In what circumstances will a court grant advance excusal of a police search warrant?

In Parsley v. Superior Court (1973) 9 Cal.3d 934, 109 Cal.Rptr. 563, 513 P.2d 611, officers conducted a search of the defendant's residence, acting under a search warrant which gave them prior authorization for non-compliance with the knock/notice requirements of sections 1531 and 844. At the preliminary hearing, the defendant attempted to learn the identity of a confidential informant who had supplied information to the police which, in turn, had formed the basis for the magistrate's decision to permit advance excusal of compliance with the knock/notice requirements.