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show: keep the document a slide backdrop was rendered from

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The backdrop cache knew its document by address only: after an edit
during the show, a new copy could reuse the freed address and get the
old backdrop. The cache now holds that document and matches on it.
(CodeRabbit review on #1161.)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
This commit is contained in:
Keresztes Barna
2026-10-22 17:59:53 +02:00
co-authored by Claude Opus 5.5
parent 31a828b027
commit df0fab857d
1 changed files with 14 additions and 7 deletions
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crates/ui-egui/src/show.rs
+14 -7
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@@ -108,6 +108,9 @@ struct BackKey {
/// than the shapes: a full-slide picture with a glow in the layout made animations crawl at 2–3
/// frames a second. `rx` brings it from another thread when it is rendered ahead of time.
struct Backdrop {
/// The presentation it was rendered from, held so its address (in `key`) can't be reused by
/// a later edit while the backdrop is kept.
doc: std::sync::Arc<Presentation>,
key: BackKey,
img: Option<std::sync::Arc<deckcraft_render::Image>>,
rx: Option<std::sync::mpsc::Receiver<deckcraft_render::Image>>,
@@ -281,8 +284,8 @@ impl Show {
/// The backdrop for `key`: kept from earlier, waited for if it is being rendered ahead of
/// time, or rendered now.
fn backdrop(&mut self, doc: &Presentation, key: BackKey) -> std::sync::Arc<deckcraft_render::Image> {
if let Some(b) = self.backdrop.as_mut().filter(|b| b.key == key) {
fn backdrop(&mut self, doc: &std::sync::Arc<Presentation>, key: BackKey) -> std::sync::Arc<deckcraft_render::Image> {
if let Some(b) = self.backdrop.as_mut().filter(|b| b.key == key && std::sync::Arc::ptr_eq(&b.doc, doc)) {
if b.img.is_none() {
b.img = b.rx.take().and_then(|rx| rx.recv().ok()).map(std::sync::Arc::new);
}
@@ -291,23 +294,23 @@ impl Show {
}
}
let img = std::sync::Arc::new(render_backdrop(doc, key, render_threads()));
self.backdrop = Some(Backdrop { key, img: Some(img.clone()), rx: None });
self.backdrop = Some(Backdrop { doc: doc.clone(), key, img: Some(img.clone()), rx: None });
img
}
/// Start rendering the backdrop for `key` on another thread, unless it is already there or
/// on its way. Nothing on the web (no threads): the first animation frame renders it.
fn prefetch_backdrop(&mut self, doc: &std::sync::Arc<Presentation>, key: BackKey) {
if cfg!(target_arch = "wasm32") || self.backdrop.as_ref().is_some_and(|b| b.key == key) {
if cfg!(target_arch = "wasm32") || self.backdrop.as_ref().is_some_and(|b| b.key == key && std::sync::Arc::ptr_eq(&b.doc, doc)) {
return;
}
let (tx, rx) = std::sync::mpsc::channel();
let doc = doc.clone();
let worker_doc = doc.clone();
let threads = render_threads().saturating_sub(1).max(1);
let spawned = std::thread::Builder::new().name("show-backdrop".into()).spawn(move || {
let _ = tx.send(render_backdrop(&doc, key, threads));
let _ = tx.send(render_backdrop(&worker_doc, key, threads));
});
self.backdrop = spawned.ok().map(|_| Backdrop { key, img: None, rx: Some(rx) });
self.backdrop = spawned.ok().map(|_| Backdrop { doc: doc.clone(), key, img: None, rx: Some(rx) });
}
fn timeline(&mut self, doc: &Presentation) -> &Timeline {
@@ -2141,6 +2144,10 @@ mod tests {
assert!(worst.is_some_and(|d| d <= 2), "frames differ by up to {worst:?}");
// The backdrop is kept, not rendered again.
assert!(std::sync::Arc::ptr_eq(&back, &show.backdrop(&doc, key)));
// Another document under the same key (an edit whose copy reused the old address) gets
// its own backdrop.
let edited = std::sync::Arc::new((*doc).clone());
assert!(!std::sync::Arc::ptr_eq(&back, &show.backdrop(&edited, key)));
}
#[test]