cas: support setting display name (#6114)

Now, the CAS server can return an attribute stating what's the desired displayname, instead of using the username directly.

changelog.d/6114.feature

@@ -0,0 +1 @@
+CAS login now provides a default display name for users if a `displayname_attribute` is set in the configuration file.

docs/sample_config.yaml

@@ -1220,6 +1220,7 @@ saml2_config:
 #   enabled: true
 #   server_url: "https://cas-server.com"
 #   service_url: "https://homeserver.domain.com:8448"
+#   #displayname_attribute: name
 #   #required_attributes:
 #   #    name: value
 

synapse/config/cas.py

@@ -30,11 +30,13 @@ class CasConfig(Config):
             self.cas_enabled = cas_config.get("enabled", True)
             self.cas_server_url = cas_config["server_url"]
             self.cas_service_url = cas_config["service_url"]
+            self.cas_displayname_attribute = cas_config.get("displayname_attribute")
             self.cas_required_attributes = cas_config.get("required_attributes", {})
         else:
             self.cas_enabled = False
             self.cas_server_url = None
             self.cas_service_url = None
+            self.cas_displayname_attribute = None
             self.cas_required_attributes = {}
 
     def generate_config_section(self, config_dir_path, server_name, **kwargs):
@@ -45,6 +47,7 @@ class CasConfig(Config):
         #   enabled: true
         #   server_url: "https://cas-server.com"
         #   service_url: "https://homeserver.domain.com:8448"
+        #   #displayname_attribute: name
         #   #required_attributes:
         #   #    name: value
         """

synapse/rest/client/v1/login.py

@@ -377,6 +377,7 @@ class CasTicketServlet(RestServlet):
         super(CasTicketServlet, self).__init__()
         self.cas_server_url = hs.config.cas_server_url
         self.cas_service_url = hs.config.cas_service_url
+        self.cas_displayname_attribute = hs.config.cas_displayname_attribute
         self.cas_required_attributes = hs.config.cas_required_attributes
         self._sso_auth_handler = SSOAuthHandler(hs)
         self._http_client = hs.get_simple_http_client()
@@ -400,6 +401,7 @@ class CasTicketServlet(RestServlet):
 
     def handle_cas_response(self, request, cas_response_body, client_redirect_url):
         user, attributes = self.parse_cas_response(cas_response_body)
+        displayname = attributes.pop(self.cas_displayname_attribute, None)
 
         for required_attribute, required_value in self.cas_required_attributes.items():
             # If required attribute was not in CAS Response - Forbidden
@@ -414,7 +416,7 @@ class CasTicketServlet(RestServlet):
                     raise LoginError(401, "Unauthorized", errcode=Codes.UNAUTHORIZED)
 
         return self._sso_auth_handler.on_successful_auth(
-            user, request, client_redirect_url
+            user, request, client_redirect_url, displayname
         )
 
     def parse_cas_response(self, cas_response_body):