What is the standard of review in a vindictive prosecution case?

The standard of review in a vindictive prosecution case is unclear in this circuit; some cases have applied an abuse of discretion standard, others have applied the clearly erroneous standard, and others have reviewed de novo. United States v. Osif, 789 F.2d 1404, 1405, 1405 n. 1 (9th Cir.1986). We affirm the district court's decision regardless of our standard of review.