Github user aarondav commented on a diff in the pull request:
https://github.com/apache/spark/pull/4016#discussion_r22920200
--- Diff: core/src/main/scala/org/apache/spark/util/EventLoop.scala ---
@@ -0,0 +1,110 @@
+/*
+ * 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.util
+
+import java.util.concurrent.{BlockingQueue, LinkedBlockingDeque}
+
+import scala.util.control.NonFatal
+
+import org.apache.spark.Logging
+
+/**
+ * An event loop to receive events from the caller and process all events
in the event thread. It
+ * will start an exclusive event thread to process all events.
+ */
+abstract class EventLoop[E](name: String) extends Logging {
+
+ private val eventQueue: BlockingQueue[E] = new LinkedBlockingDeque[E]()
+
+ private val eventThread = new Thread(name) {
+ setDaemon(true)
+
+ override def run(): Unit = {
+ try {
+ while (true) {
+ val event = eventQueue.take()
+ try {
+ onReceive(event)
+ } catch {
+ case NonFatal(e) => {
+ try {
+ onError(e)
+ } catch {
+ case NonFatal(e) => logError("Unexpected error in " +
name, e)
+ }
+ }
+ }
+ }
+ } catch {
+ case ie: InterruptedException => // exit even if eventQueue is not
empty
+ case NonFatal(e) => logError("Unexpected error in " + name, e)
+ }
+ }
+
+ }
+
+ def start(): Unit = {
+ // Call onStart before starting the event thread to make sure it
happens before onReceive
+ onStart()
+ eventThread.start()
+ }
+
+ def stop(): Unit = {
+ eventThread.interrupt()
+ eventThread.join()
+ // Call onStop after the event thread exits to make sure onReceive
happens before onStop
+ onStop()
+ }
+
+ /**
+ * Put the event into the event queue. The event thread will process it
later.
+ */
+ def post(event: E): Unit = {
+ eventQueue.put(event)
+ }
+
+ /**
+ * Return if the event thread has already been started but not yet
stopped.
+ */
+ def isActive: Boolean = eventThread.isAlive
+
+ /**
+ * Invoke when `start()` is called. It's also invoked before the event
thread starts.
+ */
+ def onStart(): Unit = {}
+
+ /**
+ * Invoke when `stop()` is called and the event thread exits.
+ */
+ def onStop(): Unit = {}
+
+ /**
+ * Invoke in the event thread when polling events from the event queue.
+ *
+ * Note: Should avoid calling blocking actions in `onReceive`, or the
event thread will be blocked
+ * and cannot process events in time. If you want to call some blocking
actions, run them in
+ * another thread.
+ */
+ def onReceive(event: E): Unit
+
+ /**
+ * Invoke if `onReceive` throws any non fatal error. `onError` must not
throw any non fatal error.
--- End diff --
The second condition is not really true, right? Such errors are simply
ignored, so I don't think we need to make a stipulation against them.
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