Bug: Event log cannot be generated when some internal accumulators use unexpected types
Status: Resolved | Priority: Major | Resolution: Fixed
Created: 06/Jun/20 23:12
Affects Version/s: 2.3.0

A user may use internal accumulators by adding the "internal.metrics." prefix to the accumulator name to hide sensitive information from UI (Accumulators except internal ones will be shown in Spark UI).

However, *org.apache.spark.util.JsonProtocol.accumValueToJson* assumes an internal accumulator has only 3 possible types: int, long, and java.util.List[(BlockId, BlockStatus)]. When an internal accumulator uses an unexpected type, it will crash.

An event log that contains such accumulator will be dropped because it cannot be converted to JSON, and it will cause weird UI issue when rendering in Spark History Server. For example, if `SparkListenerTaskEnd` is dropped because of this issue, the user will see the task is still running even if it was finished.

It's better to make *accumValueToJson* more robust.

----
How to reproduce it:

- Enable Spark event log
- Run the following command:

{code}
scala> val accu = sc.doubleAccumulator("internal.metrics.foo")
accu: org.apache.spark.util.DoubleAccumulator = DoubleAccumulator(id: 0, name: Some(internal.metrics.foo), value: 0.0)

scala> sc.parallelize(1 to 1, 1).foreach { _ => accu.add(1.0) }
20/06/06 16:11:27 ERROR AsyncEventQueue: Listener EventLoggingListener threw an exception
java.lang.ClassCastException: java.lang.Double cannot be cast to java.util.List
	at org.apache.spark.util.JsonProtocol$.accumValueToJson(JsonProtocol.scala:330)
	at org.apache.spark.util.JsonProtocol$$anonfun$accumulableInfoToJson$3.apply(JsonProtocol.scala:306)
	at org.apache.spark.util.JsonProtocol$$anonfun$accumulableInfoToJson$3.apply(JsonProtocol.scala:306)
	at scala.Option.map(Option.scala:146)
	at org.apache.spark.util.JsonProtocol$.accumulableInfoToJson(JsonProtocol.scala:306)
	at org.apache.spark.util.JsonProtocol$$anonfun$accumulablesToJson$2.apply(JsonProtocol.scala:299)
	at org.apache.spark.util.JsonProtocol$$anonfun$accumulablesToJson$2.apply(JsonProtocol.scala:299)
	at scala.collection.immutable.List.map(List.scala:284)
	at org.apache.spark.util.JsonProtocol$.accumulablesToJson(JsonProtocol.scala:299)
	at org.apache.spark.util.JsonProtocol$.taskInfoToJson(JsonProtocol.scala:291)
	at org.apache.spark.util.JsonProtocol$.taskEndToJson(JsonProtocol.scala:145)
	at org.apache.spark.util.JsonProtocol$.sparkEventToJson(JsonProtocol.scala:76)
	at org.apache.spark.scheduler.EventLoggingListener.logEvent(EventLoggingListener.scala:138)
	at org.apache.spark.scheduler.EventLoggingListener.onTaskEnd(EventLoggingListener.scala:158)
	at org.apache.spark.scheduler.SparkListenerBus$class.doPostEvent(SparkListenerBus.scala:45)
	at org.apache.spark.scheduler.AsyncEventQueue.doPostEvent(AsyncEventQueue.scala:37)
	at org.apache.spark.scheduler.AsyncEventQueue.doPostEvent(AsyncEventQueue.scala:37)
	at org.apache.spark.util.ListenerBus$class.postToAll(ListenerBus.scala:91)
	at org.apache.spark.scheduler.AsyncEventQueue.org$apache$spark$scheduler$AsyncEventQueue$$super$postToAll(AsyncEventQueue.scala:92)
	at org.apache.spark.scheduler.AsyncEventQueue$$anonfun$org$apache$spark$scheduler$AsyncEventQueue$$dispatch$1.apply$mcJ$sp(AsyncEventQueue.scala:92)
	at org.apache.spark.scheduler.AsyncEventQueue$$anonfun$org$apache$spark$scheduler$AsyncEventQueue$$dispatch$1.apply(AsyncEventQueue.scala:87)
	at org.apache.spark.scheduler.AsyncEventQueue$$anonfun$org$apache$spark$scheduler$AsyncEventQueue$$dispatch$1.apply(AsyncEventQueue.scala:87)
	at scala.util.DynamicVariable.withValue(DynamicVariable.scala:58)
	at org.apache.spark.scheduler.AsyncEventQueue.org$apache$spark$scheduler$AsyncEventQueue$$dispatch(AsyncEventQueue.scala:87)
	at org.apache.spark.scheduler.AsyncEventQueue$$anon$1$$anonfun$run$1.apply$mcV$sp(AsyncEventQueue.scala:83)
	at org.apache.spark.util.Utils$.tryOrStopSparkContext(Utils.scala:1302)
	at org.apache.spark.scheduler.AsyncEventQueue$$anon$1.run(AsyncEventQueue.scala:82)
{code}