Delegate remote_user_id mapping to the saml mapping provider (#6723)

Turns out that figuring out a remote user id for the SAML user isn't quite as obvious as it seems. Factor it out to the SamlMappingProvider so that it's easy to control.

changelog.d/6723.misc

@@ -0,0 +1 @@
+Updates to the SAML mapping provider API.

synapse/config/saml2_config.py

@@ -121,6 +121,7 @@ class SAML2Config(Config):
         required_methods = [
             "get_saml_attributes",
             "saml_response_to_user_attributes",
+            "get_remote_user_id",
         ]
         missing_methods = [
             method

synapse/handlers/saml_handler.py

@@ -135,14 +135,15 @@ class SamlHandler:
         logger.info("SAML2 response: %s", saml2_auth.origxml)
         logger.info("SAML2 mapped attributes: %s", saml2_auth.ava)
 
-        try:
-            remote_user_id = saml2_auth.ava["uid"][0]
-        except KeyError:
-            logger.warning("SAML2 response lacks a 'uid' attestation")
-            raise SynapseError(400, "'uid' not in SAML2 response")
-
         self._outstanding_requests_dict.pop(saml2_auth.in_response_to, None)
 
+        remote_user_id = self._user_mapping_provider.get_remote_user_id(
+            saml2_auth, client_redirect_url
+        )
+
+        if not remote_user_id:
+            raise Exception("Failed to extract remote user id from SAML response")
+
         with (await self._mapping_lock.queue(self._auth_provider_id)):
             # first of all, check if we already have a mapping for this user
             logger.info(
@@ -279,6 +280,20 @@ class DefaultSamlMappingProvider(object):
         self._mxid_source_attribute = parsed_config.mxid_source_attribute
         self._mxid_mapper = parsed_config.mxid_mapper
 
+        self._grandfathered_mxid_source_attribute = (
+            module_api._hs.config.saml2_grandfathered_mxid_source_attribute
+        )
+
+    def get_remote_user_id(
+        self, saml_response: saml2.response.AuthnResponse, client_redirect_url: str
+    ):
+        """Extracts the remote user id from the SAML response"""
+        try:
+            return saml_response.ava["uid"][0]
+        except KeyError:
+            logger.warning("SAML2 response lacks a 'uid' attestation")
+            raise SynapseError(400, "'uid' not in SAML2 response")
+
     def saml_response_to_user_attributes(
         self,
         saml_response: saml2.response.AuthnResponse,