T10.6: drop RGBA payload from the wire when the hardware path publishes a surface

crates/ely_app/src/services/servo_live.rs

@@ -132,13 +132,14 @@ impl ServoLiveClient {
         // Sanity bound the byte count advertised by the sidecar
         // header so a buggy or hostile sidecar can't park us on
         // `read_exact` for an arbitrarily-sized buffer. The honest
-        // upper limit is `width * height * 4` (RGBA8); anything
-        // larger is a protocol violation and we fail the request
-        // instead of allocating against it.
+        // upper limit is `width * height * 4` (RGBA8); `0` is the
+        // explicit "hardware path active, sample the IOSurface
+        // instead" signal — anything else is a protocol violation.
         let pixel_byte_count = (report.width as u64)
             .saturating_mul(report.height as u64)
             .saturating_mul(4);
-        if (report.rgba_byte_count as u64) != pixel_byte_count {
+        let advertised = report.rgba_byte_count as u64;
+        if advertised != 0 && advertised != pixel_byte_count {
             return Err(ServoLiveError::FrameBudgetExceeded {
                 advertised: report.rgba_byte_count,
                 pixel_budget: pixel_byte_count,
@@ -147,14 +148,18 @@ impl ServoLiveClient {
             });
         }
 
-        // Raw frame bytes follow the JSON header on the same pipe.
-        // `read_exact` drains BufReader's buffer first (the line read
-        // never crosses the `\n` boundary) and then pulls the rest
-        // straight from the child's stdout — no fs::read, no temp file.
+        // Raw frame bytes follow the JSON header on the same pipe
+        // ONLY when the sidecar didn't drop the payload for the
+        // hardware path. `read_exact` drains BufReader's buffer first
+        // (the line read never crosses the `\n` boundary) and then
+        // pulls the rest straight from the child's stdout — no
+        // fs::read, no temp file.
         let mut rgba_bytes = vec![0u8; report.rgba_byte_count];
-        self.stdout
-            .read_exact(&mut rgba_bytes)
-            .map_err(ServoLiveError::FrameRead)?;
+        if report.rgba_byte_count > 0 {
+            self.stdout
+                .read_exact(&mut rgba_bytes)
+                .map_err(ServoLiveError::FrameRead)?;
+        }
 
         let mut frame = ServoLiveFrame::from_parts(report, rgba_bytes);
 

crates/ely_app/src/shell/gpui_harness_tests.rs

@@ -1085,16 +1085,24 @@ fn identical_live_frames_share_render_image_arc() {
     )
     .expect("second frame builds from identical bytes");
 
+    let first_image = first
+        .image
+        .as_ref()
+        .expect("software path always produces an Arc<RenderImage>");
+    let second_image = second
+        .image
+        .as_ref()
+        .expect("software path always produces an Arc<RenderImage>");
     assert!(
-        Arc::ptr_eq(&first.image, &second.image),
+        Arc::ptr_eq(first_image, second_image),
         "TDD red: two ServoLiveFrames with byte-identical RGBA produced \
          distinct Arc<RenderImage> instances (first={:p}, second={:p}). \
          WebSurfaceFrame::from_parts must dedup the upload against the \
          previous frame's bytes, or the rendering pipeline must switch \
          to a GPU-side source of truth (IOSurface) so per-frame host \
          allocations stop entirely.",
-        Arc::as_ptr(&first.image),
-        Arc::as_ptr(&second.image),
+        Arc::as_ptr(first_image),
+        Arc::as_ptr(second_image),
     );
 }
 

crates/ely_app/src/shell/web_surface_frame.rs

@@ -57,12 +57,15 @@ pub(super) struct WebSurfaceFrame {
     content_pixel_count: u64,
     #[cfg(all(test, feature = "live-site-smoke"))]
     sample_hash: u64,
-    pub(super) image: Arc<RenderImage>,
+    /// Software-path image. `None` whenever the sidecar took the
+    /// hardware shortcut and dropped the RGBA payload from the
+    /// wire — the IOSurface in `pixel_buffer` is the source of truth
+    /// for that frame.
+    pub(super) image: Option<Arc<RenderImage>>,
     /// Hardware-path companion: when present, the view samples the
     /// IOSurface through GPUI's Metal pipeline via `gpui::surface(...)`
     /// instead of uploading the RGBA bytes again. Always `None` on
-    /// the software path; the RGBA copy in `image` is the source of
-    /// truth in that case.
+    /// the software path; `image` is the source of truth there.
     #[cfg(target_os = "macos")]
     pub(super) pixel_buffer: Option<CVPixelBuffer>,
 }
@@ -100,8 +103,21 @@ impl WebSurfaceFrame {
     }
 
