Segfault during application's exit with static QIcon instances due to a QIconEngine plugin being unloaded too early

As of qtbase's commit afaa7351c0f7f19c86796e9e48e2d16dbf5e4aa9, the following code segfaults at exit because the QSvgIconEngine is unloaded before the foo's QIcon::~QIcon() runs. At destruction time, ~QIcon accesses QIconPrivate::engine which is now a dangling pointer. It was fun to debug, valgrind didn't help much.

#include <QGuiApplication>
#include <QIcon>

static QIcon foo;

int main(int argc, char* argv[]) {
    QGuiApplication app(argc, argv); 
    foo = QIcon(QStringLiteral("/usr/share/icons/bibletime.svg"));
    return 0;
}

Using static instances of QIcon (or some other structures involving QIcon) is a common pattern when once comes around to implementing an icon cache to save some disk accesses. In my case, it was driven by a profiler's output.

As a client-side workaround, one can do something like this:

QIcon getIcon(...)
{
    static QMap<QString, QIcon> cache;
    // static to make just one connection
    static auto conn = QObject::connect(qApp, &QCoreApplication::aboutToQuit, [](){
        cache.clear();
    });

    // ... do something useful here

    return cache[...];

Please note that this is unrelated to the use of QStringLiteral; the code breaks in the exact same way if we use QString::fromUtf8.

Thiago's patch at https://codereview.qt-project.org/140750/ should fix this. QTBUG-27005 is a report against Qt 5.0.0 about this very same problem (but with a bit more complicated minimal reproducible example).

Crash visible only in Debugger