Keep track of previous started event ID

Closes #2107

Signed-off-by: Laurie O <laurie_opperman@hotmail.com>

moto/swf/models/decision_task.py

@@ -49,10 +49,11 @@ class DecisionTask(BaseModel):
             hsh["startedEventId"] = self.started_event_id
         return hsh
 
-    def start(self, started_event_id):
+    def start(self, started_event_id, previous_started_event_id=None):
         self.state = "STARTED"
         self.started_timestamp = unix_time()
         self.started_event_id = started_event_id
+        self.previous_started_event_id = previous_started_event_id
 
     def complete(self):
         self._check_workflow_execution_open()

moto/swf/models/workflow_execution.py

@@ -82,6 +82,7 @@ class WorkflowExecution(BaseModel):
         self._events = []
         # child workflows
         self.child_workflow_executions = []
+        self._previous_started_event_id = None
 
     def __repr__(self):
         return "WorkflowExecution(run_id: {0})".format(self.run_id)
@@ -295,7 +296,8 @@ class WorkflowExecution(BaseModel):
             scheduled_event_id=dt.scheduled_event_id,
             identity=identity,
         )
-        dt.start(evt.event_id)
+        dt.start(evt.event_id, self._previous_started_event_id)
+        self._previous_started_event_id = evt.event_id
 
     def complete_decision_task(
         self, task_token, decisions=None, execution_context=None

tests/test_swf/models/test_decision_task.py

@@ -30,9 +30,10 @@ def test_decision_task_full_dict_representation():
     fd["workflowExecution"].should.equal(wfe.to_short_dict())
     fd["workflowType"].should.equal(wft.to_short_dict())
 
-    dt.start(1234)
+    dt.start(1234, 1230)
     fd = dt.to_full_dict()
     fd["startedEventId"].should.equal(1234)
+    fd["previousStartedEventId"].should.equal(1230)
 
 
 def test_decision_task_first_timeout():

tests/test_swf/responses/test_decision_tasks.py

@@ -30,6 +30,30 @@ def test_poll_for_decision_task_when_one():
     )
 
 
+@mock_swf_deprecated
+def test_poll_for_decision_task_previous_started_event_id():
+    conn = setup_workflow()
+
+    resp = conn.poll_for_decision_task("test-domain", "queue")
+    assert resp["workflowExecution"]["runId"] == conn.run_id
+    assert "previousStartedEventId" not in resp
+
+    # Require a failing decision, in this case a non-existant activity type
+    attrs = {
+        "activityId": "spam",
+        "activityType": {"name": "test-activity", "version": "v1.42"},
+        "taskList": "eggs",
+    }
+    decision = {
+        "decisionType": "ScheduleActivityTask",
+        "scheduleActivityTaskDecisionAttributes": attrs,
+    }
+    conn.respond_decision_task_completed(resp["taskToken"], decisions=[decision])
+    resp = conn.poll_for_decision_task("test-domain", "queue")
+    assert resp["workflowExecution"]["runId"] == conn.run_id
+    assert resp["previousStartedEventId"] == 3
+
+
 @mock_swf_deprecated
 def test_poll_for_decision_task_when_none():
     conn = setup_workflow()