collect(event: dever.io.ReadEvent.Chunk(bytes), state: Bytes) (next: Bytes, stop: Bool) { next = dever.bytes.concat(state, bytes)
 stop = dever.bytes.length(next) >= 4 }
collect(event: dever.io.ReadEvent.Failed(message), state: Bytes) (next: Bytes, stop: Bool) { fail(dever.io.ReadEvent.Failed(message)) }

serve(listener: dever.system.Listener) () { socket = dever.net.accept(listener)
 dever.net.timeout(socket, 1000)
 bytes = reduceUntil(collect, dever.net.chunks(socket, 4), dever.bytes.fromText(""))
 dever.net.write(socket, bytes)
 dever.net.close(socket) }

echo() (answer: Bool) { listener = dever.net.listen("127.0.0.1", 0)
 pending = run(serve(listener))
 socket = dever.net.connectTimeout("127.0.0.1", dever.net.port(listener), 1000)
 dever.net.timeout(socket, 1000)
 dever.net.write(socket, dever.bytes.fromText("ping"))
 bytes = reduceUntil(collect, dever.net.chunks(socket, 4), dever.bytes.fromText(""))
 dever.net.close(socket)
 wait(pending)
 dever.net.closeListener(listener)
 answer = bytes == dever.bytes.fromText("ping") }

accepted(result: dever.net.ConnectResult.Connected(socket), count: Int) (next: Int, stop: Bool) { dever.net.close(socket)
 next = count + 1
 stop = true }
accepted(result: dever.net.ConnectResult.Failed(message), count: Int) (next: Int, stop: Bool) { fail(dever.net.ConnectResult.Failed(message)) }

connections() (answer: Bool) { listener = dever.net.listen("127.0.0.1", 0)
 port = dever.net.port(listener)
 values = dever.net.connections(listener)
 socket = dever.net.connectTimeout("127.0.0.1", port, 1000)
 count = reduceUntil(accepted, values, 0)
 remaining = reduceUntil(accepted, values, 0)
 still_open = dever.net.port(listener) == port
 dever.net.close(socket)
 dever.net.closeListener(listener)
 answer = count == 1 and remaining == 0 and still_open }

pair() (client: dever.system.Socket, server: dever.system.Socket) { listener = dever.net.listen("127.0.0.1", 0)
 client = dever.net.connectTimeout("127.0.0.1", dever.net.port(listener), 1000)
 server = dever.net.accept(listener)
 dever.net.closeListener(listener) }

reader(socket: dever.system.Socket, signal: Channel<Int>) () { send(signal, 1)
 value = dever.net.read(socket, 1) }

byteRead(value: dever.io.ReadState.Read(bytes)) (answer: Bool) { answer = bytes == dever.bytes.fromText("x") }
byteRead(value: dever.io.ReadState.End) (answer: Bool) { answer = false }

cancelVerify() (answer: Bool) { sockets = pair()
 dever.net.timeout(sockets.client, 1000)
 signal = channel(Int, 1)
 pending = run(reader(sockets.client, signal))
 started = receive(signal)
 stop(pending)
 dever.net.write(sockets.server, dever.bytes.fromText("x"))
 received = byteRead(dever.net.read(sockets.client, 1))
 dever.net.close(sockets.client)
 dever.net.close(sockets.server)
 answer = started == 1 and received }

writeFailed(value: dever.io.WriteResult.Done) (answer: Bool) recover("观察关闭别名") { answer = false }
writeFailed(value: dever.io.WriteResult.Failed(message)) (answer: Bool) recover("观察关闭别名") { answer = length(message) > 0 }

aliasClose() (answer: Bool) { sockets = pair()
 alias = sockets.client
 dever.net.close(alias)
 answer = writeFailed(result(dever.net.write(sockets.client, dever.bytes.fromText("x"))))
 dever.net.close(sockets.server) }

nullable() (socket: dever.system.Socket?) { socket = null }

invalid() () { socket = dever.net.connectTimeout("localhost", 80, 1000) }