fix(prefs): when recovering from a bad value, try only the settings' own pages one by one (#3010)

crates/engine/src/settings/prefs/document.rs

@@ -76,7 +76,15 @@ impl Prefs {
             Err(_) => {
                 let mut base = serde_json::to_value(Prefs::default()).unwrap_or_default();
                 if let (Some(b), Some(o)) = (base.as_object_mut(), v.as_object()) {
+                    // Only the settings' own pages are tried one by one (each trial parses the
+                    // whole document): any other key is kept as it is, so a document of many
+                    // unknown keys costs one pass, not one parse per key.
+                    let pages: Vec<String> = b.keys().cloned().collect();
                     for (k, x) in o {
+                        if !pages.contains(k) {
+                            b.insert(k.clone(), x.clone());
+                            continue;
+                        }
                         let mut trial = b.clone();
                         trial.insert(k.clone(), x.clone());
                         if serde_json::from_value::<Prefs>(Value::Object(trial)).is_ok() {

crates/engine/src/tests/settings.rs

@@ -40,6 +40,22 @@ fn prefs_round_trip_through_json() {
     assert_eq!(Prefs::from_json(&Prefs::default().to_json()), Prefs::default());
 }
 
+/// A document with one bad page and many unknown keys recovers its good pages in one pass per
+/// page, not one full parse per key.
+#[test]
+fn prefs_recovery_tries_only_the_settings_pages() {
+    let mut doc = serde_json::Map::new();
+    doc.insert("general".into(), json!({"undoLevels": "not a number"}));
+    doc.insert("labels".into(), serde_json::to_value(Prefs::default()).unwrap()["labels"].clone());
+    for i in 0..20_000 {
+        doc.insert(format!("unknown{i}"), json!({"x": i}));
+    }
+    let t = std::time::Instant::now();
+    let p = Prefs::from_json(&serde_json::Value::Object(doc).to_string());
+    assert!(t.elapsed() < std::time::Duration::from_secs(10), "{:?}", t.elapsed());
+    assert_eq!(p.general, Prefs::default().general);
+}
+
 #[test]
 fn prefs_migrate_old_layout_and_keep_unknown_keys() {
     let old = r#"{