Make it possible to restart kwin_wayland from a command line argument

A recent patch made it possible to restart KWin when it crashes, this
opens the possibility to do it for development purposes. So far it could
be done using `killall -9 kwin_wayland`. This patch embraces this
functionality under --replace so it's a bit more documented and under
its own semantics.
It is done by returning the process with 133 exit code and picking that
code specifically as a wish to restart.

dbusinterface.cpp

@@ -181,6 +181,11 @@ void DBusInterface::showDebugConsole()
     console->show();
 }
 
+void DBusInterface::replace()
+{
+    QCoreApplication::exit(133);
+}
+
 namespace {
 QVariantMap clientToVariantMap(const AbstractClient *c)
 {

dbusinterface.h

@@ -58,6 +58,7 @@ public Q_SLOTS: // METHODS
     QString supportInformation();
     Q_NOREPLY void unclutterDesktop();
     Q_NOREPLY void showDebugConsole();
+    Q_NOREPLY void replace();
 
     QVariantMap queryWindowInfo();
     QVariantMap getWindowInfo(const QString &uuid);

helpers/wayland_wrapper/kwin_wrapper.c

@@ -101,16 +101,21 @@ int main(int argc, char **argv)
 
         waitpid(pid, &status, 0); /* wait for child */
 
+        crashCount++;
         if (WIFEXITED(status)) {
             int exit_status = WEXITSTATUS(status);
-            fprintf(stderr, "Kwin exited with code %d\n", exit_status);
-            break;
+            if (exit_status == 133) {
+                crashCount = 0;
+                fprintf(stderr, "Compositor restarted, respawning\n");
+            } else {
+                fprintf(stderr, "KWin exited with code %d\n", exit_status);
+                break;
+            }
         } else if (WIFSIGNALED(status)) {
             // we crashed! Let's go again!
             pid = 0;
             fprintf(stderr, "Compositor crashed, respawning\n");
         }
-        crashCount++;
     }
 
     wl_socket_destroy(socket);

main_wayland.cpp

@@ -40,6 +40,7 @@
 #include <QStyle>
 #include <QDebug>
 #include <QWindow>
+#include <QDBusInterface>
 
 // system
 #if HAVE_SYS_PRCTL_H
@@ -562,11 +563,15 @@ int main(int argc, char * argv[])
                                     i18n("Wayland socket to use for incoming connections."),
                                     QStringLiteral("wayland_fd"));
 
+    QCommandLineOption replaceOption(QStringLiteral("replace"),
+                                    i18n("Exits this instance so it can be restarted by kwin_wayland_wrapper."));
+
     QCommandLineParser parser;
     a.setupCommandLine(&parser);
     parser.addOption(xwaylandOption);
     parser.addOption(waylandSocketOption);
     parser.addOption(waylandSocketFdOption);
+    parser.addOption(replaceOption);
 
     if (hasX11Option) {
         parser.addOption(x11DisplayOption);
@@ -636,6 +641,12 @@ int main(int argc, char * argv[])
     a.setUseKActivities(false);
 #endif
 
+    if (parser.isSet(replaceOption)) {
+        QDBusMessage msg = QDBusMessage::createMethodCall(QStringLiteral("org.kde.KWin"), QStringLiteral("/KWin"),
+                                         QStringLiteral("org.kde.KWin"), QStringLiteral("replace"));
+        QDBusConnection::sessionBus().call(msg, QDBus::NoBlock);
+        return 0;
+    }
     if (parser.isSet(listBackendsOption)) {
         for (const auto &plugin: availablePlugins) {
             std::cout << std::setw(40) << std::left << qPrintable(plugin.name()) << qPrintable(plugin.description()) << std::endl;

org.kde.KWin.xml

@@ -36,6 +36,7 @@
         <arg type="s" direction="out"/>
     </method>
     <method name="showDebugConsole"/>
+    <method name="replace"/>
     <method name="queryWindowInfo">
         <annotation name="org.qtproject.QtDBus.QtTypeName.Out0" value="QVariantMap"/>
         <arg type="a{sv}" direction="out"/>