Lazily initialize MessageLoop for faster thread startup

base/message_loop/message_loop.h

Index: base/message_loop/message_loop.h
diff --git a/base/message_loop/message_loop.h b/base/message_loop/message_loop.h
index 7c76616f20f9123056c9d78dfcf1c964e752d017..2c822a4e7abccb6ee3493a157e165f3ff9ecfb9e 100644
--- a/base/message_loop/message_loop.h
+++ b/base/message_loop/message_loop.h
@@ -109,14 +109,42 @@ class BASE_EXPORT MessageLoop : public MessagePump::Delegate {
 #endif  // defined(OS_ANDROID)
   };
 
+  // Init options for CreateForLazyInit().
+  struct BASE_EXPORT LazyInitOptions {
+    using MessagePumpFactory = Callback<scoped_ptr<MessagePump>()>;
+    LazyInitOptions();
+    ~LazyInitOptions();
+
+    // Specify the type of message loop that will be initialized in LazyInit.
+    // This is ignored if message_pump_factory.is_null() is false.
+    MessageLoop::Type message_loop_type;
+
+    // Specify timer slack for the message loop.
+    TimerSlack timer_slack;
+
+    // Used to create the MessagePump for the MessageLoop in LazyInit.
+    // If message_pump_factory.is_null() is true, then a MessagePump appropriate
+    // for |message_loop_type| is created. Setting this forces the
+    // MessageLoop::Type to TYPE_CUSTOM.
+    MessagePumpFactory message_pump_factory;
+  };
+
   // Normally, it is not necessary to instantiate a MessageLoop.  Instead, it
   // is typical to make use of the current thread's MessageLoop instance.
   explicit MessageLoop(Type type = TYPE_DEFAULT);
   // Creates a TYPE_CUSTOM MessageLoop with the supplied MessagePump, which must
   // be non-NULL.
-  explicit MessageLoop(scoped_ptr<base::MessagePump> pump);
+  explicit MessageLoop(scoped_ptr<MessagePump> pump);
   ~MessageLoop() override;
 
+  // Creates a MessageLoop for lazy initialization. This can be called on a
+  // different thread from the final 'current' thread of this message loop
+  // to pre-construct a message loop that accepts incoming tasks until
+  // LazyInit() is called.
+  // Note that only "PostTask" family methods can be called and posted tasks

[DaleCurtis, 2015/03/18 22:04:28]

Seems error prone; what methods exactly shouldn't be called? We'll get flaky
failures if someone accidentally calls something else, no?

[kinuko, 2015/03/19 03:08:16]

To be sure, almost all other methods (except for ScheduleWork()) are not
expected to be called on a different thread from the 'current' one, and until
this is called no task can run on the 'current' thread, so this shouldn't be
usually a problem (I think).

I elaborated the comment a little more.

+  // won't run before LazyInit() is called.
+  static MessageLoop* CreateForLazyInit(scoped_ptr<LazyInitOptions> options);
+
   // Returns the MessageLoop object for the current thread, or null if none.
   static MessageLoop* current();
 
@@ -147,6 +175,13 @@ class BASE_EXPORT MessageLoop : public MessagePump::Delegate {
     virtual ~DestructionObserver();
   };
 
+  // Configure various members for the private constructor for
+  // CreateForLazyInit().
+  // This must be called on the final 'current' thread of this message loop.
+  // It is not valid to call this unless this message loop is constructed by
+  // CreateForLazyInit().
+  void LazyInit();
+
   // Add a DestructionObserver, which will start receiving notifications
   // immediately.
   void AddDestructionObserver(DestructionObserver* destruction_observer);
@@ -407,7 +442,10 @@ class BASE_EXPORT MessageLoop : public MessagePump::Delegate {
  private:
   friend class RunLoop;
 
-  // Configures various members for the two constructors.
+  // Private constructor for CreateForLazyInit().
+  explicit MessageLoop(scoped_ptr<LazyInitOptions> options);
+
+  // Configures various members for the two public constructors.
   void Init();
 
   // Invokes the actual run loop using the message pump.
@@ -485,6 +523,9 @@ class BASE_EXPORT MessageLoop : public MessagePump::Delegate {
   bool os_modal_loop_;
 #endif
 
+  // Non-null only when this message loop is constructed by CreateForLazyInit.
+  scoped_ptr<LazyInitOptions> lazy_init_options_;
+
   std::string thread_name_;
   // A profiling histogram showing the counts of various messages and events.
   HistogramBase* message_histogram_;