Default device_display_name to null

It turns out that it's more useful to return a null device display name (and
let clients decide how to handle it: eg, falling back to device_id) than using
a constant string like "unknown device".

synapse/handlers/device.py

@@ -29,7 +29,7 @@ class DeviceHandler(BaseHandler):
 
     @defer.inlineCallbacks
     def check_device_registered(self, user_id, device_id,
-                                initial_device_display_name):
+                                initial_device_display_name = None):
         """
         If the given device has not been registered, register it with the
         supplied display name.

synapse/rest/client/v2_alpha/keys.py

@@ -130,9 +130,7 @@ class KeyUploadServlet(RestServlet):
         # old access_token without an associated device_id. Either way, we
         # need to double-check the device is registered to avoid ending up with
         # keys without a corresponding device.
-        self.device_handler.check_device_registered(
-            user_id, device_id, "unknown device"
-        )
+        self.device_handler.check_device_registered(user_id, device_id)
 
         result = yield self.store.count_e2e_one_time_keys(user_id, device_id)
         defer.returnValue((200, {"one_time_key_counts": result}))

synapse/storage/schema/delta/33/devices_for_e2e_keys.sql

@@ -16,4 +16,4 @@
 -- make sure that we have a device record for each set of E2E keys, so that the
 -- user can delete them if they like.
 INSERT INTO devices
-    SELECT user_id, device_id, 'unknown device' FROM e2e_device_keys_json;
+    SELECT user_id, device_id, NULL FROM e2e_device_keys_json;

synapse/storage/schema/delta/33/devices_for_e2e_keys_clear_unknown_device.sql

@@ -0,0 +1,20 @@
+/* Copyright 2016 OpenMarket Ltd
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+-- a previous version of the "devices_for_e2e_keys" delta set all the device
+-- names to "unknown device". This wasn't terribly helpful
+UPDATE devices
+    SET display_name = NULL
+    WHERE display_name = 'unknown device';