getObject: (-32603) Unexpected DWDS error for getObject: Unsupported operation: Only libraries, instances, classes, and scripts are supported for getObject

[Jordan-Nelson]

I am unable to view a getter in Dart's dev tools with the following app on web only. I am unsure if this is supposed to work, but I don't see why it shouldn't.

Steps to repro:

1. create a flutter app with the code below
2. run app on web
3. set break point on the print statement
4. view the local vars in dev tools

import 'package:flutter/material.dart';

class CustomHttpRequest {
  const CustomHttpRequest(this.bytes);
  final List<int> bytes;
  Stream<List<int>> get body => Stream.value(bytes);
}

void main() {
  runApp(const MyApp());
}

class MyApp extends StatefulWidget {
  const MyApp({super.key});
  @override
  State<MyApp> createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(),
        body: Center(
          child: Column(
            children: [
              FilledButton(
                onPressed: () {
                  const r = CustomHttpRequest([1, 2, 3]);
                  print('r body: ${r.body}');
                },
                child: const Text('request'),
              ),
            ],
          ),
        ),
      ),
    );
  }
}

[CoderDake]

Error thrown here:

webdev/dwds/lib/src/debugging/inspector.dart

Lines 416 to 417 in 9ada46f

	throw UnsupportedError('Only libraries, instances, classes, and scripts '
	'are supported for getObject');

We'll have to take a closer look into how we might be able to grab that information and pass it along.

[Lorenzobettega]

I encountered the same issue in my code when I attempted to cast() an Object List.
The debugger showed a generic error in the devTools for web.
my flutter doctor: [√] Chrome - develop for the web
I'm thinking that this is related to the old issue: #2297

[bkonyi]

@elliette seems to think this is an issue that has come up before. She'll share the context if she can find it, but otherwise we'll have to dig in to this to figure out what's going on.

[elliette]

This is the issue I was thinking of:

• Errors from the BatchedExpressionEvaluator on object inspection #2309
• (related) The targetId argument in evaluate might not be a library ID #2336

This seems like a separate issue though

[derekxu16]

Thank you for the reproduction instructions, @Jordan-Nelson. The problem here is that the ability to request an instance of a _ControllerStream is currently broken. Here's how I created a minimal reproduction of this problem using DWDS test cases:

1. I added final stream = Stream.value(1); after this line.

2. After this line, I added the following test case:

    test('for a stream', () async {
      final remote = await inspector.jsEvaluate(
        libraryVariableExpression('stream', compilationMode),
      );
      final instance = await inspector.instanceFor(remote);
      expect(instance!.kind, InstanceKind.kPlainInstance);
      final classRef = instance.classRef!;
      expect(classRef.name, '_ControllerStream<int>');
      expect(inspector.isDisplayableObject(instance), isTrue);
    });
   

3. After this line, I added the following test case:

    test('for a stream', () async {
      final remoteObject = await inspector.jsEvaluate(
        libraryVariableExpression('stream', compilationMode),
      );
      final ref = await inspector.instanceRefFor(remoteObject);
      expect(ref!.kind, InstanceKind.kPlainInstance);
      final classRef = ref.classRef!;
      expect(classRef.name, '_ControllerStream<int>');
      expect(
        classRef.id,
        'classes|dart:async|_ControllerStream<int>',
      );
      expect(inspector.isDisplayableObject(ref), isTrue);
    });
   

test/instances/instance_test.dart fails when run after making the changes above. Here's an excerpt from the logs:

[FINE] ClassMetadata: Could not create class metadata for {type: object, className: _ControllerStream.new, description: _ControllerStream.new, objectId: -56064532640991652.1.2}:
Unexpected error from chrome devtools:
text: Uncaught
exception:
  preview: null
  description: Error
    at Object.throw_ [as throw] (http://localhost:41141/packages/build_web_compilers/src/dev_compiler/dart_sdk.js:9456:11)
    at [_subscribe] (http://localhost:41141/packages/build_web_compilers/src/dev_compiler/dart_sdk.js:39649:21)
    at [_createSubscription] (http://localhost:41141/packages/build_web_compilers/src/dev_compiler/dart_sdk.js:36508:46)
    at _ControllerStream.new.listen (http://localhost:41141/packages/build_web_compilers/src/dev_compiler/dart_sdk.js:36474:53)
    at get length (http://localhost:41141/packages/build_web_compilers/src/dev_compiler/dart_sdk.js:23587:14)
    at Object._get (http://localhost:41141/packages/build_web_compilers/src/dev_compiler/dart_sdk.js:9356:18)
    at Object.getObjectMetadata (http://localhost:41141/packages/build_web_compilers/src/dev_compiler/dart_sdk.js:9190:23)
    at _ControllerStream.new.<anonymous> (<anonymous>:4:23)
  type: object
  value: null
attempted JS eval: `        function(arg) {
          const sdk = require('dart_sdk');
          const dart  = sdk.dart;
          return dart.getObjectMetadata(arg);
        }
      `

It looks some DDC changes will be needed to fix this. @Markzipan and @nshahan, can you take a look at this when you get the chance?

Edit: I originally didn't include step 3 in my reproduction instructions becuase I didn't think it was necessary, but it turns out that it is.

[nshahan]

@natebiggs
I don't think this is directly related to your async rewrites change but maybe your experience could help identify where this is failing and why?

[biggs0125]

I tried to repro this locally using the instructions @derekxu16 listed above but I was unable to. What version of Dart are you using when seeing this failure?

[derekxu16]

I just tried running it again and saw the same error.

❯ dart --version
Dart SDK version: 3.6.0-134.0.dev (dev) (Fri Aug 9 01:02:59 2024 -0700) on "linux_x64"

[biggs0125]

Chatted with @derekxu16 offline and we realized this failure is only reproducible if you also add the following to the test group 'instanceRef' in instance_common.dart:

test('for a stream', () async {
  final remoteObject = await inspector.jsEvaluate(
    libraryVariableExpression('stream', compilationMode),
  );
  final ref = await inspector.instanceRefFor(remoteObject);
  expect(ref!.kind, InstanceKind.kPlainInstance);
  final classRef = ref.classRef!;
  expect(classRef.name, '_ControllerStream<int>');
  expect(
    classRef.id,
    'classes|dart:async|_ControllerStream<int>',
  );
  expect(inspector.isDisplayableObject(ref), isTrue);
});

[biggs0125]

Okay I was able to figure out that the issue here is with this invocation of the length getter on the _ControllerStream object.

_ControllerStream only allows one subscription at a time and each call to length subscribes to the stream.

The issue here is that we are calling the length getter on every object being debugged. As outlined by the vm service we should only be invoking a length getter for specific types. I'm opening a CL in DDC to fix this.

[biggs0125]

Landing this fix: https://dart-review.googlesource.com/c/sdk/+/380220

This fix should get included in the next Dart release. Until then users will get errors if the same Stream object is queried multiple times.