code: public List<Long> getTimedOutSessions() {
      List<Long> ret = new ArrayList<>();
      synchronized (mSessions) {
        for (Entry<Long, SessionInfo> entry : mSessions.entrySet()) {
          if (entry.getValue().timeout()) {
            ret.add(entry.getKey());
          }
        }
      }
      return ret;
    }
docs: Gets the sessions that timed out.
  
  @return the list of session ids of sessions that timed out