Supply additional props for single request only

[Floppy012]

I have a scenario where I want to provide additional information but only for a call to a special route. The controller behind that route does some processing and I want to get the result of that processing in the frontend.

Lets say I have the following controller:

class MyController {

    public function show()
    {
        return inertia('Page', [
            'someProp' => 'someValue'
        ]);
    }

    public function process()
    {
        $result = 1 + 1;
        // How do I get the result merged into the props?
        return back();
    }
}

(The problem in the example could be solved with lazy props. The actual problem requires input parameters and has way more code so its not something that would belong into a Closure of Inertia::lazy)

When using router.post inertia does two requests. One for the actual post request, then another one to the show endpoint. So returning the result in the process function doesn't seem to work.

I have thought (and read) about storing it in the session, then extracting it in the HandleInertiaRequests middleware, however that would result in a race condition when multiple requests to the show endpoint happen at the same time.

I also cannot use render again, since the endpoint can be called from two different views.

Is there any way this is doable with Inertia?