Revert "Redirect event persistence traffic to a particular event persister worker"

Now that things have settled down in the two rooms we special-cased, we
can return the sharding config to normal.

Rich suspects that event_persister-4 being idle was the cause of
federation senders getting stuck on 2023-05-11. It was observed that the
min stream position of RoomStreamTokens was not advancing.

This reverts commit 2263c3bcd7.

synapse/config/_base.py

@@ -947,10 +947,6 @@ class ShardedWorkerHandlingConfig:
 
     instances: List[str]
 
-    # A map of key to instance name. If any of these keys are used,
-    # the associated instance is *always* returned.
-    instances_reserved_for_keys: Dict[str, str] = {}
-
     def should_handle(self, instance_name: str, key: str) -> bool:
         """Whether this instance is responsible for handling the given key."""
         # If no instances are defined we assume some other worker is handling
@@ -968,10 +964,6 @@ class ShardedWorkerHandlingConfig:
         method by default.
         """
 
-        reserved_instance = self.instances_reserved_for_keys.get(key)
-        if reserved_instance is not None:
-            return reserved_instance
-
         if not self.instances:
             raise Exception("Unknown worker")
 

synapse/config/_base.pyi

@@ -199,10 +199,7 @@ def find_config_files(search_paths: List[str]) -> List[str]: ...
 
 class ShardedWorkerHandlingConfig:
     instances: List[str]
-    instances_reserved_for_keys: Dict[str, str]
-    def __init__(
-        self, instances: List[str], instances_reserved_for_keys: Dict[str, str] = {}
-    ) -> None: ...
+    def __init__(self, instances: List[str]) -> None: ...
     def should_handle(self, instance_name: str, key: str) -> bool: ...  # noqa: F811
 
 class RoutableShardedWorkerHandlingConfig(ShardedWorkerHandlingConfig):

synapse/config/workers.py

@@ -272,19 +272,8 @@ class WorkerConfig(Config):
                 "Must only specify one instance to handle `presence` messages."
             )
 
-        # Make the event shard config point specific rooms to a specific worker.
-        # All other rooms should have events persisted by a different worker.
-        chosen_worker_instance = "event_persister-4"
-        event_instances = self.writers.events.copy()
-        event_instances.remove(chosen_worker_instance)
-
-        instances_reserved_for_keys = {
-            "!ioWEdTBHIhOGYVKWyq:libera.chat": chosen_worker_instance,
-            "!bBgnAGciIvrtPXkHkp:libera.chat": chosen_worker_instance,
-        }
-
         self.events_shard_config = RoutableShardedWorkerHandlingConfig(
-            event_instances, instances_reserved_for_keys
+            self.writers.events
         )
 
         # Handle sharded push