init: fix shutdown race

[corvink]

Summary

When calling shutdown, shutdown sends a signal to init and exits. This
causes a race condition for the waitpid function. If the signal wins the
race, wpid will be set to -1 and init calls death_single. If shutdown
wins the race, wpid will be set to the pid of the shutdown process and
the requested_transition will be ignored.

[corvink]

@trasz @imp @des @rgrimes I add you as reviewer because I found you in the git history.

[corvink]

The single_user function checks for requested_transition in it's while condition:

do {
  ...
} while (wpid != pid && !requested_transition);

if (requested_transition)
  return (state_funct_t) requested_transition

Does it make sense to use it here too?

[imp]

how does this fix the rate?

[corvink]

The race is still there. However, it's not an issue any more. This patch checks requested_transition on each exit of waitpid and not only when the signal wins the race (wpid == -1).

[imp]

Ah, it makes the race not matter. I think I understand and I agree that the change is good.