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def main(request, response):
origin = request.GET.first(b"origin");
value = request.GET.first(b"value");
# This is used to solve the race condition we have for postMessages
shouldSucceed = request.GET.first(b"loadShouldSucceed", b"false");
return ([(b"Content-Type", b"text/html")],
b"""<!DOCTYPE html>
var gotMessage = false;
window.addEventListener("message", e => {
gotMessage = true;
window.parent.postMessage(, "*");
var i = document.createElement("iframe");
i.src = "%s/x-frame-options/support/";
i.onload = _ => {
// Why 100ms timeout? Because that seems to be enough to stop the
// load event from racing with the onmessage event, and it's at least
// as long as the two renderAnimationFrame calls that used to be here.
setTimeout(_ => {
// The race condition problem we have is it is possible
// that the sub iframe is loaded before the postMessage is
// dispatched, as a result, the "Failed" message is sent
// out. So the way we fixed is we simply let the timeout
// to happen if we expect the "Loaded" postMessage to be
// sent
if (!gotMessage && %s != true) {
window.parent.postMessage("Failed", "*");
}, 100);
""" % (origin, value, shouldSucceed))