Backport from 6.8 branch which should later land in 6.7.3 in [1] for [2].

[1] https://codereview.qt-project.org/c/qt/qtwayland/+/574983
[2] https://bugs.kde.org/show_bug.cgi?id=489180

From: David Edmundson <davidedmundson@kde.org>
Date: Fri, 05 Jul 2024 16:13:40 +0100
Subject: [PATCH] Client: Improve thread safety determining window size on the render thread

updateSurface is called from both the render and GUI thread. We
therefore need every property referenced to be thread safe.

Rather than guarding each property we cache the buffer size whenever the
window geometry or scale changes and put a mutex round this one
variable.
--- a/src/hardwareintegration/client/wayland-egl/qwaylandeglwindow.cpp
+++ b/src/hardwareintegration/client/wayland-egl/qwaylandeglwindow.cpp
@@ -51,4 +51,13 @@
 void QWaylandEglWindow::ensureSize()
 {
+    // this is always called on the main thread
+    QMargins margins = mWindowDecoration ? frameMargins() : QMargins{};
+    QRect rect = geometry();
+    QSize sizeWithMargins = (rect.size() + QSize(margins.left() + margins.right(), margins.top() + margins.bottom())) * scale();
+    {
+        QWriteLocker lock(&m_bufferSizeLock);
+        m_bufferSize = sizeWithMargins;
+    }
+
     updateSurface(false);
 }
@@ -61,12 +70,15 @@
     // Just resize the wl_egl_window, the EGLSurface will be created
     // the next time makeCurrent is called.
-    updateSurface(false);
+    ensureSize();
 }
 
 void QWaylandEglWindow::updateSurface(bool create)
 {
-    QMargins margins = mWindowDecoration ? frameMargins() : QMargins{};
-    QRect rect = geometry();
-    QSize sizeWithMargins = (rect.size() + QSize(margins.left() + margins.right(), margins.top() + margins.bottom())) * scale();
+
+    QSize sizeWithMargins;
+    {
+        QReadLocker lock(&m_bufferSizeLock);
+        sizeWithMargins = m_bufferSize;
+    }
 
     // wl_egl_windows must have both width and height > 0
--- a/src/hardwareintegration/client/wayland-egl/qwaylandeglwindow_p.h
+++ b/src/hardwareintegration/client/wayland-egl/qwaylandeglwindow_p.h
@@ -61,5 +61,11 @@
 
     QSurfaceFormat m_format;
+    // Size used in the last call to wl_egl_window_resize
     QSize m_requestedSize;
+
+    // Size of the buffer used by QWaylandWindow
+    // This is always written to from the main thread, potentially read from the rendering thread
+    QReadWriteLock m_bufferSizeLock;
+    QSize m_bufferSize;
 };