QtWebEngine asserts when doing setHttpCacheType+setPersistentCookiesPolicy

QtWebEngine asserts when an application calls, in that order

page.profile()->setHttpCacheType(QWebEngineProfile::MemoryHttpCache);
page.profile()->setPersistentCookiesPolicy(QWebEngineProfile::NoPersistentCookies);

ASSERT: "m_updateHttpCache" in file /d/qt/5/kde/qtwebengine/src/core/url_request_context_getter_qt.cpp

It's a race condition (semantic race, not C++11 data race), so it doesn't happen 100% of the time, but still too often

The attached patch for qtwebengine/tests/auto/widgets/qwebengineprofile/tst_qwebengineprofile reproduces the issue. This appeared in Qt 5.9 (KMail was doing the same with 5.8 without hitting any assert).

Here's my debug output of what happens with that m_updateHttpCache boolean :

generateHttpCache: setting m_updateHttpCache to false
updateHttpCache: m_updateAllStorage is false, setting m_updateHttpCache to true
updateCookieStore: m_updateHttpCache set to true, calling generateCookieStore delayed
generateHttpCache: setting m_updateHttpCache to false
generateCookieStore: m_updateAllStorage is false, so m_updateHttpCache should be true: false ==> ASSERT!

Clearly, setting a bool and then checking it in another thread is not a valid programming practice, another call in the main thread can have changed that bool again meanwhile.

But I'm not sure how this should be fixed.