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..976550fc938d0593dbef232ff254c91a6b153830 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()
@@ -103,6 +105,16 @@ 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 StartAndWait();
+
+  // Blocks until the thread starts running. Supposed to be used for testing.
+  bool WaitUntilThreadStarted();
+
   // 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).
@@ -203,6 +215,9 @@ class BASE_EXPORT Thread : PlatformThread::Delegate {
   // PlatformThread::Delegate methods:
   void ThreadMain() override;
 
+  // Returns true while inside of Run().
+  bool IsInRunLoop() const;
+
 #if defined(OS_WIN)
   // Whether this thread needs to initialize COM, and if so, in what mode.
   ComStatus com_status_;
@@ -216,12 +231,9 @@ class BASE_EXPORT Thread : PlatformThread::Delegate {
   bool stopping_;
 
   // True while inside of Run().
+  mutable base::Lock running_lock_;
   bool running_;
 
-  // Used to pass data to ThreadMain.
-  struct StartupData;
-  StartupData* startup_data_;
-
   // The thread's handle.
   PlatformThreadHandle thread_;
 
@@ -235,6 +247,8 @@ class BASE_EXPORT Thread : PlatformThread::Delegate {
   // The name of the thread.  Used for debugging purposes.
   std::string name_;
 
+  scoped_ptr<WaitableEvent> start_event_;
+
   friend void ThreadQuitHelper();
 
   DISALLOW_COPY_AND_ASSIGN(Thread);