sarutak opened a new pull request #34583:
URL: https://github.com/apache/spark/pull/34583
### What changes were proposed in this pull request?
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This PR is to mitigate a `ConcurrentModificationException` sometimes thrown
from a test.
Recently, I notice the exception is thrown from the following part of the
test `pipeline read/write events` in `MLEventSuite` when Scala 2.13 is used.
```
events.map(JsonProtocol.sparkEventToJson).foreach { event =>
assert(JsonProtocol.sparkEventFromJson(event).isInstanceOf[MLEvent])
}
```
We can also find this issue from the scheduled build.
https://github.com/apache/spark/runs/4196812399?check_suite_focus=true#step:9:17616
I think the root cause is the `ArrayBuffer` (`events`) is updated
asynchronously by the following part.
```
private val listener: SparkListener = new SparkListener {
override def onOtherEvent(event: SparkListenerEvent): Unit = event match {
case e: MLEvent => events.append(e)
case _ =>
}
}
```
You can easily reproduce this issue by applying the following diff to the
commit hash 4d29becee1ee8afa990f91684d84d717.
```
diff --git a/mllib/src/test/scala/org/apache/spark/ml/MLEventsSuite.scala
b/mllib/src/test/scala/org/apache/spark/ml/MLEventsSuite.scala
index f2343b7a88..ff63639e00 100644
--- a/mllib/src/test/scala/org/apache/spark/ml/MLEventsSuite.scala
+++ b/mllib/src/test/scala/org/apache/spark/ml/MLEventsSuite.scala
@@ -42,7 +42,9 @@ class MLEventsSuite
private val events = mutable.ArrayBuffer.empty[MLEvent]
private val listener: SparkListener = new SparkListener {
override def onOtherEvent(event: SparkListenerEvent): Unit = event
match {
- case e: MLEvent => events.append(e)
+ case e: MLEvent =>
+ events.append(e)
+ Thread.sleep(500)
case _ =>
}
}
@@ -235,11 +237,13 @@ class MLEventsSuite
}
// Test if they can be ser/de via JSON protocol.
assert(events.nonEmpty)
- events.map(JsonProtocol.sparkEventToJson).foreach { event =>
- assert(JsonProtocol.sparkEventFromJson(event).isInstanceOf[MLEvent])
- }
+ events.map { x =>
+ Thread.sleep(500)
+ JsonProtocol.sparkEventToJson(x)
+ }.foreach { event =>
+
assert(JsonProtocol.sparkEventFromJson(event).isInstanceOf[MLEvent])
+ }
sc.listenerBus.waitUntilEmpty(timeoutMillis = 10000)
-
events.clear()
val pipelineReader = Pipeline.read
assert(events.isEmpty)
```
This is a kind of race condition but I think we can mitigate by retrying.
Actually, I have never seen this issue when I used Scala 2.13.5 and recently
we upgrade to 2.13.7.
Scala 2.13.7 includes an update to detect `ConcurrentModificationException`
more precisely.
https://github.com/scala/scala/pull/9786
### Why are the changes needed?
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For test stability.
### Does this PR introduce _any_ user-facing change?
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No.
### How was this patch tested?
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I manually modified the test code, inserting sleep like the diff shown
above, and confirmed no ConcurrentModificationException is thrown.
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