SAMLBackend check context for discovery service base URL

The SAMLBackend start_auth() and disco_query() methods have been
modified so that disco_query() can check the context for the base URL
to use for the discovery service. This allows request microservices
the opportunity to set the base URL for the discovery service to use.

Author: skoranda

src/satosa/backends/saml2.py

@@ -40,6 +40,7 @@ class SAMLBackend(BackendModule, SAMLBaseModule):
     A saml2 backend module (acting as a SP).
     """
     KEY_DISCO_SRV = 'disco_srv'
+    KEY_SAML_DISCOVERY_SERVICE_URL = 'saml_discovery_service_url'
     KEY_SP_CONFIG = 'sp_config'
     VALUE_ACR_COMPARISON_DEFAULT = 'exact'
 
@@ -102,9 +103,9 @@ def start_auth(self, context, internal_req):
             entity_id = idps[0]
             return self.authn_request(context, entity_id)
 
-        return self.disco_query()
+        return self.disco_query(context)
 
-    def disco_query(self):
+    def disco_query(self, context):
         """
         Makes a request to the discovery server
 
@@ -116,8 +117,17 @@ def disco_query(self):
         :param internal_req: The request
         :return: Response
         """
-        return_url = self.sp.config.getattr("endpoints", "sp")["discovery_response"][0][0]
-        loc = self.sp.create_discovery_service_request(self.discosrv, self.sp.config.entityid, **{"return": return_url})
+        endpoints = self.sp.config.getattr("endpoints", "sp")
+        return_url = endpoints["discovery_response"][0][0]
+
+        disco_url = context.get_decoration(self.KEY_SAML_DISCOVERY_SERVICE_URL)
+        if not disco_url:
+            disco_url = self.discosrv
+
+        loc = self.sp.create_discovery_service_request(
+                    disco_url,
+                    self.sp.config.entityid, **{"return": return_url})
+
         return SeeOther(loc)
 
     def construct_requested_authn_context(self, entity_id):