Make 'event.redacts' never raise. (#6771)

There are quite a few places that we assume that a redaction event has a
corresponding `redacts` key, which is not always the case. So lets
cheekily make it so that event.redacts just returns None instead.

changelog.d/6771.bugfix

@@ -0,0 +1 @@
+Fix persisting redaction events that have been redacted (or otherwise don't have a redacts key).

synapse/events/__init__.py

@@ -116,16 +116,32 @@ def is_redacted(self):
         return getattr(self, "redacted", False)
 
 
-def _event_dict_property(key):
+_SENTINEL = object()
+
+
+def _event_dict_property(key, default=_SENTINEL):
+    """Creates a new property for the given key that delegates access to
+    `self._event_dict`.
+
+    The default is used if the key is missing from the `_event_dict`, if given,
+    otherwise an AttributeError will be raised.
+
+    Note: If a default is given then `hasattr` will always return true.
+    """
+
     # We want to be able to use hasattr with the event dict properties.
     # However, (on python3) hasattr expects AttributeError to be raised. Hence,
     # we need to transform the KeyError into an AttributeError
-    def getter(self):
+
+    def getter_raises(self):
         try:
             return self._event_dict[key]
         except KeyError:
             raise AttributeError(key)
 
+    def getter_default(self):
+        return self._event_dict.get(key, default)
+
     def setter(self, v):
         try:
             self._event_dict[key] = v
@@ -138,7 +154,11 @@ def delete(self):
         except KeyError:
             raise AttributeError(key)
 
-    return property(getter, setter, delete)
+    if default is _SENTINEL:
+        # No default given, so use the getter that raises
+        return property(getter_raises, setter, delete)
+    else:
+        return property(getter_default, setter, delete)
 
 
 class EventBase(object):
@@ -165,7 +185,7 @@ def __init__(
     origin = _event_dict_property("origin")
     origin_server_ts = _event_dict_property("origin_server_ts")
     prev_events = _event_dict_property("prev_events")
-    redacts = _event_dict_property("redacts")
+    redacts = _event_dict_property("redacts", None)
     room_id = _event_dict_property("room_id")
     sender = _event_dict_property("sender")
     user_id = _event_dict_property("sender")

synapse/storage/data_stores/main/events.py

@@ -951,7 +951,7 @@ def _update_metadata_tables_txn(
             elif event.type == EventTypes.Message:
                 # Insert into the event_search table.
                 self._store_room_message_txn(txn, event)
-            elif event.type == EventTypes.Redaction:
+            elif event.type == EventTypes.Redaction and event.redacts is not None:
                 # Insert into the redactions table.
                 self._store_redaction(txn, event)
             elif event.type == EventTypes.Retention:

synapse/storage/data_stores/main/events_worker.py

@@ -287,7 +287,7 @@ def get_events_as_list(
             # we have to recheck auth now.
 
             if not allow_rejected and entry.event.type == EventTypes.Redaction:
-                if not hasattr(entry.event, "redacts"):
+                if entry.event.redacts is None:
                     # A redacted redaction doesn't have a `redacts` key, in
                     # which case lets just withhold the event.
                     #

tests/storage/test_redaction.py

@@ -398,3 +398,38 @@ def test_redact_redaction(self):
         self.get_success(
             self.store.get_event(first_redact_event.event_id, allow_none=True)
         )
+
+    def test_store_redacted_redaction(self):
+        """Tests that we can store a redacted redaction.
+        """
+
+        self.get_success(
+            self.inject_room_member(self.room1, self.u_alice, Membership.JOIN)
+        )
+
+        builder = self.event_builder_factory.for_room_version(
+            RoomVersions.V1,
+            {
+                "type": EventTypes.Redaction,
+                "sender": self.u_alice.to_string(),
+                "room_id": self.room1.to_string(),
+                "content": {"reason": "foo"},
+            },
+        )
+
+        redaction_event, context = self.get_success(
+            self.event_creation_handler.create_new_client_event(builder)
+        )
+
+        self.get_success(
+            self.storage.persistence.persist_event(redaction_event, context)
+        )
+
+        # Now lets jump to the future where we have censored the redaction event
+        # in the DB.
+        self.reactor.advance(60 * 60 * 24 * 31)
+
+        # We just want to check that fetching the event doesn't raise an exception.
+        self.get_success(
+            self.store.get_event(redaction_event.event_id, allow_none=True)
+        )