nxos_ssh devices with Netmiko 3.1 fail on is_alive() call

[xschlef]

Description of Issue/Question

This is the same issue as #1128 but with nxos_ssh.
Downgrading to netmiko 2.4.2 fixed the issue, but I think the logic has to be fixed here as well.

Setup

napalm version

3.0

Network operating system version

nxos

Steps to Reproduce the Issue

Error Traceback

Traceback (most recent call last):
  File "/usr/local/lib/python3.7/dist-packages/paramiko/channel.py", line 699, in recv
    out = self.in_buffer.read(nbytes, self.timeout)
  File "/usr/local/lib/python3.7/dist-packages/paramiko/buffered_pipe.py", line 164, in read
    raise PipeTimeout()
paramiko.buffered_pipe.PipeTimeout

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/local/lib/python3.7/dist-packages/netmiko/base_connection.py", line 550, in _read_channel_expect
    new_data = self.remote_conn.recv(MAX_BUFFER)
  File "/usr/local/lib/python3.7/dist-packages/paramiko/channel.py", line 701, in recv
    raise socket.timeout()
socket.timeout

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
...
  File "/usr/local/lib/python3.7/dist-packages/napalm/nxos_ssh/nxos_ssh.py", line 509, in is_alive
    self._send_command(null)
  File "/usr/local/lib/python3.7/dist-packages/napalm/nxos_ssh/nxos_ssh.py", line 447, in _send_command
    return self.device.send_command(command)
  File "/usr/local/lib/python3.7/dist-packages/netmiko/utilities.py", line 347, in wrapper_decorator
    return func(self, *args, **kwargs)
  File "/usr/local/lib/python3.7/dist-packages/netmiko/base_connection.py", line 1389, in send_command
    new_data = self.read_until_pattern(pattern=re.escape(cmd))
  File "/usr/local/lib/python3.7/dist-packages/netmiko/base_connection.py", line 627, in read_until_pattern
    return self._read_channel_expect(*args, **kwargs)
  File "/usr/local/lib/python3.7/dist-packages/netmiko/base_connection.py", line 561, in _read_channel_expect
    "Timed-out reading channel, data not available."
netmiko.ssh_exception.NetmikoTimeoutException: Timed-out reading channel, data not available.

[ktbyers]

@xschlef Can you post your NAPALM code corresponding to the exception. Thanks.

The exception above happens in the is_alive() call.

[xschlef]

I have wrapped all calls which checks if the connection is still alive.

def connection_alive(f):
    """ decorator for all functions that require a working connection """

    @wraps(f)
    def wrapper(*args, **kwargs):
        if "hostname" not in kwargs:
            raise Exception("Supply a hostname.")
        if kwargs['hostname'] not in CONNECTIONS:
            raise Exception(
                "There is no existing connection to %s." % kwargs['hostname'])
        if not CONNECTIONS[kwargs['hostname']].is_alive():
            raise Exception("No alive connection to %s." % kwargs['hostname'])

        return f(*args, **kwargs)

    return wrapper

[ktbyers]

Proposed fix:

#1214