Revert of Use SequenceChecker to allow Timer to run in SequencedWorkerPool

base/timer/timer.cc

Index: base/timer/timer.cc
diff --git a/base/timer/timer.cc b/base/timer/timer.cc
index effac5a80145deabb8814ca8246bc299ff840e1f..fa6b8cd2724a034ecf7d5fea48bad4e41884ee97 100644
--- a/base/timer/timer.cc
+++ b/base/timer/timer.cc
@@ -8,13 +8,15 @@
 
 #include "base/logging.h"
 #include "base/memory/ref_counted.h"
-#include "base/sequenced_task_runner.h"
-#include "base/threading/sequenced_task_runner_handle.h"
+#include "base/single_thread_task_runner.h"
+#include "base/thread_task_runner_handle.h"
+#include "base/threading/platform_thread.h"
 
 namespace base {
 
 // BaseTimerTaskInternal is a simple delegate for scheduling a callback to
-// Timer in the task runner. It also handles the following edge cases:
+// Timer in the thread's default task runner. It also handles the following
+// edge cases:
 // - deleted by the task runner.
 // - abandoned (orphaned) by Timer.
 class BaseTimerTaskInternal {
@@ -59,15 +61,10 @@
 
 Timer::Timer(bool retain_user_task, bool is_repeating)
     : scheduled_task_(NULL),
-      was_scheduled_(false),
+      thread_id_(0),
       is_repeating_(is_repeating),
       retain_user_task_(retain_user_task),
       is_running_(false) {
-  // It is safe for the timer to be created on a different thread/sequence
-  // than the one from which the timer APIs are called. The first call to the
-  // checker's CalledOnValidSequencedThread() method will re-bind the checker,
-  // and later calls will verify that the same task runner is used.
-  origin_sequence_checker_.DetachFromSequence();
 }
 
 Timer::Timer(const tracked_objects::Location& posted_from,
@@ -78,16 +75,13 @@
       posted_from_(posted_from),
       delay_(delay),
       user_task_(user_task),
-      was_scheduled_(false),
+      thread_id_(0),
       is_repeating_(is_repeating),
       retain_user_task_(true),
       is_running_(false) {
-  // See comment in other constructor.
-  origin_sequence_checker_.DetachFromSequence();
 }
 
 Timer::~Timer() {
-  DCHECK(origin_sequence_checker_.CalledOnValidSequencedThread());
   StopAndAbandon();
 }
 
@@ -99,10 +93,10 @@
   return delay_;
 }
 
-void Timer::SetTaskRunner(scoped_refptr<SequencedTaskRunner> task_runner) {
+void Timer::SetTaskRunner(scoped_refptr<SingleThreadTaskRunner> task_runner) {
   // Do not allow changing the task runner once something has been scheduled.
-  DCHECK(!was_scheduled_);
-  destination_task_runner_ = std::move(task_runner);
+  DCHECK_EQ(thread_id_, 0);
+  task_runner_.swap(task_runner);
 }
 
 void Timer::Start(const tracked_objects::Location& posted_from,
@@ -155,8 +149,6 @@
 
 void Timer::PostNewScheduledTask(TimeDelta delay) {
   DCHECK(scheduled_task_ == NULL);
-  DCHECK(origin_sequence_checker_.CalledOnValidSequencedThread());
-  was_scheduled_ = true;
   is_running_ = true;
   scheduled_task_ = new BaseTimerTaskInternal(this);
   if (delay > TimeDelta::FromMicroseconds(0)) {
@@ -169,15 +161,21 @@
         base::Bind(&BaseTimerTaskInternal::Run, base::Owned(scheduled_task_)));
     scheduled_run_time_ = desired_run_time_ = TimeTicks();
   }
-}
-
-scoped_refptr<SequencedTaskRunner> Timer::GetTaskRunner() {
-  return destination_task_runner_.get() ? destination_task_runner_
-                                        : SequencedTaskRunnerHandle::Get();
+  // Remember the thread ID that posts the first task -- this will be verified
+  // later when the task is abandoned to detect misuse from multiple threads.
+  if (!thread_id_) {
+    DCHECK(GetTaskRunner()->BelongsToCurrentThread());
+    thread_id_ = static_cast<int>(PlatformThread::CurrentId());
+  }
+}
+
+scoped_refptr<SingleThreadTaskRunner> Timer::GetTaskRunner() {
+  return task_runner_.get() ? task_runner_ : ThreadTaskRunnerHandle::Get();
 }
 
 void Timer::AbandonScheduledTask() {
-  DCHECK(origin_sequence_checker_.CalledOnValidSequencedThread());
+  DCHECK(thread_id_ == 0 ||
+         thread_id_ == static_cast<int>(PlatformThread::CurrentId()));
   if (scheduled_task_) {
     scheduled_task_->Abandon();
     scheduled_task_ = NULL;