Parent process hanging in multiprocessing if children terminate unexpectedly

[gdb]

BPO	9205
Nosy	@jcea, @brianquinlan, @pitrou, @vstinner, @vlasovskikh
Dependencies	bpo-11743: Rewrite PipeConnection and Connection in pure Python bpo-12040: Expose a Process.sentinel property (and fix polling loop in Process.join())
Files	termination.patch: Patch to deal with unexpected termination of children shutdown.patch: Patch to catch exceptions raised upon interpeter shutdown pickling_error.patch: Inform an application that is has failed if a task is unpickleable multiprocessing-trunk@82502-termination3.patch multiprocessing-trunk@82502-termination-trackjobs.patch assign-tasks.patch: Record the assignment of tasks to workers multiprocessing-trunk@82502-termination-trackjobs2.patch multiprocessing-trunk@82502-termination-trackjobs3.patch sentinels.patch sentinels2.patch sentinels3.patch sentinels4.patch sentinels5.patch multiproc_broken_pool.diff: Raise BrokenProcessPool if a process unexecptedly dies in a multiprocessing.Pool

^{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 = None
closed_at = <Date 2011-06-08.15:25:30.134>
created_at = <Date 2010-07-08.20:00:44.723>
labels = ['type-bug', 'library']
title = 'Parent process hanging in multiprocessing if children terminate unexpectedly'
updated_at = <Date 2014-09-11.22:34:12.788>
user = 'https://bugs.python.org/gdb'

bugs.python.org fields:

activity = <Date 2014-09-11.22:34:12.788>
actor = 'dan.oreilly'
assignee = 'jnoller'
closed = True
closed_date = <Date 2011-06-08.15:25:30.134>
closer = 'pitrou'
components = ['Library (Lib)']
creation = <Date 2010-07-08.20:00:44.723>
creator = 'gdb'
dependencies = ['11743', '12040']
files = ['17905', '17934', '17987', '18015', '18026', '18513', '18657', '18664', '21865', '21923', '21928', '21937', '22266', '36454']
hgrepos = []
issue_num = 9205
keywords = ['patch']
message_count = 78.0
messages = ['109585', '109867', '109885', '109910', '109922', '109936', '110129', '110136', '110139', '110142', '110152', '110169', '110174', '110197', '110207', '110256', '110283', '110285', '110288', '110353', '110366', '110369', '110370', '110386', '110387', '110399', '110428', '110979', '111025', '111028', '111124', '111559', '111690', '111696', '111706', '111915', '113828', '114423', '114449', '115065', '115107', '115114', '115118', '115125', '115128', '132646', '132660', '132661', '132664', '132665', '132666', '135013', '135021', '135025', '135026', '135027', '135028', '135461', '135493', '135502', '135513', '135544', '135624', '135895', '135896', '135901', '135904', '135968', '136090', '137767', '137855', '137912', '137913', '138065', '225833', '226699', '226794', '226806']
nosy_count = 16.0
nosy_names = ['jcea', 'bquinlan', 'pitrou', 'vstinner', 'jnoller', 'hongqn', 'asksol', 'vlasovskikh', 'neologix', 'gdb', 'Albert.Strasheim', 'aljungberg', 'python-dev', 'sbt', 'gkcn', 'dan.oreilly']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue9205'
versions = ['Python 3.3']

[gdb]

I have recently begun using multiprocessing for a variety of batch
jobs. It's a great library, and it's been quite useful. However, I have been bitten several times by situations where a worker process in a Pool will unexpectedly die, leaving multiprocessing hanging in a wait. A simple example of this is produced by the following:
"""
#!/usr/bin/env python
import multiprocessing, sys
def foo(x):
sys.exit(1)
multiprocessing.Pool(1).apply(foo, [1])
"""
The child will exit and the parent will hang forever. A similar occurrence happens if one pushes C-c while a child process is running (this special case is noted in http://bugs.python.org/issue8296) or killed by a signal.

Attached is a patch to handle unexpected terminations of children
processes and prevent the parent process from hanging. A test case is included. (Developed and tested on 64-bit Ubuntu.) Please let me know what you think. Thanks!

