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, 14 insertions, 0 deletions

diff --git a/src/plugins/platforms/xcb/qxcbconnection_basic.cpp b/src/plugins/platforms/xcb/qxcbconnection_basic.cpp
index af72285135..9a028e5a7e 100644
--- a/src/plugins/platforms/xcb/qxcbconnection_basic.cpp
+++ b/src/plugins/platforms/xcb/qxcbconnection_basic.cpp
@@ -134,6 +134,7 @@ QXcbBasicConnection::QXcbBasicConnection(const char *displayName)
 
     m_setup = xcb_get_setup(m_xcbConnection);
     m_xcbAtom.initialize(m_xcbConnection);
+    m_maximumRequestLength = xcb_get_maximum_request_length(m_xcbConnection);
 
     xcb_extension_t *extensions[] = {
         &xcb_shm_id, &xcb_xfixes_id, &xcb_randr_id, &xcb_shape_id, &xcb_sync_id,
@@ -178,6 +179,14 @@ QXcbBasicConnection::~QXcbBasicConnection()
     }
 }
 
+size_t QXcbBasicConnection::maxRequestDataBytes(size_t requestSize) const
+{
+    if (hasBigRequest())
+        requestSize += 4; // big-request encoding adds 4 bytes
+
+    return m_maximumRequestLength * 4 - requestSize;
+}
+
 xcb_atom_t QXcbBasicConnection::internAtom(const char *name)
 {
     if (!name || *name == 0)
@@ -199,6 +208,11 @@ QByteArray QXcbBasicConnection::atomName(xcb_atom_t atom)
     return QByteArray();
 }
 
+bool QXcbBasicConnection::hasBigRequest() const
+{
+    return m_maximumRequestLength > m_setup->maximum_request_length;
+}
+
 #if QT_CONFIG(xcb_xinput)
 // Starting from the xcb version 1.9.3 struct xcb_ge_event_t has changed:
 // - "pad0" became "extension"