TINKERPOP-2096 Throw GremlinServerError if server disconnects before …

…read completes

Prior to this change we'd get a more obscure looking AttributeError which didn't really explain what had happened. CTR

CHANGELOG.asciidoc

@@ -37,6 +37,7 @@ This release also includes changes from <<release-3-2-11, 3.2.11>>.
 * Deprecated `TraversalSource#GREMLIN_REMOTE` and `TraversalSource#GREMLIN_REMOTE_CONNECTION_CLASS` moving them to `RemoteConnection`.
 * Fixed the setting of the default label for a `ReferenceVertex` when the original vertex was of type `ComputerAdjacentVertex`.
 * Changed Java driver to expect a generic `RemoteTraverser` object rather than the specific `DefaultRemoteTraverser`.
+* Better handled server disconnect condition for the `gremlin-python` driver by throwing a clear exception.
 * Display the remote stack trace in the Gremlin Console when scripts sent to the server fail.
 * Added `AnonymousTraversalSource` which provides a more unified means of constructing a `TraversalSource`.
 * Added `DriverRemoteConnection.using(Client)` to provide users better control over the number of connections being created.

gremlin-python/src/main/jython/gremlin_python/driver/protocol.py

@@ -66,6 +66,10 @@ def write(self, request_id, request_message):
         self._transport.write(message)
 
     def data_received(self, message, results_dict):
+        # if Gremlin Server cuts off then we get a None for the message
+        if message is None:
+            raise GremlinServerError("Server disconnected - please try to reconnect")
+
         message = self._message_serializer.deserialize_message(json.loads(message.decode('utf-8')))
         request_id = message['requestId']
         result_set = results_dict[request_id]