Rewrite the user_filter migration again (#6184)

you can't plausibly ALTER TABLE in sqlite, so we create the new table with the
right schema to start with.

changelog.d/6184.misc

+Update `user_filters` table to have a unique index, and non-null columns. Thanks to @pik for contributing this.
\ No newline at end of file

synapse/storage/schema/delta/56/unique_user_filter_index.py

@@ -5,42 +5,48 @@ from synapse.storage.engines import PostgresEngine
 logger = logging.getLogger(__name__)
 
 
+"""
+This migration updates the user_filters table as follows:
+
+ - drops any (user_id, filter_id) duplicates
+ - makes the columns NON-NULLable
+ - turns the index into a UNIQUE index
+"""
+
+
 def run_upgrade(cur, database_engine, *args, **kwargs):
+    pass
+
+
+def run_create(cur, database_engine, *args, **kwargs):
     if isinstance(database_engine, PostgresEngine):
         select_clause = """
-        CREATE TEMPORARY TABLE user_filters_migration AS
             SELECT DISTINCT ON (user_id, filter_id) user_id, filter_id, filter_json
-            FROM user_filters;
+            FROM user_filters
         """
     else:
         select_clause = """
-        CREATE TEMPORARY TABLE user_filters_migration AS
-            SELECT * FROM user_filters GROUP BY user_id, filter_id;
+            SELECT * FROM user_filters GROUP BY user_id, filter_id
         """
-    sql = (
-        """
-        BEGIN;
-            %s
-            DROP INDEX user_filters_by_user_id_filter_id;
-            DELETE FROM user_filters;
-            ALTER TABLE user_filters
-               ALTER COLUMN user_id SET NOT NULL,
-               ALTER COLUMN filter_id SET NOT NULL,
-               ALTER COLUMN filter_json SET NOT NULL;
-            INSERT INTO user_filters(user_id, filter_id, filter_json)
-                SELECT * FROM user_filters_migration;
-            DROP TABLE user_filters_migration;
-            CREATE UNIQUE INDEX user_filters_by_user_id_filter_id_unique
-                ON user_filters(user_id, filter_id);
-        END;
-    """
-        % select_clause
+    sql = """
+            DROP TABLE IF EXISTS user_filters_migration;
+            DROP INDEX IF EXISTS user_filters_unique;
+            CREATE TABLE user_filters_migration (
+                user_id TEXT NOT NULL,
+                filter_id BIGINT NOT NULL,
+                filter_json BYTEA NOT NULL
+            );
+            INSERT INTO user_filters_migration (user_id, filter_id, filter_json)
+                %s;
+            CREATE UNIQUE INDEX user_filters_unique ON user_filters_migration
+                (user_id, filter_id);
+            DROP TABLE user_filters;
+            ALTER TABLE user_filters_migration RENAME TO user_filters;
+        """ % (
+        select_clause,
     )
+
     if isinstance(database_engine, PostgresEngine):
         cur.execute(sql)
     else:
         cur.executescript(sql)
-
-
-def run_create(cur, database_engine, *args, **kwargs):
-    pass