✨ [#4398] Pass initial_data_reference from auth start to return view

the initial data reference is passed from the SDK to the backend as part of the login URL. In order to pass it as part of the submission create body, it must be propagated from the authentication start/return views back to the SDK

src/openforms/authentication/views.py

@@ -166,6 +166,11 @@ class AuthenticationStartView(AuthenticationFlowBaseView):
     register = register
 
     def get(self, request: Request, slug: str, plugin_id: str):
+        # Store the initial data reference in the session, so it can be passed back
+        # in the `AuthenticationReturnView`
+        request.session["initial_data_reference"] = request.query_params.get(
+            "initial_data_reference"
+        )
         form = self.get_object()
         try:
             plugin = self.register[plugin_id]
@@ -300,6 +305,7 @@ def _handle_return(self, request: Request, slug: str, plugin_id: str):
         plugin. We must define ``get`` and ``post`` to have them properly show up and
         be documented in the OAS.
         """
+        initial_data_reference = request.session.pop("initial_data_reference", None)
         form = self.get_object()
         try:
             plugin = self.register[plugin_id]
@@ -339,6 +345,14 @@ def _handle_return(self, request: Request, slug: str, plugin_id: str):
         if hasattr(request, "session") and FORM_AUTH_SESSION_KEY in request.session:
             authentication_success.send(sender=self.__class__, request=request)
 
+        if initial_data_reference:
+            # Pass the initial data reference back to the SDK, to allow sending it
+            # with the submission create call
+            return HttpResponseRedirect(
+                furl(response.url)
+                .add({"initial_data_reference": initial_data_reference})
+                .url
+            )
         return response
 
     def _handle_co_sign(self, form: Form, plugin: BasePlugin) -> None: