Bugs item #1731717, was opened at 2007-06-05 17:19
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Category: Python Library
Group: Python 2.4
Status: Open
Resolution: None
Priority: 5
Private: No
Submitted By: dsagal (dsagal)
Assigned to: Nobody/Anonymous (nobody)
Summary: race condition in subprocess module
Initial Comment:
Python's subprocess module has a race condition: Popen() constructor has a call
to global "_cleanup()" function on whenever a Popen object gets created, and
that call causes a check for all pending Popen objects whether their subprocess
has exited - i.e. the poll() method is called for every active Popen object.
Now, if I create Popen object "foo" in one thread, and then a.wait(), and
meanwhile I create another Popen object "bar" in another thread, then a.poll()
might get called by _clean() right at the time when my first thread is running
a.wait(). But those are not synchronized - each calls os.waitpid() if
returncode is None, but the section which checks returncode and calls
os.waitpid() is not protected as a critical section should be.
The following code illustrates the problem, and is known to break reasonably
consistenly with Python2.4. Changes to subprocess in Python2.5 seems to address
a somewhat related problem, but not this one.
import sys, os, threading, subprocess, time
class X(threading.Thread):
def __init__(self, *args, **kwargs):
super(X, self).__init__(*args, **kwargs)
self.start()
def tt():
s = subprocess.Popen(("/bin/ls", "-a", "/tmp"), stdout=subprocess.PIPE,
universal_newlines=True)
# time.sleep(1)
s.communicate()[0]
for i in xrange(1000):
X(target = tt)
This typically gives a few dozen errors like these:
Exception in thread Thread-795:
Traceback (most recent call last):
File "/usr/lib/python2.4/threading.py", line 442, in __bootstrap
self.run()
File "/usr/lib/python2.4/threading.py", line 422, in run
self.__target(*self.__args, **self.__kwargs)
File "z.py", line 21, in tt
s.communicate()[0]
File "/usr/lib/python2.4/subprocess.py", line 1083, in communicate
self.wait()
File "/usr/lib/python2.4/subprocess.py", line 1007, in wait
pid, sts = os.waitpid(self.pid, 0)
OSError: [Errno 10] No child processes
Note that uncommenting time.sleep(1) fixes the problem. So does wrapping
subprocess.poll() and wait() with a lock. So does removing the call to global
_cleanup() in Popen constructor.
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