tez-runtime-library/src/main/java/org/apache/tez/dag/library/vertexmanager/ShuffleVertexManagerBase.java [663:677]:
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  private boolean preconditionsSatisfied() {
    if (!onVertexStartedDone.get()) {
      // vertex not started yet
      return false;
    }

    if (!sourceVerticesScheduled && !canScheduleTasks()) {
      if (LOG.isDebugEnabled()) {
        LOG.debug("Defer scheduling tasks for vertex: {} as one task needs " +
            "to be completed per source vertex", getContext().getVertexName());
      }
      return false;
    }
    return true;
  }
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tez-dag/src/main/java/org/apache/tez/dag/app/dag/impl/RootInputVertexManager.java [347:361]:
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  private boolean preconditionsSatisfied() {
    if (!onVertexStartedDone.get()) {
      // vertex not started yet
      return false;
    }

    if (!sourceVerticesScheduled && !canScheduleTasks()) {
      if (LOG.isDebugEnabled()) {
        LOG.debug("Defer scheduling tasks for vertex: {} as one task needs " +
            "to be completed per source vertex", getContext().getVertexName());
      }
      return false;
    }
    return true;
  }
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