Python子进程.Popen管道IO意外阻塞

Question

I am trying to use subprocess.Popen to control an ssh process and interact with it via pipes, like so:

p=subprocess.Popen(['ssh','-tt','LOGIN@HOSTNAME'], stdin=subprocess.PIPE,
                    stdout=subprocess.PIPE, stderr=subprocess.PIPE,
                    universal_newlines=True)

while True:
    (out_stdout,out_stderr)=p.communicate(timeout=10)
    if out_stderr:
        print(out_stderr)
    if not out_stdout:
        raise EOFError
    print(out_stdout)

This works fine without the '-tt' option to ssh. However the program I need to interact with on the remote side of the ssh breaks if there is no pseudo tty allocated, so I am forced to use it.

What this seems to do is that the p.communicate reads then block indefinitely (or until timeout), even if input is available.

I have rewritten this using lower level calls to io.read, select.select etc to avoid going through Popen.communicate. Select will actually return the file descriptor as ready, but a subsequent io.read to that file descriptor will also block. If I disable 'universal newlines' and set 'bufsize=0' in the Popen call it then works fine, but then I am forced to do binary/unicode conversion and line ending processing myself.

It's worth saying though, that disabling universal_newlines in the p.communicate version also blocks indefinitely, so its not just that.

Any advice on how I can get line buffered input working properly here without having to reimplement everything?

Answer 1

There are library alternatives to subprocess and the SSH binary that are better suited for such tasks.

parallel-ssh :

from pssh.pssh2_client import ParalellSSHClient
client = ParallelSSHClient(['HOSTNAME'], user='LOGIN')
output = client.run_command('echo', use_pty=True)

for host, host_output in output.items():
    for line in host_output.stdout:
        print(line)

Replace echo with the command you need to run, or leave as-is if no command is required. The libraries require that something is passed in as command even if the remote side executes something automatically.

See also documentation for the single host SSHClient of the same project.

Per documentation, line parsing and encoding are handled by the library which is also cross-platform.

There are others like paramiko and ssh2-python that are lower level and need more code for the equivalent above - see their respective home pages for examples.