Python subprocesses experience mysterious delay in receiving stdin EOF
jeanmichel at sequans.com
Wed Feb 9 11:28:55 EST 2011
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)
>>> if verbose:
>>> for chunk in iter(lambda: sys.stdin.read(4096), ''):
>>> print>> sys.stderr, '..', time(), sys.argv, 'read',
>>> print>> sys.stderr, elapsed(), '..', sys.argv, 'done reading'
>>> print msg
>>> 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'
>>> print elapsed(), i, 'closing'
>>> print elapsed(), i, 'reading'
>>> 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
> Good point - I didn't specify that I'm seeing this on Linux (Ubuntu
> 10.04's Python 2.6).
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)
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