PoC for filtering spammy events (#2456)

Demonstration of how you might add some hooks to filter out spammy events.

synapse/events/spamcheck.py

@@ -0,0 +1,38 @@
+# -*- coding: utf-8 -*-
+# Copyright 2017 New Vector 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.
+
+
+def check_event_for_spam(event):
+    """Checks if a given event is considered "spammy" by this server.
+
+    If the server considers an event spammy, then it will be rejected if
+    sent by a local user. If it is sent by a user on another server, then
+    users
+
+    Args:
+        event (synapse.events.EventBase): the event to be checked
+
+    Returns:
+        bool: True if the event is spammy.
+    """
+    if not hasattr(event, "content") or "body" not in event.content:
+        return False
+
+    # for example:
+    #
+    # if "the third flower is green" in event.content["body"]:
+    #    return True
+
+    return False