feature: add client side cookie to expose login info

[tkurki]

Webapps are often used in multiple windows and the browser's login status in session cookie may change in another window or in another webapp (Admin UI vs Freeboard, Kip etc). The regular session cookie is set to be httpOnly, so it is not available for clientside code.

This adds a separate cookie that is set and removed by the server when the user's login status changes. A webapp can poll for changes of this cookie to react. One notable use case for this is applicationData that is available only when the user/browser is logged in.

[cmotelet]

@tkurki Do you have a piece of webapp code to test/see the use case ?
My timeout is 365d and I don't log out, so I haven't really encountered the problem.

But in theory it sounds like a good idea.

[tkurki]

The use case is to detect login status changes in one browser tab when the user logs in or out in another window. Not your everyday incidence, but since there is currently no way to detect that.

This could be more specific to this cookie, but works for any cookie changing:

window.setInterval(
      (() => {
        let lastCookie = document.cookie
        return () => {
          const currentCookie = document.cookie

          if (currentCookie != lastCookie) {
            updateLoginInfo() //should fetch actual login status from the server
            lastCookie = currentCookie
          }
        }
      })(),
      1000
    )