Indexeddb: Cache IDBCursor.key conversion to preserve object identity (#44161)

Testing: Covered by idbcursor-key.any.html test.
part of https://github.com/servo/servo/issues/40983

---------

Signed-off-by: Taym Haddadi <haddadi.taym@gmail.com>

components/script/dom/indexeddb/idbcursor.rs

@@ -56,6 +56,10 @@ pub(crate) struct IDBCursor {
     /// <https://www.w3.org/TR/IndexedDB-3/#cursor-key>
     #[no_trace]
     key: DomRefCell<Option<IndexedDBKeyType>>,
+    #[ignore_malloc_size_of = "mozjs"]
+    cached_key: DomRefCell<Option<Heap<JSVal>>>,
+    #[ignore_malloc_size_of = "mozjs"]
+    cached_primary_key: DomRefCell<Option<Heap<JSVal>>>,
     /// <https://www.w3.org/TR/IndexedDB-3/#cursor-value>
     #[ignore_malloc_size_of = "mozjs"]
     value: Heap<JSVal>,
@@ -89,6 +93,8 @@ impl IDBCursor {
             direction,
             position: DomRefCell::new(None),
             key: DomRefCell::new(None),
+            cached_key: DomRefCell::new(None),
+            cached_primary_key: DomRefCell::new(None),
             value: Heap::default(),
             got_value: Cell::new(got_value),
             object_store_position: DomRefCell::new(None),
@@ -124,15 +130,30 @@ impl IDBCursor {
     }
 
     fn set_position(&self, position: Option<IndexedDBKeyType>) {
+        let changed = *self.position.borrow() != position;
         *self.position.borrow_mut() = position;
+        if changed {
+            *self.cached_primary_key.borrow_mut() = None;
+        }
     }
 
     fn set_key(&self, key: Option<IndexedDBKeyType>) {
+        let key_changed = {
+            let current_key = self.key.borrow();
+            current_key.as_ref() != key.as_ref()
+        };
         *self.key.borrow_mut() = key;
+        if key_changed {
+            *self.cached_key.borrow_mut() = None;
+        }
     }
 
     fn set_object_store_position(&self, object_store_position: Option<IndexedDBKeyType>) {
+        let changed = *self.object_store_position.borrow() != object_store_position;
         *self.object_store_position.borrow_mut() = object_store_position;
+        if changed {
+            *self.cached_primary_key.borrow_mut() = None;
+        }
     }
 
     pub(crate) fn set_request(&self, request: &IDBRequest) {
@@ -172,18 +193,50 @@ impl IDBCursorMethods<crate::DomTypeHolder> for IDBCursor {
 
     /// <https://www.w3.org/TR/IndexedDB-3/#dom-idbcursor-key>
     fn Key(&self, cx: &mut JSContext, mut value: MutableHandleValue) {
+        // The key getter steps are to return the result of converting a key to a value with the cursor’s current key.
+        //
+        // NOTE: If key returns an object (e.g. a Date or Array), it returns the
+        // same object instance every time it is inspected, until the cursor’s key is changed.
+        // This means that if the object is modified, those modifications will be seen by
+        // anyone inspecting the value of the cursor. However modifying such an object does not
+        // modify the contents of the database.
+        if let Some(cached) = &*self.cached_key.borrow() {
+            value.set(cached.get());
+            return;
+        }
+
         match self.key.borrow().as_ref() {
-            Some(key) => key_type_to_jsval(cx, key, value),
+            Some(key) => key_type_to_jsval(cx, key, value.reborrow()),
             None => value.set(UndefinedValue()),
         }
+
+        *self.cached_key.borrow_mut() = Some(Heap::default());
+        self.cached_key.borrow().as_ref().unwrap().set(value.get());
     }
 
     /// <https://www.w3.org/TR/IndexedDB-3/#dom-idbcursor-primarykey>
     fn PrimaryKey(&self, cx: &mut JSContext, mut value: MutableHandleValue) {
+        // NOTE: If primaryKey returns an object (e.g. a Date or Array),
+        // it returns the same object instance every time it is inspected,
+        // until the cursor’s effective key is changed. This means that if the object is modified,
+        // those modifications will be seen by anyone inspecting the value of the cursor.
+        // However modifying such an object does not modify the contents of the database.
+        if let Some(cached) = &*self.cached_primary_key.borrow() {
+            value.set(cached.get());
+            return;
+        }
+
         match self.effective_key() {
-            Some(effective_key) => key_type_to_jsval(cx, &effective_key, value),
+            Some(effective_key) => key_type_to_jsval(cx, &effective_key, value.reborrow()),
             None => value.set(UndefinedValue()),
         }
+
+        *self.cached_primary_key.borrow_mut() = Some(Heap::default());
+        self.cached_primary_key
+            .borrow()
+            .as_ref()
+            .unwrap()
+            .set(value.get());
     }
 
     /// <https://w3c.github.io/IndexedDB/#dom-idbcursor-request>

tests/wpt/meta/IndexedDB/idbcursor-key.any.js.ini

@@ -1,15 +1,5 @@
 [idbcursor-key.any.sharedworker.html]
   expected: ERROR
 
-[idbcursor-key.any.worker.html]
-  [IDBCursor.key 2]
-    expected: FAIL
-
-
-[idbcursor-key.any.html]
-  [IDBCursor.key 2]
-    expected: FAIL
-
-
 [idbcursor-key.any.serviceworker.html]
   expected: ERROR