Refuse to start when faster_joins is enabled on a worker deployment (#13531)

Synapse does not currently support faster room joins on deployments with
workers.

Signed-off-by: Sean Quah <[email protected]>

Author: squahtx

changelog.d/13531.misc

@@ -0,0 +1 @@
+Faster room joins: Refuse to start when faster joins is enabled on a deployment with workers, since worker configurations are not currently supported.

synapse/app/generic_worker.py

@@ -441,6 +441,13 @@ def start(config_options: List[str]) -> None:
         "synapse.app.user_dir",
     )
 
+    if config.experimental.faster_joins_enabled:
+        raise ConfigError(
+            "You have enabled the experimental `faster_joins` config option, but it is "
+            "not compatible with worker deployments yet. Please disable `faster_joins` "
+            "or run Synapse as a single process deployment instead."
+        )
+
     synapse.events.USE_FROZEN_DICTS = config.server.use_frozen_dicts
     synapse.util.caches.TRACK_MEMORY_USAGE = config.caches.track_memory_usage