Fetch client: Cancelling response subscriptions waits for next chunk #1820
Description
Consider an HTTP server that sends a line every 10 seconds:
import 'dart:convert';
import 'dart:io';
void main() async {
final server = (await HttpServer.bind('localhost', 0))
..listen((request) async {
await request.drain<void>();
request.response.headers.set('Access-Control-Allow-Origin', '*');
request.response.headers.set('Content-Type', 'text/plain');
request.response.bufferOutput = false;
request.response.add(utf8.encode('0\n'));
await request.response.addStream(
Stream.periodic(const Duration(seconds: 10), (i) => '${i + 1}\n')
.transform(utf8.encoder),
);
});
print('listening on http://localhost:${server.port}/');
}Now, let's say we have a client that inspects the first line and, after a second of deliberation, decides it's done with the response:
import 'dart:async';
import 'dart:convert';
import 'package:http/http.dart';
void main() async {
final client = Client();
final response =
await client.send(Request('GET', Uri.parse('http://localhost:<port>')));
print('has response');
late StreamSubscription<String> sub;
sub = response.stream
.transform(utf8.decoder)
.transform(const LineSplitter())
.listen((line) async {
print('has line: $line');
await Future<void>.delayed(const Duration(seconds: 1));
print('cancelling subscription');
final sw = Stopwatch()..start();
await sub.cancel();
sw.stop();
print('subscription cancelled: ${sw.elapsed}');
});
}
With the dart:io implementation, cancelling the stream subscription is fast:
has response
listening
has line: 0
cancelling subscription
subscription cancelled: 0:00:00.003603
With the fetch client however, cancelling the stream after a line waits for the next one:
has response
listening
has line: 0
cancelling subscription
subscription cancelled: 0:00:08.981000
The issue is that, because the client is implemented with async*, it can't react to cancellation requests while in this line:
http/pkgs/http/lib/src/browser_client.dart
Line 172 in ef05b37
I think the only solution is to rewrite that logic with stream controllers, and I'm happy to contribute a fix (unless I'm doing something wrong or there's a better solution). I know that I could use Abortable for that, but I think we should also be able to cancel responses like any other stream.