Github user JoshRosen commented on a diff in the pull request:
https://github.com/apache/spark/pull/3518#discussion_r21062475
--- Diff: core/src/main/scala/org/apache/spark/util/RDDWalker.scala ---
@@ -0,0 +1,66 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements. See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+// Created by Ilya Ganelin
+
+package org.apache.spark.util
+
+import java.util
+import org.apache.spark.rdd.RDD
+import scala.language.existentials
+
+/**
+ * This class allows execution of a function on an RDD and all of its
dependencies. This is
+ * accomplished by walking the object graph linking these RDDs. This is
useful for debugging
+ * internal RDD references. See SPARK-3694.
+ */
+object RDDWalker {
+
+ // Keep track of both the RDD and its depth in the traversal graph.
+ val walkQueue = new util.ArrayDeque[(RDD[_], Int)]
+ var visited = new util.HashSet[RDD[_]]
+
+
+ /**
+ *
+ * Execute the passed function on the underlying RDD
+ * @param rddToWalk - The RDD to traverse along with its dependencies
+ * @param func - The function to execute on each node. Returns a string
+ * @return Array[String] - An array of results generated by the
traversal function
+ * TODO Can there be cycles in RDD dependencies?
--- End diff --
No; the RDD DAG will not contain directed cycles.
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