JS_FreeRuntime SIGABRT when Dart GC finalizes JsAsyncRuntime (non-empty gc_obj_list)

[wytzepiet]

Problem

When a JsAsyncRuntime Dart object is garbage collected (e.g. during Flutter hot restart), the Rust Drop calls QuickJS's JS_FreeRuntime, which hits this assertion:

Assertion failed: (list_empty(&rt->gc_obj_list)), function JS_FreeRuntime, file quickjs.c, line 2308.

This is a SIGABRT — it kills the process. It happens because QuickJS still has objects on its GC list (modules, bridge closures, built-in objects from JsBuiltinOptions.essential()) that weren't freed before the runtime was dropped.

Reproduction

1. Create a runtime, context, and engine with a bridge
2. Evaluate some JS (especially modules via evaluateModule)
3. Let the JsAsyncRuntime Dart object get garbage collected (e.g. via Flutter hot restart, or by dropping all references)

final runtime = await JsAsyncRuntime.withOptions(
  builtin: JsBuiltinOptions.essential(),
);
final context = await JsAsyncContext.from(runtime: runtime);
final engine = JsEngine(context: context);
await engine.init(bridge: (v) async => const JsResult.ok(JsValue.none()));
await engine.evaluateModule(
  module: JsModule.code(module: '/test', code: 'export default 1;'),
);
// Later: runtime goes out of scope → Dart GC → Rust Drop → JS_FreeRuntime → SIGABRT

Calling engine.dispose() first doesn't help — the assertion still fires when the runtime is eventually dropped.

Workaround

Bump the Arc ref count so the Rust Drop never runs:

import 'package:flutter_rust_bridge/src/misc/rust_opaque.dart';

void preventNativeDrop(Object obj) {
  if (obj is RustOpaque) {
    obj.frbInternalCstEncode(move: false); // increments Arc strong count
  }
}

// After creation:
preventNativeDrop(runtime);

This permanently leaks the QuickJS runtime (the Arc never reaches 0), but avoids the crash.

Suggested Fix

Before calling JS_FreeRuntime in the Rust Drop, run JS_RunGC and/or iterate rt->gc_obj_list to free remaining objects. Alternatively, skip the assertion and clean up gracefully — QuickJS's assert is unconditional (not behind #ifdef DEBUG).

[iota9star]

Please try version 2.2.0, I have made some adjustments to the APIs for controlling the lifecycle, which should resolve this issue.

[wytzepiet]

fjs 2.2.0 still trips this assertion. No hot restart needed — the crash happens on the first run as soon as the engine local is garbage collected after runApp() returns:

import 'package:fjs/fjs.dart';
import 'package:flutter/material.dart';

void main() async {
  WidgetsFlutterBinding.ensureInitialized();
  await LibFjs.init();

  final engine = await JsEngine.create();
  await engine.init(
    bridge: (_) async => const JsResult.ok(JsValue.none()),
  );
  await engine.eval(source: JsCode.code('1 + 1'));

  runApp(const MaterialApp(home: Scaffold(body: Center(child: Text('minimal repro')))));
}

Output:

Assertion failed: (list_empty(&rt->gc_obj_list)), function JS_FreeRuntime, file quickjs.c, line 2308.
Process 23929 stopped
* thread #21, name = 'DartWorker', stop reason = signal SIGABRT

The DartWorker thread name shows it's Dart's FFI finalizer running the native Drop, not anything user-triggered. close() is never called because garbage collection finalization and hot restart have no hook to run it.

It looks like the 2.2.0 lifecycle API fix only covers the explicit close() path; Drop still calls JS_FreeRuntime without draining the runtime first. Would it be possible to run JS_RunGC (or an object-free pass) inside Drop before JS_FreeRuntime, so objects left on gc_obj_list are cleaned up regardless of whether close() was called?

Thanks for the great package!