Fix openDecisionTasks counter not updated when we complete a DecisionTask

moto/swf/models/workflow_execution.py

@@ -279,10 +279,10 @@ class WorkflowExecution(object):
         Handles a Decision according to SWF docs.
         See: http://docs.aws.amazon.com/amazonswf/latest/apireference/API_Decision.html
         """
-        # 'decisions' can be None per boto.swf defaults, so better exiting
-        # directly for falsy values
+        # 'decisions' can be None per boto.swf defaults, so replace it with something iterable
         if not decisions:
-            return
+            decisions = []
+
         # handle each decision separately, in order
         for decision in decisions:
             decision_type = decision["decisionType"]
@@ -306,6 +306,9 @@ class WorkflowExecution(object):
                 # TODO: implement Decision type: StartTimer
                 raise NotImplementedError("Cannot handle decision: {}".format(decision_type))
 
+        # finally decrement counter if and only if everything went well
+        self.open_counts["openDecisionTasks"] -= 1
+
     def complete(self, event_id, result=None):
         self.execution_status = "CLOSED"
         self.close_status = "COMPLETED"

tests/test_swf/test_decision_tasks.py

@@ -76,6 +76,15 @@ def test_count_pending_decision_tasks_on_non_existent_task_list():
     resp = conn.count_pending_decision_tasks("test-domain", "non-existent")
     resp.should.equal({"count": 0, "truncated": False})
 
+@mock_swf
+def test_count_pending_decision_tasks_after_decision_completes():
+    conn = setup_workflow()
+    resp = conn.poll_for_decision_task("test-domain", "queue")
+    conn.respond_decision_task_completed(resp["taskToken"])
+
+    resp = conn.count_pending_decision_tasks("test-domain", "queue")
+    resp.should.equal({"count": 0, "truncated": False})
+
 
 # RespondDecisionTaskCompleted endpoint
 @mock_swf