'Release a cache lock for this tile.
Lock is implemented as a row in the "locks" table.'

def unlock(self, layer, coord, format):

    sys.stderr.write(('unlock    %d/%d/%d,    %s' % (coord.zoom, coord.column, coord.row, format)))
    db = connect(self.dbpath, isolation_level='EXCLUSIVE').cursor()
    db.execute('DELETE    FROM    locks\n                                                                                        WHERE    row=?    AND    column=?    AND    zoom=?    AND    format=?', (coord.row, coord.column, coord.zoom, format))
    db.connection.commit()
    db.connection.close()
