Major Description
At the end of checkpoint BackupNode tries to convergeJournalSpool() and calls revertFileStreams(). The latter closes each file stream, and tries to rename the corresponding file to its permanent location current/edits. If for any reason the rename fails processIOError() is called for failed streams. processIOError() will try to close the stream again and will get NPE in EditLogFileOutputStream.close() because bufCurrent was set to null by the previous close.