wasm: prevent thread cross-talk when logging

Emscriptens implementation of fprintf does not provide
mutal exclusion when called from multiple threads, for
the emsdk versions Qt 5.15, Qt 6.2, and current dev
is using.

Pick-to: 5.15 6.2
Change-Id: Ied92730b735b11e4e5e85442de48fc25cbad0611
Reviewed-by: Lorn Potter <lorn.potter@gmail.com>

src/corelib/global/qlogging.cpp

@@ -1748,6 +1748,13 @@ static void stderr_message_handler(QtMsgType type, const QMessageLogContext &con
     if (formattedMessage.isNull())
         return;
 
+#ifdef Q_OS_WASM
+    // Prevent thread cross-talk, which causes Emscripten to log
+    // non-valid UTF-8. FIXME: remove once we upgrade to emsdk > 2.0.30
+    static QBasicMutex m;
+    auto locker = qt_unique_lock(m);
+#endif
+
     fprintf(stderr, "%s\n", formattedMessage.toLocal8Bit().constData());
     fflush(stderr);
 }