Use token to correlate "requests" and "responses". (#210)

**Issue:** MQTT is not a request/response protocol.  Messages for the Shadow service have a "client token" field to help users correlate "request" messages to "response" messages, but this sample script wasn't using the token field. This led to confusion when running Device Advisor tests which send several `shadow/update` "request" messages during setup. The sample was receiving the `shadow/update/accepted` "response" messages unexpectedly leading to crashes.

**Changes:** Set token on all "request" messages. Ignore any "response" message with an unexpected token.

samples/shadow.py

@@ -66,6 +66,7 @@ def __init__(self):
         self.lock = threading.Lock()
         self.shadow_value = None
         self.disconnect_called = False
+        self.request_tokens = set()
 
 locked_data = LockedData()
 
@@ -95,9 +96,15 @@ def on_disconnected(disconnect_future):
 def on_get_shadow_accepted(response):
     # type: (iotshadow.GetShadowResponse) -> None
     try:
-        print("Finished getting initial shadow state.")
-
         with locked_data.lock:
+            # check that this is a response to a request from this session
+            try:
+                locked_data.request_tokens.remove(response.client_token)
+            except KeyError:
+                print("Ignoring get_shadow_accepted message due to unexpected token.")
+                return
+
+            print("Finished getting initial shadow state.")
             if locked_data.shadow_value is not None:
                 print("  Ignoring initial query because a delta event has already been received.")
                 return
@@ -126,12 +133,24 @@ def on_get_shadow_accepted(response):
 
 def on_get_shadow_rejected(error):
     # type: (iotshadow.ErrorResponse) -> None
-    if error.code == 404:
-        print("Thing has no shadow document. Creating with defaults...")
-        change_shadow_value(SHADOW_VALUE_DEFAULT)
-    else:
-        exit("Get request was rejected. code:{} message:'{}'".format(
-            error.code, error.message))
+    try:
+        # check that this is a response to a request from this session
+        with locked_data.lock:
+            try:
+                locked_data.request_tokens.remove(error.client_token)
+            except KeyError:
+                print("Ignoring get_shadow_rejected message due to unexpected token.")
+                return
+
+        if error.code == 404:
+            print("Thing has no shadow document. Creating with defaults...")
+            change_shadow_value(SHADOW_VALUE_DEFAULT)
+        else:
+            exit("Get request was rejected. code:{} message:'{}'".format(
+                error.code, error.message))
+
+    except Exception as e:
+        exit(e)
 
 def on_shadow_delta_updated(delta):
     # type: (iotshadow.ShadowDeltaUpdatedEvent) -> None
@@ -164,15 +183,39 @@ def on_publish_update_shadow(future):
 def on_update_shadow_accepted(response):
     # type: (iotshadow.UpdateShadowResponse) -> None
     try:
-        print("Finished updating reported shadow value to '{}'.".format(response.state.reported[shadow_property])) # type: ignore
-        print("Enter desired value: ") # remind user they can input new values
-    except:
-        exit("Updated shadow is missing the target property.")
+        # check that this is a response to a request from this session
+        with locked_data.lock:
+            try:
+                locked_data.request_tokens.remove(response.client_token)
+            except KeyError:
+                print("Ignoring update_shadow_accepted message due to unexpected token.")
+                return
+
+        try:
+            print("Finished updating reported shadow value to '{}'.".format(response.state.reported[shadow_property])) # type: ignore
+            print("Enter desired value: ") # remind user they can input new values
+        except:
+            exit("Updated shadow is missing the target property.")
+
+    except Exception as e:
+        exit(e)
 
 def on_update_shadow_rejected(error):
     # type: (iotshadow.ErrorResponse) -> None
-    exit("Update request was rejected. code:{} message:'{}'".format(
-        error.code, error.message))
+    try:
+        # check that this is a response to a request from this session
+        with locked_data.lock:
+            try:
+                locked_data.request_tokens.remove(error.client_token)
+            except KeyError:
+                print("Ignoring update_shadow_rejected message due to unexpected token.")
+                return
+
+        exit("Update request was rejected. code:{} message:'{}'".format(
+            error.code, error.message))
+
+    except Exception as e:
+        exit(e)
 
 def set_local_value_due_to_initial_query(reported_value):
     with locked_data.lock:
@@ -189,16 +232,25 @@ def change_shadow_value(value):
         print("Changed local shadow value to '{}'.".format(value))
         locked_data.shadow_value = value
 
-    print("Updating reported shadow value to '{}'...".format(value))
-    request = iotshadow.UpdateShadowRequest(
-        thing_name=thing_name,
-        state=iotshadow.ShadowState(
-            reported={ shadow_property: value },
-            desired={ shadow_property: value },
+        print("Updating reported shadow value to '{}'...".format(value))
+
+        # use a unique token so we can correlate this "request" message to
+        # any "response" messages received on the /accepted and /rejected topics
+        token = str(uuid4())
+
+        request = iotshadow.UpdateShadowRequest(
+            thing_name=thing_name,
+            state=iotshadow.ShadowState(
+                reported={ shadow_property: value },
+                desired={ shadow_property: value },
+            ),
+            client_token=token,
         )
-    )
-    future = shadow_client.publish_update_shadow(request, mqtt.QoS.AT_LEAST_ONCE)
-    future.add_done_callback(on_publish_update_shadow)
+        future = shadow_client.publish_update_shadow(request, mqtt.QoS.AT_LEAST_ONCE)
+
+        locked_data.request_tokens.add(token)
+
+        future.add_done_callback(on_publish_update_shadow)
 
 def user_input_thread_fn():
     while True:
@@ -318,14 +370,22 @@ def user_input_thread_fn():
         # Wait for subscription to succeed
         delta_subscribed_future.result()
 
-        # The rest of the sample runs asyncronously.
+        # The rest of the sample runs asynchronously.
 
         # Issue request for shadow's current state.
         # The response will be received by the on_get_accepted() callback
         print("Requesting current shadow state...")
-        publish_get_future = shadow_client.publish_get_shadow(
-            request=iotshadow.GetShadowRequest(thing_name=args.thing_name),
-            qos=mqtt.QoS.AT_LEAST_ONCE)
+
+        with locked_data.lock:
+            # use a unique token so we can correlate this "request" message to
+            # any "response" messages received on the /accepted and /rejected topics
+            token = str(uuid4())
+
+            publish_get_future = shadow_client.publish_get_shadow(
+                request=iotshadow.GetShadowRequest(thing_name=args.thing_name, client_token=token),
+                qos=mqtt.QoS.AT_LEAST_ONCE)
+
+            locked_data.request_tokens.add(token)
 
         # Ensure that publish succeeds
         publish_get_future.result()