Python Subprocess readline() hangs; can't use normal options

user_2308300 0
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To start, I'm aware this looks like a duplicate. I've been reading:

Python subprocess readlines() hangs

Python Subprocess readline hangs() after reading all input

subprocess readline hangs waiting for EOF

But these options either straight don't work or I can't use them.

The Problem

# Obviously, swap HOSTNAME1 and HOSTNAME2 with something real
cmd = "ssh -N -f -L 1111:<HOSTNAME1>:80 <HOSTNAME2>"

p = subprocess.Popen(cmd, shell=True, stdout=subprocess.PIPE, stderr=subprocess.STDOUT, env=os.environ)
while True:
    out = p.stdout.readline()
    # Hangs here ^^^^^^^ forever

    out = out.decode('utf-8')
    if out:
        print(out)
    if p.poll() is not None:
        break

My dilemma is that the function calling the subprocess.Popen() is a library function for running bash commands, so it needs to be very generic and has the following restrictions:

  • Must display output as it comes in; not block and then spam the screen all at once
  • Can't use multiprocessing in case the parent caller is multiprocessing the library function (Python doesn't allow child processes to have child processes)
  • Can't use signal.SIGALRM for the same reason as multiprocessing; the parent caller may be trying to set their own timeout
  • Can't use third party non-built-in modules
  • Threading straight up doesn't work. When the readline() call is in a thread, thread.join(timeout=1)lets the program continue, but ctrl+c doesn't work on it at all, and calling sys.exit() doesn't exit the program, since the thread is still open. And as you know, you can't kill a thread in python by design.
  • No manner of bufsize or other subprocess args seems to make a difference; neither does putting readline() in an iterator.

I would have a workable solution if I could kill a thread, but that's super taboo, even though this is definitely a legitimate use case.

I'm open to any ideas.

subprocess multithreading python
2 Answers

One option is to use a thread to publish to a queue. Then you can block on the queue with a timeout. You can make the reader thread a daemon so it won't prevent system exit. Here's a sketch:

import subprocess
from threading import Thread
from queue import Queue

def reader(stream, queue):
    while True:
        line = stream.readline()
        queue.put(line)
        if not line:
            break

p = subprocess.Popen(cmd, stdout=subprocess.PIPE, ...)
queue = Queue()
thread = Thread(target=reader, args=(p.stdout, queue))
thread.daemon = True
thread.start()
while True:
    out = queue.get(timeout=1)  # timeout is optional
    if not out:  # Reached end of stream
        break
    ...  # Do whatever with output

# Output stream was closed but process may still be running
p.wait()

Note that you should adapt this answer to your particular use case. For example, you may want to add a way to signal to the reader thread to stop running before reaching the end of stream.

Another option would be to poll the input stream, like in this question: timeout on subprocess readline in python

user_25724310

I finally got a working solution; the key piece of information I was missing was thread.daemon = True, which @augurar pointed out in their answer.

Setting thread.daemon = True allows the thread to be terminated when the main process terminates; therefore unblocking my use of a sub-thread to monitor readline().

Here is a sample implementation of my solution; I used a Queue() object to pass strings to the main process, and I implemented a 3 second timer for cases like the original problem I was trying to solve where the subprocess has finished and terminated, but the readline() is hung for some reason.

This also helps avoid a race condition between which thing finishes first.

This works for both Python 2 and 3.

import sys
import threading
import subprocess
from datetime import datetime

try:
    import queue
except:
    import Queue as queue # Python 2 compatibility


def _monitor_readline(process, q):
    while True:
        bail = True
        if process.poll() is None:
            bail = False
        out = ""
        if sys.version_info[0] >= 3:
            out = process.stdout.readline().decode('utf-8')
        else:
            out = process.stdout.readline()
        q.put(out)
        if q.empty() and bail:
            break

def bash(cmd):
    # Kick off the command
    process = subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.STDOUT, shell=True)

    # Create the queue instance
    q = queue.Queue()
    # Kick off the monitoring thread
    thread = threading.Thread(target=_monitor_readline, args=(process, q))
    thread.daemon = True
    thread.start()
    start = datetime.now()
    while True:
        bail = True
        if process.poll() is None:
            bail = False
            # Re-set the thread timer
            start = datetime.now()
        out = ""
        while not q.empty():
            out += q.get()
        if out:
            print(out)

        # In the case where the thread is still alive and reading, and
        # the process has exited and finished, give it up to 3 seconds
        # to finish reading
        if bail and thread.is_alive() and (datetime.now() - start).total_seconds() < 3:
            bail = False
        if bail:
            break

# To demonstrate output in realtime, sleep is called in between these echos
bash("echo lol;sleep 2;echo bbq")
user_23083000
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