Ensure activity and decision tasks cannot progress on a closed workflow

This is a second barrier because I'm a little nervous about this and I
don't want moto/swf to make any activity progress while in the real
world service, it's strictly impossible once the execution is closed.
Python doesn't seem to have any nice way of freezing an object so here
we go with a manual boundary...

moto/swf/models/decision_task.py

@@ -2,6 +2,7 @@ from __future__ import unicode_literals
 from datetime import datetime
 import uuid
 
+from ..exceptions import SWFWorkflowExecutionClosedError
 from ..utils import now_timestamp
 
 
@@ -21,6 +22,10 @@ class DecisionTask(object):
         self.scheduled_at = datetime.now()
         self.timeout_type = None
 
+    def _check_workflow_execution_open(self):
+        if not self.workflow_execution.open:
+            raise SWFWorkflowExecutionClosedError()
+
     def to_full_dict(self, reverse_order=False):
         events = self.workflow_execution.events(reverse_order=reverse_order)
         hsh = {
@@ -42,6 +47,7 @@ class DecisionTask(object):
         self.started_event_id = started_event_id
 
     def complete(self):
+        self._check_workflow_execution_open()
         self.state = "COMPLETED"
 
     def has_timedout(self):
@@ -54,5 +60,9 @@ class DecisionTask(object):
 
     def process_timeouts(self):
         if self.has_timedout():
-            self.state = "TIMED_OUT"
-            self.timeout_type = "START_TO_CLOSE"
+            self.timeout()
+
+    def timeout(self):
+        self._check_workflow_execution_open()
+        self.state = "TIMED_OUT"
+        self.timeout_type = "START_TO_CLOSE"