Python subprocesses experience mysterious delay in receiving stdin EOF

Yang Zhang yanghatespam at gmail.com
Sat Feb 19 07:15:01 CET 2011


After way too much time, I figured it out, after a quote from [this
post](http://fixunix.com/questions/379652-sending-eof-named-pipe.html)
jumped out at me:

> See the "I/O on Pipes and FIFOs" section of pipe(7) ("man 7 pipe")
>
> "If all file descriptors referring to the write end of a pipe have
> been closed, then an attempt to read(2) from the pipe will see
> end-of-file (read(2) will return 0)."

I should've known this, but it never occurred to me - had nothing to
do with Python in particular. What was happening was: the subprocesses
were getting forked with open (writer) file descriptors to each
others' pipes. As long as there are open writer file descriptors to a
pipe, readers won't see EOF.

E.g.:

    p1=Popen(..., stdin=PIPE, ...) # creates a pipe the parent process
can write to
    p2=Popen(...) # inherits the writer FD - as long as p2 exists, p1
won't see EOF

Turns out there's a `close_fds` parameter to `Popen`, so the solution
is to pass `close_fds=True`. All simple and obvious in hindsight, but
still managed to cost at least a couple eyeballs good chunks of time.

On Sun, Feb 13, 2011 at 11:52 PM, Yang Zhang <yanghatespam at gmail.com> wrote:
> Anybody else see this issue?
>
> On Thu, Feb 10, 2011 at 10:37 AM, Yang Zhang <yanghatespam at gmail.com> wrote:
>> On Thu, Feb 10, 2011 at 12:28 AM, Jean-Michel Pichavant
>> <jeanmichel at sequans.com> wrote:
>>> Yang Zhang wrote:
>>>>
>>>> On Wed, Feb 9, 2011 at 11:01 AM, MRAB <python at mrabarnett.plus.com> wrote:
>>>>
>>>>>
>>>>> On 09/02/2011 01:59, Yang Zhang wrote:
>>>>>
>>>>>>
>>>>>> I reduced a problem I was seeing in my application down into the
>>>>>> following test case. In this code, a parent process concurrently
>>>>>> spawns 2 (you can spawn more) subprocesses that read a big message
>>>>>> from the parent over stdin, sleep for 5 seconds, and write something
>>>>>> back. However, there's unexpected waiting happening somewhere, causing
>>>>>> the code to complete in 10 seconds instead of the expected 5.
>>>>>>
>>>>>> If you set `verbose=True`, you can see that the straggling subprocess
>>>>>> is receiving most of the messages, then waiting for the last chunk of
>>>>>> 3 chars---it's not detecting that the pipe has been closed.
>>>>>> Furthermore, if I simply don't do anything with the second process
>>>>>> (`doreturn=True`), the first process will *never* see the EOF.
>>>>>>
>>>>>> Any ideas what's happening? Further down is some example output.
>>>>>> Thanks in advance.
>>>>>>
>>>>>>    from subprocess import *
>>>>>>    from threading import *
>>>>>>    from time import *
>>>>>>    from traceback import *
>>>>>>    import sys
>>>>>>    verbose = False
>>>>>>    doreturn = False
>>>>>>    msg = (20*4096+3)*'a'
>>>>>>    def elapsed(): return '%7.3f' % (time() - start)
>>>>>>    if sys.argv[1:]:
>>>>>>      start = float(sys.argv[2])
>>>>>>      if verbose:
>>>>>>        for chunk in iter(lambda: sys.stdin.read(4096), ''):
>>>>>>          print>>  sys.stderr, '..', time(), sys.argv[1], 'read',
>>>>>> len(chunk)
>>>>>>      else:
>>>>>>        sys.stdin.read()
>>>>>>      print>>  sys.stderr, elapsed(), '..', sys.argv[1], 'done reading'
>>>>>>      sleep(5)
>>>>>>      print msg
>>>>>>    else:
>>>>>>      start = time()
>>>>>>      def go(i):
>>>>>>        print elapsed(), i, 'starting'
>>>>>>        p = Popen(['python','stuckproc.py',str(i), str(start)],
>>>>>> stdin=PIPE, stdout=PIPE)
>>>>>>        if doreturn and i == 1: return
>>>>>>        print elapsed(), i, 'writing'
>>>>>>        p.stdin.write(msg)
>>>>>>        print elapsed(), i, 'closing'
>>>>>>        p.stdin.close()
>>>>>>        print elapsed(), i, 'reading'
>>>>>>        p.stdout.read()
>>>>>>        print elapsed(), i, 'done'
>>>>>>      ts = [Thread(target=go, args=(i,)) for i in xrange(2)]
>>>>>>      for t in ts: t.start()
>>>>>>      for t in ts: t.join()
>>>>>>
>>>>>> Example output:
>>>>>>
>>>>>>      0.001 0 starting
>>>>>>      0.003 1 starting
>>>>>>      0.005 0 writing
>>>>>>      0.016 1 writing
>>>>>>      0.093 0 closing
>>>>>>      0.093 0 reading
>>>>>>      0.094 1 closing
>>>>>>      0.094 1 reading
>>>>>>      0.098 .. 1 done reading
>>>>>>      5.103 1 done
>>>>>>      5.108 .. 0 done reading
>>>>>>     10.113 0 done
>>>>>>
>>>>>>
>>>>>
>>>>> I changed 'python' to the path of python.exe and 'stuckproc.py' to its
>>>>> full path and tried it with Python 2.7 on Windows XP Pro. It worked as
>>>>> expected.
>>>>>
>>>>
>>>> Good point - I didn't specify that I'm seeing this on Linux (Ubuntu
>>>> 10.04's Python 2.6).
>>>>
>>>>
>>>
>>> python test.py   0.000 0 starting
>>>  0.026 0 writing
>>>  0.026 0 closing
>>>  0.026 0 reading
>>>  0.029 .. 0 done reading
>>>  0.030 1 starting
>>>  0.038 1 writing
>>>  0.058 1 closing
>>>  0.058 1 reading
>>>  0.061 .. 1 done reading
>>>  5.026 0 done
>>>  5.061 1 done
>>>
>>> on debian lenny (Python 2.5)
>>>
>>> JM
>>>
>>
>> FWIW, this is consistently reproduce-able across all the Ubuntu 10.04s
>> I've tried. You may need to increase the message size so that it's
>> large enough for your system.
>>
>> --
>> Yang Zhang
>> http://yz.mit.edu/
>>
>
>
>
> --
> Yang Zhang
> http://yz.mit.edu/
>



-- 
Yang Zhang
http://yz.mit.edu/



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