Currently in FCBIS we call killCursor on the backup cursor in "fire and forget" mode when copying the files from the sync source is done. There is a theoretical race condition where WiredTiger may have internally closed the backup cursor (e.g. due to a slow refresh) and the last file may be corrupt. The documented way to detect this is to check the result on the killCursor when this happens. For FCBIS and any other users of DocumentSourceBackupFile, it is possible and more convenient (and less error-prone) to return an error from DocumentSourceBackupFile if the backup cursor is not still open after reading the data.