     fn from_parts(parts: WebSurfaceFrameParts) -> Result<Self, WebSurfaceError> {
-        let bytes_hash = rgba_hash(&parts.rgba_bytes);
-        let image = resolve_render_image(parts.width, parts.height, parts.rgba_bytes, bytes_hash)?;
+        // Hardware path frames arrive with `rgba_bytes` empty — the
+        // sidecar dropped the 8 MB payload from the wire and the
+        // receiver samples the IOSurface directly. In that case the
+        // RGBA hash + LAST_FRAME_IMAGE dedup are skipped entirely.
+        let image = if parts.rgba_bytes.is_empty() {
+            None
+        } else {
+            let bytes_hash = rgba_hash(&parts.rgba_bytes);
+            Some(resolve_render_image(
+                parts.width,
+                parts.height,
+                parts.rgba_bytes,
+                bytes_hash,
+            )?)
+        };
 
         Ok(Self {
             requested_url: parts.requested_url,

crates/ely_app/src/shell/web_surface_view.rs

@@ -18,9 +18,10 @@ pub(super) fn render_ready_web_surface(
     // IOSurface and we successfully imported it into a CVPixelBuffer.
     // GPUI's `surface(...)` hands the buffer to its Blade Metal
     // renderer, which samples the IOSurface directly — no
-    // RGBA→texture upload, no LAST_FRAME_IMAGE dedup needed. Falls
-    // back to the software RGBA image when the buffer is missing
-    // (software webview, import failure, non-macOS host).
+    // RGBA→texture upload, no LAST_FRAME_IMAGE dedup needed. The
+    // sidecar drops the RGBA payload from the wire whenever it
+    // populates `pixel_buffer`, so `image` is `None` on this branch
+    // and there's no software memcpy to fall back on.
     #[cfg(target_os = "macos")]
     if let Some(pixel_buffer) = frame.pixel_buffer.as_ref() {
         return render_web_surface(
@@ -29,11 +30,20 @@ pub(super) fn render_ready_web_surface(
             surface(pixel_buffer.clone()).size_full().object_fit(ObjectFit::Fill),
         );
     }
-    render_web_surface(
-        tab,
-        state_entity,
-        img(ImageSource::Render(frame.image.clone())).size_full().object_fit(ObjectFit::Fill),
-    )
+    // Software path: the sidecar streamed the RGBA payload and the
+    // receiver decoded it into an Arc<RenderImage>.
+    if let Some(image) = frame.image.as_ref() {
+        return render_web_surface(
+            tab,
+            state_entity,
+            img(ImageSource::Render(image.clone())).size_full().object_fit(ObjectFit::Fill),
+        );
+    }
+    // Both image variants empty: shouldn't happen because the sidecar
+    // only drops RGBA when it has a hardware surface to publish, but
+    // a blank canvas is the honest user-facing fallback if it ever
+    // does.
+    render_web_surface(tab, state_entity, div().size_full())
 }
 
 pub(super) fn render_loading_web_surface(

crates/ely_servo_host/src/bin/ely_servo_sidecar/live.rs

@@ -225,11 +225,24 @@ fn write_outcome(
     if let Some(summary) = pending_summary.take() {
         outcome.response.perf = Some(summary);
     }
+    // Hardware path: receiver samples the IOSurface directly through
+    // its CVPixelBuffer cache, so the raw RGBA payload is dead
+    // weight. Drop it from the wire (and zero the byte count in the
+    // header so the client knows nothing follows). At 1080p × 60 fps
+    // that's 8 MB × 60 = ~480 MB/s of pipe traffic eliminated.
+    let drop_rgba_payload = outcome.response.current_surface_id.is_some();
+    if drop_rgba_payload {
+        if let Some(report) = outcome.response.frame.as_mut() {
+            report.rgba_byte_count = 0;
+        }
+    }
     let write_started_at = Instant::now();
     serde_json::to_writer(&mut *stdout, &outcome.response)?;
     stdout.write_all(b"\n")?;
-    if let Some(frame) = outcome.frame.as_ref() {
-        stdout.write_all(frame.rgba_bytes())?;
+    if !drop_rgba_payload {
+        if let Some(frame) = outcome.frame.as_ref() {
+            stdout.write_all(frame.rgba_bytes())?;
+        }
     }
     stdout.flush()?;
     if frame_present {

