pzhdfy commented on issue #6690: bugfix: when building materialized-view, if taskCount>1, may cause concurrentModificationException URL: https://github.com/apache/incubator-druid/pull/6690#issuecomment-459931967 Yes, this is guarded by taskLock . This is just the WRONG USE of hashmap. if we remove an entry from a hashmap by remove(key), while iterating it, this will throw a concurrentModificationException, even in a single thread.  ref:https://stackoverflow.com/questions/602636/concurrentmodificationexception-and-a-hashmap ```java for (Map.Entry<Interval, HadoopIndexTask> entry : runningTasks.entrySet()) { Optional<TaskStatus> taskStatus = taskStorage.getStatus(entry.getValue().getId()); if (!taskStatus.isPresent() || !taskStatus.get().isRunnable()) { runningTasks.remove(entry.getKey()); runningVersion.remove(entry.getKey()); } } ```
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