{"action":"created","comment":{"url":"https://api.github.com/repos/apache/spark/pulls/comments/64139990","id":64139990,"diff_hunk":"@@ -437,7 +438,9 @@ class ListAccumulator[T] extends AccumulatorV2[T, java.util.List[T]] {\n       s\"Cannot merge ${this.getClass.getName} with ${other.getClass.getName}\")\n   }\n \n-  override def value: java.util.List[T] = java.util.Collections.unmodifiableList(_list)\n+  override def value: java.util.List[T] = _list.synchronized {\n+    java.util.Collections.unmodifiableList(new ArrayList[T](_list))","path":"core/src/main/scala/org/apache/spark/util/AccumulatorV2.scala","position":23,"original_position":23,"commit_id":"3af08bd7c417520854971d2d14fb4ae608a9522a","original_commit_id":"3af08bd7c417520854971d2d14fb4ae608a9522a","user":{"login":"srowen","id":822522,"avatar_url":"https://avatars.githubusercontent.com/u/822522?v=3","gravatar_id":"","url":"https://api.github.com/users/srowen","html_url":"https://github.com/srowen","followers_url":"https://api.github.com/users/srowen/followers","following_url":"https://api.github.com/users/srowen/following{/other_user}","gists_url":"https://api.github.com/users/srowen/gists{/gist_id}","starred_url":"https://api.github.com/users/srowen/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/srowen/subscriptions","organizations_url":"https://api.github.com/users/srowen/orgs","repos_url":"https://api.github.com/users/srowen/repos","events_url":"https://api.github.com/users/srowen/events{/privacy}","received_events_url":"https://api.github.com/users/srowen/received_events","type":"User","site_admin":false},"body":"One last thought ... now that this is cloned, does it need to be in an unmodifiable wrapper? maybe it's still a good idea so that the caller doesn't somehow think modifying the list modifies the accumulator","created_at":"2016-05-21T20:42:49Z","updated_at":"2016-05-21T20:42:49Z","html_url":"https://github.com/apache/spark/pull/13211#discussion_r64139990","pull_request_url":"https://api.github.com/repos/apache/spark/pulls/13211","_links":{"self":{"href":"https://api.github.com/repos/apache/spark/pulls/comments/64139990"},"html":{"href":"https://github.com/apache/spark/pull/13211#discussion_r64139990"},"pull_request":{"href":"https://api.github.com/repos/apache/spark/pulls/13211"}}},"pull_request":{"url":"https://api.github.com/repos/apache/spark/pulls/13211","id":70799754,"html_url":"https://github.com/apache/spark/pull/13211","diff_url":"https://github.com/apache/spark/pull/13211.diff","patch_url":"https://github.com/apache/spark/pull/13211.patch","issue_url":"https://api.github.com/repos/apache/spark/issues/13211","number":13211,"state":"open","locked":false,"title":"[SPARK-15430][SQL] Fix potential ConcurrentModificationException for ListAccumulator","user":{"login":"viirya","id":68855,"avatar_url":"https://avatars.githubusercontent.com/u/68855?v=3","gravatar_id":"","url":"https://api.github.com/users/viirya","html_url":"https://github.com/viirya","followers_url":"https://api.github.com/users/viirya/followers","following_url":"https://api.github.com/users/viirya/following{/other_user}","gists_url":"https://api.github.com/users/viirya/gists{/gist_id}","starred_url":"https://api.github.com/users/viirya/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/viirya/subscriptions","organizations_url":"https://api.github.com/users/viirya/orgs","repos_url":"https://api.github.com/users/viirya/repos","events_url":"https://api.github.com/users/viirya/events{/privacy}","received_events_url":"https://api.github.com/users/viirya/received_events","type":"User","site_admin":false},"body":"## What changes were proposed in this pull request?\r\n\r\nIn `ListAccumulator` we create an unmodifiable view for underlying list. However, it doesn't prevent the underlying to be modified further. So as we access the unmodifiable list, the underlying list can be modified in the same time. It could cause `java.util.ConcurrentModificationException`. We can observe such exception in recent tests.\r\n\r\nTo fix it, we can copy a list of the underlying list and then create the unmodifiable view of this list instead.\r\n\r\n## How was this patch tested?\r\nThe exception might be difficult to test. Existing tests should be 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