def add_source(self, filename, env_filename):
    """Add a source to the PEX environment.

    :param filename: The source filename to add to the PEX; None to create an empty file at
      `env_filename`.
    :param env_filename: The destination filename in the PEX.  This path
      must be a relative path.
    """
    self._ensure_unfrozen('Adding source')
    self._copy_or_link(filename, env_filename, "source")