Resolve xcb_poll_for_queued_event at run-time.

By not making this a compile time decision we ensure forward
compatibility for older xcb versions if the xcb plugin is built against
a newer xcb.

Change-Id: I744777d53bf7b8deb6eff372494f4403d19d364c
Reviewed-by: Kai Koehne <kai.koehne@digia.com>

3 files changed, 49 insertions, 29 deletions

diff --git a/src/plugins/platforms/xcb/qxcbconnection.cpp b/src/plugins/platforms/xcb/qxcbconnection.cpp
index e35954caf9..266491d5d9 100644
--- a/src/plugins/platforms/xcb/qxcbconnection.cpp
+++ b/src/plugins/platforms/xcb/qxcbconnection.cpp
@@ -58,6 +58,7 @@
 #include <QTimer>
 #include <QByteArray>
 
+#include <dlfcn.h>
 #include <stdio.h>
 #include <errno.h>
 #include <xcb/shm.h>
@@ -281,18 +282,16 @@ QXcbConnection::QXcbConnection(QXcbNativeInterface *nativeInterface, const char
         qFatal("QXcbConnection: Could not connect to display %s", m_displayName.constData());
 
     m_reader = new QXcbEventReader(this);
-#ifdef XCB_POLL_FOR_QUEUED_EVENT
     connect(m_reader, SIGNAL(eventPending()), this, SLOT(processXcbEvents()), Qt::QueuedConnection);
     connect(m_reader, SIGNAL(finished()), this, SLOT(processXcbEvents()));
-    m_reader->start();
-#else
-    QSocketNotifier *notifier = new QSocketNotifier(xcb_get_file_descriptor(xcb_connection()), QSocketNotifier::Read, this);
-    connect(notifier, SIGNAL(activated(int)), this, SLOT(processXcbEvents()));
+    if (!m_reader->startThread()) {
+        QSocketNotifier *notifier = new QSocketNotifier(xcb_get_file_descriptor(xcb_connection()), QSocketNotifier::Read, this);
+        connect(notifier, SIGNAL(activated(int)), this, SLOT(processXcbEvents()));
 
-    QAbstractEventDispatcher *dispatcher = QGuiApplicationPrivate::eventDispatcher;
-    connect(dispatcher, SIGNAL(aboutToBlock()), this, SLOT(processXcbEvents()));
-    connect(dispatcher, SIGNAL(awake()), this, SLOT(processXcbEvents()));
-#endif
+        QAbstractEventDispatcher *dispatcher = QGuiApplicationPrivate::eventDispatcher;
+        connect(dispatcher, SIGNAL(aboutToBlock()), this, SLOT(processXcbEvents()));
+        connect(dispatcher, SIGNAL(awake()), this, SLOT(processXcbEvents()));
+    }
 
     xcb_extension_t *extensions[] = {
         &xcb_shm_id, &xcb_xfixes_id, &xcb_randr_id, &xcb_shape_id, &xcb_sync_id,
@@ -364,10 +363,11 @@ QXcbConnection::~QXcbConnection()
     finalizeXInput2();
 #endif
 
-#ifdef XCB_POLL_FOR_QUEUED_EVENT
-    sendConnectionEvent(QXcbAtom::_QT_CLOSE_CONNECTION);
-    m_reader->wait();
-#endif
+    if (m_reader->isRunning()) {
+        sendConnectionEvent(QXcbAtom::_QT_CLOSE_CONNECTION);
+        m_reader->wait();
+    }
+
     delete m_reader;
 
 #ifdef XCB_USE_EGL
@@ -808,14 +808,37 @@ void QXcbConnection::addPeekFunc(PeekFunc f)
     m_peekFuncs.append(f);
 }
 
