Add a configuration setting for the dummy event threshold (#7422)

Add dummy_events_threshold which allows configuring the number of forward extremities a room needs for Synapse to send forward extremities in it.

changelog.d/7422.feature

@@ -0,0 +1 @@
+Add a configuration setting to tweak the threshold for dummy events.

docs/sample_config.yaml

@@ -253,6 +253,18 @@ listeners:
   #  bind_addresses: ['::1', '127.0.0.1']
   #  type: manhole
 
+# Forward extremities can build up in a room due to networking delays between
+# homeservers. Once this happens in a large room, calculation of the state of
+# that room can become quite expensive. To mitigate this, once the number of
+# forward extremities reaches a given threshold, Synapse will send an
+# org.matrix.dummy_event event, which will reduce the forward extremities
+# in the room.
+#
+# This setting defines the threshold (i.e. number of forward extremities in the
+# room) at which dummy events are sent. The default value is 10.
+#
+#dummy_events_threshold: 5
+
 
 ## Homeserver blocking ##
 

synapse/config/server.py

@@ -505,6 +505,9 @@ class LimitRemoteRoomsConfig(object):
             "cleanup_extremities_with_dummy_events", True
         )
 
+        # The number of forward extremities in a room needed to send a dummy event.
+        self.dummy_events_threshold = config.get("dummy_events_threshold", 10)
+
         self.enable_ephemeral_messages = config.get("enable_ephemeral_messages", False)
 
         # Inhibits the /requestToken endpoints from returning an error that might leak
@@ -823,6 +826,18 @@ def generate_config_section(
           #  bind_addresses: ['::1', '127.0.0.1']
           #  type: manhole
 
+        # Forward extremities can build up in a room due to networking delays between
+        # homeservers. Once this happens in a large room, calculation of the state of
+        # that room can become quite expensive. To mitigate this, once the number of
+        # forward extremities reaches a given threshold, Synapse will send an
+        # org.matrix.dummy_event event, which will reduce the forward extremities
+        # in the room.
+        #
+        # This setting defines the threshold (i.e. number of forward extremities in the
+        # room) at which dummy events are sent. The default value is 10.
+        #
+        #dummy_events_threshold: 5
+
 
         ## Homeserver blocking ##
 

synapse/handlers/message.py

@@ -419,6 +419,8 @@ def __init__(self, hs):
 
         self._ephemeral_events_enabled = hs.config.enable_ephemeral_messages
 
+        self._dummy_events_threshold = hs.config.dummy_events_threshold
+
     @defer.inlineCallbacks
     def create_event(
         self,
@@ -1085,7 +1087,7 @@ async def _send_dummy_events_to_fill_extremities(self):
         """
         self._expire_rooms_to_exclude_from_dummy_event_insertion()
         room_ids = await self.store.get_rooms_with_many_extremities(
-            min_count=10,
+            min_count=self._dummy_events_threshold,
             limit=5,
             room_id_filter=self._rooms_to_exclude_from_dummy_event_insertion.keys(),
         )