Auto merge of #22798 - jdm:webgl-trace, r=nox

Report all messages processed by the webgl thread.

This might be useful for figuring out what a WebGL-based page is doing when it's not behaving correctly.

---
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes do not require tests because they are debug-logging changes only.

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/22798)
<!-- Reviewable:end -->

components/canvas/webgl_thread.rs

@@ -117,6 +117,7 @@ impl<VR: WebVRRenderHandler + 'static> WebGLThread<VR> {
     /// Handles a generic WebGLMsg message
     #[inline]
     fn handle_msg(&mut self, msg: WebGLMsg, webgl_chan: &WebGLChan) -> bool {
+        trace!("processing {:?}", msg);
         match msg {
             WebGLMsg::CreateContext(version, size, attributes, result_sender) => {
                 let result = self.create_webgl_context(version, size, attributes);
@@ -1104,7 +1105,7 @@ impl WebGLImpl {
                     alpha_treatment,
                     y_axis_treatment,
                     pixel_format,
-                    Cow::Borrowed(data),
+                    Cow::Borrowed(&*data),
                 );
 
                 ctx.gl()
@@ -1144,7 +1145,7 @@ impl WebGLImpl {
                     alpha_treatment,
                     y_axis_treatment,
                     pixel_format,
-                    Cow::Borrowed(data),
+                    Cow::Borrowed(&*data),
                 );
 
                 ctx.gl()

components/canvas_traits/webgl.rs

@@ -9,7 +9,9 @@ use offscreen_gl_context::{GLContextAttributes, GLLimits};
 use pixels::PixelFormat;
 use serde_bytes::ByteBuf;
 use std::borrow::Cow;
+use std::fmt;
 use std::num::NonZeroU32;
+use std::ops::Deref;
 use webrender_api::{DocumentId, ImageKey, PipelineId};
 
 /// Helper function that creates a WebGL channel (WebGLSender, WebGLReceiver) to be used in WebGLCommands.
@@ -173,6 +175,33 @@ impl WebGLMsgSender {
     }
 }
 
+#[derive(Deserialize, Serialize)]
+pub struct TruncatedDebug<T>(T);
+
+impl<T> From<T> for TruncatedDebug<T> {
+    fn from(v: T) -> TruncatedDebug<T> {
+        TruncatedDebug(v)
+    }
+}
+
+impl<T: fmt::Debug> fmt::Debug for TruncatedDebug<T> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        let mut s = format!("{:?}", self.0);
+        if s.len() > 20 {
+            s.truncate(20);
+            s.push_str("...");
+        }
+        write!(f, "{}", s)
+    }
+}
+
+impl<T> Deref for TruncatedDebug<T> {
+    type Target = T;
+    fn deref(&self) -> &T {
+        &self.0
+    }
+}
+
 /// WebGL Commands for a specific WebGLContext
 #[derive(Debug, Deserialize, Serialize)]
 pub enum WebGLCommand {
@@ -284,7 +313,7 @@ pub enum WebGLCommand {
         alpha_treatment: Option<AlphaTreatment>,
         y_axis_treatment: YAxisTreatment,
         pixel_format: Option<PixelFormat>,
-        data: IpcSharedMemory,
+        data: TruncatedDebug<IpcSharedMemory>,
     },
     TexSubImage2D {
         target: u32,
@@ -300,7 +329,7 @@ pub enum WebGLCommand {
         alpha_treatment: Option<AlphaTreatment>,
         y_axis_treatment: YAxisTreatment,
         pixel_format: Option<PixelFormat>,
-        data: IpcSharedMemory,
+        data: TruncatedDebug<IpcSharedMemory>,
     },
     DrawingBufferWidth(WebGLSender<i32>),
     DrawingBufferHeight(WebGLSender<i32>),

components/script/dom/webglrenderingcontext.rs

@@ -708,7 +708,7 @@ impl WebGLRenderingContext {
             alpha_treatment,
             y_axis_treatment,
             pixel_format: pixels.pixel_format,
-            data: pixels.data,
+            data: pixels.data.into(),
         });
 
         if let Some(fb) = self.bound_framebuffer.get() {
@@ -783,7 +783,7 @@ impl WebGLRenderingContext {
             alpha_treatment,
             y_axis_treatment,
             pixel_format: pixels.pixel_format,
-            data: pixels.data,
+            data: pixels.data.into(),
         });
     }