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...

tests/test_swf/models/test_activity_task.py

@@ -1,6 +1,7 @@
 from freezegun import freeze_time
 from sure import expect
 
+from moto.swf.exceptions import SWFWorkflowExecutionClosedError
 from moto.swf.models import (
     ActivityTask,
     ActivityType,
@@ -123,3 +124,21 @@ def test_activity_task_cannot_timeout_on_closed_workflow_execution():
         wfe.has_timedout().should.equal(True)
         wfe.process_timeouts()
         task.has_timedout().should.equal(False)
+
+def test_activity_task_cannot_change_state_on_closed_workflow_execution():
+    wfe = make_workflow_execution()
+    wfe.start()
+
+    task = ActivityTask(
+        activity_id="my-activity-123",
+        activity_type="foo",
+        input="optional",
+        scheduled_event_id=117,
+        timeouts=ACTIVITY_TASK_TIMEOUTS,
+        workflow_execution=wfe,
+    )
+    wfe.complete(123)
+
+    task.timeout.when.called_with().should.throw(SWFWorkflowExecutionClosedError)
+    task.complete.when.called_with().should.throw(SWFWorkflowExecutionClosedError)
+    task.fail.when.called_with().should.throw(SWFWorkflowExecutionClosedError)

tests/test_swf/models/test_decision_task.py

@@ -1,6 +1,7 @@
 from freezegun import freeze_time
 from sure import expect
 
+from moto.swf.exceptions import SWFWorkflowExecutionClosedError
 from moto.swf.models import DecisionTask
 
 from ..utils import make_workflow_execution
@@ -61,3 +62,13 @@ def test_decision_task_cannot_timeout_on_closed_workflow_execution():
         wfe.has_timedout().should.equal(True)
         wfe.process_timeouts()
         dt.has_timedout().should.equal(False)
+
+def test_decision_task_cannot_change_state_on_closed_workflow_execution():
+    wfe = make_workflow_execution()
+    wfe.start()
+    task = DecisionTask(wfe, 123)
+
+    wfe.complete(123)
+
+    task.timeout.when.called_with().should.throw(SWFWorkflowExecutionClosedError)
+    task.complete.when.called_with().should.throw(SWFWorkflowExecutionClosedError)