Test server extended to start Xwayland server

Proof-of-concept code illustrating how to start a Wayland server
before creating the QGuiApplication and also starting an Xwayland
server before creating the QGuiApplication. Thus the QGuiApplication
is able to connect to the Xwayland server started in the same
application.

src/wayland/tests/waylandservertest.cpp

@@ -23,24 +23,93 @@ License along with this library.  If not, see <http://www.gnu.org/licenses/>.
 #include "../src/server/seat_interface.h"
 #include "../src/server/shell_interface.h"
 
-#include <QCoreApplication>
+#include <QGuiApplication>
+#include <QFile>
+
+#include <unistd.h>
+#include <iostream>
+
+static int startXServer()
+{
+    const QByteArray process = QByteArrayLiteral("Xwayland");
+    int pipeFds[2];
+    if (pipe(pipeFds) != 0) {
+        std::cerr << "FATAL ERROR failed to create pipe to start X Server "
+                  << process.constData()
+                  << std::endl;
+        exit(1);
+    }
+
+    pid_t pid = fork();
+    if (pid == 0) {
+        // child process - should be turned into Xwayland
+        // writes to pipe, closes read side
+        close(pipeFds[0]);
+        char fdbuf[16];
+        sprintf(fdbuf, "%d", pipeFds[1]);
+        execlp(process.constData(), process.constData(), "-displayfd", fdbuf, (char *)0);
+        close(pipeFds[1]);
+        exit(20);
+    }
+    // parent process - this is the wayland server
+    // reads from pipe, closes write side
+    close(pipeFds[1]);
+    return pipeFds[0];
+}
+
+static void readDisplayFromPipe(int pipe)
+{
+    QFile readPipe;
+    if (!readPipe.open(pipe, QIODevice::ReadOnly)) {
+        std::cerr << "FATAL ERROR failed to open pipe to start X Server XWayland" << std::endl;
+        exit(1);
+    }
+    QByteArray displayNumber = readPipe.readLine();
+
+    displayNumber.prepend(QByteArray(":"));
+    displayNumber.remove(displayNumber.size() -1, 1);
+    std::cout << "X-Server started on display " << displayNumber.constData() << std::endl;
+
+    setenv("DISPLAY", displayNumber.constData(), true);
+
+    // close our pipe
+    close(pipe);
+}
 
 int main(int argc, char **argv)
 {
     using namespace KWayland::Server;
-    QCoreApplication app(argc, argv);
 
+    // first create the Server and setup with minimum to get an XWayland connected
     Display display;
     display.start();
+    CompositorInterface *compositor = display.createCompositor(&display);
+    compositor->create();
+    ShellInterface *shell = display.createShell();
+    shell->create();
+
+    // starts XWayland by forking and opening a pipe
+    const int pipe = startXServer();
+    if (pipe == -1) {
+        exit(1);
+    }
+
+    // need four roundtrips to dispatch events
+    for (int i = 0; i < 4; i++) {
+        display.dispatchEvents(1000);
+    }
+
+    // now Xwayland is ready and we can read the pipe to get the display
+    readDisplayFromPipe(pipe);
+
+    QGuiApplication app(argc, argv);
+    display.startLoop();
+
+    display.createShm();
     OutputInterface *output = display.createOutput(&display);
     output->setPhysicalSize(QSize(10, 10));
     output->addMode(QSize(1024, 768));
     output->create();
-    CompositorInterface *compositor = display.createCompositor(&display);
-    compositor->create();
-    display.createShm();
-    ShellInterface *shell = display.createShell();
-    shell->create();
     SeatInterface *seat = display.createSeat();
     seat->setName(QStringLiteral("testSeat0"));
     seat->create();