读取“watch”命令 output 时，readline 在 paramiko.Channel 上挂起

Question

I am testing this code to read the output of watch command. I suspect it has to do with how watch works, but I can't figure out what's wrong or how to work around it:

import paramiko

host = "micro"
# timeout = 2  # Succeeds
timeout = 3  # Hangs!
command = 'ls / && watch -n2 \'touch "f$(date).txt"\''

ssh_client = paramiko.SSHClient()
ssh_client.set_missing_host_key_policy(paramiko.AutoAddPolicy())
ssh_client.connect(host, password='', look_for_keys=False)

transport = ssh_client.get_transport()
channel = transport.open_session()
channel.get_pty()
channel.settimeout(timeout)
channel.set_combine_stderr(True)
stdout = channel.makefile()
channel.exec_command(command)

for line in stdout:  # Hangs here
    print(line.strip())

There are several similar issues, some of them quite old ( 1 , 2 , and probably others)

This does not happen with other commands that don't use watch either.

Does someone know what's special about this particular command and / or how to reliably set a timeout for the read operation?

(Tested on Python 3.4.2 and paramiko 1.15.1)

Edit 1 : I incorporated channel.set_combine_stderr(True) as suggested in this answer to a related question , but still didn't do the trick. However, watch does produce a lot of output, so perhaps the problem is exactly that. In fact, using this command removed the hanging:

command = 'ls / && watch -n2 \'touch "f$(date).txt"\' > /dev/null'

So, probably this question is almost a duplicate of Paramiko ssh die/hang with big output , but makes me wonder if there's really no way to use .readline() (called through __next__ in this case) and one has to resort to read with a fixed buffer size and assemble the lines manually.

Answer 1

This probably hangs because watch does not produce newlines. If one replaces

for line in stdout:
    print(line.strip())

with a busy loop with

stdout.readline(some_fixed_size)

it can be seen that the bytes never contain a newline character. Therefore, this is a very special case and is not related to other hangs reported in other issues and SO questions.