opening TCP/IP connection to invalid IP blocks forever

[iliis]

While resource_manager.open_resource('TCPIP::127.0.0.1') immediately raises a ConnectionRefusedError, connecting to an IP that doesn't exist (e.g. doing resource_manager.open_resource('TCPIP::192.255.255.42') or something) will block forever.

This is an issue in https://github.com/hgrecco/pyvisa-py/blob/master/pyvisa-py/protocols/rpc.py#L339: socket.connect() blocks as the socket is in blocking mode. Adding for example socket.settimeout(5) immediately before this puts the socket into timeout mode and the connect() will timeout after five seconds.

I haven't checked, but as long as the socket is in blocking mode other operations might behave similarly and block forever (or timeout after a very long time).

I would suggest to at least set some default timeout. Pyvisa actually has a parameter for that in open_resource() called open_timeout (see http://pyvisa.readthedocs.io/en/stable/api/resourcemanager.html#pyvisa.highlevel.ResourceManager.open_resource).

See also https://docs.python.org/3/library/socket.html#socket-timeouts

[skrchnavy]

Different approach implemented in TCPIP INSTR and TPCIP SOCKET.

• TCPIP INSTR:

    def connect(self):
        logger.debug('RawTCPClient: connecting to socket at (%s, %s)', self.host, self.port)
        self.sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        self.sock.connect((self.host, self.port))

• TPCIP SOCKET:

        self.interface = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        self.interface.setblocking(0)

        try:
            self.interface.connect_ex((self.parsed.host_address, int(self.parsed.port)))
        except Exception as e:
            raise Exception("could not create socket: %s" % e)

None is correct, handling of connect timeout is not provided. In SOCKET mode, also there is no check, if connect was succesful.

[skrchnavy]

#115 implemented connect timeout in TCPIP SOCKET session.
@iliis Is same expected also in TCPIP INSTR session? (I don't have access to such device so can't test).

[iliis]

@skrchnavy Uhm, I'm not sure. What's the difference? I'm sending GPIB commands to the device via TCP, so its INSTR?
Does it actually matter? Connecting should never block indefinitely, so I would say, yes, the same behaviour is expected in both cases.
And you don't need a device to test the timeout-behaviour itself for a non-existing devce ;) If you tell me what to test I can do that too.

[skrchnavy]

@iliis :

There are 2 types of devices:

• TCPIP0::1.2.3.4::999::SOCKET
  Raw TCP/IP access to port 999 at the specified IP address.
• TCPIP::dev.company.com::INSTR
  A TCP/IP device using VXI-11 or LXI located at the specified address. This uses the default LAN Device Name of inst0.
  when TCPIP::1.2.3.4 is asked then it is equivalent to TCPIP::1.2.3.4::INSTR.

Device shall not block in case, when timeout != VI_TMO_INFINITE (http://zone.ni.com/reference/en-XX/help/370131S-01/ni-visa/vi_attr_tmo_value/).
But handling in both cases is different, SOCKET only initializes connection but INSTR is more complex protocol and it communicates during open resource phase.

Anyway I have INSTR changes in progress. Do you have access to a device for testing?