code: def UpdateTaskAsPendingMerge(self, task):
      
      with self._lock:
        is_abandoned = task.identifier in self._tasks_abandoned
        is_processing = task.identifier in self._tasks_processing
        is_queued = task.identifier in self._tasks_queued
  
        if not is_queued and not is_processing and not is_abandoned:
          raise KeyError('Status of task {0:s} is unknown.'.format(
              task.identifier))
  
        if is_abandoned and task.has_retry:
          raise KeyError('Will not merge a task {0:s} with retry task.'.format(
              task.identifier))
  
        if is_queued:
          logger.debug('Task {0:s} was queued, now merging.'.format(
              task.identifier))
          del self._tasks_queued[task.identifier]
  
        if is_processing:
          logger.debug('Task {0:s} was processing, now merging.'.format(
              task.identifier))
          del self._tasks_processing[task.identifier]
  
        if is_abandoned:
          logger.debug('Task {0:s} was abandoned, now merging.'.format(
              task.identifier))
          del self._tasks_abandoned[task.identifier]
  
        self._tasks_pending_merge.PushTask(task)
  
        self.SampleTaskStatus(task, 'pending_merge')
  
        task.UpdateProcessingTime()
        self._UpdateLatestProcessingTime(task)
docs: Updates the task manager to reflect the task is ready to be merged.
  
      Args:
        task (Task): task.
  
      Raises:
        KeyError: if the task was not queued, processing or abandoned, or
            the task was abandoned and has a retry task.