Worker threads get stuck when using caller_runs policy

[jonmast]

Worker threads get stuck when using caller_runs policy

[chrisseaton]

Yes looks like it's run while holding the lock.

concurrent-ruby/lib/concurrent-ruby/concurrent/executor/ruby_executor_service.rb

Lines 19 to 24 in 7b7900a

	synchronize do
	# If the executor is shut down, reject this task
	return handle_fallback(*args, &task) unless running?
	ns_execute(*args, &task)
	true
	end

[chrisseaton]

Task 4 running on 'main'
Task 1 running on 'worker-1'
Task 4 complete
Task 5 running on 'main'
Task 1 complete
Task 2 running on 'worker-1'
Task 5 complete
Task 7 running on 'main'
Task 2 complete
Task 3 running on 'worker-1'
Task 7 complete
Task 9 running on 'main'
Task 3 complete
Task 6 running on 'worker-1'
Task 9 complete
Task 6 complete
Task 8 running on 'worker-1'
Task 8 complete
Task 10 running on 'worker-1'
Task 10 complete

That's what we want, isn't it?

[chrisseaton]

Here's a deterministic test.

require 'concurrent-ruby'

log = Queue.new

executor = Concurrent::FixedThreadPool.new(1, max_queue: 2, fallback_policy: :caller_runs)

worker_unblocker = Concurrent::CountDownLatch.new(1)
executor_unblocker = Concurrent::CountDownLatch.new(1)

# Block the worker thread
executor << proc { worker_unblocker.wait }

# Fill the queue
executor << proc { log.push :queued }
executor << proc { log.push :queued }

t = Thread.new {
  # Block in a caller_runs job
  executor << proc { executor_unblocker.wait; log.push :unblocked }
}

# Wait until the caller_runs job is blocked
Thread.pass until t.status == 'sleep'

# Now unblock the worker thread
worker_unblocker.count_down
executor_unblocker.count_down

executor.shutdown
executor.wait_for_termination

# Ideally, we will see the queued jobs run before the caller_runs job unblocks
p [log.pop, log.pop, log.pop]

# [:unblocked, :queued, :queued] bad
# [:queued, :queued, :unblocked] good

Phew that was hard.

I have a PR that achieves that, if I get feedback that is is the desired behaviour.

[jonmast]

Sorry I'm so slow to follow up but this is perfect, thanks @chrisseaton