LibJS: Avoid internal assertion accessing detached TA internal slots

This defers accessing TA internal slots until we know we have a valid,
attached TA. Our implementation has assertions that guard against this.

Libraries/LibJS/Runtime/AtomicsObject.cpp

@@ -290,12 +290,6 @@ static ThrowCompletionOr<Value> atomic_compare_exchange_impl(VM& vm, TypedArrayB
     // 1. Let byteIndexInBuffer be ? ValidateAtomicAccessOnIntegerTypedArray(typedArray, index).
     auto byte_index_in_buffer = TRY(validate_atomic_access_on_integer_typed_array(vm, typed_array, index));
 
-    // 2. Let buffer be typedArray.[[ViewedArrayBuffer]].
-    auto* buffer = typed_array.viewed_array_buffer();
-
-    // 3. Let block be buffer.[[ArrayBufferData]].
-    auto& block = buffer->buffer();
-
     Value expected;
     Value replacement;
 
@@ -319,6 +313,15 @@ static ThrowCompletionOr<Value> atomic_compare_exchange_impl(VM& vm, TypedArrayB
     // 6. Perform ? RevalidateAtomicAccess(typedArray, byteIndexInBuffer).
     TRY(revalidate_atomic_access(vm, typed_array, byte_index_in_buffer));
 
+    // NOTE: We defer steps 2 and 3 to ensure we have revalidated the TA before accessing these internal slots.
+    //       In our implementation, accessing [[ArrayBufferData]] on a detached buffer will fail assertions.
+
+    // 2. Let buffer be typedArray.[[ViewedArrayBuffer]].
+    auto* buffer = typed_array.viewed_array_buffer();
+
+    // 3. Let block be buffer.[[ArrayBufferData]].
+    auto& block = buffer->buffer();
+
     // 7. Let elementType be TypedArrayElementType(typedArray).
     // 8. Let elementSize be TypedArrayElementSize(typedArray).
 

Libraries/LibJS/Tests/builtins/Atomics/Atomics.compareExchange.js

@@ -21,6 +21,24 @@ test("error cases", () => {
         const array = new Int32Array(4);
         Atomics.compareExchange(array, 100, 0, 0);
     }).toThrow(RangeError);
+
+    expect(() => {
+        const array = new Int32Array(4);
+
+        function detachArrayWhileAccessingIndex(array) {
+            return {
+                valueOf() {
+                    detachArrayBuffer(array.buffer);
+                    return 0;
+                },
+            };
+        }
+
+        Atomics.compareExchange(array, detachArrayWhileAccessingIndex(array), 0, 0);
+    }).toThrowWithMessage(
+        TypeError,
+        "TypedArray contains a property which references a value at an index not contained within its buffer's bounds"
+    );
 });
 
 test("basic functionality (non-BigInt)", () => {