Make _Scheduler submit right away if delay is 0

This is required by upcoming LimitedConcurrencyShardConnectionBackoffPolicy.

Author: dkropachev

cassandra/cluster.py

@@ -4456,7 +4456,10 @@ def shutdown(self):
         self.join()
 
     def schedule(self, delay, fn, *args, **kwargs):
-        self._insert_task(delay, (fn, args, tuple(kwargs.items())))
+        if delay:
+            self._insert_task(delay, (fn, args, tuple(kwargs.items())))
+        else:
+            self._executor.submit(fn, *args, **kwargs)
 
     def schedule_unique(self, delay, fn, *args, **kwargs):
         task = (fn, args, tuple(kwargs.items()))

tests/unit/test_cluster.py

@@ -148,8 +148,8 @@ def test_event_delay_timing(self, *_):
         PYTHON-473
         """
         sched = _Scheduler(None)
-        sched.schedule(0, lambda: None)
-        sched.schedule(0, lambda: None)  # pre-473: "TypeError: unorderable types: function() < function()"t
+        sched.schedule(1, lambda: None)
+        sched.schedule(1, lambda: None)  # pre-473: "TypeError: unorderable types: function() < function()"t
 
 
 class SessionTest(unittest.TestCase):