Fix event filtering for frozen events

Looks like this one was introduced by #11194.

synapse/api/filtering.py

@@ -22,6 +22,7 @@
     Dict,
     Iterable,
     List,
+    Mapping,
     Optional,
     Set,
     TypeVar,
@@ -361,10 +362,10 @@ def _check(self, event: FilterEvent) -> bool:
             return self._check_fields(field_matchers)
         else:
             content = event.get("content")
-            # Content is assumed to be a dict below, so ensure it is. This should
+            # Content is assumed to be a mapping below, so ensure it is. This should
             # always be true for events, but account_data has been allowed to
             # have non-dict content.
-            if not isinstance(content, dict):
+            if not isinstance(content, Mapping):
                 content = {}
 
             sender = event.get("sender", None)

tests/api/test_filtering.py

@@ -18,6 +18,7 @@
 from unittest.mock import patch
 
 import jsonschema
+from frozendict import frozendict
 
 from synapse.api.constants import EventContentFields
 from synapse.api.errors import SynapseError
@@ -327,6 +328,15 @@ def test_filter_labels(self):
 
         self.assertFalse(Filter(self.hs, definition)._check(event))
 
+        # check it works with frozendicts too
+        event = MockEvent(
+            sender="@foo:bar",
+            type="m.room.message",
+            room_id="!secretbase:unknown",
+            content=frozendict({EventContentFields.LABELS: ["#fun"]}),
+        )
+        self.assertTrue(Filter(self.hs, definition)._check(event))
+
     def test_filter_not_labels(self):
         definition = {"org.matrix.not_labels": ["#fun"]}
         event = MockEvent(