What is the test for certifying a motion for habeas corpus?

"A district court's denial of a habeas corpus petition may not be affirmed unless the record on appeal indicates that the court independently reviewed all relevant portions of the state court record." Chaney v. Lewis, 801 F.2d 1191, 1193 (9th Cir. 1986), cert. denied, 481 U.S. 1023 (1987). "While the district court is under no duty to obtain sua sponte and review the state court record of either purely factual or purely legal questions, it does have such a duty as to mixed questions of fact and law." Id. at 1194.