Fix bugs in handling clientRedirectUrl, and improve OIDC tests (#9127, #9128)

* Factor out a common TestHtmlParser

Looks like I'm doing this in a few different places.

* Improve OIDC login test

Complete the OIDC login flow, rather than giving up halfway through.

* Ensure that OIDC login works with multiple OIDC providers

* Fix bugs in handling clientRedirectUrl

 - don't drop duplicate query-params, or params with no value
 - allow utf-8 in query-params

2 files changed, 3 insertions, 3 deletions

diff --git a/synapse/handlers/auth.py b/synapse/handlers/auth.py
index 18cd2b62f0..0e98db22b3 100644
--- a/synapse/handlers/auth.py
+++ b/synapse/handlers/auth.py
@@ -1504,8 +1504,8 @@ class AuthHandler(BaseHandler):
     @staticmethod
     def add_query_param_to_url(url: str, param_name: str, param: Any):
         url_parts = list(urllib.parse.urlparse(url))
-        query = dict(urllib.parse.parse_qsl(url_parts[4]))
-        query.update({param_name: param})
+        query = urllib.parse.parse_qsl(url_parts[4], keep_blank_values=True)
+        query.append((param_name, param))
         url_parts[4] = urllib.parse.urlencode(query)
         return urllib.parse.urlunparse(url_parts)
 
diff --git a/synapse/handlers/oidc_handler.py b/synapse/handlers/oidc_handler.py
index 5e5fda7b2f..ba686d74b2 100644
--- a/synapse/handlers/oidc_handler.py
+++ b/synapse/handlers/oidc_handler.py
@@ -85,7 +85,7 @@ class OidcHandler:
         self._token_generator = OidcSessionTokenGenerator(hs)
         self._providers = {
             p.idp_id: OidcProvider(hs, self._token_generator, p) for p in provider_confs
-        }
+        }  # type: Dict[str, OidcProvider]
 
     async def load_metadata(self) -> None:
         """Validate the config and load the metadata from the remote endpoint.