[jnoller]

thanks greg; I'm going to take a look and think about this. I'd like to resolve bug 9207 first though

[gdb]

Cool, thanks.  I'll note that with this patch applied, using the test program from 9207 I consistently get the following exception:
"""
Exception in thread Thread-1 (most likely raised during interpreter shutdown):
Traceback (most recent call last):
  File "/usr/lib/python2.6/threading.py", line 532, in __bootstrap_inner
  File "/usr/lib/python2.6/threading.py", line 484, in run
  File "/home/gdb/repositories/multiprocessing/pool.py", line 312, in _handle_workers
  File "/home/gdb/repositories/multiprocessing/pool.py", line 190, in _maintain_pool
  File "/home/gdb/repositories/multiprocessing/pool.py", line 158, in _join_exited_workers
<type 'exceptions.TypeError'>: 'NoneType' object is not callable
"""

This is line 148 in the unpatched source, namely the 'reversed(range(len(self._pool)))' line of _join_exited_workers. Looks like the same issue, where instead reversed/range/len have been set to None.

So I think by changing how much time the worker_handler spends in various functions, I've made it possible (or just more likely?) that if we lose the race with interpreter shutdown the worker_handler will be in the middle of _join_exited_workers. This may mean that someone should keep around a local reference to reversed/range/len... not sure if there's a better solution.

[jnoller]

Ugh. I'm going to have to think about the cleanest way of handling this case of functions vanishing from us since this is going to be more widespread inside the code. Suggestions welcome.

[gdb]

What about just catching the exception? See e.g. the attached patch. (Disclaimer: not heavily tested).

[jnoller]

A+ for creativity; I wouldn't have thought of that ;)

[asksol]

termination.patch, in the result handler you've added:

while cache and thread._state != TERMINATE and not failed

why are you terminating the second pass after finding a failed process?

Unpickleable errors and other errors occurring in the worker body are not exceptional cases, at least not now that the pool is supervised by _handle_workers. I think the result should be set also in this case, so the user can inspect the exception after the fact.

I have some other suggestions too, so I will review this patch tomorrow.

For shutdown.patch, I thought this only happened in the worker handler, but you've enabled this for the result handler too? I don't care about the worker handler, but with the result handler I'm worried that I don't know what ignoring these exceptions actually means. For example, is there a possibility that we may lose results at shutdown?

[gdb]

Thanks much for taking a look at this!

why are you terminating the second pass after finding a failed
process?
Unfortunately, if you've lost a worker, you are no longer guaranteed that cache will eventually be empty. In particular, you may have lost a task, which could result in an ApplyResult waiting forever for a _set call.

More generally, my chief assumption that went into this is that the unexpected death of a worker process is unrecoverable. It would be nice to have a better workaround than just aborting everything, but I couldn't see a way to do that.

Unpickleable errors and other errors occurring in the worker body are
not exceptional cases, at least not now that the pool is supervised
by _handle_workers.
I could be wrong, but that's not what my experiments were indicating. In particular, if an unpickleable error occurs, then a task has been lost, which means that the relevant map, apply, etc. will wait forever for completion of the lost task.

I think the result should be set also in this case, so the user can
inspect the exception after the fact.
That does sound useful. Although, how can you determine the job (and the value of i) if it's an unpickleable error? It would be nice to be able to retrieve job/i without having to unpickle the rest.

For shutdown.patch, I thought this only happened in the worker
handler, but you've enabled this for the result handler too? I don't
care about the worker handler, but with the result handler I'm
worried that I don't know what ignoring these exceptions actually
means.
You have a good point. I didn't think about the patch very hard. I've only seen these exceptions from the worker handler, but AFAICT there's no guarantee that bad luck with the scheduler wouldn't result in the same problem in the result handler. One option would be to narrow the breadth of the exceptions caught by _make_shutdown_safe (do we need to catch anything but TypeErrors?). Another option would be to enable only for the worker handler. I don't have a particularly great sense of what the Right Thing to do here is.

[jnoller]

Greg - I asked Ask to take a look - his celery package is a huge consumer of multiprocessing, and so I tend to run things past him as well.

