xuanyuanking commented on a change in pull request #28941:
URL: https://github.com/apache/spark/pull/28941#discussion_r447481083



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File path: core/src/main/scala/org/apache/spark/util/JsonProtocol.scala
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@@ -1078,7 +1078,10 @@ private[spark] object JsonProtocol {
         val blockManagerAddress = blockManagerIdFromJson(json \ "Block Manager 
Address")
         val shuffleId = (json \ "Shuffle ID").extract[Int]
         val mapId = (json \ "Map ID").extract[Long]
-        val mapIndex = (json \ "Map Index").extract[Int]
+        val mapIndex = (json \ "Map Index") match {
+          case JNothing => 0

Review comment:
       nit: 0 is a legal value for the map index here. Since it's for backward 
compatibility, an illegal value here should make more sense. Also, it's better 
to comment here for emphasizing it's for backward compatibility, otherwise the 
fetch failed event will be dropped when SHS loading old version logs.




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