base: Re-apply WeakPtr support for SequencedWorkerPools, fixing deadlock

base/sequence_checker_impl.h

Index: base/sequence_checker_impl.h
diff --git a/base/sequence_checker_impl.h b/base/sequence_checker_impl.h
index ccd119831f5abe9eb2c53a8606430610cf4c97cc..741aafee645946791a95515cad1d1833f10aaec8 100644
--- a/base/sequence_checker_impl.h
+++ b/base/sequence_checker_impl.h
@@ -7,48 +7,42 @@
 
 #include "base/base_export.h"
 #include "base/basictypes.h"
-#include "base/memory/ref_counted.h"
 #include "base/synchronization/lock.h"
+#include "base/threading/sequenced_worker_pool.h"
 #include "base/threading/thread_checker_impl.h"
 
 namespace base {
 
-class SequencedTaskRunner;
-
 // SequenceCheckerImpl is used to help verify that some methods of a
 // class are called in sequence -- that is, called from the same
 // SequencedTaskRunner. It is a generalization of ThreadChecker; in
-// particular, it behaves exactly like ThreadChecker if its passed a
-// NULL SequencedTaskRunner.
+// particular, it behaves exactly like ThreadChecker if constructed
+// on a thread that is not part of a SequencedWorkerPool.
 class BASE_EXPORT SequenceCheckerImpl {
  public:
-  // |sequenced_task_runner| can be NULL.  In that case, this object
-  // behaves exactly like a ThreadChecker bound to the current thread,
-  // i.e.  CalledOnValidSequence() behaves like CalledOnValidThread().
-  explicit SequenceCheckerImpl(
-      const scoped_refptr<SequencedTaskRunner>& sequenced_task_runner);
+  SequenceCheckerImpl();
   ~SequenceCheckerImpl();
 
-  // Returns whether the we are being called on the underyling
-  // SequencedTaskRunner.  If we're not bound to a
-  // |sequenced_task_runner|, returns whether we are being called on
-  // the underlying ThreadChecker's thread.
-  bool CalledOnValidSequence() const;
+  // Returns whether the we are being called on the same sequence token
+  // as previous calls. If there is no associated sequence, then returns
+  // whether we are being called on the underlying ThreadChecker's thread.
+  bool CalledOnValidSequencedThread() const;
 
-  // Changes the underyling SequencedTaskRunner.
-  // |sequenced_task_runner| can be NULL.  In that case, this object
-  // behaves exactly like a ThreadChecker that has been detached from
-  // its thread, i.e. we will be bound to the thread on which we next
-  // call CalledOnValidSequence().
-  void ChangeSequence(
-      const scoped_refptr<SequencedTaskRunner>& sequenced_task_runner);
+  // Unbinds the checker from the currently associated sequence. The
+  // checker will be re-bound on the next call to CalledOnValidSequence().
+  void DetachFromSequence();
 
  private:
+  void EnsureSequenceTokenAssigned() const;
+
   // Guards all variables below.
   mutable Lock lock_;
-  scoped_refptr<SequencedTaskRunner> sequenced_task_runner_;
-  // Used if |sequenced_task_runner_| is NULL.
+
+  // Used if |sequence_token_| is not valid.
   ThreadCheckerImpl thread_checker_;
+  mutable bool sequence_token_assigned_;
+
+  mutable SequencedWorkerPool::SequenceToken sequence_token_;
 
   DISALLOW_COPY_AND_ASSIGN(SequenceCheckerImpl);
 };