fix(indexeddb): preserve first primary keys in reverse unique cursors

Deduplicate the ascending index entries before reversing unique cursor traversal. Previously prevunique chose the last primary key in each duplicate index-key group. Preserve ordinary reverse traversal and cover multiple duplicated index keys in native cursor tests. Update the existing JavaScript cursor and collection regressions to assert the corrected primary-key choice.

moli-indexeddb/src/cursor.rs

@@ -38,12 +38,14 @@ pub fn apply_cursor_direction_by_key<T>(
     direction: CursorDirection,
     mut key_for: impl FnMut(&T) -> &Key,
 ) -> Vec<T> {
-    if direction.is_reverse() {
-        entries.reverse();
-    }
+    // Unique index cursors select the lowest primary key for each index key,
+    // including when the distinct index keys are visited in reverse order.
     if direction.is_unique() {
         entries.dedup_by(|right, left| key_for(left) == key_for(right));
     }
+    if direction.is_reverse() {
+        entries.reverse();
+    }
     entries
 }
 

moli-indexeddb/src/tests.rs

@@ -60,7 +60,7 @@ fn cursor_direction_helpers_reverse_and_deduplicate_by_key() {
     );
     assert_eq!(
         apply_cursor_direction_by_key(entries, CursorDirection::PrevUnique, |entry| &entry.0),
-        vec![(Key::from("b"), 3), (Key::from("a"), 2)]
+        vec![(Key::from("b"), 3), (Key::from("a"), 1)]
     );
 }
 
@@ -1661,3 +1661,25 @@ fn auto_increment_rejects_exhausted_safe_integer_range() {
         .expect_err("generate_key should fail once the safe integer range is exhausted");
     assert!(matches!(error, IndexedDbError::Constraint(_)));
 }
+
+#[test]
+fn reverse_unique_index_cursor_keeps_first_primary_key_for_each_index_key() {
+    let entries = vec![
+        (Key::from("a"), 1),
+        (Key::from("a"), 2),
+        (Key::from("b"), 3),
+        (Key::from("b"), 4),
+        (Key::from("c"), 5),
+        (Key::from("c"), 6),
+    ];
+    let actual =
+        apply_cursor_direction_by_key(entries, CursorDirection::PrevUnique, |entry| &entry.0);
+    assert_eq!(
+        actual,
+        vec![
+            (Key::from("c"), 5),
+            (Key::from("b"), 3),
+            (Key::from("a"), 1)
+        ]
+    );
+}

moli-renderer-v8/src/script_vm/tests/indexed_db.rs

@@ -4888,7 +4888,7 @@ fn indexed_db_cursors_support_prev_directions() {
         .eval_after_selected_page_tasks("String(globalThis.__indexedDbCursorPrevResult)")
         .expect("indexeddb cursor prev result should be readable");
 
-    assert_eq!(result, "c,b,a|tech:c,news:b");
+    assert_eq!(result, "c,b,a|tech:c,news:a");
 }
 
 #[test]

moli-wpt-compat/fixtures/wpt/ported/indexeddb/idb-collection-query-conversion-basic.html

@@ -121,7 +121,7 @@ promise_test(async () => {
   );
   assert_array_equals(
     await wait_request(index.getAllKeys({ direction: "prevunique" })),
-    [3, 2],
+    [3, 1],
     "index.getAllKeys() applies options prevunique direction"
   );