xcb: fix bitmap cursor loading performance regression

... introduced by 422838685c31d9b57133a8711bfd5db92095d96d.
Instead of completely droping caching for bitmap cursors we can do
the same what is done in libXcursor - have a fixed size cache, with
oldest entries eventually being replaced with new bitmaps. This
fixes the original issue, where the hash was growing indefinitely
until running out of file descriptors and won't have the performance
penalty as in 422838685c31d9b57133a8711bfd5db92095d96d.

Task-number: QTBUG-66897
Change-Id: I14f80b46f97fd0e2c920e17a31ffbc0441cd9d22
Reviewed-by: Uli Schlachter <psychon@znc.in>
Reviewed-by: Robin Burchell <robin.burchell@crimson.no>
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>

1 files changed, 23 insertions, 19 deletions

diff --git a/src/plugins/platforms/xcb/qxcbcursor.cpp b/src/plugins/platforms/xcb/qxcbcursor.cpp
index 34b7d0d236..70138abede 100644
--- a/src/plugins/platforms/xcb/qxcbcursor.cpp
+++ b/src/plugins/platforms/xcb/qxcbcursor.cpp
@@ -301,6 +301,9 @@ QXcbCursorCacheKey::QXcbCursorCacheKey(const QCursor &c)
 QXcbCursor::QXcbCursor(QXcbConnection *conn, QXcbScreen *screen)
     : QXcbObject(conn), m_screen(screen), m_gtkCursorThemeInitialized(false)
 {
+    // see NUM_BITMAPS in libXcursor/src/xcursorint.h
+    m_bitmapCache.setMaxCost(8);
+
     if (cursorCount++)
         return;
 
@@ -351,37 +354,38 @@ QXcbCursor::~QXcbCursor()
 }
 
 #ifndef QT_NO_CURSOR
-void QXcbCursor::changeCursor(QCursor *cursor, QWindow *widget)
+void QXcbCursor::changeCursor(QCursor *cursor, QWindow *window)
 {
-    QXcbWindow *w = 0;
-    if (widget && widget->handle())
-        w = static_cast<QXcbWindow *>(widget->handle());
-    else
-        // No X11 cursor control when there is no widget under the cursor
+    if (!window || !window->handle())
         return;
 
     xcb_cursor_t c = XCB_CURSOR_NONE;
-    bool isBitmapCursor = false;
-
     if (cursor) {
+        const QXcbCursorCacheKey key(*cursor);
         const Qt::CursorShape shape = cursor->shape();
-        isBitmapCursor = shape == Qt::BitmapCursor;
 
-        if (!isBitmapCursor) {
-            const QXcbCursorCacheKey key(*cursor);
-            CursorHash::iterator it = m_cursorHash.find(key);
-            if (it == m_cursorHash.end()) {
-                it = m_cursorHash.insert(key, createFontCursor(shape));
+        if (shape == Qt::BitmapCursor) {
+            auto *bitmap = m_bitmapCache.object(key);
+            if (bitmap) {
+                c = bitmap->cursor;
+            } else {
+                c = createBitmapCursor(cursor);
+                m_bitmapCache.insert(key, new CachedCursor(xcb_connection(), c));
             }
-            c = it.value();
         } else {
-            // Do not cache bitmap cursors, as otherwise they have unclear
-            // lifetime (we effectively leak xcb_cursor_t).
-            c = createBitmapCursor(cursor);
+            auto it = m_cursorHash.find(key);
+            if (it == m_cursorHash.end()) {
+                c = createFontCursor(shape);
+                m_cursorHash.insert(key, c);
+            } else {
+                c = it.value();
+            }
         }
     }
 
-    w->setCursor(c, isBitmapCursor);
+    auto *w = static_cast<QXcbWindow *>(window->handle());
+    xcb_change_window_attributes(xcb_connection(), w->xcb_window(), XCB_CW_CURSOR, &c);
+    xcb_flush(xcb_connection());
 }
 
 static int cursorIdForShape(int cshape)