Add experimental support for sharding event persister. Again. (#8294)

This is *not* ready for production yet. Caveats:

1. We should write some tests...
2. The stream token that we use for events can get stalled at the minimum position of all writers. This means that new events may not be processed and e.g. sent down sync streams if a writer isn't writing or is slow.

synapse/config/workers.py

@@ -13,12 +13,24 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from typing import List, Union
+
 import attr
 
 from ._base import Config, ConfigError, ShardedWorkerHandlingConfig
 from .server import ListenerConfig, parse_listener_def
 
 
+def _instance_to_list_converter(obj: Union[str, List[str]]) -> List[str]:
+    """Helper for allowing parsing a string or list of strings to a config
+    option expecting a list of strings.
+    """
+
+    if isinstance(obj, str):
+        return [obj]
+    return obj
+
+
 @attr.s
 class InstanceLocationConfig:
     """The host and port to talk to an instance via HTTP replication.
@@ -33,11 +45,13 @@ class WriterLocations:
     """Specifies the instances that write various streams.
 
     Attributes:
-        events: The instance that writes to the event and backfill streams.
-        events: The instance that writes to the typing stream.
+        events: The instances that write to the event and backfill streams.
+        typing: The instance that writes to the typing stream.
     """
 
-    events = attr.ib(default="master", type=str)
+    events = attr.ib(
+        default=["master"], type=List[str], converter=_instance_to_list_converter
+    )
     typing = attr.ib(default="master", type=str)
 
 
@@ -105,15 +119,18 @@ class WorkerConfig(Config):
         writers = config.get("stream_writers") or {}
         self.writers = WriterLocations(**writers)
 
-        # Check that the configured writer for events and typing also appears in
+        # Check that the configured writers for events and typing also appears in
         # `instance_map`.
         for stream in ("events", "typing"):
-            instance = getattr(self.writers, stream)
-            if instance != "master" and instance not in self.instance_map:
-                raise ConfigError(
-                    "Instance %r is configured to write %s but does not appear in `instance_map` config."
-                    % (instance, stream)
-                )
+            instances = _instance_to_list_converter(getattr(self.writers, stream))
+            for instance in instances:
+                if instance != "master" and instance not in self.instance_map:
+                    raise ConfigError(
+                        "Instance %r is configured to write %s but does not appear in `instance_map` config."
+                        % (instance, stream)
+                    )
+
+        self.events_shard_config = ShardedWorkerHandlingConfig(self.writers.events)
 
     def generate_config_section(self, config_dir_path, server_name, **kwargs):
         return """\