Properly wait to request buffer mapping until after the command to copy the screenshot texture to the buffer has been submitted.

CHANGELOG.md

@@ -14,6 +14,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Removed
 
 ### Fixed
+- Fixed bug that would sometimes cause taking a screenshot to panic because a buffer was mapped a the wrong time.
 
 ## [0.12.0] - 2022-02-19
 

voxygen/src/render/renderer/drawer.rs

@@ -553,11 +553,11 @@ impl<'frame> Drop for Drawer<'frame> {
         // If taking a screenshot and the blit pipeline is available
         // NOTE: blit pipeline should always be available for now so we don't report an
         // error if it isn't
-        if let Some((screenshot, blit)) = self
+        let download_and_handle_screenshot = self
             .taking_screenshot
             .take()
             .zip(self.borrow.pipelines.blit())
-        {
+            .map(|(screenshot, blit)| {
                 // Image needs to be copied from the screenshot texture to the swapchain texture
                 let mut render_pass = encoder.scoped_render_pass(
                     "screenshot blit",
@@ -579,16 +579,22 @@ impl<'frame> Drop for Drawer<'frame> {
                 render_pass.set_bind_group(0, screenshot.bind_group(), &[]);
                 render_pass.draw(0..3, 0..1);
                 drop(render_pass);
-            // Issues a command to copy from the texture to a buffer and then sends the
+                // Issues a command to copy from the texture to a buffer and returns a closure
-            // buffer off to another thread to be mapped and processed
+                // that will send the buffer off to another thread to be mapped
-            screenshot.download_and_handle(&mut encoder);
+                // and processed.
-        }
+                screenshot.copy_to_buffer(&mut encoder)
+            });
 
         let (mut encoder, profiler) = encoder.end_scope();
         profiler.resolve_queries(&mut encoder);
 
         // It is recommended to only do one submit per frame
         self.borrow.queue.submit(std::iter::once(encoder.finish()));
+        // Need to call this after submit so the async mapping doesn't occur before
+        // copying the screenshot to the buffer which will be mapped.
+        if let Some(f) = download_and_handle_screenshot {
+            f();
+        }
 
         profiler
             .end_frame()

voxygen/src/render/renderer/screenshot.rs

@@ -88,9 +88,13 @@ impl TakeScreenshot {
     /// swapchain image
     pub fn bind_group(&self) -> &wgpu::BindGroup { &self.bind_group.bind_group }
 
-    /// NOTE: spawns thread
     /// Call this after rendering to the screenshot texture
-    pub fn download_and_handle(self, encoder: &mut wgpu::CommandEncoder) {
+    ///
+    /// Issues a command to copy from the texture to a buffer and returns a
+    /// closure that needs to be called after submitting the encoder
+    /// to the queue. When called, the closure will spawn a new thread for
+    /// async mapping of the buffer and downloading of the screenshot.
+    pub fn copy_to_buffer(self, encoder: &mut wgpu::CommandEncoder) -> impl FnOnce() {
         // Calculate padded bytes per row
         let padded_bytes_per_row = padded_bytes_per_row(self.width, self.bytes_per_pixel);
         // Copy image to a buffer
@@ -114,6 +118,8 @@ impl TakeScreenshot {
                 depth_or_array_layers: 1,
             },
         );
+
+        move || {
             // Send buffer to another thread for async mapping, downloading, and passing to
             // the given handler function (which probably saves it to the disk)
             std::thread::Builder::new()
@@ -123,6 +129,7 @@ impl TakeScreenshot {
                 })
                 .expect("Failed to spawn screenshot thread");
         }
+    }
 
     fn download_and_handle_internal(self) {
         // Calculate padded bytes per row