"InternalError: No batch result object ID" error when inspecting getters

[Reprevise]

I'm using VSCode, Dart 3.6.0 (Flutter 3.27.1) and sometimes when debugging I get InternalError: No batch result object ID. errors instead of the computed getter values. This is happening when making a Flutter web app.

[dart-github-bot]

Summary: Debugging Flutter web app in VS Code (Dart 3.6.0) yields InternalError: No batch result object ID when inspecting getter values. Inconsistently reproducible.

[biggs0125]

Hi @Reprevise, would you mind including a code snippet where you've run into this to help us reproduce the issue?

[Reprevise]

Hi @biggs0125. I can reproduce the problem in this code sample:

import 'package:flutter/material.dart';

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

class MainApp extends StatelessWidget {
  const MainApp({super.key});

  Iterable<int> _numbers() sync* {
    for (var i = 1; i <= 10; i++) {
      yield i;
    }
  }

  @override
  Widget build(BuildContext context) {
    final _ = _numbers();

    return MaterialApp(
      title: 'Sample App',
      home: const Column(
        children: [
          Text('Sample'),
        ],
      ),
    );
  }
}

Logs in VS Code's Debug Console:

ChromeProxyService: Failed to evaluate expression 'x.iterator': InternalError: No batch result object ID..
ChromeProxyService: Failed to evaluate expression 'x.length': InternalError: No batch result object ID..
ChromeProxyService: Failed to evaluate expression 'x.isEmpty': InternalError: No batch result object ID..
ChromeProxyService: Failed to evaluate expression 'x.isNotEmpty': InternalError: No batch result object ID..
ChromeProxyService: Failed to evaluate expression 'x.first': InternalError: No batch result object ID..
ChromeProxyService: Failed to evaluate expression 'x.last': InternalError: No batch result object ID..
ChromeProxyService: Failed to evaluate expression 'x.single': InternalError: No batch result object ID..
ChromeProxyService: Failed to evaluate expression 'x.hashCode': InternalError: No batch result object ID..
ChromeProxyService: Failed to evaluate expression 'x.runtimeType': InternalError: No batch result object ID..

[biggs0125]

Thanks for the snippet, this helped me reproduce this locally! For posterity I added a breakpoint after _numbers was called. Then in the variables panel expanding the fields on the resulting _SyncStarIterable reproduced the errors above.

I made some local changes to dwds (the debug service talking to the Chrome debugger API) and was able to get more information on these errors:
For efficiency, it batches requests to the Chrome debugger API into a single expression evaluation (by wrapping all the subexpressions in a JS array). So the expression it asks Chrome to evaluate is really something like [x.iterator, x.length, x.isEmpty,...]. The problem occurs when any of these subexpressions throws. This causes the whole batch of expressions to fail.

In the case of the example you provided, the failing expression is x.single. The iterable has more than one element so that throws.

There's really no way to know if an expression is going to throw without trying to evaluate it first. But we can add a retry mechanism that unbatches them if the batched version fails.

I've opened a issue for this in the dwds package: dart-lang/webdev#2551

I also have a fix I'll work on landing there but it may take some time for that change to roll into Flutter tools.

Workaround:
In the meantime I recommend using the "Watch" panel to evaluate specific getters you care about on variables that fail. This will avoid the batching issue.

[biggs0125]

Marking as closed and tracking fix in dart-lang/webdev#2551