WIP to announce deleted devices over federation

Previously we queued up the poke correctly when the device was deleted,
but then the actual EDU wouldn't get sent, as the device was no longer known.
Instead, we now send EDUs for deleted devices too if there's a poke for them.

synapse/storage/end_to_end_keys.py

@@ -64,12 +64,17 @@ class EndToEndKeyStore(SQLBaseStore):
         )
 
     @defer.inlineCallbacks
-    def get_e2e_device_keys(self, query_list, include_all_devices=False):
+    def get_e2e_device_keys(
+        self, query_list, include_all_devices=False,
+        include_deleted_devices=False
+    ):
         """Fetch a list of device keys.
         Args:
             query_list(list): List of pairs of user_ids and device_ids.
             include_all_devices (bool): whether to include entries for devices
                 that don't have device keys
+            include_deleted_devices (bool): whether to include null entries for
+                devices which no longer exist (but where in the query_list)
         Returns:
             Dict mapping from user-id to dict mapping from device_id to
             dict containing "key_json", "device_display_name".
@@ -82,10 +87,19 @@ class EndToEndKeyStore(SQLBaseStore):
             query_list, include_all_devices,
         )
 
+        if include_deleted_devices:
+            deleted_devices = set(query_list)
+
         for user_id, device_keys in iteritems(results):
             for device_id, device_info in iteritems(device_keys):
+                if include_deleted_devices:
+                    deleted_devices -= (user_id, device_id)
                 device_info["keys"] = json.loads(device_info.pop("key_json"))
 
+        if include_deleted_devices:
+            for user_id, device_id in deleted_devices:
+                results.setdefault(user_id, {})[device_id] = None
+
         defer.returnValue(results)
 
     def _get_e2e_device_keys_txn(self, txn, query_list, include_all_devices):