Python subprocesses experience mysterious delay in receiving stdin EOF
yanghatespam at gmail.com
Tue Feb 8 20:59:18 EST 2011
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 *
verbose = False
doreturn = False
msg = (20*4096+3)*'a'
def elapsed(): return '%7.3f' % (time() - start)
start = float(sys.argv)
for chunk in iter(lambda: sys.stdin.read(4096), ''):
print >> sys.stderr, '..', time(), sys.argv, 'read', len(chunk)
print >> sys.stderr, elapsed(), '..', sys.argv, 'done reading'
start = time()
print elapsed(), i, 'starting'
p = Popen(['python','stuckproc.py',str(i), str(start)],
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()
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
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