Preserve document and history when Fade cannot allocate its buffers. (#4593)

crates/engine/src/edit_menu_cmds.rs

@@ -151,14 +151,14 @@ fn surface_of(doc: &Document, id: LayerId, mask: bool) -> Option<&Surface> {
 
 /// Blend `after` back towards `before` over `area`: `before + opacity × (B(before, after) − before)`
 /// with blend mode `mode` (premultiplied, so alpha changes fade too).
-pub fn fade_surface(before: &Surface, after: &mut Surface, area: Rect, mode: BlendMode, opacity: f32) {
+pub fn fade_surface(before: &Surface, after: &mut Surface, area: Rect, mode: BlendMode, opacity: f32) -> Result<()> {
     let fmt = after.format();
     let n = fmt.channels();
     if area.is_empty() || n == 0 {
-        return;
+        return Ok(());
     }
-    let b = before.read_region(area);
-    let mut a = after.read_region(area);
+    let b = crate::allocation::read_region(before, area, "fading pixels")?;
+    let mut a = crate::allocation::read_region(after, area, "fading pixels")?;
     let k = opacity.clamp(0.0, 1.0);
     let lerp_premul = |pb: &[f32], pa: &[f32], out: &mut [f32], alpha: bool| {
         let m = out.len();
@@ -176,7 +176,7 @@ pub fn fade_surface(before: &Surface, after: &mut Surface, area: Rect, mode: Ble
         }
         out[m - 1] = ao;
     };
-    let mut tmp = vec![0.0f32; n];
+    let mut tmp = crate::allocation::filled(n, 0.0f32, "fading pixels")?;
     for (pa, pb) in a.chunks_exact_mut(n).zip(b.chunks_exact(n)) {
         if mode == BlendMode::Normal || n == 1 {
             lerp_premul(pb, pa, &mut tmp, fmt.alpha);
@@ -192,7 +192,8 @@ pub fn fade_surface(before: &Surface, after: &mut Surface, area: Rect, mode: Ble
             pa.copy_from_slice(&enc[..n]);
         }
     }
-    after.write_region(area, &a);
+    after.try_write_region(area, &a)?;
+    Ok(())
 }
 
 fn fade(s: &mut Session, p: &Value) -> Result<Value> {
@@ -212,7 +213,7 @@ fn fade(s: &mut Session, p: &Value) -> Result<Value> {
     s.edit(&label, |doc, _| {
         let l = doc.layer_mut(src.layer).ok_or(EngineError::NoLayer(src.layer))?;
         let surf = if src.mask { l.mask.as_mut().map(|m| &mut m.surface) } else { l.surface_mut() }.ok_or(EngineError::NoLayer(src.layer))?;
-        fade_surface(&before, surf, area, mode, opacity);
+        fade_surface(&before, surf, area, mode, opacity)?;
         surf.prune();
         Ok(())
     })?;

crates/engine/src/edit_menu_cmds/tests.rs

@@ -544,3 +544,23 @@ fn backward_change_find_moves_past_the_replaced_text() {
     assert_eq!((r["changed"]["start"].as_u64(), r["changed"]["end"].as_u64()), (Some(0), Some(3)), "{r}");
     assert_eq!((r["found"]["start"].as_u64(), r["found"]["end"].as_u64()), (Some(8), Some(11)), "{r}");
 }
+
+#[test]
+fn fade_allocation_failure_keeps_document_and_history() {
+    for depth in [8, 16, 32] {
+        let mut s = session(depth);
+        s.execute("edit.fill", json!({"color": "#000000"})).unwrap();
+        let before = s.active().unwrap().doc.clone();
+        let history = s.active().unwrap().history.entries();
+        crate::allocation::fail_next_for_test();
+        let result = s.execute("edit.fade", json!({"opacity": 25}));
+        let _ = crate::allocation::capacity::<u8>(0, "reset failpoint");
+        assert!(result.is_err());
+        assert_eq!(*s.active().unwrap().doc, *before);
+        assert_eq!(s.active().unwrap().history.entries(), history);
+        assert!(s.is_enabled("edit.fade"));
+        s.execute("edit.fade", json!({"opacity": 25})).unwrap();
+        assert!(s.undo());
+        assert_eq!(*s.active().unwrap().doc, *before);
+    }
+}