race condition in subprocess module

[dsagal]

BPO	1731717
Nosy	@dbaarda, @gpshead, @pitrou, @vstinner, @giampaolo, @tiran, @matejcik, @benjaminp, @djc, @bitdancer, @jonashaag, @florentx, @kmoza
Files	mylib.c main.c exim_local_scan2.py

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = 'https://github.com/gpshead'
closed_at = <Date 2010-12-14.15:17:05.636>
created_at = <Date 2007-06-05.22:19:54.000>
labels = ['type-bug', 'library']
title = 'race condition in subprocess module'
updated_at = <Date 2016-02-18.13:05:37.382>
user = 'https://bugs.python.org/dsagal'

bugs.python.org fields:

activity = <Date 2016-02-18.13:05:37.382>
actor = 'vstinner'
assignee = 'gregory.p.smith'
closed = True
closed_date = <Date 2010-12-14.15:17:05.636>
closer = 'gregory.p.smith'
components = ['Library (Lib)']
creation = <Date 2007-06-05.22:19:54.000>
creator = 'dsagal'
dependencies = []
files = ['14086', '14087', '14134']
hgrepos = []
issue_num = 1731717
keywords = []
message_count = 59.0
messages = ['32210', '32211', '32212', '32213', '32214', '32215', '32216', '32217', '32218', '32219', '32220', '32221', '32222', '56245', '57716', '57888', '57890', '57892', '57894', '64180', '64181', '64421', '64422', '64424', '64426', '64428', '64660', '64737', '65610', '70471', '82663', '88406', '88617', '88619', '88621', '88623', '88625', '92772', '100430', '100523', '100643', '100646', '100647', '100648', '100650', '100651', '100657', '100659', '106519', '115210', '117985', '122556', '123946', '123951', '123957', '142215', '251268', '260455', '260457']
nosy_count = 30.0
nosy_names = ['nnorwitz', 'collinwinter', 'abo', 'gregory.p.smith', 'astrand', 'siemer', 'exarkun', 'gjb1002', 'oefe', 'pitrou', 'vstinner', 'giampaolo.rodola', 'christian.heimes', 'dsagal', 'jyasskin', 'matejcik', 'tom_culliton', 'benjamin.peterson', 'djc', 'grossetti', 'r.david.murray', 'yonas', 'jonash', 'flox', 'kanaka', 'santoni', 'shaphiro', 'jlamanna', 'vitaly', 'Krishna Oza']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'needs patch'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue1731717'
versions = ['Python 2.6', 'Python 3.1', 'Python 2.7', 'Python 3.2']

[dsagal]

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.

[nnorwitz]

Peter, could you take a look at this? Thanks.

[dbaarda]

It appears that subprocess calls a module global "_cleanup()" whenever opening a new subprocess. This method is meant to reap any child processes that have terminated and have not explicitly cleaned up. These are processes you would expect to be cleaned up by GC, however, subprocess keeps a list of of all spawned subprocesses in _active until they are reaped explicitly so it can cleanup any that nolonger referenced anywhere else.

The problem is lots of methods, including poll() and wait(), check self.returncode and then modify it. Any non-atomic read/modify action is inherently non-threadsafe. And _cleanup() calls poll() on all un-reaped child processes. If two threads happen to try and spawn subprocesses at once, these _cleanup() calls collide..

The way to fix this depends on how thread-safe you want to make it. If you want to share popen objects between threads to wait()/poll() with impunity from any thread, you should add a recursive lock attribute to the Popen instance and have it lock/release it at the start/end of every method call.

If you only care about using popen objects in one thread at a time, then all you need to fix is the nasty "every popen created calls poll() on every other living popen object regardless of what thread started them,
and poll() is not threadsafe" behaviour.

Removing _cleanup() is one way, but it will then not reap child processes that you del'ed all references to (except the one in subprocess._active) before you checked they were done.

Probably another good idea is to not append and remove popen objects to _active directly, instead and add a popen.__del__() method that defers GC'ing of non-finished popen objects by adding them to _active. This
way, _active only contains un-reaped child processes that were due to be GC'ed. _cleanup() will then be responsible for polling and removing these popen objects from _active when they are done.

However, this alone will not fix things because you are still calling _cleanup() from different threads, it is still calling poll() on all these un-reaped processes, and poll() is not threadsafe. So... you either have to make poll() threadsafe (lock/unlock at the beginning/end of the method), or make _cleanup() threadsafe. The reason you can get away with making only _cleanup() threadsafe this way is _active will contain a list of processes that are not referenced anywhere else, so you know the only thing that will call poll() on them is the _cleanup() method.

