Separate the create and start phases in DelayedTaskManager.

base/task_scheduler/delayed_task_manager.h

Index: base/task_scheduler/delayed_task_manager.h
diff --git a/base/task_scheduler/delayed_task_manager.h b/base/task_scheduler/delayed_task_manager.h
index 55cf8e521d0568eb30dff66fc9ca170f434eca8f..c9d487c7d4c80f6009f5c0589e96557816ea76d8 100644
--- a/base/task_scheduler/delayed_task_manager.h
+++ b/base/task_scheduler/delayed_task_manager.h
@@ -6,11 +6,18 @@
 #define BASE_TASK_SCHEDULER_DELAYED_TASK_MANAGER_H_
 
 #include <memory>
+#include <utility>
+#include <vector>
 
 #include "base/base_export.h"
-#include "base/callback_forward.h"
+#include "base/callback.h"
 #include "base/macros.h"
+#include "base/memory/ptr_util.h"
 #include "base/memory/ref_counted.h"
+#include "base/synchronization/atomic_flag.h"
+#include "base/task_scheduler/scheduler_lock.h"
+#include "base/time/default_tick_clock.h"
+#include "base/time/tick_clock.h"
 
 namespace base {
 
@@ -20,26 +27,51 @@ namespace internal {
 
 struct Task;
 
-// The DelayedTaskManager forwards tasks to various scheduler components when
-// they become ripe for execution. This class is thread-safe.
+// The DelayedTaskManager forwards tasks to post task callbacks when they become
+// ripe for execution. Tasks are not forwarded before Start() is called. This
+// class is thread-safe.
 class BASE_EXPORT DelayedTaskManager {
  public:
   // Posts |task| for execution immediately.
-  using PostTaskNowCallback = Callback<void(std::unique_ptr<Task> task)>;
+  using PostTaskNowCallback = OnceCallback<void(std::unique_ptr<Task> task)>;
 
+  // |tick_clock| can be specified for testing.
+  DelayedTaskManager(
+      std::unique_ptr<TickClock> tick_clock = MakeUnique<DefaultTickClock>());
+  ~DelayedTaskManager();
+
+  // Starts the delayed task manager, allowing past and future tasks to be
+  // forwarded to their callbacks as they become ripe for execution.
   // |service_thread_task_runner| posts tasks to the TaskScheduler service
   // thread.
-  explicit DelayedTaskManager(
-      scoped_refptr<TaskRunner> service_thread_task_runner);
-  ~DelayedTaskManager();
+  void Start(scoped_refptr<TaskRunner> service_thread_task_runner);
 
-  // Calls |post_task_now_callback| with |task| when |task| is ripe for
-  // execution.
+  // Schedules a call to |post_task_now_callback| with |task| as argument when
+  // |task| is ripe for execution and Start() has been called.
   void AddDelayedTask(std::unique_ptr<Task> task,
-                      const PostTaskNowCallback& post_task_now_callback);
+                      PostTaskNowCallback post_task_now_callback);
 
  private:
-  const scoped_refptr<TaskRunner> service_thread_task_runner_;
+  // Schedules a call to |post_task_now_callback| with |task| as argument when
+  // |delay| expires. Start() must have been called before this.
+  void AddDelayedTaskNow(std::unique_ptr<Task> task,
+                         TimeDelta delay,
+                         PostTaskNowCallback post_task_now_callback);
+
+  const std::unique_ptr<TickClock> tick_clock_;
+
+  AtomicFlag started_;
+
+  // Synchronizes access to all members below before |started_| is set. Once
+  // |started_| is set:
+  // - |service_thread_task_runner| doest not change, so it can be read without
+  //   holding the lock.
+  // - |tasks_added_before_start_| isn't accessed anymore.
+  SchedulerLock lock_;
+
+  scoped_refptr<TaskRunner> service_thread_task_runner_;
+  std::vector<std::pair<std::unique_ptr<Task>, PostTaskNowCallback>>
+      tasks_added_before_start_;
 
   DISALLOW_COPY_AND_ASSIGN(DelayedTaskManager);
 };