saml: implement a2_hook_good_next_url for SAML 2.0 SPs (#21769)

src/authentic2/saml/__init__.py

@@ -0,0 +1,11 @@
+from django.apps import AppConfig
+
+default_app_config = 'authentic2.saml.A2SAMLAppConfig'
+
+
+class A2SAMLAppConfig(AppConfig):
+    name = 'authentic2.saml'
+
+    def a2_hook_good_next_url(self, next_url):
+        from .utils import saml_good_next_url
+        return saml_good_next_url(next_url)

src/authentic2/saml/utils.py

@@ -0,0 +1,21 @@
+from authentic2.decorators import GlobalCache
+
+
+@GlobalCache(timeout=60)
+def get_entity_ids():
+    from .models import LibertyProvider
+
+    return LibertyProvider.objects.values_list('entity_id', flat=True)
+
+
+@GlobalCache(timeout=60)
+def saml_good_next_url(next_url):
+    from authentic2.utils import same_origin
+
+    entity_ids = get_entity_ids()
+    for entity_id in entity_ids:
+        if same_origin(entity_id, next_url):
+            return True
+    return None
+
+