A person offending against any of the provisions of this chapter shall be a competent witness against any other person violating any provisions of this chapter, and may be compelled to attend and testify upon any trial, hearing, proceeding, or investigation in the same manner as any other person, but the testimony so given shall not be used in any prosecution or proceeding, civil or criminal, against the person so testifying, except for perjury in giving such testimony, and a person so testifying shall not thereafter be liable to indictment, prosecution, or punishment for the offense with reference to which such testimony was given, and may plead or prove the giving of the testimony accordingly in bar of such an indictment or prosecution.