Expected response.statusCode == 500, but got HttpException: Connection closed

[nielsenko]

The following code simulates an error while processing a HttpRequest server side.

import 'dart:convert';
import 'dart:io';
import 'dart:isolate';

extension on HttpResponse {
  Future<void> flushAndClose() async {
    await flush();
    await close();
  }
}

Future<void> main(List<String> arguments) async {
  const port = 8080;
  await Isolate.spawn((port) async {
    final server = await HttpServer.bind(InternetAddress.loopbackIPv6, port);
    print('[Server] Started on port $port');
    await for (var request in server) {
      print('[Server] Request received: $request');

      final response = request.response;
      response.headers.contentType = ContentType(
        'text',
        'plain',
        charset: 'utf-8',
      );

      try {
        // INNER TRY BLOCK
        await for (var data in request) {
          print('[Server] Data received: $data');
          // throw without draining the request data stream
          throw Exception('This is an internal server error');
          request.response.add(data);
        }
        await request.response.flushAndClose();
      } catch (e) {
        print('Error: $e');
        response.statusCode = HttpStatus.internalServerError;
        response.write('Internal server error: $e');
        await request.response.flushAndClose();
      }
    }
  }, port);

  final request = await HttpClient().post('::1', port, '/');
  request.headers.contentType = ContentType('text', 'plain', charset: 'utf-8');
  request.add(utf8.encode('Hello, world!'));

  final response = await request.close();
  print('[Client] Response: ${await response.transform(utf8.decoder).join()}');
  print('[Client] Response status: ${response.statusCode}');

  exit(response.statusCode == HttpStatus.ok ? 0 : response.statusCode);
}

However this will not report a 500 on the client as expected, instead we get connection closed:

[Server] Started on port 8080
[Server] Request received: Instance of '_HttpRequest'
[Server] Data received: [72, 101, 108, 108, 111, 44, 32, 119, 111, 114, 108, 100, 33]
Error: Exception: This is an internal server error
Unhandled exception:
HttpException: Connection closed while receiving data, uri = http://[::1]:8080/
#0      _HttpIncoming.listen.<anonymous closure> (dart:_http/http_impl.dart:433:11)
#1      Stream.handleError.<anonymous closure> (dart:async/stream.dart:959:16)
#2      _HandleErrorStream._handleError (dart:async/stream_pipe.dart:303:17)
#3      _ForwardingStreamSubscription._handleError (dart:async/stream_pipe.dart:188:13)
#4      _RootZone.runBinaryGuarded (dart:async/zone.dart:1790:10)
...

I found a workaround, which is to ensure that the request stream is drained before raising the exception,
ie. replace the INNER TRY BLOCK with:

        var error = false;
        await for (var data in request) {
          print('[Server] Data received: $data');
          error = true;
          if (error) continue; // always drain request stream
          request.response.add(data);
        }
        if (error) throw Exception('This is an internal server error');
        await request.response.flushAndClose();

which will give the expected flow:

[Server] Started on port 8080
[Server] Request received: Instance of '_HttpRequest'
[Server] Data received: [72, 101, 108, 108, 111, 44, 32, 119, 111, 114, 108, 100, 33]
Error: Exception: This is an internal server error
[Client] Response: Internal server error: Exception: This is an internal server error
[Client] Response status: 500

Exited (244).

But the code feels clumsy, and I don't like that I have to drain the request stream first. Why would we waste time traversing the full stream, when we have already decided to fault.

• The Dart version and tooling diagnostic info (dart info)

bin/dart info

If providing this information as part of reporting a bug, please review the information
below to ensure it only contains things you're comfortable posting publicly.

#### General info

- Dart 3.7.0 (stable) (Wed Feb 5 04:53:58 2025 -0800) on "macos_x64"
- on macos / Version 15.3.1 (Build 24D70)
- locale is en-DK

#### Process info

| Memory |  CPU | Elapsed time | Command line                                                                     |
| -----: | ---: | -----------: | -------------------------------------------------------------------------------- |
| 435 MB | 0.0% |     01:05:26 | dart language-server --protocol=lsp --client-id=VS-Code --client-version=3.106.0 |
| 507 MB | 0.0% |     03:01:58 | dart language-server --protocol=lsp --client-id=VS-Code --client-version=3.106.0 |
|  75 MB | 0.0% |     01:05:26 | dart tooling-daemon --machine                                                    |
|  27 MB | 0.0% |     03:01:58 | dart tooling-daemon --machine                                                    |
|  41 MB | 0.0% |     02:17:14 | serverpod_cli.dart-3.7.0-323.0.dev.snapshot language-server --stdio              |