That said - to both of you - the fundamental problem the shutdown patch is trying to scratch is located in bpo-9207 - greg's termination patch just exposes the problem in 9207 a lot more.

Focusing specifically on the shutdown patch; our issue is that during interpreter shutdown, sys.modules is iterated, and entries are set to None - for threads which "live on" well into that cycle can end up losing imported functions/modules/etc. The multiple daemon threads in the Pool code are exposing this as code which executed imported functions (such as the debug() statement in handle_workers) which will fire after the pool has exited and the interpreter is shut down.

We can work around the shutdown issue (really, bug 9207) by ignoring the exception such as shutdown.patch does, or passing in references/adding references to the functions those methods need. Or (as Brett suggested) converting them to class methods and adding references to the class. Or passing them in via the signature like this _handle_workers(arg, _debug=debug), etc.

[asksol]

Unfortunately, if you've lost a worker, you are no
longer guaranteed that cache will eventually be empty.
In particular, you may have lost a task, which could
result in an ApplyResult waiting forever for a _set call.

More generally, my chief assumption that went into this
is that the unexpected death of a worker process is
unrecoverable. It would be nice to have a better workaround
than just aborting everything, but I couldn't see a way
to do that.

It would be a problem if the process simply disappeared,
But in this case you have the ability to put a result on the queue,
so it doesn't have to wait forever.

For processes disappearing (if that can at all happen), we could solve
that by storing the jobs a process has accepted (started working on),
so if a worker process is lost, we can mark them as failed too.

I could be wrong, but that's not what my experiments
were indicating. In particular, if an unpickleable error occurs,
then a task has been lost, which means that the relevant map,
apply, etc. will wait forever for completion of the lost task.

It's lost now, but not if we handle the error...
For a single map operation this behavior may make sense, but what about
someone running the pool as s long-running service for users to submit map operations to? Errors in this context are expected to happen, even unpickleable errors.

I guess that the worker handler works as a supervisor is a side effect,
as it was made for the maxtasksperchild feature, but for me it's a welcome one. With the supervisor in place, multiprocessing.pool is already fairly stable to be used for this use case, and there's not much to be done to make it solid (Celery is already running for months without issue, unless there's a pickling error...)

That does sound useful. Although, how can you determine the
job (and the value of i) if it's an unpickleable error?
It would be nice to be able to retrieve job/i without having
to unpickle the rest.

I was already working on this issue last week actually, and I managed
to do that in a way that works well enough (at least for me):
http://github.com/ask/celery/commit/eaa4d5ddc06b000576a21264f11e6004b418bda1#diff-1

[gdb]

For processes disappearing (if that can at all happen), we could solve
that by storing the jobs a process has accepted (started working on),
so if a worker process is lost, we can mark them as failed too.
Sure, this would be reasonable behavior. I had considered it but decided it as a larger change than I wanted to make without consulting the devs.

I was already working on this issue last week actually, and I managed
to do that in a way that works well enough (at least for me):
If I'm reading this right, you catch the exception upon pickling the result (at which point you have the job/i information already; totally reasonable). I'm worried about the case of unpickling the task failing. (Namely, the "task = get()" line of the "worker" method.) Try running the following:
"""
#!/usr/bin/env python
import multiprocessing
p = multiprocessing.Pool(1)
def foo(x):
pass
p.apply(foo, [1])
"""
And if "task = get()" fails, then the worker doesn't know what the relevant job/i values are.

Anyway, so I guess the question that is forming in my mind is, what sorts of errors do we want to handle, and how do we want to handle them? My answer is I'd like to handle all possible errors with some behavior that is not "hang forever". This includes handling children processes dying by signals or os._exit, raising unpickling errors, etc.

I believe my patch provides this functionality. By adding the extra mechanism that you've written/proposed, we can improve the error handling in specific recoverable cases (which probably constitute the vast majority of real-world cases).

[asksol]

I think I misunderstood the purpose of the patch. This is about handling errors on get(), not on put() like I was working on. So sorry for that confusion.

What kind of errors are you having that makes the get() call fail?

If the queue is not working, then I guess the only sensible approach is to shutdown the pool like suggested. I'll open up another issue for unpickleable errors then.