xcb: respect big-request encoding in max request size

From big-request specification:

"This extension defines a mechanism for extending the length
field beyond 16 bits. If the normal 16-bit length field of the protocol
request is zero, then an additional 32-bit field containing the actual
length (in 4-byte units) is inserted into the request, immediately
following the 16-bit length field."

Meaning that the request requires 4 additional bytes. This patch provides
a convenience API for calculating maximum request data size.

Besides fixing QTBUG-73044, it was also discovered that calculations
for xcb_image_put (in QXcbBackingStoreImage::flushPixmap) were wrong.
The code assumed that xcb_get_maximum_request_length() returns bytes,
but what it actually returns is length which is measured in four-byte
units. This means that we were sending 4x less bytes than allowed by
the protocol. Furthermore, use the actual 'stride' (bytes per line) value
when calculating rows_per_put. The new stride value was introduced
by 760b2929a3b268e2edf14a561329bdb78fbdc26e, but was not updated in
rows_per_put calculations.

Fixes: QTBUG-73044
Done-with: JiDe Zhang <zccrs@live.com>
Done-with: Mikhail Svetkin <mikhail.svetkin@qt.io>
Change-Id: I06beb6082da3e8bc78225a87603914e796fe5878
Reviewed-by: Błażej Szczygieł <spaz16@wp.pl>
Reviewed-by: JiDe Zhang <zccrs@live.com>
Reviewed-by: Mikhail Svetkin <mikhail.svetkin@qt.io>
Reviewed-by: Uli Schlachter <psychon@znc.in>
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>

1 files changed, 2 insertions, 2 deletions

diff --git a/src/plugins/platforms/xcb/qxcbclipboard.h b/src/plugins/platforms/xcb/qxcbclipboard.h
index 26d3b3b395..51ae0dc1ee 100644
--- a/src/plugins/platforms/xcb/qxcbclipboard.h
+++ b/src/plugins/platforms/xcb/qxcbclipboard.h
@@ -113,7 +113,7 @@ public:
     xcb_window_t getSelectionOwner(xcb_atom_t atom) const;
     QByteArray getSelection(xcb_atom_t selection, xcb_atom_t target, xcb_atom_t property, xcb_timestamp_t t = 0);
 
-    int increment() const { return m_increment; }
+    int increment() const { return m_maxPropertyRequestDataBytes; }
     int clipboardTimeout() const { return clipboard_timeout; }
 
     void removeTransaction(xcb_window_t window) { m_transactions.remove(window); }
@@ -137,7 +137,7 @@ private:
 
     static const int clipboard_timeout;
 
-    int m_increment = 0;
+    int m_maxPropertyRequestDataBytes = 0;
     bool m_clipboard_closing = false;
     xcb_timestamp_t m_incr_receive_time = 0;