Fix loading xcursor themes with invalid Inherits field

Xcursor loading can get stuck in an infinite recursion if index.theme
file indicates that the theme inherits itself.

In order to prevent that, keep track of the loaded so far themes and
avoid loading already loaded themes.

BUG: 457926

src/xcursortheme.cpp

@@ -12,7 +12,9 @@
 
 #include <QDir>
 #include <QFile>
+#include <QSet>
 #include <QSharedData>
+#include <QStack>
 #include <QStandardPaths>
 
 namespace KWin
@@ -157,22 +159,36 @@ static QStringList searchPaths()
 void KXcursorThemePrivate::load(const QString &themeName, int size, qreal devicePixelRatio)
 {
     const QStringList paths = searchPaths();
-    QStringList inherits;
 
-    for (const QString &path : paths) {
-        const QDir dir(path + QLatin1Char('/') + themeName);
-        if (!dir.exists()) {
+    QStack<QString> stack;
+    QSet<QString> loaded;
+
+    stack.push(themeName);
+
+    while (!stack.isEmpty()) {
+        const QString themeName = stack.pop();
+        if (loaded.contains(themeName)) {
             continue;
         }
-        loadCursors(dir.filePath(QStringLiteral("cursors")), size, devicePixelRatio);
-        if (inherits.isEmpty()) {
-            const KConfig config(dir.filePath(QStringLiteral("index.theme")), KConfig::NoGlobals);
-            inherits << KConfigGroup(&config, "Icon Theme").readEntry("Inherits", QStringList());
+
+        QStringList inherits;
+
+        for (const QString &path : paths) {
+            const QDir dir(path + QLatin1Char('/') + themeName);
+            if (!dir.exists()) {
+                continue;
+            }
+            loadCursors(dir.filePath(QStringLiteral("cursors")), size, devicePixelRatio);
+            if (inherits.isEmpty()) {
+                const KConfig config(dir.filePath(QStringLiteral("index.theme")), KConfig::NoGlobals);
+                inherits << KConfigGroup(&config, "Icon Theme").readEntry("Inherits", QStringList());
+            }
         }
-    }
 
-    for (const QString &inherit : inherits) {
-        load(inherit, size, devicePixelRatio);
+        loaded.insert(themeName);
+        for (auto it = inherits.crbegin(); it != inherits.crend(); ++it) {
+            stack.push(*it);
+        }
     }
 }