From be4c3575aa6a82328850d7aa67151fbf74cd73cd Mon Sep 17 00:00:00 2001
From: Dmitry Stepanov <d1maxa@yandex.ru>
Date: Sat, 31 Aug 2024 22:15:37 +0300
Subject: [PATCH] use `glClientWaitSync` with timeout=0 instead of
 `glGetSynciv`

- fixes GPU stalls when transferring data from GPU to CPU
---
 fyrox-impl/src/renderer/framework/pixel_buffer.rs | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/fyrox-impl/src/renderer/framework/pixel_buffer.rs b/fyrox-impl/src/renderer/framework/pixel_buffer.rs
index 3307c024f..79513b181 100644
--- a/fyrox-impl/src/renderer/framework/pixel_buffer.rs
+++ b/fyrox-impl/src/renderer/framework/pixel_buffer.rs
@@ -157,6 +157,8 @@ impl<T> PixelBuffer<T> {
                 PixelPackData::BufferOffset(0),
             );
 
+            state.gl.bind_buffer(glow::PIXEL_PACK_BUFFER, None);
+
             self.request = Some(ReadRequest {
                 fence: state
                     .gl
@@ -164,8 +166,6 @@ impl<T> PixelBuffer<T> {
                     .unwrap(),
             });
 
-            state.gl.bind_buffer(glow::PIXEL_PACK_BUFFER, None);
-
             Ok(())
         }
     }
@@ -183,7 +183,10 @@ impl<T> PixelBuffer<T> {
         let mut buffer = vec![T::default(); self.pixel_count];
 
         unsafe {
-            if state.gl.get_sync_status(request.fence) == glow::SIGNALED {
+            // For some reason, glGetSynciv still blocks execution and produces GPU stall, ruining
+            // the performance. glClientWaitSync with timeout=0 does not have this issue.
+            let fence_state = state.gl.client_wait_sync(request.fence, 0, 0);
+            if fence_state != glow::TIMEOUT_EXPIRED && fence_state != glow::WAIT_FAILED {
                 self.read_internal(state, &mut buffer);
 
                 state.gl.delete_sync(request.fence);