Lock session list in CdmEngine OnTimerEvent (master)

Cherry pick of
https://widevine-internal-review.googlesource.com/#/c/12935/

Change-Id: I029d36b2b6d092ae938fca2a7f6d893814c25a8a

libwvdrmengine/cdm/core/include/cdm_engine.h

@@ -138,9 +138,16 @@ class CdmEngine {
   virtual bool IsKeyLoaded(const KeyId& key_id);
   virtual bool FindSessionForKey(const KeyId& key_id, CdmSessionId* sessionId);
 
-  // Event listener related methods
+  // Event listener related methods.
+
+  // We assume that the WvCdmEventListener is asynchronous -- i.e. an event
+  // should be dispatched to another thread which actually does the work. In
+  // particular, if a syncrhonous listener calls OpenSession or CloseSession,
+  // the thread will dead lock.
+  // Returns false if listener already attached.
   virtual bool AttachEventListener(const CdmSessionId& session_id,
                                    WvCdmEventListener* listener);
+  // Returns true if listener was detached.
   virtual bool DetachEventListener(const CdmSessionId& session_id,
                                    WvCdmEventListener* listener);
 
@@ -148,7 +155,8 @@ class CdmEngine {
   virtual void NotifyResolution(const CdmSessionId& session_id, uint32_t width,
                                 uint32_t height);
 
-  // Timer expiration method
+  // Timer expiration method. This method is not re-entrant -- there can be
+  // only one timer.
   virtual void OnTimerEvent();
 
  private:
@@ -175,11 +183,13 @@ class CdmEngine {
   scoped_ptr<UsagePropertySet> usage_property_set_;
   int64_t last_usage_information_update_time_;
 
+  // Locks the list of sessions, |sessions_|, for the event timer.  It will be
+  // locked in OpenSession, CloseSession. It is also locked in OnTimerEvent and
+  // OnKeyReleaseEvent while the list of event listeners is being generated.
+  // The layer above the CDM implementation is expected to handle thread
+  // synchronization to make sure other functions that access sessions do not
+  // occur simultaneously with OpenSession or CloseSession.
   Lock session_list_lock_;
-  // If the timer is currently active -- do not delete any sessions.
-  bool timer_event_active_;
-  // Sessions to be deleted after the timer has finished.
-  std::vector<CdmSession*> dead_sessions_;
 
   CORE_DISALLOW_COPY_AND_ASSIGN(CdmEngine);
 };