What is the test for dismissing a petition for habeas corpus?

Rule 4 of the Habeas Rules requires the court to summarily dismiss a habeas petition "[i]f it plainly appears from the petition and any attached exhibits that the petitioner is not entitled to relief in the district court." "[A] petition for habeas corpus should not be dismissed without leave to amend unless it appears that no tenable claim for relief can be pleaded were such leave granted." Jarvis v. Nelson, 440 F.2d 13, 14 (9th Cir. 1971).