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3441 :: net/Socket recv behavior

• erhlee_bird (orginal) [2017-12-29T01:02:16+01:00] view original

  I'm trying to set up a simple example for Socket programming and I've run into a confusing situation. I'm on Linux with 0.17.3 built from devel at some point today. Also tried on a different Linux machine with 0.17.2.

  I set up a simple server for my socket read example.

  
  #!/bin/sh
  
  socat TCP4-LISTEN:5001,fork EXEC:"echo hi"
  Run

  import net
  
  var sock = newSocket(AF_INET, SOCK_STREAM, IPPROTO_TCP)
  sock.connect("127.0.0.1", Port(5001))
  sock.send("Hello")
  echo("Received: ")
  echo(sock.recv(4000))
  Run

  The call to recv hangs.

  Watching with strace, the recvfrom syscall is called twice and hangs on the second call.

  This can be traced to net.nim and a call to waitFor in recv.

  I rewrote the example with asyncdispatch and got different behavior.

  import asyncdispatch, asyncnet, net
  
  proc test() {.async.} =
    var sock = newAsyncSocket(AF_INET, SOCK_STREAM, IPPROTO_TCP)
    await sock.connect("127.0.0.1", Port(5001))
    await sock.send("Hello")
    echo("Received: ")
    echo(await sock.recv(4000))
  
  waitFor test()
  Run

  A rough translation to Python behaves the same way as the async code snippet above and not the normal net-based code.

  import socket
  
  s = socket.socket(socket.AF_INET, socket.SOCK_STREAM, socket.IPPROTO_TCP)
  s.connect(("127.0.0.1", 5001))
  s.send(b"Hello!")
  print("Received: ")
  print(s.recv(4000))
  Run

  strace shows a very similar chain of syscalls without the repeated recvfrom.

  Am I doing something wrong with the original Socket code?

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