Fix window decorations not being uninstallable

BUG: 414570
FIXED-IN: 5.79

src/kpackage-install-handlers/kns/main.cpp

 /*
     This file is part of the KDE libraries
     SPDX-FileCopyrightText: 2016 Aleix Pol Gonzalez <aleixpol@kde.org>
+    SPDX-FileCopyrightText: 2021 Alexander Lohnau <alexander.lohnau@gmx.de>
 
     SPDX-License-Identifier: LGPL-2.0-only OR LGPL-3.0-only OR LicenseRef-KDE-Accepted-LGPL
 */
@@ -12,6 +13,7 @@
 #include <QDebug>
 #include <QStandardPaths>
 #include <QFile>
+#include <QFileInfo>
 
 #include <KLocalizedString>
 
@@ -22,8 +24,37 @@
 
 #include "knshandlerversion.h"
 
+/**
+ * Unfortunately there are two knsrc files for the window decorations, but only one is used in the KCM.
+ * But both are used by third parties, consequently we can not remove one. To solve this we create a symlink
+ * which links the old cache file to the new cache file, which is exposed on the GUI.
+ * This way users can again remove window decorations that are installed as a dependency of a global theme.
+ * BUG: 414570
+ */
+void createSymlinkForWindowDecorations()
+{
+    QFileInfo info(QStandardPaths::writableLocation(QStandardPaths::GenericDataLocation) + QLatin1String("/knewstuff3/aurorae.knsregistry"));
+    // If we have created the symbolic link already we can exit the function here
+    if (info.isSymbolicLink()) {
+        return;
+    }
+    // Delete this file, it the KNS entries are not exposed in any GUI
+    if (info.exists()) {
+        QFile::remove(info.absoluteFilePath());
+    }
+    QFileInfo newFileInfo(QStandardPaths::writableLocation(QStandardPaths::GenericDataLocation) + QLatin1String("/knewstuff3/window-decorations.knsregistry"));
+    QFile file(newFileInfo.absoluteFilePath());
+    // Make sure that the file exists
+    if (!newFileInfo.exists()) {
+        file.open(QFile::WriteOnly);
+        file.close();
+    }
+    file.link(info.absoluteFilePath());
+}
+
 int main(int argc, char** argv)
 {
+    createSymlinkForWindowDecorations();
     QCoreApplication app(argc, argv);
     app.setApplicationName(QLatin1String("kpackage-knshandler"));
     app.setApplicationVersion(knshandlerversion);