Backoff deadlock with low number of threads

[mratsim]

There seems to be a race condition here:

weave/weave/thieves.nim

Lines 187 to 204 in 42b0f80

	proc lastStealAttemptFailure*(req: sink StealRequest) =
	## If it's the last theft attempt per emitted steal requests
	## - if we are the lead thread, we know that every other threads are idle/waiting for work
	## but there is none --> termination
	## - if we are a worker thread, we message our parent and
	## passively wait for it to send us work or tell us to shutdown.
	if myID() == LeaderID:
	detectTermination()
	forget(req)
	else:
	req.state = Waiting
	debugTermination:
	log("Worker %2d: sends state passively WAITING to its parent worker %d\n", myID(), myWorker().parent)
	sendShare(req)
	ascertain: not myWorker().isWaiting
	myWorker().isWaiting = true
	myParking().wait() # Thread is blocked here until woken up.

The child sends the steal request and then goes idle

But what if the parent checks the steal request and sends a signal to wakeup to the child before it has time to actually sleep, for example to signal termination:

weave/weave/signals.nim

Lines 42 to 56 in 42b0f80

	proc signalTerminate*(_: pointer) =
	preCondition: not localCtx.signaledTerminate
	# 1. Terminating means everyone ran out of tasks
	# so their cache for task channels should be full
	# if there were sufficiently more tasks than workers
	# 2. Since they have an unique parent, no one else sent them a signal (checked in asyncSignal)
	if myWorker().left != Not_a_worker:
	# Send the terminate signal
	asyncSignal(signalTerminate, globalCtx.com.tasks[myWorker().left].access(0))
	# Wake the worker up so that it can process the terminate signal
	wakeup(myWorker().left)
	if myWorker().right != Not_a_worker:
	asyncSignal(signalTerminate, globalCtx.com.tasks[myWorker().right].access(0))
	wakeup(myWorker().right)

The parent exits, is deadlocked at the exit barrier and the child worker is deadlocked sleeping forever.

[mratsim]

closed by #28