package main

...

func (app *application) userLogoutPost(w http.ResponseWriter, r *http.Request) {
    // Use the RenewToken() method on the current session to change the session
    // ID again.
    err := app.sessionManager.RenewToken(r.Context())
    if err != nil {
        app.serverError(w, r, err)
        return
    }

    // Remove the authenticatedUserID from the session data so that the user is
    // 'logged out'.
    app.sessionManager.Remove(r.Context(), "authenticatedUserID")

    // Add a flash message to the session to confirm to the user that they've been
    // logged out.
    app.sessionManager.Put(r.Context(), "flash", "You've been logged out successfully!")

    // Redirect the user to the application home page.
    http.Redirect(w, r, "/", http.StatusSeeOther)
}