Support hotplugging of input devices with XInput2

Since we only scan for XInput2 devices on application start, we will
currently miss any devices plugged in while the application is running.

This patch makes QXcbConnection listen for XInput2 hierachyChanged
events and use them to trigger a rescan of XInput2 devices.

This fixes a regression in Qt 5.3, where the scroll wheel on hot-
plugged mice does not work until the Qt application is restarted.

Change-Id: I2cdc7ca24d3ab00716cedc4b22355b6e4935b184
Reviewed-by: Shawn Rutledge <shawn.rutledge@digia.com>

src/plugins/platforms/xcb/qxcbconnection.h

@@ -496,8 +496,10 @@ private:
 #ifdef XCB_USE_XINPUT2
     void initializeXInput2();
     void finalizeXInput2();
+    void xi2SetupDevices();
     XInput2DeviceData *deviceForId(int id);
     void xi2HandleEvent(xcb_ge_event_t *event);
+    void xi2HandleHierachyEvent(void *event);
     int m_xiOpCode, m_xiEventBase, m_xiErrorBase;
 #ifndef QT_NO_TABLETEVENT
     struct TabletData {

src/plugins/platforms/xcb/qxcbconnection_xi2.cpp

@@ -73,10 +73,6 @@ void QXcbConnection::initializeXInput2()
 {
     debug_xinput = qEnvironmentVariableIsSet("QT_XCB_DEBUG_XINPUT");
     debug_xinput_devices = qEnvironmentVariableIsSet("QT_XCB_DEBUG_XINPUT_DEVICES");
-#ifndef QT_NO_TABLETEVENT
-    m_tabletData.clear();
-#endif
-    m_scrollingDevices.clear();
     Display *xDisplay = static_cast<Display *>(m_xlib_display);
     if (XQueryExtension(xDisplay, "XInputExtension", &m_xiOpCode, &m_xiEventBase, &m_xiErrorBase)) {
         int xiMajor = 2;
@@ -97,6 +93,23 @@ void QXcbConnection::initializeXInput2()
 #else
                 qDebug("XInput version %d.%d is available and Qt supports 2.0", xiMajor, m_xi2Minor);
 #endif
+        }
+
+        xi2SetupDevices();
+    }
+}
+
+void QXcbConnection::xi2SetupDevices()
+{
+#ifndef QT_NO_TABLETEVENT
+    m_tabletData.clear();
+#endif
+    m_scrollingDevices.clear();
+
+    if (!m_xi2Enabled)
+        return;
+
+    Display *xDisplay = static_cast<Display *>(m_xlib_display);
     int deviceCount = 0;
     XIDeviceInfo *devices = XIQueryDevice(xDisplay, XIAllDevices, &deviceCount);
     for (int i = 0; i < deviceCount; ++i) {
@@ -211,8 +224,6 @@ void QXcbConnection::initializeXInput2()
         }
     }
     XIFreeDeviceInfo(devices);
-        }
-    }
 }
 
 void QXcbConnection::finalizeXInput2()
@@ -303,6 +314,16 @@ void QXcbConnection::xi2Select(xcb_window_t window)
 #else
     Q_UNUSED(xiBitMask);
 #endif
+
+    {
+        // Listen for hotplug events
+        XIEventMask xiEventMask;
+        bitMask = XI_HierarchyChangedMask;
+        xiEventMask.deviceid = XIAllDevices;
+        xiEventMask.mask_len = sizeof(bitMask);
+        xiEventMask.mask = xiBitMask;
+        XISelectEvents(xDisplay, window, &xiEventMask, 1);
+    }
 }
 
 XInput2DeviceData *QXcbConnection::deviceForId(int id)
@@ -402,6 +423,10 @@ void QXcbConnection::xi2HandleEvent(xcb_ge_event_t *event)
     if (xi2PrepareXIGenericDeviceEvent(event, m_xiOpCode)) {
         xXIGenericDeviceEvent *xiEvent = reinterpret_cast<xXIGenericDeviceEvent *>(event);
 
+        if (xiEvent->evtype == XI_HierarchyChanged) {
+            xi2HandleHierachyEvent(xiEvent);
+            return;
+        }
 #ifndef QT_NO_TABLETEVENT
         for (int i = 0; i < m_tabletData.count(); ++i) {
             if (m_tabletData.at(i).deviceId == xiEvent->deviceid) {
@@ -561,6 +586,19 @@ void QXcbConnection::xi2HandleEvent(xcb_ge_event_t *event)
     }
 }
 
+void QXcbConnection::xi2HandleHierachyEvent(void *event)
+{
+    xXIHierarchyEvent *xiEvent = reinterpret_cast<xXIHierarchyEvent *>(event);
+    // We only care about hotplugged devices
+    if (!(xiEvent->flags & (XISlaveRemoved | XISlaveAdded)))
+        return;
+    xi2SetupDevices();
+    // Reselect events for all event-listening windows.
+    Q_FOREACH (xcb_window_t window, m_mapper.keys()) {
+        xi2Select(window);
+    }
+}
+
 void QXcbConnection::handleEnterEvent(const xcb_enter_notify_event_t *)
 {
 #ifdef XCB_USE_XINPUT21