Github user shivaram commented on a diff in the pull request:
https://github.com/apache/spark/pull/16154#discussion_r91625917
--- Diff: core/src/main/scala/org/apache/spark/api/r/JVMObjectTracker.scala
---
@@ -0,0 +1,87 @@
+/*
+ * 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.api.r
+
+import java.util.concurrent.atomic.AtomicInteger
+import java.util.concurrent.ConcurrentHashMap
+
+/** JVM object ID wrapper */
+private[r] case class JVMObjectId(id: String) {
+ require(id != null, "Object ID cannot be null.")
+}
+
+/**
+ * Counter that tracks JVM objects returned to R.
+ * This is useful for referencing these objects in RPC calls.
+ */
+private[r] class JVMObjectTracker {
+
+ private[this] val objMap = new ConcurrentHashMap[JVMObjectId, Object]()
+ private[this] val objCounter = new AtomicInteger()
+
+ /**
+ * Returns the JVM object associated with the input key or None if not
found.
+ */
+ final def get(id: JVMObjectId): Option[Object] = this.synchronized {
+ if (objMap.containsKey(id)) {
+ Some(objMap.get(id))
+ } else {
+ None
+ }
+ }
+
+ /**
+ * Returns the JVM object associated with the input key or throws an
exception if not found.
+ */
+ @throws[NoSuchElementException]("if key does not exist.")
+ final def apply(id: JVMObjectId): Object = {
+ get(id).getOrElse(
+ throw new NoSuchElementException(s"$id does not exist.")
+ )
+ }
+
+ /**
+ * Adds a JVM object to track and returns assigned ID, which is unique
within this tracker.
+ */
+ final def addAndGetId(obj: Object): JVMObjectId = {
+ val id = JVMObjectId(objCounter.getAndIncrement().toString)
--- End diff --
The counter is atomic - but the map insert is not ?
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