crates/ely_servo_host/tests/live_perf_bench.rs

@@ -129,6 +129,10 @@ fn run_live_bench() -> Result<(), Box<dyn Error>> {
     print_summaries(&kind, frames, &outcome.summaries);
     print_surface_handles(&kind, &outcome.surface_handles);
     print_current_surface_summary(&kind, &outcome.current_surface_ids);
+    eprintln!(
+        "\n=== ELY_PERF_KIND={kind} rgba_bytes_received={} ===",
+        outcome.rgba_bytes_received
+    );
     assert!(
         !outcome.summaries.is_empty(),
         "expected at least one FramePerfSummary across {frames} frames"
@@ -154,6 +158,18 @@ fn run_live_bench() -> Result<(), Box<dyn Error>> {
             !outcome.current_surface_ids.is_empty(),
             "hardware path must report current_surface_id on every frame"
         );
+        // T10.6: once the receiver samples the IOSurface directly,
+        // the sidecar drops the RGBA payload. The initial navigate
+        // response may still carry bytes (no current_surface_id yet
+        // because surfman hasn't bound the painted surface), but the
+        // steady-state per-frame cost must be zero.
+        assert!(
+            outcome.rgba_bytes_received < (frames as u64) * 1_024,
+            "hardware path leaked {} RGBA bytes across {} frames \
+             (expected ~0 — the wire-drop optimisation regressed)",
+            outcome.rgba_bytes_received,
+            frames + 1,
+        );
     } else {
         assert!(
             outcome.surface_handles.is_empty(),
@@ -163,6 +179,15 @@ fn run_live_bench() -> Result<(), Box<dyn Error>> {
             outcome.current_surface_ids.is_empty(),
             "software path must never report current_surface_id"
         );
+        // Software path keeps streaming pixels — every frame must
+        // carry a full RGBA payload.
+        let viewport_bytes = (1024u64) * (768u64) * 4;
+        assert!(
+            outcome.rgba_bytes_received >= viewport_bytes,
+            "software path delivered only {} bytes — expected at least one full frame ({})",
+            outcome.rgba_bytes_received,
+            viewport_bytes,
+        );
     }
     Ok(())
 }
@@ -171,6 +196,7 @@ struct BenchOutcome {
     summaries: Vec<FramePerfSummary>,
     surface_handles: Vec<BenchSurfaceHandle>,
     current_surface_ids: Vec<u64>,
+    rgba_bytes_received: u64,
 }
 
 fn spawn_sidecar(kind: &str, profile_data_dir: &PathBuf) -> Result<Child, Box<dyn Error>> {
@@ -199,6 +225,7 @@ fn drive_bench(
     let mut summaries = Vec::new();
     let mut surface_handles = Vec::new();
     let mut current_surface_ids = Vec::new();
+    let mut rgba_bytes_received: u64 = 0;
 
     let navigate = build_ensure(tab, profile_id, url, 0, 0, false);
     write_request(stdin, &navigate)?;
@@ -206,6 +233,7 @@ fn drive_bench(
     record_summary(&response, kind, &mut summaries);
     record_surface_handle(&response, kind, &mut surface_handles);
     record_current_surface_id(&response, &mut current_surface_ids);
+    rgba_bytes_received += response.frame.as_ref().map_or(0, |f| f.rgba_byte_count as u64);
 
     let mut accumulated_scroll = 0;
     for frame_index in 0..frames {
@@ -224,10 +252,11 @@ fn drive_bench(
         record_summary(&response, kind, &mut summaries);
         record_surface_handle(&response, kind, &mut surface_handles);
         record_current_surface_id(&response, &mut current_surface_ids);
+        rgba_bytes_received += response.frame.as_ref().map_or(0, |f| f.rgba_byte_count as u64);
     }
     let _ = accumulated_scroll;
 
-    Ok(BenchOutcome { summaries, surface_handles, current_surface_ids })
+    Ok(BenchOutcome { summaries, surface_handles, current_surface_ids, rgba_bytes_received })
 }
 
 fn record_surface_handle(