-#ifdef XCB_POLL_FOR_QUEUED_EVENT
+QXcbEventReader::QXcbEventReader(QXcbConnection *connection)
+    : m_connection(connection)
+    , m_xcb_poll_for_queued_event(0)
+{
+#ifdef RTLD_DEFAULT
+    m_xcb_poll_for_queued_event = (XcbPollForQueuedEventFunctionPointer)dlsym(RTLD_DEFAULT, "xcb_poll_for_queued_event");
+#endif
+
+#ifdef Q_XCB_DEBUG
+    if (m_xcb_poll_for_queued_event)
+        qDebug("Using threaded event reader with xcb_poll_for_queued_event");
+#endif
+}
+
+bool QXcbEventReader::startThread()
+{
+    if (m_xcb_poll_for_queued_event) {
+        QThread::start();
+        return true;
+    }
+
+    return false;
+}
+
 void QXcbEventReader::run()
 {
     xcb_generic_event_t *event;
     while (m_connection && (event = xcb_wait_for_event(m_connection->xcb_connection()))) {
         m_mutex.lock();
         addEvent(event);
-        while (m_connection && (event = xcb_poll_for_queued_event(m_connection->xcb_connection())))
+        while (m_connection && (event = m_xcb_poll_for_queued_event(m_connection->xcb_connection())))
             addEvent(event);
         m_mutex.unlock();
         emit eventPending();
@@ -824,7 +847,6 @@ void QXcbEventReader::run()
     for (int i = 0; i < m_events.size(); ++i)
         free(m_events.at(i));
 }
-#endif
 
 void QXcbEventReader::addEvent(xcb_generic_event_t *event)
 {
@@ -837,10 +859,10 @@ void QXcbEventReader::addEvent(xcb_generic_event_t *event)
 QXcbEventArray *QXcbEventReader::lock()
 {
     m_mutex.lock();
-#ifndef XCB_POLL_FOR_QUEUED_EVENT
-    while (xcb_generic_event_t *event = xcb_poll_for_event(m_connection->xcb_connection()))
-        m_events << event;
-#endif
+    if (!m_xcb_poll_for_queued_event) {
+        while (xcb_generic_event_t *event = xcb_poll_for_event(m_connection->xcb_connection()))
+            m_events << event;
+    }
     return &m_events;
 }
 
diff --git a/src/plugins/platforms/xcb/qxcbconnection.h b/src/plugins/platforms/xcb/qxcbconnection.h
index 8b2315c67e..f466928360 100644
--- a/src/plugins/platforms/xcb/qxcbconnection.h
+++ b/src/plugins/platforms/xcb/qxcbconnection.h
@@ -278,18 +278,15 @@ class QXcbEventReader : public QThread
 {
     Q_OBJECT
 public:
-    QXcbEventReader(QXcbConnection *connection)
-        : m_connection(connection)
-    {
-    }
+    QXcbEventReader(QXcbConnection *connection);
 
-#ifdef XCB_POLL_FOR_QUEUED_EVENT
     void run();
-#endif
 
     QXcbEventArray *lock();
     void unlock();
 
+    bool startThread();
+
 signals:
     void eventPending();
 
@@ -299,6 +296,9 @@ private:
     QMutex m_mutex;
     QXcbEventArray m_events;
     QXcbConnection *m_connection;
+
+    typedef xcb_generic_event_t * (*XcbPollForQueuedEventFunctionPointer)(xcb_connection_t *c);
+    XcbPollForQueuedEventFunctionPointer m_xcb_poll_for_queued_event;
 };
 
 class QAbstractEventDispatcher;
diff --git a/src/plugins/platforms/xcb/xcb.pro b/src/plugins/platforms/xcb/xcb.pro
index 34f7c74675..8a3e9e8e5e 100644
--- a/src/plugins/platforms/xcb/xcb.pro
+++ b/src/plugins/platforms/xcb/xcb.pro
@@ -40,9 +40,7 @@ HEADERS = \
         qxcbimage.h \
         qxlibconvenience.h
 
-contains(QT_CONFIG, xcb-poll-for-queued-event) {
-    DEFINES += XCB_POLL_FOR_QUEUED_EVENT
-}
+LIBS += -ldl
 
 # needed by GLX, Xcursor, XLookupString, ...
 contains(QT_CONFIG, xcb-xlib) {