sS googleSignIn_signInButton(S verifyURL) {
  ret googleSignIn_signInButton(verifyURL, "", "");
}

sS googleSignIn_signInButton(S verifyURL, S onSuccess, S onSignOut) {
  ret [[<div class="g-signin2" data-onsuccess="onSignIn"></div>
    <script>
    function onSignIn(googleUser) {
      console.log("Signing in");
      var profile = googleUser.getBasicProfile();
      var email = profile.getEmail();
      if (email != "") {
        var token = googleUser.getAuthResponse().id_token;
        console.log("Email: " + email + "\nToken: " + token);
        var myKeyVals = { token: token };
        $.ajax({
          type: "POST",
          url: "$VERIFYURL",
          data: myKeyVals,
          dataType: "text",
          success: function(data) {
            $ONSUCCESS
          }
        });
      }
    }

    function signOut() {
      var auth2 = gapi.auth2.getAuthInstance();
      auth2.signOut().then(function () { $ONSIGNOUT});
    }
    </script>
  ]].replace("$VERIFYURL", verifyURL)
    .replace("$ONSUCCESS", onSuccess)
    .replace("$ONSIGNOUT", onSignOut);
}

Began life as a copy of #1013823