Actually, a Glock isn't completely cocked, the trigger finishes cocking it and releases it. It's still not a double action, but it really isn't a single action either.

The striker firing mechanism has a spring-loaded firing pin that is cocked in two stages, powered by the firing pin spring. When the pistol is charged, the firing pin is in the half-cock position. As the trigger is pulled, the striker is then fully cocked.
http://en.wikipedia.org/wiki/Glock_pistol