Ensure drawable size atomicity within a frame

Revert surfacePixelSize() to be a getter only. With Metal this will
mean returning the "live" layer size (and so not the
layer.drawableSize), which is in line with what we expect with other
backends.

Instead, we leave it to the swapchain's buildOrResize() to "commit"
the size by setting drawableSize on the layer. With typical
application or Qt Quick logic this ensures that layer.drawableSize is
set once and stays static until we get to process the next resize - on
the rendering thread.

This of course would still mean that there was a race when a client
queries surfacePixelSize() to set the depth-stencil buffer size that
is associated with a swapchain. (because that must happen before
calling buildOrResize() according to the current semantics)

That can however be solved in a quite elegant way, it turns out,
because we already have a flag that indicates if a QRhiRenderBuffer is
used in combination with (and only in combination with) a
swapchain. If we simply say that setting the UsedWithSwapChainOnly
flag provides automatic sizing as well (so no setPixelSize() call is
needed), clients can simply get rid of the problematic
surfacePixelSize() query and everything works.

Task-number: QTBUG-78641
Change-Id: Ib1bfc9ef8531bcce033d1f1e5d4d5b4984d6d69f
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

1 files changed, 3 insertions, 7 deletions

diff --git a/tests/manual/rhi/hellominimalcrossgfxtriangle/hellominimalcrossgfxtriangle.cpp b/tests/manual/rhi/hellominimalcrossgfxtriangle/hellominimalcrossgfxtriangle.cpp
index 3c39ff1719..ea1fefc308 100644
--- a/tests/manual/rhi/hellominimalcrossgfxtriangle/hellominimalcrossgfxtriangle.cpp
+++ b/tests/manual/rhi/hellominimalcrossgfxtriangle/hellominimalcrossgfxtriangle.cpp
@@ -298,7 +298,7 @@ void Window::init()
     m_sc = m_r->newSwapChain();
     // allow depth-stencil, although we do not actually enable depth test/write for the triangle
     m_ds = m_r->newRenderBuffer(QRhiRenderBuffer::DepthStencil,
-                                QSize(), // no need to set the size yet
+                                QSize(), // no need to set the size here, due to UsedWithSwapChainOnly
                                 1,
                                 QRhiRenderBuffer::UsedWithSwapChainOnly);
     releasePool << m_ds;
@@ -376,16 +376,12 @@ void Window::releaseResources()
 
 void Window::resizeSwapChain()
 {
-    const QSize outputSize = m_sc->surfacePixelSize();
-
-    m_ds->setPixelSize(outputSize);
-    m_ds->build(); // == m_ds->release(); m_ds->build();
-
-    m_hasSwapChain = m_sc->buildOrResize();
+    m_hasSwapChain = m_sc->buildOrResize(); // also handles m_ds
 
     m_elapsedMs = 0;
     m_elapsedCount = 0;
 
+    const QSize outputSize = m_sc->currentPixelSize();
     m_proj = m_r->clipSpaceCorrMatrix();
     m_proj.perspective(45.0f, outputSize.width() / (float) outputSize.height(), 0.01f, 100.0f);
     m_proj.translate(0, 0, -4);