CODEWITHSUNDEEP
pythonsubprocesspopen
MrGRafael
MrGRafael

Reputation: 55

Popen communicate is not working

I have a script that has been working properly for the past 3 months. The Server went down last Monday and since then my script stopped working. The script hangs at coords = p.communicate()[0].split().

Here's a part of the script:

class SelectByLatLon(GridSelector):
def __init__(self, from_lat, to_lat, from_lon, to_lon):
self.from_lat = from_lat
self.to_lat = to_lat
self.from_lon = from_lon
self.to_lon = to_lon

def get_selection(self, file):
p = subprocess.Popen(
        [
    os.path.join(module_root, 'bin/points_from_latlon.tcl'), 
    file, 
    str(self.from_lat), str(self.to_lat), str(self.from_lon), str(self.to_lon)
    ],
        stdout = subprocess.PIPE
    )
    coords = p.communicate()[0].split()
    return ZGridSelection(int(coords[0]), int(coords[1]), int(coords[2]), int(coords[3]))

When I run the script on another server everything works just fine. Can I use something else instead of p.communicate()[0].split() ?

Upvotes: 1

Views: 1531

Answers (1)

jfs
jfs

Reputation: 414079

You might have previously run your server without daemonization i.e., you had functional stdin, stdout, stderr streams. To fix, you could redirect the streams to DEVNULL for the subprocess:

import os
from subprocess import Popen, PIPE

DEVNULL = os.open(os.devnull, os.O_RDWR)
p = Popen(tcl_cmd, stdin=DEVNULL, stdout=PIPE, stderr=DEVNULL, close_fds=True)
os.close(DEVNULL)

.communicate() may wait for EOF on stdout even if tcl_cmd already exited: the tcl script might have spawned a child process that inherited the standard streams and outlived its parent.

If you know that you don't need any stdout after the tcl_cmd exits then you could kill the whole process tree when you detect that tcl_cmd is done.

You might need start_new_session=True analog to be able to kill the whole process tree:

import os
import signal
from threading import Timer

def kill_tree_on_exit(p):
    p.wait() # wait for tcl_cmd to exit
    os.killpg(p.pid, signal.SIGTERM)

t = Timer(0, kill_tree_on_exit, [p])
t.start()
coords = p.communicate()[0].split()
t.cancel()

See How to terminate a python subprocess launched with shell=True

Upvotes: 1

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