Support for half-open connections

[sigurdm]

A tcp-socket can be closed in one direction (the client can close one direction after it is done sending the request, and still listen for more response). We should support this in package:http2.

Currently this program:

import 'dart:io';
import 'dart:async';
import 'dart:isolate';
import 'package:http2/http2.dart';

client(port) async {
  Socket s = await Socket.connect('localhost', port);
  final ctc = ClientTransportConnection.viaSocket(s);
  final cts = ctc.makeRequest([Header.ascii("a", "b")]);
  
  cts.incomingMessages.listen(print);
  await Future.delayed(Duration(seconds: 2));
  print("closing client socket outgoing");
  s.close();
}

spawnClient(port) async {
  await Isolate.spawn(client, port);
}

main() async {
  ServerSocket ss = await ServerSocket.bind('localhost', 0);
  spawnClient(ss.port);
  Socket s = await ss.first;
  ServerTransportStream ts;
  print("Listening");
  new ServerTransportConnection.viaSocket(s).incomingStreams.listen((stream) {
    print("got stream");
    ts = stream;
  }, onDone: () {
    print("done");
    ts.sendHeaders([Header.ascii("foo", "bar")]);
  });
}

prints:

Listening
got stream
closing client socket
Unhandled exception:
Bad state: StreamSink is bound to a stream
#0      _StreamSinkImpl.close (dart:io/io_sink.dart:218:7)
dart-lang/http#1342      _Socket.close (dart:io/runtime/binsocket_patch.dart:1627:27)
dart-lang/http#1343      client (file:///usr/local/google/home/sigurdm/projects/blah/bin/http2sample.dart:14:5)
<asynchronous suspension>
dart-lang/http#1344      _startIsolate.<anonymous closure> (dart:isolate/runtime/libisolate_patch.dart:292:17)
dart-lang/http#1345      _RawReceivePortImpl._handleMessage (dart:isolate/runtime/libisolate_patch.dart:171:12)
done
Unhandled exception:
Bad state: Cannot add event after closing
#0      _StreamController.add (dart:async/stream_controller.dart:585:24)
dart-lang/http#1342      _StreamSinkWrapper.add (dart:async/stream_controller.dart:858:13)
dart-lang/http#1343      TransportStream.sendHeaders (package:http2/transport.dart:145:10)
dart-lang/http#1344      main.<anonymous closure> (file:///usr/local/google/home/sigurdm/projects/blah/bin/http2sample.dart:32:8)
dart-lang/http#1345      _RootZone.runGuarded (dart:async/zone.dart:1302:10)
dart-lang/http#1346      _BufferingStreamSubscription._sendDone.sendDone (dart:async/stream_impl.dart:389:13)
dart-lang/http#1347      _BufferingStreamSubscription._sendDone (dart:async/stream_impl.dart:399:15)