`co19/LanguageFeatures/FinalizationRegistry/ffi/Finalizer/Finalizer_A01_t04` failed on `vm-aot-win-release-x64`

[derekxu16]

stderr:
Unhandled exception:
Expect.equals(expected: <0>, actual: <1>) fails.
#0      _fail (file:///C:/b/s/w/ir/tests/co19/src/Utils/expect.dart:18)
#1      Expect.equals (file:///C:/b/s/w/ir/tests/co19/src/Utils/expect_common.dart:15)
#2      attachToFinalizer.<anonymous closure> (file:///C:/b/s/w/ir/tests/co19/src/LanguageFeatures/FinalizationRegistry/ffi/Finalizer/Finalizer_A01_t04.dart:40)
<asynchronous suspension>

https://ci.chromium.org/ui/p/dart/builders/ci.sandbox/vm-aot-win-release-x64/4440/overview

@dcharkes
@blaugold

[dcharkes]

Thanks @derekxu16!

It's failing on all AOT configurations: https://dart-current-results.web.app/#/filter=co19/LanguageFeatures/FinalizationRegistry/ffi/Finalizer/Finalizer_A01_t04&showAll

I'll take a look.

[dcharkes]

My hunch is that the optimizer in AOT is being smart here:

accessibleLink is not read in the async callback, nor in main. So, my guess is that the object is no longer alive after object = null.

Because Finalizer does not have require Finalizables in the same way that NativeFinalizer does, there are no guarantees that objects stay alive until a certain point. The optimizer is free to optimize as much as doesn't change the language semantics, and since the object is never accessed again, it may even already forget about object immediately after finalizer.attach(object, 123).

/// @description Checks that finalizer object can be created and its callback
/// will never run if attached object is accessible.

So this comment is wrong. Finalizables if they are accessible the callback will not be run. But for non-Finalizables the callback may run as soon as the object will never be accessed anymore. So for non-Finalizables a more precise description would be:

/// @description Checks that finalizer object can be created and its callback
/// will never run if attached object is accessed.

Possible fix, but this might it redundant with respect to other tests:

// Copyright (c) 2022, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

/// @assertion Finalizer<T> constructor (Null safety)
/// Finalizer<T>(void callback(T))
/// Creates a finalizer with the given finalization callback.
///
/// The callback is bound to the current zone when the [Finalizer] is created,
/// and will run in that zone when called.
///
/// @description Checks that finalizer object can be created and its callback
/// will never run if attached object is accessed.
/// @author iarkh@unipro.ru

import 'dart:async';
import '../../gc_utils_lib.dart';
import '../../../../Utils/expect.dart';

int called = 0;
Object? accessedLink;

final Finalizer finalizer = Finalizer((token) {
  Expect.equals(123, token);
  called++;
});

Completer<void> completer = Completer<void>();

@pragma('vm:never-inline')
attachToFinalizer() async {
  Object? object = Object();
  finalizer.attach(object, 123);
  await triggerGcWithDelay();
  accessedLink = object;
  object = null;
  Expect.equals(0, called);
  () async {
    await triggerGcWithDelay();
    Expect.equals(0, called);
    print(accessedLink)
    completer.complete();
  }();
}

main() async {
  await attachToFinalizer();
  await completer.future;
  accessedLink = null;
  // Previous triggerGc move some objects to old space. Do multiple GCs to
  // force all objects to old space.
  await triggerGcWithDelay(repeat: 3);
  Expect.equals(1, called);
  await triggerGcWithDelay();
  Expect.equals(1, called);
}

cc @sgrekhov

[blaugold]

Maybe this issue can be closed since dart-lang/co19#3042 has been merged.

[dcharkes]

Green on all configurations: https://dart-current-results.web.app/#/filter=co19/LanguageFeatures/FinalizationRegistry/ffi/Finalizer/Finalizer_A01_t04&showAll