What is the test for a writ of habeas corpus ad testificandum to bring a state prisoner witness into federal court?

In determining whether to issue a writ of habeas corpus ad testificandum to bring a state prisoner witness into federal court, a district court must exercise its discretion based upon consideration of such factors as whether the prisoner's presence will substantially further the resolution of the case, the security risks presented by the prisoner's presence, the expense of the prisoner's transportation and safekeeping, and whether the suit can be stayed until the prisoner is released without prejudice to the cause asserted. See Wiggins v. Alameda County, 717 F.2d 466, 468 at n.1 (9th Cir. 1995), cert. denied, 465 U.S. 1070 (1984) (citing Ballard v. Spradley, 557 F.2d 476, 480 (5th Cir. 1977)).