> Where is Action's execute method called in the event the queue is not
> empty?
If the agent's queue was non-empty from the perspective of enqueue(),
that means one of the following is true:
(1) A previous enqueue() has already scheduled the execution of an
action, but it has not yet begun running.
(2) doRun() is running but has not yet completed its CAS loop at the end.
In either case, once doRun() runs and reaches its CAS loop, it will
detect that the queue is non-empty and schedule the next action. This
is the:
if(error == null && next.q.count() > 0)
((Action) next.q.peek()).execute();
}
which appears towards the end of doRun(). Next is the atomically
obtained (through CAS looping) queue, after it popped itself.
Another way to look at it might be that the queue can be in two
relevant states at any point in time:
* empty
* non-empty
In the non-empty state, doRun() is continuously responsible for
re-scheduling itself. In the empty state, enqueue() is responsible for
scheduling it. Because of the use of CAS loops, the transition from
"empty" to "non-empty" can be atomically detected by enqueue(),
allowing it to determine whether or not it was responsible for such a
state transition, in which case it schedules the action for execution.
--
/ Peter Schuller
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