Support trying multiple localparts for OpenID Connect.

synapse/handlers/oidc_handler.py

@@ -883,6 +883,12 @@ async def oidc_response_to_user_attributes(failures):
                     userinfo, token, failures
                 )
             else:
+                # If the mapping provider does not support processing failures,
+                # do not continually generate the same Matrix ID since this will
+                # likely continue to fail.
+                if failures:
+                    raise RuntimeError("Mapping provider does not support de-duplicating Matrix IDs")
+
                 attributes = await self._user_mapping_provider.map_user_attributes(  # type: ignore
                     userinfo, token
                 )

tests/handlers/test_oidc.py

@@ -705,7 +705,8 @@ def test_map_userinfo_to_user(self):
             MappingException,
         )
         self.assertEqual(
-            str(e.value), "Unable to generate a Matrix ID from the SSO response"
+            str(e.value),
+            "Could not extract user attributes from SSO response: Mapping provider does not support de-duplicating Matrix IDs",
         )
 
     @override_config({"oidc_config": {"allow_existing_users": True}})