/threading/thread.h
+++ b/base/threading/thread.h
@@ -13,11 +13,13 @@
 #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
@@ -45,7 +47,7 @@ class BASE_EXPORT Thread : PlatformThread::Delegate {
     // 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()
@@ -81,7 +83,7 @@ class BASE_EXPORT Thread : PlatformThread::Delegate {
   // 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
@@ -103,6 +105,17 @@ class BASE_EXPORT Thread : PlatformThread::Delegate {
   // 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 StartAndWaitForTesting();
+
+  // Blocks until the thread starts running. Called within
+  // StartAndWaitForTesting(). Supposed to be used only for testing.
+  bool WaitUntilThreadStartedForTesting();
+
   // 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).
@@ -166,7 +179,7 @@ class BASE_EXPORT Thread : PlatformThread::Delegate {
   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;
@@ -208,19 +221,13 @@ class BASE_EXPORT Thread : PlatformThread::Delegate {
   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().
   bool running_;
-
-  // Used to pass data to ThreadMain.
-  struct StartupData;
-  StartupData* startup_data_;
+  mutable base::Lock lock_;  // Protects running_.
 
   // The thread's handle.
   PlatformThreadHandle thread_;
@@ -229,12 +236,14 @@ class BASE_EXPORT Thread : PlatformThread::Delegate {
   // by the created thread.
   MessageLoop* message_loop_;
 
-  // Our thread's ID.
-  PlatformThreadId thread_id_;
+  TimerSlack message_loop_timer_slack_;

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

Add a comment what this is for. ("Stores Options::timer_slack_ until the message
loop has been bound to a thread." or something like that)

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

Done.

 
   // 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);