Python subprocesses experience mysterious delay in receiving stdin EOF

[Yang Zhang]

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/