The fetch process hangs when a microtask invokes fetch via document.write

[trflynn89]

Repro HTML:

<iframe id="iframe"></iframe>
<script>
    queueMicrotask(() => {
        iframe.contentDocument.write("<script src=empty.js><\/script>");
        iframe.contentDocument.close();
    });
</script>

Where empty.js is an empty file.

---

All promise callbacks are executed on the microtask queue by way of vm.host_enqueue_promise_job. Only one microtask is allowed to execute at a time (see EventLoop::perform_a_microtask_checkpoint).

So, when we write to the iframe, we will eventually fetch the script's source URL in HTMLScriptElement::prepare_script. The call to document.write will block until this fetch is complete here:

ladybird/Libraries/LibWeb/HTML/Parser/HTMLParser.cpp

Lines 3035 to 3039 in 66519af

	// spin the event loop until the parser's Document has no style sheet that is blocking scripts
	// and the script's ready to be parser-executed becomes true.
	main_thread_event_loop().spin_until(GC::create_function(heap(), [&] {
	return !m_document->has_a_style_sheet_that_is_blocking_scripts() && the_script->is_ready_to_be_parser_executed();
	}));

But because we are already in a microtask, the promises involved in this inner fetch are not able to complete, as those microtasks are effectively blocked.

This is reached in the following WPT tests:
https://wpt.fyi/results/upgrade-insecure-requests/gen/iframe-blank-inherit.meta/unset/fetch.https.html?diff=&filter=ADC&run_id=5161602225799168&run_id=5177547862310912