[dbaarda]

Having just gone through that waffly description of the problems and various levels of fix, I think there are really only two fixes worth considering;

1. Make Popen instances fully threadsafe. Give them a recursive lock attribute and have every method acquire the lock at the start, and release it at the end.

2. Decide the "try to reap abandoned children at each Popen" idea was an ugly hack and abandon it. Remove _active and _cleanup(), and document that any child process not explicitly handled to completion will result in zombie child processes.

[gvanrossum]

I like #2. I don't see any use for threadsafe Popen instances, and I think that any self-respecting long-running server using Popen should properly manage its subprocesses anyway. (And for short-running processes it doesn't really matter if you have a few zombies.)

One could add a __del__ method that registers zombies to be reaped later, but I don't think it's worth it, and __del__ has some serious issues of its own. (If you really want to do this, use a weak reference callback instead of __del__ to do the zombie registration.)

[astrand]

Suddenly starting to leave Zombies is NOT an option for me. To prevent zombies, all applications using the subprocess module would need to be changed. This is a major breakage of backwards compatibility, IMHO. subprocess (as well as the popen2 module) has prevented zombies automatically from the beginning and I believe they should continue to do so (at least by default).

A little bit of history: When I wrote the subprocess module, I stole the idea of calling _cleanup whenever a new instance is created from the popen2 module, since I thought it was nice, lightweight and avoided having a __del__ method (which I have some bad experience with, GC-related). This worked great for a long time. At 2006-04-10, however, martin.v.loewis committed patch 1467770 (revision r45234), to solve bug 1460493. It introduces a __del__ method and some more complexity. I must admit that I didn't review this patch in detail, but it looked like thread safeness was being taken care of. But apparently it isn't.

Perhaps reverting to the original popen2 approach would solve the problem? Or does the popen2 module suffer from this bug as well?

I think that we need to fix this once and for all, this time. We should probably also consider adding the option of not having subprocess auto-wait while we are at it, for those what would like to wait() themselves (there should be a tracker item for this, but I can't find it right now).

Are we tight on time for fixing this bug? I'm out in the forest right now...

[dbaarda]

I just looked at popen2 and it has the same bug. I don't think __del__() related changes have anything to do with this. Popen.poll() and _cleanup() are non-threadsafe. When __init__() is called to create subprocesses simultaniously in two different threads, they both call _cleanup() and trample all over each other.

Removing _cleanup() will not leave zombies for popened processes that are correctly handled to completion. Using methods like communicate() and wait() will handle the process to completion and reap the child.

Unfortunately, I think a fairly common use-case is people using popen to fire-and-forget subprocesses without using communicate() or wait(). These will not get reaped, and will end up as zombies.

I cannot think of an easy way to reap abandoned popen instances that is thread safe without using locks. At times like this I wish that the GIL was exposed as a recursive lock... we could have __cleanup__() acquire/release the GIL and make it atomic... no more race condition :-)

[dbaarda]

Actually, after thinking about it, there may be a way to make _cleanup() threadsafe without using explicit locks by leveraging off the GIL. If _active.pop() and _active.append() are atomic because of the GIL, then _cleanup() can be made threadsafe. To be truely threadsafe it also needs to make sure that _active contains only abandoned popen instances so that _cleanup() doesn't have it's inst.poll() calls collide with other threads that are still using those popen instances. This can be done by havin the popen instances added to _active only by Popen.__del__(), and only removed by _cleanup() when they are done.

[gvanrossum]

If you want this fixed in 2.5.x, a new release is just around the corner, and time is very tight. Otherwise, 2.6a1 is half a year off.

[dbaarda]

I just looked at subprocess in svs trunk and it looks like it might already be fixed in both subprocess.py and popen2.py. The part where __del__() adds abandoned Popen instances to _active and _cleanup() removes them is already there. _cleanup() has been made more thread resistant by catching and ignoring exceptions that arise when two _cleanups() try to remove the same instances. Popen.poll() has been made more robust by having it set self.returncode to an optional _deadstate argument value when poll gets an os.error from os.pidwait() on a child that gets cleaned up by another thread. I think there are still a few minor race conditions around Popen.poll(), but it will only affect what non-zero returncode you get... it's not so much "thread safe" as "thread robust".

I think the _deadstate argument approach to try and make Popen.poll() thread-robust is a bit hacky. I would rather see _cleanup() be properly threadsafe by having it remove the inst from _active before processing it and re-adding it back if it is still not finished. However, as it is now it looks like it is fixed...

[dbaarda]

I can create a patch to make current head a bit cleaner, if anyone is interested...