Simplify the flow for SSO UIA (#8881)

* SsoHandler: remove inheritance from BaseHandler

* Simplify the flow for SSO UIA

We don't need to do all the magic for mapping users when we are doing UIA, so
let's factor that out.

synapse/handlers/oidc_handler.py

@@ -674,6 +674,21 @@ class OidcHandler(BaseHandler):
                 self._sso_handler.render_error(request, "invalid_token", str(e))
                 return
 
+        # first check if we're doing a UIA
+        if ui_auth_session_id:
+            try:
+                remote_user_id = self._remote_id_from_userinfo(userinfo)
+            except Exception as e:
+                logger.exception("Could not extract remote user id")
+                self._sso_handler.render_error(request, "mapping_error", str(e))
+                return
+
+            return await self._sso_handler.complete_sso_ui_auth_request(
+                self._auth_provider_id, remote_user_id, ui_auth_session_id, request
+            )
+
+        # otherwise, it's a login
+
         # Pull out the user-agent and IP from the request.
         user_agent = request.get_user_agent("")
         ip_address = self.hs.get_ip_from_request(request)
@@ -698,14 +713,9 @@ class OidcHandler(BaseHandler):
             extra_attributes = await get_extra_attributes(userinfo, token)
 
         # and finally complete the login
-        if ui_auth_session_id:
-            await self._auth_handler.complete_sso_ui_auth(
-                user_id, ui_auth_session_id, request
-            )
-        else:
-            await self._auth_handler.complete_sso_login(
-                user_id, request, client_redirect_url, extra_attributes
-            )
+        await self._auth_handler.complete_sso_login(
+            user_id, request, client_redirect_url, extra_attributes
+        )
 
     def _generate_oidc_session_token(
         self,
@@ -856,14 +866,11 @@ class OidcHandler(BaseHandler):
             The mxid of the user
         """
         try:
-            remote_user_id = self._user_mapping_provider.get_remote_user_id(userinfo)
+            remote_user_id = self._remote_id_from_userinfo(userinfo)
         except Exception as e:
             raise MappingException(
                 "Failed to extract subject from OIDC response: %s" % (e,)
             )
-        # Some OIDC providers use integer IDs, but Synapse expects external IDs
-        # to be strings.
-        remote_user_id = str(remote_user_id)
 
         # Older mapping providers don't accept the `failures` argument, so we
         # try and detect support.
@@ -933,6 +940,19 @@ class OidcHandler(BaseHandler):
             grandfather_existing_users,
         )
 
+    def _remote_id_from_userinfo(self, userinfo: UserInfo) -> str:
+        """Extract the unique remote id from an OIDC UserInfo block
+
+        Args:
+            userinfo: An object representing the user given by the OIDC provider
+        Returns:
+            remote user id
+        """
+        remote_user_id = self._user_mapping_provider.get_remote_user_id(userinfo)
+        # Some OIDC providers use integer IDs, but Synapse expects external IDs
+        # to be strings.
+        return str(remote_user_id)
+
 
 UserAttributeDict = TypedDict(
     "UserAttributeDict", {"localpart": str, "display_name": Optional[str]}