Guard against non-dict Payload in is_step_function_event (#846)

Some Step Functions integrations pass a string Payload, which previously
caused incorrect behavior when checking for step function event fields.

Author: purple4reina

datadog_lambda/trigger.py

@@ -461,6 +461,8 @@ def is_step_function_event(event):
     The actual event must contain "Execution", "StateMachine", and "State" fields.
     """
     event = event.get("Payload", event)
+    if not isinstance(event, dict):
+        return False
 
     # JSONPath style
     if "Execution" in event and "StateMachine" in event and "State" in event:

tests/test_trigger.py

@@ -757,3 +757,7 @@ def test_is_step_function_event_dd_header(self):
             }
         }
         self.assertFalse(is_step_function_event(event))
+
+    def test_is_step_function_event_payload_not_dict(self):
+        event = {"Payload": "This is a string, not a dict"}
+        self.assertFalse(is_step_function_event(event))