Testing whether a user is signed in to Google
Recently I've been in a couple of conversations where the idea of testing whether a user is logged in to Google came up. This can be helpful for tuning the experience when presenting sign-in options: you can highlight the Google+ Sign-In button on the basis the user was already signed-in to Google, so should just need to consent. It's also one way of responding to the fact that signed-in users typically are going across search using HTTPS, so you don't get information about the search terms a user used to reach you. By highlighting the benefits of signing in, the users may choose to do that, and hence give much more ability to personalise and so onThe (slightly arcane) method for doing this is checkSessionState. This is a bit of Google oAuth 2.0 plumbing that allows cheaply checking whether things have changed without round-tripping to the server in many cases. There is a session state, which is kind of a hash of various aspects of the user's signed in status, locally …