"Cannot have multiple async operations in flight at once" during QApplication initialization

[fwcd]

I am currently porting a moderately-sized Qt app to the web (Mixxx) and have found that enabling -sASSERTIONS (with Qt 6.6.2) will crash the app at launch with

mixxx.js:12662 Aborted(Assertion failed: Cannot have multiple async operations in flight at once)

The issue seems to be that Mixxx's own asynchronous operations (mostly QDialog::exec) seem to clash with QWasmClipboard::initClipboardPermissions, which is invoked internally during QApplication's initialization:

abort
assert
whenDone
promiseCallbackFinally2
Promise.finally (async)		
methodCaller<(emscripten::val) => emscripten::val>
__emval_call_method
imports.<computed>
$std::__2::__function::__func<qstdweb::(anonymous namespace)::WebPromiseManager::adoptPromise(emscripten::val, qstdweb::PromiseCallbacks)::$_0, std::__2::allocator<qstdweb::(anonymous namespace)::WebPromiseManager::adoptPromise(emscripten::val, qstdweb::PromiseCallbacks)::$_0>, void (std::__2::unique_ptr<qstdweb::(anonymous namespace)::ThunkPool::ThunkAllocation, std::__2::default_delete<qstdweb::(anonymous namespace)::ThunkPool::ThunkAllocation>>)>::operator()(std::__2::unique_ptr<qstdweb::(anonymous namespace)::ThunkPool::ThunkAllocation, std::__2::default_delete<qstdweb::(anonymous namespace)::ThunkPool::ThunkAllocation>>&&)
$qstdweb::Promise::adoptPromise(emscripten::val, qstdweb::PromiseCallbacks)
$void qstdweb::Promise::make<emscripten::val>(emscripten::val, QString, qstdweb::PromiseCallbacks, emscripten::val)
$QWasmClipboard::initClipboardPermissions()
$QWasmClipboard::QWasmClipboard()
$QWasmIntegration::QWasmIntegration()
$QWasmIntegrationPlugin::create(QString const&, QList<QString> const&)
$QPlatformIntegrationPlugin::create(QString const&, QList<QString> const&, int&, char**)
$QPlatformIntegrationFactory::create(QString const&, QList<QString> const&, int&, char**, QString const&)
$init_platform(QString const&, QString const&, QString const&, int&, char**)
$QGuiApplicationPrivate::createPlatformIntegration()
$QGuiApplicationPrivate::createEventDispatcher()
$QApplicationPrivate::createEventDispatcher()
$QCoreApplicationPrivate::init()
$QGuiApplicationPrivate::init()
$QApplicationPrivate::init()
$QApplication::QApplication(int&, char**, int)
$MixxxApplication::MixxxApplication(int&, char**)
$main

My question is whether there is some way to avoid having QApplication perform such async operations internally, specifically if e.g. the clipboard initialization could be deferred or disabled, ideally without having to rearchitect the Mixxx side too much?

I am pretty new to the Qt/Wasm backend, so any help would be greatly appreciated.

[fwcd]

For testing I have patched out QWasmClipboard::initClipboardPermissions and bootstrapCheckPermissions (which also spawns async operations), now I am getting the same error for qtStdWebEventCallbackActivate, which looks like this bug:

• https://github.com/emscripten-core/emscripten/issues/18412
• https://bugreports.qt.io/browse/QTBUG-102827