Getting live output from asyncio subprocess

I'm trying to use Python asyncio subprocesses to start an interactive SSH session and automatically input the password. The actual use case doesn't matter but it helps illustrate my problem. This is my code:

    proc = await asyncio.create_subprocess_exec(
        'ssh', '[email protected]',
        stdout=asyncio.subprocess.PIPE,
        stderr=asyncio.subprocess.STDOUT,
        stdin=asyncio.subprocess.PIPE,
    )

    # This loop could be replaced by async for, I imagine
    while True:
        buf = await proc.stdout.read()
        if not buf:
            break
        print(f'stdout: { buf }')

I expected it to work something like asyncio streams, where I can create two tasks/subroutines/futures, one to listen to the StreamReader (in this case given by proc.stdout), the other to write to StreamWriter (proc.stdin).

However, it doesn't work as expected. The first few lines of output from the ssh command are printed directly to the terminal, until it gets to the password prompt (or host key prompt, as the case may be) and waits for manual input. I expected to be able to read the first few lines, check whether it was asking for password or the host prompt, and write to the StreamReader accordingly.

The only time it runs the line print(f'stdout: { buf }') is after I press enter, when it prints, obviously, that "stderr: b'Host key verification failed.\r\n'".

I also tried the recommended proc.communicate(), which isn't as neat as using StreamReader/Writer, but it has the same problem: Execution freezes while it waits for manual input.

How is this actually supposed to work? If it's not how I imagined, why not, and is there any way to achieve this without resorting to some sort of busy loop in a thread?

PS: I'm explaining using ssh just for clarity. I ended up using plink for what I wanted, but I want to understand how to do this with python to run arbitrary commands.

Solution

This isn't a problem specific to asyncio. The ssh process does not interact with the stdin and stdout streams, but rather accesses the TTY device directly, in order to ensure that password entry is properly secured.

You have three options to work around this:

Don't use ssh, but some other SSH client, one that doesn't expect to a TTY to control. For asyncio, you could use the asyncssh library. This library directly implements the SSH protocol and so doesn't require a separate process, and it accepts username and password credentials directly.
Provide a pseudo-tty for SSH to talk to, one your Python program controls. The pexpect library provides a high-level API that does this for you and can be used to fully control the ssh command.
Set up an alternative password prompter for ssh to use. The ssh program can let something else handle password entry if there is no TTY, via the SSH_ASKPASS environment variable. Most versions of ssh are quite picky about when they'll accept SSH_ASKPASS however, you need to set DISPLAY too, use the -n command-line switch for ssh and use the setsid command to run ssh in a new session, disconnected from any TTY.

I've previously described how to use SSH_ASKPASS with asyncio in an answer to a question about git and ssh.

The path of least resistance is to use pexpect, as it supports asyncio natively (any method that accepts async_=True can be used as a coroutine):

import pexpect

proc = pexpect.spawn('ssh [email protected]')
await child.expect('password:', timeout=120, async_=True) 
child.sendline(password_for_user)