XdgDesktopPortal: Prevent c'tor re-entry

XdgDesktopPortal requires a base platform theme, which is created in
its c'tor. If XdgDesktopPortal was shipped as a plugin of its own,
it would find itself as a plugin, create itself as its own base theme
and thus end up in an infinite loop.

Factor out the key check from main.cpp as a static bool in
QXdgDesktopPortalTheme.
Ignore XdgDesktop keys when scanning for plugins suitable as a base
theme.

Fixes: QTBUG-134703
Pick-to: 6.9 6.8 6.5
Change-Id: I4aab1e77a48251581369f371e4c9af308ef0a02b
Reviewed-by: Liang Qi <liang.qi@qt.io>

src/plugins/platformthemes/xdgdesktopportal/main.cpp

@@ -20,12 +20,7 @@ public:
 QPlatformTheme *QXdgDesktopPortalThemePlugin::create(const QString &key, const QStringList &params)
 {
     Q_UNUSED(params);
-    if (!key.compare("xdgdesktopportal"_L1, Qt::CaseInsensitive) ||
+    return QXdgDesktopPortalTheme::isXdgPlugin(key) ? new QXdgDesktopPortalTheme : nullptr;
-        !key.compare("flatpak"_L1, Qt::CaseInsensitive) ||
-        !key.compare("snap"_L1, Qt::CaseInsensitive))
-        return new QXdgDesktopPortalTheme;
-
-    return nullptr;
 }
 
 QT_END_NAMESPACE

src/plugins/platformthemes/xdgdesktopportal/qxdgdesktopportaltheme.cpp

@@ -88,6 +88,8 @@ QXdgDesktopPortalTheme::QXdgDesktopPortalTheme()
     themeNames += QGuiApplicationPrivate::platform_integration->themeNames();
     // 1) Look for a theme plugin.
     for (const QString &themeName : std::as_const(themeNames)) {
+        if (QXdgDesktopPortalTheme::isXdgPlugin(themeName))
+            continue;
         d->baseTheme = QPlatformThemeFactory::create(themeName, nullptr);
         if (d->baseTheme)
             break;
@@ -97,6 +99,8 @@ QXdgDesktopPortalTheme::QXdgDesktopPortalTheme()
     // create a theme
     if (!d->baseTheme) {
         for (const QString &themeName : std::as_const(themeNames)) {
+            if (QXdgDesktopPortalTheme::isXdgPlugin(themeName))
+                continue;
             d->baseTheme = QGuiApplicationPrivate::platform_integration->createPlatformTheme(themeName);
             if (d->baseTheme)
                 break;
@@ -263,6 +267,13 @@ QString QXdgDesktopPortalTheme::standardButtonText(int button) const
     return d->baseTheme->standardButtonText(button);
 }
 
+bool QXdgDesktopPortalTheme::isXdgPlugin(const QString &key)
+{
+    return key.compare("xdgdesktopportal"_L1, Qt::CaseInsensitive) == 0 ||
+           key.compare("flatpak"_L1, Qt::CaseInsensitive) == 0 ||
+           key.compare("snap"_L1, Qt::CaseInsensitive) == 0;
+}
+
 QT_END_NAMESPACE
 
 #include "qxdgdesktopportaltheme.moc"

src/plugins/platformthemes/xdgdesktopportal/qxdgdesktopportaltheme.h

@@ -48,6 +48,8 @@ public:
 
     QString standardButtonText(int button) const override;
 
+    static bool isXdgPlugin(const QString &key);
+
 private:
     QScopedPointer<QXdgDesktopPortalThemePrivate> d_ptr;
     Q_DISABLE_COPY_MOVE(QXdgDesktopPortalTheme)