Fix counting of available execution threads

lib/good_job/scheduler.rb

@@ -16,18 +16,17 @@ class Scheduler
       max_threads: Concurrent.processor_count,
       auto_terminate: true,
       idletime: 60,
-      max_queue: 0,
-      fallback_policy: :abort, # shouldn't matter -- 0 max queue
+      max_queue: -1,
+      fallback_policy: :discard,
     }.freeze
 
     def initialize(performer, timer_options: {}, pool_options: {})
       raise ArgumentError, "Performer argument must implement #next" unless performer.respond_to?(:next)
 
       @performer = performer
-      @pool = Concurrent::ThreadPoolExecutor.new(DEFAULT_POOL_OPTIONS.merge(pool_options))
+      @pool = ThreadPoolExecutor.new(DEFAULT_POOL_OPTIONS.merge(pool_options))
       @timer = Concurrent::TimerTask.new(DEFAULT_TIMER_OPTIONS.merge(timer_options)) do
-        idle_threads = @pool.max_length - @pool.length
-        create_thread if idle_threads.positive?
+        create_thread
       end
       @timer.add_observer(self, :timer_observer)
       @timer.execute
@@ -58,6 +57,8 @@ def shutdown?
     end
 
     def create_thread
+      return false unless @pool.ready_worker_count.positive?
+
       future = Concurrent::Future.new(args: [@performer], executor: @pool) do |performer|
         output = nil
         Rails.application.executor.wrap { output = performer.next }
@@ -75,5 +76,17 @@ def task_observer(time, output, thread_error)
       ActiveSupport::Notifications.instrument("finished_job_task.good_job", { result: output, error: thread_error, time: time })
       create_thread if output
     end
+
+    class ThreadPoolExecutor < Concurrent::ThreadPoolExecutor
+      # https://github.com/ruby-concurrency/concurrent-ruby/issues/684#issuecomment-427594437
+      def ready_worker_count
+        synchronize do
+          workers_still_to_be_created = @max_length - @pool.length
+          workers_created_but_waiting = @ready.length
+
+          workers_still_to_be_created + workers_created_but_waiting
+        end
+      end
+    end
   end
 end