title: Gracefully handling InvalidAuthenticityToken exceptions in Rails 4
body: <p>I've just upgraded an app from Rails 3 to Rails 4, and I'm seeing a bunch of InvalidAuthenticityToken exceptions popping up. Digging in it looks like it is fairly common for our users to have multiple long-lived tabs open on our site. So I think what's happening is something like this: user Alice has three tabs open and her session expires. She logs back in on one of the tabs, which updates the authenticity token stored in her session. Then she returns to one of the other open tabs and tries to submit data, but she gets a 500 error from the InvalidAuthenticityToken error we raised. </p>
  
  <p>It would clearly be nice to do some error handling for Alice so she doesn't get a 500 error. I'm wondering about best practices for this kind of situation. What would be a nice way to handle Alice's submission from the expired tab? I don't want to expire the current session, because that would be super annoying from the user's perspective ("I <em>just</em> logged in, you dolt!"). Ideally, I just want the user to reload the page, which would result in the correct authenticity token being present in the form. Or should I be doing something different so that the long-lived tabs that are open notice that the session has expired and force a reload? This would probably be sub-optimal from the user's point of view, because they liked having that page ready and easily accessible to keep referencing, which is why they left it open in the tab in the first place.</p>