docs: def _copy_stream(src, dest, length=0):
      """Similar to shutil.copyfileobj, but supports limiting data size.
  
      As for why this is required, refer to
      https://www.python.org/dev/peps/pep-0333/#input-and-error-streams
  
      Yes, there are WSGI implementations which do not support EOFs, and
      believe me, you don't want to debug this.
  
      Args:
          src: source file-like object
          dest: destination file-like object
          length: optional file size hint
              If not 0, exactly length bytes will be written.
              If 0, write will continue until EOF is encountered.
      """
      if length == 0:
          shutil.copyfileobj(src, dest)
          return
  
      bytes_left = length
      while bytes_left > 0:
          buf_size = min(_BUFFER_SIZE, bytes_left)
          buf = src.read(buf_size)
          dest.write(buf)
          bytes_left -= buf_size