[SPARK-14679][UI] Fix UI DAG visualization OOM.

## What changes were proposed in this pull request?

The DAG visualization can cause an OOM when generating the DOT file.
This happens because clusters are not correctly deduped by a contains
check because they use the default equals implementation. This adds a
working equals implementation.

## How was this patch tested?

This adds a test suite that checks the new equals implementation.

Author: Ryan Blue <blue@apache.org>

Closes #12437 from rdblue/SPARK-14679-fix-ui-oom.

core/src/main/scala/org/apache/spark/ui/scope/RDDOperationGraph.scala

@@ -17,6 +17,8 @@
 
 package org.apache.spark.ui.scope
 
+import java.util.Objects
+
 import scala.collection.mutable
 import scala.collection.mutable.{ListBuffer, StringBuilder}
 
@@ -72,6 +74,22 @@ private[ui] class RDDOperationCluster(val id: String, private var _name: String)
   def getCachedNodes: Seq[RDDOperationNode] = {
     _childNodes.filter(_.cached) ++ _childClusters.flatMap(_.getCachedNodes)
   }
+
+  def canEqual(other: Any): Boolean = other.isInstanceOf[RDDOperationCluster]
+
+  override def equals(other: Any): Boolean = other match {
+    case that: RDDOperationCluster =>
+      (that canEqual this) &&
+          _childClusters == that._childClusters &&
+          id == that.id &&
+          _name == that._name
+    case _ => false
+  }
+
+  override def hashCode(): Int = {
+    val state = Seq(_childClusters, id, _name)
+    state.map(Objects.hashCode).foldLeft(0)((a, b) => 31 * a + b)
+  }
 }
 
 private[ui] object RDDOperationGraph extends Logging {

core/src/test/scala/org/apache/spark/ui/scope/RDDOperationGraphSuite.scala

@@ -0,0 +1,37 @@
+/*
+ * 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.
+ */
+
+package org.apache.spark.ui.scope
+
+import org.apache.spark.SparkFunSuite
+
+class RDDOperationGraphSuite extends SparkFunSuite {
+  test("Test simple cluster equals") {
+    // create a 2-cluster chain with a child
+    val c1 = new RDDOperationCluster("1", "Bender")
+    val c2 = new RDDOperationCluster("2", "Hal")
+    c1.attachChildCluster(c2)
+    c1.attachChildNode(new RDDOperationNode(3, "Marvin", false, "collect!"))
+
+    // create an equal cluster, but without the child node
+    val c1copy = new RDDOperationCluster("1", "Bender")
+    val c2copy = new RDDOperationCluster("2", "Hal")
+    c1copy.attachChildCluster(c2copy)
+
+    assert(c1 == c1copy)
+  }
+}