Bad state: Header already sent when upgrading to a Websocket connection

[nicobritos]

When using this server (v1.0.0) with socket.io-client-dart (v1.0.1) an exception is thrown when making a connection. This happens because the socket is upgraded to a Websocket socket via its transport and the payload is sent, but the polling transport tries to send the packet again.

Stacktrace:

SHOUT: 2021-09-02 16:22:59.467450:Bad state: Header already sent
#0      _HttpResponse.statusCode= (dart:_http/http_impl.dart:1188:35)
#1      PollingTransport.doWrite.<anonymous closure> (package:socket_io/src/engine/transport/polling_transport.dart:270:13)
#2      PollingTransport.doWrite (package:socket_io/src/engine/transport/polling_transport.dart:306:14)
#3      PollingTransport.write (package:socket_io/src/engine/transport/polling_transport.dart:245:5)
#4      PollingTransport.send.<anonymous closure> (package:socket_io/src/engine/transport/polling_transport.dart:234:12)
#5      PacketParser.encodePayload.<anonymous closure> (package:socket_io_common/src/engine/parser/parser.dart:212:22)
#6      PacketParser.map.<anonymous closure> (package:socket_io_common/src/engine/parser/parser.dart:229:25)
#7      StackZoneSpecification._registerUnaryCallback.<anonymous closure>.<anonymous closure> (package:stack_trace/src/stack_zone_specification.dart:126:26)