base: Make SequencedWorkerPool issue globally unique SequenceTokens.

base/threading/sequenced_worker_pool.h

Index: base/threading/sequenced_worker_pool.h
diff --git a/base/threading/sequenced_worker_pool.h b/base/threading/sequenced_worker_pool.h
index 7e04b071428c838dfcec0faa647f82baf2932045..c41085ba781e01834aa502f7c6a51dab2aa499d7 100644
--- a/base/threading/sequenced_worker_pool.h
+++ b/base/threading/sequenced_worker_pool.h
@@ -126,6 +126,11 @@ class BASE_EXPORT SequencedWorkerPool : public TaskRunner {
       return id_ == other.id_;
     }
 
+    // An zero |id_| is a sentinel value internally for unsequenced tasks.

[darin (slow to review), 2013/07/05 05:24:19]

nit: An -> A, and I think you also meant to put "used" after "value"

That said, this comment seems a bit odd on a public method as consumers probably
don't need to know about implementation details.  What do unsequenced tasks have
to do with a SequenceToken, which is all about sequential-izing tasks?   Who is
expected to call this method?

I think this is about supporting the case where the current thread is executing
an unsequenced task dispatched through a sequenced worker pool.  In other words,
there is no sequence token for such a thread.  The comment for IsValid should
probably just state something like this.

[tommycli, 2013/07/08 17:42:15]

Done.

+    bool IsValid() const {
+      return id_ != 0;
+    }
+
    private:
     friend class SequencedWorkerPool;
 
@@ -143,6 +148,11 @@ class BASE_EXPORT SequencedWorkerPool : public TaskRunner {
     virtual void OnDestruct() = 0;
   };
 
+  // Gets the SequencedToken of the current thread.
+  // If current thread is not a SequencedWorkerPool worker thread or is running
+  // an unsequenced task, returns an invalid SequenceToken.
+  static SequenceToken CurrentThreadSequenceToken();

[darin (slow to review), 2013/07/05 05:24:19]

nit: GetSequenceTokenForCurrentThread might be a slightly better name.  It
matches your first sentence.

[tommycli, 2013/07/08 17:42:15]

Done.

+
   // When constructing a SequencedWorkerPool, there must be a
   // MessageLoop on the current thread unless you plan to deliberately
   // leak it.
@@ -159,7 +169,7 @@ class BASE_EXPORT SequencedWorkerPool : public TaskRunner {
                       TestingObserver* observer);
 
   // Returns a unique token that can be used to sequence tasks posted to
-  // PostSequencedWorkerTask(). Valid tokens are alwys nonzero.
+  // PostSequencedWorkerTask(). Valid tokens are always nonzero.
   SequenceToken GetSequenceToken();
 
   // Returns the sequence token associated with the given name. Calling this