[issue13422] Subprocess: children hang due to open pipes

[Felix Steffenhagen]

New submission from Felix Steffenhagen <felix at sutus.com>:

subprocess.Popen.communicate() hangs for daemonized subprocesses that leave a pipe open. The python caller, invoking the daemon subprocess, runs as long as the daemon process. This happens on POSIX platforms with python 2.7 as well as 3.2. 

Please find attached a simple python daemon, which leaves stderr pipe open. It runs for 10 sec and then exits.

To reproduce the problem run the following start script:
##################
# startpydaemon.py
import subprocess

cmd = 'python pydaemon.py'
print("Starting daemon that runs for 10 sec")
daemon = subprocess.Popen(cmd, 
                          stdout=subprocess.PIPE,
                          stderr=subprocess.PIPE,
                          shell=True)
stdout, stderr = daemon.communicate()
print("Stdout:")
print(stdout)
print("Stderr:")
print(stderr)
##################

Expected Behavior:
------------------
Daemon starter should exit immediately after the daemon process is initialized.

Actual Behavior:
----------------
Daemon starter waits for daemon process to be finished due to left open error pipe. 

Workaround:
-----------
One workaround is to not capture output from the left-open pipe. This way.

----------
components: Library (Lib)
files: pydaemon.py
messages: 147820
nosy: fsteffenhagen
priority: normal
severity: normal
status: open
title: Subprocess: children hang due to open pipes
versions: Python 2.6, Python 3.2
Added file: http://bugs.python.org/file23719/pydaemon.py

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue13422>
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