Major Description
Introduced by SPARK-19946.
DebugFilesystem.assertNoOpenStreams gets the size of the openStreams ConcurrentHashMap and then later, if the size was > 0, accesses the first element in openStreams.values. But, the ConcurrentHashMap might be cleared by another thread between getting its size and accessing it, resulting in an exception when trying to call .head on an empty collection.
