Add comment explaining use of createTempShuffleBlock().

core/src/main/scala/org/apache/spark/util/collection/ExternalSorter.scala

@@ -264,6 +264,9 @@ private[spark] class ExternalSorter[K, V, C](
   private def spillToMergeableFile(collection: SizeTrackingPairCollection[(Int, K), C]): Unit = {
     assert(!bypassMergeSort)
 
+    // Because these files may be read during shuffle, they must be compressed using
+    // spark.shuffle.compress instead of spark.shuffle.spill.compress, so we need to use
+    // createTempShuffleBlock here; see SPARK-3426 for more context.
     val (blockId, file) = diskBlockManager.createTempShuffleBlock()
     curWriteMetrics = new ShuffleWriteMetrics()
     var writer = blockManager.getDiskWriter(blockId, file, ser, fileBufferSize, curWriteMetrics)
@@ -343,6 +346,9 @@ private[spark] class ExternalSorter[K, V, C](
     if (partitionWriters == null) {
       curWriteMetrics = new ShuffleWriteMetrics()
       partitionWriters = Array.fill(numPartitions) {
+        // Because these files may be read during shuffle, they must be compressed using
+        // spark.shuffle.compress instead of spark.shuffle.spill.compress, so we need to use
+        // createTempShuffleBlock here; see SPARK-3426 for more context.
         val (blockId, file) = diskBlockManager.createTempShuffleBlock()
         blockManager.getDiskWriter(blockId, file, ser, fileBufferSize, curWriteMetrics).open()
       }