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3549 :: Socket file descriptor leaks?

• Mahlon (orginal) [2018-02-14T22:08:10+01:00] view original

  Hello nimmers!

  Long time lurker, first time poster.

  I had a random problem at $dayjob that I decided would be a fun and easy fit to use nim to solve.

  There's a daemon that streams line data over a TCP socket at an interval. There are no delimiters in the stream, and it's an unknown amount of data per interval. After putting something together fairly quickly, I hit an exception after a few hours:

  
  Error: unhandled exception: Too many open files [OSError]
  Run

  Sure enough, I appear to be leaking closed sockets with each connection.

  
  test    57537 root    4u  IPv4 0xfffff80024e4b410      0t0     TCP *:* (CLOSED)
  test    57537 root    5u  IPv4 0xfffff8001a1e7000      0t0     TCP *:* (CLOSED)
  test    57537 root    6u  IPv4 0xfffff8001ad2a410      0t0     TCP *:* (CLOSED)
  test    57537 root    7u  IPv4 0xfffff800142bd820      0t0     TCP *:* (CLOSED)
  test    57537 root    8u  IPv4 0xfffff80003e07410      0t0     TCP *:* (CLOSED)
  test    57537 root    9u  IPv4 0xfffff80003e07000      0t0     TCP *:* (CLOSED)
  .... (etc etc etc)
  Run

  I've gutted everything down to the simplest test code that reproduces the problem.

  import net
  
  var server = newSocket()
  server.set_sock_opt( OptReuseAddr, true )
  server.bind_addr( Port(14865) )
  server.listen()
  
  
  while true:
      var
          client  = newSocket()
          address = ""
          result  = ""
      
      server.acceptAddr( client, address ) # block for a client
      
      try:
          result = client.recv_line( timeout=500 ) & "\n"
          while result != "":
              result = result & client.recv_line( timeout=500 ) & "\n"
      except TimeoutError:
          echo "Done receiving data from " & address
      finally:
          client.close
      
      echo address & " said: " & result
  Run

  Am I misunderstanding something about close()? (I thought that freed the FD to the kernel?) Something else naive I'm missing (some sockopt)?

  Help/ideas/cluebats all equally appreciated. This is Nim 0.17.2, same behavior on both Linux and FreeBSD.

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• dom96 (orginal) [2018-02-14T22:23:13+01:00] view original

  You're creating a new socket FD for the client and acceptAddr then overwrites it with another FD without closing the socket you created.

  I've seen this gotcha happen to at least two people now. Please report it as an issue on github :)

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• Mahlon (orginal) [2018-02-14T22:37:04+01:00] view original

  Bingo! Thanks for the quick reply.

  I was using the example at https://nim-lang.org/docs/net.html, so maybe this is just a docs update ticket.

  I can't simply declare the var type in advance without an assignment:

  
  var client: Socket
  server.acceptAddr( client, address )
  ...
  
  Traceback (most recent call last)
  test.nim(21)             test
  net.nim(867)             close
  SIGSEGV: Illegal storage access. (Attempt to read from nil?)
  Run

  However, I can create the socket and immediately close it.

  
  var client = newSocket()
  client.close
  server.acceptAddr( client, address )
  Run

  ... which works, without leaking, though definitely isn't an obvious path. Fixed.

  What is the "right" way do do this?

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• Mahlon (orginal) [2018-02-14T22:56:03+01:00] view original

  Ah, looking a little closer: The example has the "client" variable defined once outside of the loop, not within it. Whoops.

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• dom96 (orginal) [2018-02-16T23:56:49+01:00] view original

  This example is actually wrong. You should just use new instead of newSocket, this is described in the docs for accept/acceptAddr (https://nim-lang.org/docs/net.html#acceptAddr,Socket,Socket,string).

  Edit: I created an issue to fix this gotcha btw: https://github.com/nim-lang/Nim/issues/7227

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