How to make the parent thread wait until a step in a fork is completed?

Scenario is this:

1. User navigates to home page.
2. A large resource is requested but the user can log in asynchronously.
3. User waits after login for resource to finish loading before proceeding.

I want to use a fork to request the resource, but how can I make the parent thread wait until the child request finishes loading (or times out?)