Github user srowen commented on a diff in the pull request:
https://github.com/apache/spark/pull/11242#discussion_r59050309
--- Diff: core/src/main/scala/org/apache/spark/rdd/UnionRDD.scala ---
@@ -62,8 +62,14 @@ class UnionRDD[T: ClassTag](
var rdds: Seq[RDD[T]])
extends RDD[T](sc, Nil) { // Nil since we implement getDependencies
+ // visible for testing
+ private[spark] val isPartitionEvalParallel: Boolean =
+ rdds.length > conf.getInt("spark.rdd.parallelListingThreshold", 10)
--- End diff --
Heh, yeah can you see the long collapsed conversation above? that was
indeed the concern. Here, you would have to have source whose entire class was
not thread-safe (i.e. you can't concurrently access two unrelated instances),
which would be pretty broken. My contention is that such a source would break
in other places in Spark, so trying to fix it here doesn't do anything. It also
sounds like there's no known instance of this (?) so that's why I'm not sure if
this is a meaningful option.
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