code: private ArrayList<Thread> getActiveThreads(int namespaceId, Block block) {
      lock.writeLock().lock();
      try {
        //check ongoing create threads
        final ActiveFile activefile = volumeMap.getOngoingCreates(namespaceId, block);
        if (activefile != null && !activefile.threads.isEmpty()) {
          //remove dead threads
          for(Iterator<Thread> i = activefile.threads.iterator(); i.hasNext(); ) {
            final Thread t = i.next();
            if (!t.isAlive()) {
              i.remove();
            }
          }
    
          //return living threads
          if (!activefile.threads.isEmpty()) {
            return new ArrayList<Thread>(activefile.threads);
          }
        }
      } finally {
        lock.writeLock().unlock();
      }
      return null;
    }
docs: Return a list of active writer threads for the given block.
  @return null if there are no such threads or the file is
  not being created