xcb: lock-free event processing

For details how this works refer to the documentation in the patch.

The follow-up patches will switch to calling processXcbEvents() on every
event loop iteration. With the existing code that would mean frequent
locking of shared data (event queue). Acquiring a lock is fast, but
lock contention isn't. To avoid potential problems, reimplement xcb event
processing to be lock-free. Besides theoretical performance benefits,
this definitally improves code readability in qxcbconnection.cpp. Thanks
to Mikhail Svetkin for questioning the design of the existing code.

Done-with: Mikhail Svetkin <mikhail.svetkin@qt.io>
Change-Id: I935f2b6ca802580f5c80205aef7b2f9afc172d26
Reviewed-by: Mikhail Svetkin <mikhail.svetkin@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 7 insertions, 7 deletions

diff --git a/src/plugins/platforms/xcb/qxcbclipboard.cpp b/src/plugins/platforms/xcb/qxcbclipboard.cpp
index 84831cdbe5..3fd14a659e 100644
--- a/src/plugins/platforms/xcb/qxcbclipboard.cpp
+++ b/src/plugins/platforms/xcb/qxcbclipboard.cpp
@@ -807,18 +807,18 @@ xcb_generic_event_t *QXcbClipboard::waitForClipboardEvent(xcb_window_t window, i
 {
     QElapsedTimer timer;
     timer.start();
+    QXcbEventQueue *queue = connection()->eventQueue();
     do {
-        auto e = connection()->checkEvent([window, type](xcb_generic_event_t *event, int eventType) {
+        auto e = queue->peek([window, type](xcb_generic_event_t *event, int eventType) {
             if (eventType != type)
                 return false;
             if (eventType == XCB_PROPERTY_NOTIFY) {
                 auto propertyNotify = reinterpret_cast<xcb_property_notify_event_t *>(event);
-                if (propertyNotify->window == window)
-                    return true;
-            } else if (eventType == XCB_SELECTION_NOTIFY) {
+                return propertyNotify->window == window;
+            }
+            if (eventType == XCB_SELECTION_NOTIFY) {
                 auto selectionNotify = reinterpret_cast<xcb_selection_notify_event_t *>(event);
-                if (selectionNotify->requestor == window)
-                    return true;
+                return selectionNotify->requestor == window;
             }
             return false;
         });
@@ -833,7 +833,7 @@ xcb_generic_event_t *QXcbClipboard::waitForClipboardEvent(xcb_window_t window, i
 
         // process other clipboard events, since someone is probably requesting data from us
         auto clipboardAtom = atom(QXcbAtom::CLIPBOARD);
-        e = connection()->checkEvent([clipboardAtom](xcb_generic_event_t *event, int type) {
+        e = queue->peek([clipboardAtom](xcb_generic_event_t *event, int type) {
             xcb_atom_t selection = XCB_ATOM_NONE;
             if (type == XCB_SELECTION_REQUEST)
                 selection = reinterpret_cast<xcb_selection_request_event_t *>(event)->selection;