In People v. McShann (1958) 50 Cal.2d 802, the California Supreme Court discussed when disclosure is required. "Disclosure is not limited to the informer who participates in the crime alleged. the information elicited from an informer may be 'relevant and helpful to the defense of the accused or essential to a fair determination of a cause' even though the informer was not a participant. For example, the testimony of an eyewitness-nonparticipant informer that would vindicate the innocence of the accused or lessen the risk of false testimony would obviously be relevant and helpful to the defense of the accused and essential to a fair determination of the cause. "Disclosure is frequently a problem in such cases as the present one involving violations of the narcotics laws, when the so-called informer is also a material witness on the issue of guilt. A mere informer has a limited role. 'When such a person is truly an informant he simply points the finger of suspicion toward a person who has violated the law. He puts the wheels in motion which cause the defendant to be suspected and perhaps arrested, but he plays no part in the criminal act with which the defendant is later charged.' His identity is ordinarily not necessary to the defendant's case, and the privilege against disclosure properly applies.... Thus, when it appears from the evidence that the informer is a material witness on the issue of guilt and the accused seeks disclosure on cross-examination, the People must either disclose his identity or incur a dismissal." (McShann, supra, 50 Cal.2d at p. 808.)