python communicate() hangs after process terminate()

Question

The below code tries to run multiple commands in parallel and each command has a timeout. If processing is not completed by timeout it is to be stopped (I am using terminate()).

The issue is after termination (returncode is set to -ve) the communicate() method hangs and when forced exit (Ctrl+C) then following error is displayed.

(stdout, stderr) = proc.communicate()
File "python3.7/subprocess.py", line 926, in communicate
stdout = self.stdout.read()

Code

procList = []
for app in appList:
    try:
        p = subprocess.Popen(app['command'], stdout=subprocess.PIPE, stderr=subprocess.STDOUT, shell=True)
        procList.append((app, p))
    except Exception as e:
        print(e)

start = time.time()
while len(procList):
    time.sleep(30)
    try:
        for app, proc in procList:
            if (time.time() - start > app['timeoutSec']):
                proc.terminate()
            if proc.poll() is not None and app['mailSent'] == 0:
                (stdout, stderr) = proc.communicate() #Hangs here is the process is terminated
                send_results_mail('Execution Completed or Terminated')
                app['mailSent'] = 1
    except subprocess.SubprocessError as e:
        print(e)
    procList = [(app, proc) for (app, proc) in procList if app['mailSent'] == 0]

Answer 1

EDIT: Here is a working example, using kill(), with a misbehaving child (that won't respond to terminate()) but I'm not sure of the nature of your child process. Hope this helps get closer to a solution for you!

Simple child program, Python 3:

import signal
import time

def sighandler(signal, _stack):
    print(f"Ignoring signal {signal}")

signal.signal(signal.SIGTERM, sighandler)
signal.signal(signal.SIGINT, sighandler)

secs = 10
print(f"Sleeping {secs} seconds...")
time.sleep(secs)
print("Exiting...")

Revised parent program, Python 3:

import subprocess
import time

start = time.time()
appList = [
   {'command': 'python ./child.py', 'timeoutSec': 2, 'mailSent': 0},
   {'command': 'python ./child.py', 'timeoutSec': 2, 'mailSent': 0},
   {'command': 'python ./child.py', 'timeoutSec': 2, 'mailSent': 0},
]

def logmsg(msg):
    elap = time.time()-start
    print(f"{elap:2.1f} secs: {msg}")

def send_results_mail(result):
    logmsg(f"Result: {result}")

procList = []
for app in appList:
    try:
        p = subprocess.Popen(app['command'], stdout=subprocess.PIPE, stderr=subprocess.STDOUT, shell=True)
        procList.append((app, p))
    except Exception as e:
        logmsg(e)
    logmsg(f"Launching child... {p.pid}")

start = time.time()
while len(procList):
    time.sleep(1)
    try:
        for app, proc in procList:
            if (time.time() - start > app['timeoutSec']):
                logmsg(f"Trying to terminate()...{proc.pid}")
                proc.terminate()
                proc.kill()
            if proc.poll() is not None and app['mailSent'] == 0:
                proc.kill()
                logmsg(f"Trying to communicate()...{proc.pid}")
                (stdout, stderr) = proc.communicate()
                send_results_mail('Execution Completed or Terminated')
                app['mailSent'] = 1
    except subprocess.SubprocessError as e:
        logmsg(e)
    procList = [(app, proc) for (app, proc) in procList if app['mailSent'] == 0]

Output:

$ python parent.py
0.0 secs: Launching child... 537567
0.0 secs: Launching child... 537568
0.0 secs: Launching child... 537569
2.0 secs: Trying to terminate()...537567
2.0 secs: Trying to terminate()...537568
2.0 secs: Trying to terminate()...537569
3.0 secs: Trying to terminate()...537567
3.0 secs: Trying to communicate()...537567
3.0 secs: Result: Execution Completed or Terminated
3.0 secs: Trying to terminate()...537568
3.0 secs: Trying to communicate()...537568
3.0 secs: Result: Execution Completed or Terminated
3.0 secs: Trying to terminate()...537569
3.0 secs: Trying to communicate()...537569
3.0 secs: Result: Execution Completed or Terminated

Answer 2

Thanks for all the suggestions and responses, but nothing solved the problem as for me the processes I was spanning were creating multiple levels of sub-processes.

So I had to terminate the processes recursively and for that I used the solution using psutil

Pls see the post below for details

https://stackoverflow.com/a/27034438/2393961

once all the child and grandchild processes are killed communicate() works fine.

Another, piece I learnt is even though htop can show you the tree structure of the process but each process is independent, killing a parent does not automatically kill its decedents. Thanks to my friend Noga for pointing that out