wayland: Bump default max buffer size to 1 MiB

This is to prevent disconnecting clients that sometimes struggle with
keeping up with the events that the compositor sends. 1 MiB was eyeballed.

Wayland's wire protocol consists of a stream of messages. Every message
consists of a header (2 words: 1 word for object id, the other word
specifies the message size + event/request opcode) and the arguments.

Considering the compositor sending motion events generated by a mouse
with 1000Hz polling rate, we have

- wl_pointer.motion event size: (4 bytes for serial + 4 bytes for x +
  4 bytes for y + 8 bytes for header) = 20 bytes
- wl_pointer.frame event size: 8 bytes for header

So in total, if the client freezes for a second, the client buffer size
should be at least (20 + 8) * 1000 / 1024 = 27 KiB. 1 MiB corresponds to
the client not processing the motion events for approximately 30 seconds.

CMakeLists.txt

@@ -196,6 +196,11 @@ set_package_properties(Wayland PROPERTIES
     TYPE REQUIRED
     PURPOSE "Required for building KWin with Wayland support"
 )
+if (Wayland_VERSION VERSION_GREATER_EQUAL 1.23)
+    set(HAVE_WL_DISPLAY_SET_DEFAULT_MAX_BUFFER_SIZE 1)
+else()
+    set(HAVE_WL_DISPLAY_SET_DEFAULT_MAX_BUFFER_SIZE 0)
+endif()
 
 find_package(WaylandProtocols 1.34)
 set_package_properties(WaylandProtocols PROPERTIES

src/config-kwin.h.cmake

@@ -31,3 +31,4 @@ constexpr QLatin1String BREEZE_KDECORATION_PLUGIN_ID("${BREEZE_KDECORATION_PLUGI
 #cmakedefine01 HAVE_XWAYLAND_ENABLE_EI_PORTAL
 #cmakedefine01 HAVE_GLX
 #cmakedefine01 HAVE_DL_LIBRARY
+#cmakedefine01 HAVE_WL_DISPLAY_SET_DEFAULT_MAX_BUFFER_SIZE

src/wayland/display.cpp

@@ -5,6 +5,9 @@
     SPDX-License-Identifier: LGPL-2.1-only OR LGPL-3.0-only OR LicenseRef-KDE-Accepted-LGPL
 */
 #include "display.h"
+
+#include "config-kwin.h"
+
 #include "clientconnection.h"
 #include "display_p.h"
 #include "linuxdmabufv1clientbuffer_p.h"
@@ -243,6 +246,13 @@ GraphicsBuffer *Display::bufferForResource(wl_resource *resource)
     }
 }
 
+void Display::setDefaultMaxBufferSize(size_t max)
+{
+#if HAVE_WL_DISPLAY_SET_DEFAULT_MAX_BUFFER_SIZE
+    wl_display_set_default_max_buffer_size(d->display, max);
+#endif
+}
+
 SecurityContext::SecurityContext(Display *display, FileDescriptor &&listenFd, FileDescriptor &&closeFd, const QString &appId)
     : QObject(display)
     , m_display(display)

src/wayland/display.h

@@ -116,6 +116,12 @@ public:
      */
     static GraphicsBuffer *bufferForResource(wl_resource *resource);
 
+    /**
+     * Sets the default maximum size for connection buffers of new clients. The size is in bytes.
+     * The minimum buffer size is 4096.
+     */
+    void setDefaultMaxBufferSize(size_t max);
+
 private Q_SLOTS:
     void flush();
 

src/wayland_server.cpp

@@ -188,10 +188,24 @@ public:
     }
 };
 
+static size_t mibToBytes(size_t mib)
+{
+    return mib * (size_t(1) << 20);
+}
+
+static size_t defaultMaxBufferSize()
+{
+    if (int hint = qEnvironmentVariableIntValue("KWIN_WAYLAND_DEFAULT_MAX_CONNECTION_BUFFER_SIZE"); hint > 0) {
+        return hint;
+    }
+    return mibToBytes(1);
+}
+
 WaylandServer::WaylandServer(QObject *parent)
     : QObject(parent)
     , m_display(new KWinDisplay(this))
 {
+    m_display->setDefaultMaxBufferSize(defaultMaxBufferSize());
 }
 
 WaylandServer::~WaylandServer()