wayland: Truncate strings sent via plasmawindowmanager interface

Wayland has a limited size for strings in arguments. We should truncate
any strings that come from user-defined data before sending.

We keep the full title around within kwin as it keeps logic simpler.
Size length logic is lifted from QWaylandWindow's setTitle

BUG: 465775

src/wayland/autotests/client/test_wayland_windowmanagement.cpp

@@ -36,6 +36,7 @@ private Q_SLOTS:
     void init();
 
     void testWindowTitle();
+    void testReallyLongTitle();
     void testMinimizedGeometry();
     void testUseAfterUnmap();
     void testServerDelete();
@@ -166,6 +167,18 @@ void TestWindowManagement::testWindowTitle()
     QCOMPARE(m_window->title(), QString::fromUtf8("Test Title"));
 }
 
+void TestWindowManagement::testReallyLongTitle()
+{
+    QString title;
+    title.fill(QLatin1Char('t'), 500000);
+    m_windowInterface->setTitle(title);
+
+    QSignalSpy titleSpy(m_window, &KWayland::Client::PlasmaWindow::titleChanged);
+
+    QVERIFY(titleSpy.wait());
+    QVERIFY(m_window->title().startsWith("t"));
+}
+
 void TestWindowManagement::testMinimizedGeometry()
 {
     m_window->setMinimizedGeometry(m_surface, QRect(5, 10, 100, 200));

src/wayland/plasmawindowmanagement_interface.cpp

@@ -25,6 +25,19 @@ namespace KWaylandServer
 static const quint32 s_version = 16;
 static const quint32 s_activationVersion = 1;
 
+// any strings that come from user-defined sources
+// that could exceed the maximum wayland length (i.e xwayland clients)
+// need to be truncated to the maximumWaylandBufferSize
+static QString truncate(const QString &stringIn)
+{
+    const int libwaylandMaxBufferSize = 4096;
+    // Some parts of the buffer is used for metadata, so subtract 100 to be on the safe side.
+    // Also, QString is in utf-16, which means that in the worst case each character will be
+    // three bytes when converted to utf-8 (which is what libwayland uses), so divide by three.
+    const int maxLength = libwaylandMaxBufferSize / 3 - 100;
+    return stringIn.left(maxLength);
+}
+
 class PlasmaWindowManagementInterfacePrivate : public QtWaylandServer::org_kde_plasma_window_management
 {
 public:
@@ -347,13 +360,13 @@ void PlasmaWindowInterfacePrivate::org_kde_plasma_window_bind_resource(Resource
         }
     }
     if (!m_appId.isEmpty()) {
-        send_app_id_changed(resource->handle, m_appId);
+        send_app_id_changed(resource->handle, truncate(m_appId));
     }
     if (m_pid != 0) {
         send_pid_changed(resource->handle, m_pid);
     }
     if (!m_title.isEmpty()) {
-        send_title_changed(resource->handle, m_title);
+        send_title_changed(resource->handle, truncate(m_title));
     }
     if (!m_appObjectPath.isEmpty() || !m_appServiceName.isEmpty()) {
         send_application_menu(resource->handle, m_appServiceName, m_appObjectPath);
@@ -393,7 +406,7 @@ void PlasmaWindowInterfacePrivate::setAppId(const QString &appId)
     const auto clientResources = resourceMap();
 
     for (auto resource : clientResources) {
-        send_app_id_changed(resource->handle, m_appId);
+        send_app_id_changed(resource->handle, truncate(m_appId));
     }
 }
 
@@ -503,7 +516,7 @@ void PlasmaWindowInterfacePrivate::setTitle(const QString &title)
     const auto clientResources = resourceMap();
 
     for (auto resource : clientResources) {
-        send_title_changed(resource->handle, m_title);
+        send_title_changed(resource->handle, truncate(m_title));
     }
 }