Add a configuration setting for the dummy event threshold

changelog.d/7422.feature

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

docs/sample_config.yaml

@@ -424,6 +424,14 @@ retention:
 #
 #request_token_inhibit_3pid_errors: true
 
+# Number of forward extremities in a room at which Synapse will send a dummy
+# events.
+# This is to prevent forward extremities from building up in the room, which
+# could lead to poor performance.
+# The default value is 10.
+#
+#dummy_event_threshold: 5
+
 
 ## TLS ##
 

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_event_threshold = config.get("dummy_event_threshold", 10)
+
         self.enable_ephemeral_messages = config.get("enable_ephemeral_messages", False)
 
         # Inhibits the /requestToken endpoints from returning an error that might leak
@@ -993,6 +996,14 @@ def generate_config_section(
         # act as if no error happened and return a fake session ID ('sid') to clients.
         #
         #request_token_inhibit_3pid_errors: true
+
+        # Number of forward extremities in a room at which Synapse will send a dummy
+        # events.
+        # This is to prevent forward extremities from building up in the room, which
+        # could lead to poor performance.
+        # The default value is 10.
+        #
+        #dummy_event_threshold: 5
         """
             % locals()
         )

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_event_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(),
         )