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, 2 insertions, 2 deletions

diff --git a/src/plugins/platforms/xcb/qxcbdrag.cpp b/src/plugins/platforms/xcb/qxcbdrag.cpp
index 2b8e507f30..aa329d8cb7 100644
--- a/src/plugins/platforms/xcb/qxcbdrag.cpp
+++ b/src/plugins/platforms/xcb/qxcbdrag.cpp
@@ -794,7 +794,7 @@ void QXcbDrag::handlePosition(QPlatformWindow * w, const xcb_client_message_even
 {
     xcb_client_message_event_t *lastEvent = const_cast<xcb_client_message_event_t *>(event);
     ClientMessageScanner scanner(atom(QXcbAtom::XdndPosition));
-    while (auto nextEvent = connection()->checkEvent(scanner)) {
+    while (auto nextEvent = connection()->eventQueue()->peek(scanner)) {
         if (lastEvent != event)
             free(lastEvent);
         lastEvent = reinterpret_cast<xcb_client_message_event_t *>(nextEvent);
@@ -846,7 +846,7 @@ void QXcbDrag::handleStatus(const xcb_client_message_event_t *event)
     xcb_client_message_event_t *lastEvent = const_cast<xcb_client_message_event_t *>(event);
     xcb_generic_event_t *nextEvent;
     ClientMessageScanner scanner(atom(QXcbAtom::XdndStatus));
-    while ((nextEvent = connection()->checkEvent(scanner))) {
+    while ((nextEvent = connection()->eventQueue()->peek(scanner))) {
         if (lastEvent != event)
             free(lastEvent);
         lastEvent = (xcb_client_message_event_t *)nextEvent;