Python multiprocessing: synchronizing file-like object

Question

I'm trying to make a file like object which is meant to be assigned to sys.stdout/sys.stderr during testing to provide deterministic output. It's not meant to be fast, just reliable. What I have so far almost works, but I need some help getting rid of the last few edge-case errors.

Here is my current implementation.

try:
    from cStringIO import StringIO
except ImportError:
    from StringIO import StringIO

from os import getpid
class MultiProcessFile(object):
    """
    helper for testing multiprocessing

    multiprocessing poses a problem for doctests, since the strategy
    of replacing sys.stdout/stderr with file-like objects then
    inspecting the results won't work: the child processes will
    write to the objects, but the data will not be reflected
    in the parent doctest-ing process.

    The solution is to create file-like objects which will interact with
    multiprocessing in a more desirable way.

    All processes can write to this object, but only the creator can read.
    This allows the testing system to see a unified picture of I/O.
    """
    def __init__(self):
        # per advice at:
        #    http://docs.python.org/library/multiprocessing.html#all-platforms
        from multiprocessing import Queue
        self.__master = getpid()
        self.__queue = Queue()
        self.__buffer = StringIO()
        self.softspace = 0

    def buffer(self):
        if getpid() != self.__master:
            return

        from Queue import Empty
        from collections import defaultdict
        cache = defaultdict(str)
        while True:
            try:
                pid, data = self.__queue.get_nowait()
            except Empty:
                break
            cache[pid] += data
        for pid in sorted(cache):
            self.__buffer.write( '%s wrote: %r\n' % (pid, cache[pid]) )
    def write(self, data):
        self.__queue.put((getpid(), data))
    def __iter__(self):
        "getattr doesn't work for iter()"
        self.buffer()
        return self.__buffer
    def getvalue(self):
        self.buffer()
        return self.__buffer.getvalue()
    def flush(self):
        "meaningless"
        pass

... and a quick test script:

#!/usr/bin/python2.6

from multiprocessing import Process
from mpfile import MultiProcessFile

def printer(msg):
    print msg

processes = []
for i in range(20):
    processes.append( Process(target=printer, args=(i,), name='printer') )

print 'START'
import sys
buffer = MultiProcessFile()
sys.stdout = buffer

for p in processes:
    p.start()
for p in processes:
    p.join()

for i in range(20):
    print i,
print

sys.stdout = sys.__stdout__
sys.stderr = sys.__stderr__
print 
print 'DONE'
print
buffer.buffer()
print buffer.getvalue()

This works perfectly 95% of the time, but it has three edge-case problems. I have to run the test script in a fast while-loop to reproduce these.

1. 3% of the time, the parent process output isn't completely reflected. I assume this is because the data is being consumed before the Queue-flushing thread can catch up. I haven't though of a way to wait for the thread without deadlocking.
2. .5% of the time, there's a traceback from the multiprocess.Queue implementation
3. .01% of the time, the PIDs wrap around, and so sorting by PID gives the wrong ordering.

In the very worst case (odds: one in 70 million), the output would look like this:

START

DONE

302 wrote: '19\n'
32731 wrote: '0 1 2 3 4 5 6 7 8 '
32732 wrote: '0\n'
32734 wrote: '1\n'
32735 wrote: '2\n'
32736 wrote: '3\n'
32737 wrote: '4\n'
32738 wrote: '5\n'
32743 wrote: '6\n'
32744 wrote: '7\n'
32745 wrote: '8\n'
32749 wrote: '9\n'
32751 wrote: '10\n'
32752 wrote: '11\n'
32753 wrote: '12\n'
32754 wrote: '13\n'
32756 wrote: '14\n'
32757 wrote: '15\n'
32759 wrote: '16\n'
32760 wrote: '17\n'
32761 wrote: '18\n'

Exception in thread QueueFeederThread (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 "/usr/lib/python2.6/multiprocessing/queues.py", line 233, in _feed
<type 'exceptions.TypeError'>: 'NoneType' object is not callable

In python2.7 the exception is slightly different:

Exception in thread QueueFeederThread (most likely raised during interpreter shutdown):
Traceback (most recent call last):
  File "/usr/lib/python2.7/threading.py", line 552, in __bootstrap_inner
  File "/usr/lib/python2.7/threading.py", line 505, in run
  File "/usr/lib/python2.7/multiprocessing/queues.py", line 268, in _feed
<type 'exceptions.IOError'>: [Errno 32] Broken pipe

How do I get rid of these edge cases?

Answer 1

The solution came in two parts. I've successfully run the test program 200 thousand times without any change in output.

The easy part was to use multiprocessing.current_process()._identity to sort the messages. This is not a part of the published API, but it is a unique, deterministic identifier of each process. This fixed the problem with PIDs wrapping around and giving a bad ordering of output.

The other part of the solution was to use multiprocessing.Manager().Queue() rather than the multiprocessing.Queue. This fixes problem #2 above because the manager lives in a separate Process, and so avoids some of the bad special cases when using a Queue from the owning process. #3 is fixed because the Queue is fully exhausted and the feeder thread dies naturally before python starts shutting down and closes stdin.