Restore "Do not create instance of QPlatformIntegration for invalid displays"

This patch is amended version of 67cc8fea10,
which was temporary reverted to simplify integration of conflicting
patches. What was amended:

- Dropped the factory interface. It is sufficiently clean to check for
QXcbConnection::isConnected().

Task-number: QTBUG-68859
Change-Id: I810897b3ea20e356fc4d62e6f01231fd287962dc
Reviewed-by: Johan Helsing <johan.helsing@qt.io>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

src/plugins/platforms/xcb/qxcbintegration.cpp

@@ -195,14 +195,17 @@ QXcbIntegration::QXcbIntegration(const QStringList &parameters, int &argc, char
 
     const int numParameters = parameters.size();
     m_connections.reserve(1 + numParameters / 2);
-    auto conn = new QXcbConnection(m_nativeInterface.data(), m_canGrab, m_defaultVisualId, displayName);
-    if (conn->isConnected())
-        m_connections << conn;
-    else
-        delete conn;
 
+    auto conn = new QXcbConnection(m_nativeInterface.data(), m_canGrab, m_defaultVisualId, displayName);
+    if (!conn->isConnected()) {
+        delete conn;
+        return;
+    }
+    m_connections << conn;
+
+    // ### Qt 6 (QTBUG-52408) remove this multi-connection code path
     for (int i = 0; i < numParameters - 1; i += 2) {
-        qCDebug(lcQpaScreen) << "connecting to additional display: " << parameters.at(i) << parameters.at(i+1);
+        qCDebug(lcQpaXcb) << "connecting to additional display: " << parameters.at(i) << parameters.at(i+1);
         QString display = parameters.at(i) + QLatin1Char(':') + parameters.at(i+1);
         conn = new QXcbConnection(m_nativeInterface.data(), m_canGrab, m_defaultVisualId, display.toLatin1().constData());
         if (conn->isConnected())
@@ -211,11 +214,6 @@ QXcbIntegration::QXcbIntegration(const QStringList &parameters, int &argc, char
             delete conn;
     }
 
-    if (m_connections.isEmpty()) {
-        qCritical("Could not connect to any X display.");
-        exit(1);
-    }
-
     m_fontDatabase.reset(new QGenericUnixFontDatabase());
 
 #if QT_CONFIG(xcb_native_painting)

src/plugins/platforms/xcb/qxcbintegration.h

@@ -102,6 +102,7 @@ public:
     QPlatformTheme *createPlatformTheme(const QString &name) const override;
     QVariant styleHint(StyleHint hint) const override;
 
+    bool hasDefaultConnection() const { return !m_connections.isEmpty(); }
     QXcbConnection *defaultConnection() const { return m_connections.first(); }
 
     QByteArray wmClass() const;

src/plugins/platforms/xcb/qxcbmain.cpp

@@ -52,10 +52,16 @@ public:
 
 QPlatformIntegration* QXcbIntegrationPlugin::create(const QString& system, const QStringList& parameters, int &argc, char **argv)
 {
-    if (!system.compare(QLatin1String("xcb"), Qt::CaseInsensitive))
-        return new QXcbIntegration(parameters, argc, argv);
+    if (!system.compare(QLatin1String("xcb"), Qt::CaseInsensitive)) {
+        auto xcbIntegration = new QXcbIntegration(parameters, argc, argv);
+        if (!xcbIntegration->hasDefaultConnection()) {
+            delete xcbIntegration;
+            return nullptr;
+        }
+        return xcbIntegration;
+    }
 
-    return 0;
+    return nullptr;
 }
 
 QT_END_NAMESPACE