code: def _readBlock(self):
          
  
          if self.interrupted or self.fp is None:
              if self.debug:
                  log.msg('WorkerFileDownloadCommand._readBlock(): end')
              return True
  
          length = self.blocksize
          if self.bytes_remaining is not None and length > self.bytes_remaining:
              length = self.bytes_remaining
  
          if length <= 0:
              if self.stderr is None:
                  self.stderr = "Maximum filesize reached, truncating file '{0}'".format(
                      self.path)
                  self.rc = 1
              return True
          else:
              d = self.reader.callRemote('read', length)
              d.addCallback(self._writeData)
              return d
docs: Read a block of data from the remote reader.