Add some type annotations in synapse.storage (#6987)

I cracked, and added some type definitions in synapse.storage.

synapse/storage/types.py

@@ -0,0 +1,65 @@
+# -*- coding: utf-8 -*-
+# Copyright 2020 The Matrix.org Foundation C.I.C.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from typing import Any, Iterable, Iterator, List, Tuple
+
+from typing_extensions import Protocol
+
+
+"""
+Some very basic protocol definitions for the DB-API2 classes specified in PEP-249
+"""
+
+
+class Cursor(Protocol):
+    def execute(self, sql: str, parameters: Iterable[Any] = ...) -> Any:
+        ...
+
+    def executemany(self, sql: str, parameters: Iterable[Iterable[Any]]) -> Any:
+        ...
+
+    def fetchall(self) -> List[Tuple]:
+        ...
+
+    def fetchone(self) -> Tuple:
+        ...
+
+    @property
+    def description(self) -> Any:
+        return None
+
+    @property
+    def rowcount(self) -> int:
+        return 0
+
+    def __iter__(self) -> Iterator[Tuple]:
+        ...
+
+    def close(self) -> None:
+        ...
+
+
+class Connection(Protocol):
+    def cursor(self) -> Cursor:
+        ...
+
+    def close(self) -> None:
+        ...
+
+    def commit(self) -> None:
+        ...
+
+    def rollback(self, *args, **kwargs) -> None:
+        ...