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, 16 insertions, 19 deletions

diff --git a/src/plugins/platforms/xcb/qxcbwindow.cpp b/src/plugins/platforms/xcb/qxcbwindow.cpp
index 69fc6c2951..772309c6ae 100644
--- a/src/plugins/platforms/xcb/qxcbwindow.cpp
+++ b/src/plugins/platforms/xcb/qxcbwindow.cpp
@@ -1818,21 +1818,20 @@ void QXcbWindow::handleExposeEvent(const xcb_expose_event_t *event)
     m_exposeRegion |= rect;
 
     bool pending = true;
-    xcb_generic_event_t *e = nullptr;
-    do { // compress expose events
-        e = connection()->checkEvent([this, &pending](xcb_generic_event_t *event, int type) {
-            if (type != XCB_EXPOSE)
-                return false;
-            auto expose = reinterpret_cast<xcb_expose_event_t *>(event);
-            if (expose->window != m_window)
-                return false;
-            if (expose->count == 0)
-                pending = false;
-            m_exposeRegion |= QRect(expose->x, expose->y, expose->width, expose->height);
-            return true;
-        });
-        free(e);
-    } while (e);
+
+    connection()->eventQueue()->peek(QXcbEventQueue::PeekRemoveMatchContinue,
+                                     [this, &pending](xcb_generic_event_t *event, int type) {
+        if (type != XCB_EXPOSE)
+            return false;
+        auto expose = reinterpret_cast<xcb_expose_event_t *>(event);
+        if (expose->window != m_window)
+            return false;
+        if (expose->count == 0)
+            pending = false;
+        m_exposeRegion |= QRect(expose->x, expose->y, expose->width, expose->height);
+        free(expose);
+        return true;
+    });
 
     // if count is non-zero there are more expose events pending
     if (event->count == 0 || !pending) {
@@ -2152,13 +2151,11 @@ void QXcbWindow::handleLeaveNotifyEvent(int root_x, int root_y,
         return;
 
     // check if enter event is buffered
-    auto event = connection()->checkEvent([](xcb_generic_event_t *event, int type) {
+    auto event = connection()->eventQueue()->peek([](xcb_generic_event_t *event, int type) {
         if (type != XCB_ENTER_NOTIFY)
             return false;
         auto enter = reinterpret_cast<xcb_enter_notify_event_t *>(event);
-        if (ignoreEnterEvent(enter->mode, enter->detail))
-            return false;
-        return true;
+        return !ignoreEnterEvent(enter->mode, enter->detail);
     });
     auto enter = reinterpret_cast<xcb_enter_notify_event_t *>(event);
     QXcbWindow *enterWindow = enter ? connection()->platformWindowFromId(enter->event) : nullptr;