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

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

synapse/storage/engines/_base.py

@@ -12,7 +12,94 @@
 # 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.
+import abc
+from typing import Generic, TypeVar
+
+from synapse.storage.types import Connection
 
 
 class IncorrectDatabaseSetup(RuntimeError):
     pass
+
+
+ConnectionType = TypeVar("ConnectionType", bound=Connection)
+
+
+class BaseDatabaseEngine(Generic[ConnectionType], metaclass=abc.ABCMeta):
+    def __init__(self, module, database_config: dict):
+        self.module = module
+
+    @property
+    @abc.abstractmethod
+    def single_threaded(self) -> bool:
+        ...
+
+    @property
+    @abc.abstractmethod
+    def can_native_upsert(self) -> bool:
+        """
+        Do we support native UPSERTs?
+        """
+        ...
+
+    @property
+    @abc.abstractmethod
+    def supports_tuple_comparison(self) -> bool:
+        """
+        Do we support comparing tuples, i.e. `(a, b) > (c, d)`?
+        """
+        ...
+
+    @property
+    @abc.abstractmethod
+    def supports_using_any_list(self) -> bool:
+        """
+        Do we support using `a = ANY(?)` and passing a list
+        """
+        ...
+
+    @abc.abstractmethod
+    def check_database(
+        self, db_conn: ConnectionType, allow_outdated_version: bool = False
+    ) -> None:
+        ...
+
+    @abc.abstractmethod
+    def check_new_database(self, txn) -> None:
+        """Gets called when setting up a brand new database. This allows us to
+        apply stricter checks on new databases versus existing database.
+        """
+        ...
+
+    @abc.abstractmethod
+    def convert_param_style(self, sql: str) -> str:
+        ...
+
+    @abc.abstractmethod
+    def on_new_connection(self, db_conn: ConnectionType) -> None:
+        ...
+
+    @abc.abstractmethod
+    def is_deadlock(self, error: Exception) -> bool:
+        ...
+
+    @abc.abstractmethod
+    def is_connection_closed(self, conn: ConnectionType) -> bool:
+        ...
+
+    @abc.abstractmethod
+    def lock_table(self, txn, table: str) -> None:
+        ...
+
+    @abc.abstractmethod
+    def get_next_state_group_id(self, txn) -> int:
+        """Returns an int that can be used as a new state_group ID
+        """
+        ...
+
+    @property
+    @abc.abstractmethod
+    def server_version(self) -> str:
+        """Gets a string giving the server version. For example: '3.22.0'
+        """
+        ...