Merge pull request #8535 from matrix-org/rav/third_party_events_updates

Support modifying event content from ThirdPartyRules modules

2 files changed, 19 insertions, 6 deletions

diff --git a/synapse/events/__init__.py b/synapse/events/__init__.py
index 7a51d0a22f..65df62107f 100644
--- a/synapse/events/__init__.py
+++ b/synapse/events/__init__.py
@@ -312,6 +312,12 @@ class EventBase(metaclass=abc.ABCMeta):
         """
         return [e for e, _ in self.auth_events]
 
+    def freeze(self):
+        """'Freeze' the event dict, so it cannot be modified by accident"""
+
+        # this will be a no-op if the event dict is already frozen.
+        self._dict = freeze(self._dict)
+
 
 class FrozenEvent(EventBase):
     format_version = EventFormatVersions.V1  # All events of this type are V1
diff --git a/synapse/events/third_party_rules.py b/synapse/events/third_party_rules.py
index 1535cc5339..77fbd3f68a 100644
--- a/synapse/events/third_party_rules.py
+++ b/synapse/events/third_party_rules.py
@@ -12,7 +12,8 @@
 # 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.
-from typing import Callable
+
+from typing import Callable, Union
 
 from synapse.events import EventBase
 from synapse.events.snapshot import EventContext
@@ -44,15 +45,20 @@ class ThirdPartyEventRules:
 
     async def check_event_allowed(
         self, event: EventBase, context: EventContext
-    ) -> bool:
+    ) -> Union[bool, dict]:
         """Check if a provided event should be allowed in the given context.
 
+        The module can return:
+            * True: the event is allowed.
+            * False: the event is not allowed, and should be rejected with M_FORBIDDEN.
+            * a dict: replacement event data.
+
         Args:
             event: The event to be checked.
             context: The context of the event.
 
         Returns:
-            True if the event should be allowed, False if not.
+            The result from the ThirdPartyRules module, as above
         """
         if self.third_party_rules is None:
             return True
@@ -63,9 +69,10 @@ class ThirdPartyEventRules:
         events = await self.store.get_events(prev_state_ids.values())
         state_events = {(ev.type, ev.state_key): ev for ev in events.values()}
 
-        # The module can modify the event slightly if it wants, but caution should be
-        # exercised, and it's likely to go very wrong if applied to events received over
-        # federation.
+        # Ensure that the event is frozen, to make sure that the module is not tempted
+        # to try to modify it. Any attempt to modify it at this point will invalidate
+        # the hashes and signatures.
+        event.freeze()
 
         return await self.third_party_rules.check_event_allowed(event, state_events)