Fix schema delta for servers that have not backfilled (#8396)

Fixes #8395.

changelog.d/8396.feature

+Add experimental support for sharding event persister.

synapse/storage/databases/main/schema/delta/58/14events_instance_name.sql.postgres

@@ -21,6 +21,8 @@ SELECT setval('events_stream_seq', (
 
 CREATE SEQUENCE IF NOT EXISTS events_backfill_stream_seq;
 
+-- If the server has never backfilled a room then doing `-MIN(...)` will give
+-- a negative result, hence why we do `GREATEST(...)`
 SELECT setval('events_backfill_stream_seq', (
-    SELECT COALESCE(-MIN(stream_ordering), 1) FROM events
+    SELECT GREATEST(COALESCE(-MIN(stream_ordering), 1), 1) FROM events
 ));

synapse/storage/util/id_generators.py

@@ -287,8 +287,12 @@ class MultiWriterIdGenerator:
         min_stream_id = min(self._current_positions.values(), default=None)
 
         if min_stream_id is None:
+            # We add a GREATEST here to ensure that the result is always
+            # positive. (This can be a problem for e.g. backfill streams where
+            # the server has never backfilled).
             sql = """
-                SELECT COALESCE(%(agg)s(%(id)s), 1) FROM %(table)s
+                SELECT GREATEST(COALESCE(%(agg)s(%(id)s), 1), 1)
+                FROM %(table)s
             """ % {
                 "id": id_column,
                 "table": table,