Let auth providers get to the database

Somewhat open to abuse, but also somewhat unavoidable :/

synapse/handlers/auth.py

@@ -730,6 +730,7 @@ def __init__(self, hs, check_user_exists):
         self.hs = hs
 
         self._check_user_exists = check_user_exists
+        self._store = hs.get_datastore()
 
     def check_user_exists(self, user_id):
         """Check if user exissts.
@@ -747,3 +748,18 @@ def register(self, localpart):
         """
         reg = self.hs.get_handlers().registration_handler
         return reg.register(localpart=localpart)
+
+    def run_db_interaction(self, desc, func, *args, **kwargs):
+        """Run a function with a database connection
+
+        Args:
+            desc (str): description for the transaction, for metrics etc
+            func (func): function to be run. Passed a database cursor object
+                as well as *args and **kwargs
+            *args: positional args to be passed to func
+            **kwargs: named args to be passed to func
+
+        Returns:
+            Deferred[object]: result of func
+        """
+        return self._store.runInteraction(desc, func, *args, **kwargs)