code: PendingCheckpointStats reportPendingCheckpoint(
  			long checkpointId,
  			long triggerTimestamp,
  			CheckpointProperties props) {
  
  		ConcurrentHashMap<JobVertexID, TaskStateStats> taskStateStats = createEmptyTaskStateStatsMap();
  
  		PendingCheckpointStats pending = new PendingCheckpointStats(
  				checkpointId,
  				triggerTimestamp,
  				props,
  				totalSubtaskCount,
  				taskStateStats,
  				new PendingCheckpointStatsCallback());
  
  		statsReadWriteLock.lock();
  		try {
  			counts.incrementInProgressCheckpoints();
  			history.addInProgressCheckpoint(pending);
  
  			dirty = true;
  		} finally {
  			statsReadWriteLock.unlock();
  		}
  
  		return pending;
  	}
docs: Creates a new pending checkpoint tracker.
  
  @param checkpointId ID of the checkpoint.
  @param triggerTimestamp Trigger timestamp of the checkpoint.
  @param props The checkpoint properties.
  @return Tracker for statistics gathering.