What is the test for qualified immunity from civil liability in the context of civil litigation?

It is well established that qualified immunity shields government officials performing discretionary functions from civil damages liability "insofar as their conduct does not violate clearly established statutory or constitutional rights of which a reasonable person would have known." Harlow v. Fitzgerald, 457 U.S. 800, 818, 102 S.Ct. 2727, 2738, 73 L.Ed.2d 396 (1982). Thus, the existence of immunity "turns on the 'objective legal reasonableness' of the action ... assessed in light of the legal rules that were 'clearly established' at the time it was taken." Anderson v. Creighton, 483 U.S. 635, 639, 107 S.Ct. 3034, 3038, 97 L.Ed.2d 523 (1987) (quoting Harlow, 457 U.S. at 819, 818, 102 S.Ct. at 2738, 2738).