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Skia render: an error late in the render surfaces only after unfinished segments drain their x264 lookahead (looks like a hang) #209
Description
What happens
When render_frame fails late in a full render (here: a frame without a root <svg>, so ParserError(NoRootNode)), the Skia pipeline stops, but the error is printed only after SegmentWriter is dropped. Dropping every unfinished segment flushes its encoder on the main thread, and with a slow x264 preset that drains the whole lookahead of each segment, one segment after another. Meanwhile one core sits at ~99 %, the progress output has stopped and nothing is printed, so it looks like a hang.
| Case (fframes 1.2.0, Skia Metal, M3 Pro) | Time to exit |
|---|---|
| Repro below, no bad frame | 12.5 s (success) |
Repro, BAD_FRAME=290 (of 300) |
22 s, then ParserError(NoRootNode) |
Repro, BAD_FRAME=30 or 150 |
< 1 s (fine) |
Real project (1080p, heavy SkSL pass, preset=slow, tune=grain), no bad frame |
28 s (success) |
| Same project, bad frame at 170 of 180 | 86 s, then ParserError(NoRootNode) |
A short range render containing the same frame (render 160..180) fails within seconds.
Repro
[dependencies]
fframes = { version = "=1.2.0", features = ["cli", "h264", "libav-agree-gpl"] }
fframes_skia_renderer = { version = "=1.2.0", features = ["metal"] }use fframes::{AudioMap, Color, Duration, EncoderOptions, FFramesContext, Frame, RenderOptions, Svgr, Video, cli};
use fframes_skia_renderer::{SkiaFFramesRenderer, SkiaPipelineConfig, metal::SkiaMetalCtx};
struct Repro { bad_frame: Option<usize> }
impl Video for Repro {
const FPS: usize = 30;
const WIDTH: usize = 1920;
const HEIGHT: usize = 1080;
const BACKGROUND_COLOR: Color = Color::BLACK;
fn duration(&self) -> Duration<'_> { Duration::Frames(300) }
fn audio(&self) -> AudioMap<'_> { AudioMap::none() }
fn render_frame<'a>(&'a self, frame: Frame, _ctx: &FFramesContext<'a, '_>) -> Svgr<'a> {
if self.bad_frame.is_some_and(|n| frame.index >= n) {
return Svgr::empty(); // no root <svg>
}
// busy, changing content so the encoder has real work
let dots: Vec<Svgr> = (0..1500usize).map(|i| {
let h = (i * 7919 + frame.index * 104_729) % 99_991;
let (x, y) = ((h * 31) % 1920, (h * 17) % 1080);
let fill = format!("#{:06x}", h.wrapping_mul(2_654_435_761) & 0xff_ffff);
fframes::svgr!(<rect x={x} y={y} width="14" height="14" fill={fill} />)
}).collect();
fframes::svgr!(<svg xmlns="http://www.w3.org/2000/svg" width="1920" height="1080">
<rect width="1920" height="1080" fill="#101820" />
{dots}
</svg>)
}
}
fn main() -> std::process::ExitCode {
let video = Repro { bad_frame: std::env::var("BAD_FRAME").ok().and_then(|v| v.parse().ok()) };
let options = RenderOptions {
video_encoder_options: EncoderOptions {
preferred_encoder: Some("libx264"),
codec_params: Some(&[("crf", "16"), ("preset", "slow")]),
..Default::default()
},
..Default::default()
};
let gpu = SkiaMetalCtx::new(1920, 1080).expect("GPU context");
let backend = SkiaFFramesRenderer::new_metal(&gpu, SkiaPipelineConfig::default()).expect("skia");
cli::new(&video, options).backend(backend).run()
}cargo run --release -- render -o ok.mp4 # 12.5 s
BAD_FRAME=290 cargo run --release -- render -o bad.mp4 # 22 s, then the errorWhere the time goes
sample of the stuck process (main thread, 2462/2462 samples):
fframes_skia_renderer::skia_pipeline::render
core::ptr::drop_in_place<fframes::renderer::segment_writer::SegmentWriter>
alloc::sync::Arc<T,A>::drop_slow
core::ptr::drop_in_place<UnsafeCell<Option<segment_writer::Segment>>>
libx264.165.dylib (encoding)
Encoder::drop (fframes/src/renderer/encoder.rs) calls avcodec_flush_buffers on the video encoder. For libx264 that drains every delayed frame. A segment that finished normally was already drained by flush_stream in SegmentWriter::submit_with, so the drop is cheap there. Only a segment that never finished, whose file is thrown away anyway, pays for the drain.
Possible fix
On the error path, discard open segments without draining them, for example a SegmentWriter::abort() that drops the open encoders with a flag that skips avcodec_flush_buffers and av_write_trailer and removes the temp files. Then skia_pipeline::render calls it before returning Err. I'm happy to send a PR if this direction is OK.
Related docs note
skills/fframes-video/references/api.md shows let Some(clip) = frame.get_synced_video_frame(..) else { return Svgr::empty() };. That's fine inside a Scene, but at the Video::render_frame level it produces exactly this NoRootNode error. A one-line hint ("at the video level return a root <svg>, e.g. a black rect") would save the next person the search.
Environment: fframes 1.2.0, fframes_skia_renderer 1.2.0 (Metal), macOS 26.6.2, Apple M3 Pro, rustc 1.91.1, Homebrew FFmpeg 9.0.1, libx264.165. On 1.0.0 I saw the same stop with 0 % CPU and no exit after 10 min; I did not re-check 1.0.0 after finding the cause.
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