[issue4112] Subprocess: Popen'ed children hang due to open pipes
report at bugs.python.org
Mon Oct 13 12:36:19 CEST 2008
New submission from Boye Høverstad <boyeah at gmail.com>:
subprocess.Popen.wait() hangs if you have spawned multiple child
processes that do not terminate until their standard input is closed
('cat' is example of such a process). This happens on POSIX platforms.
I'm using Python 2.5.1, but I believe the issue is present in the SVN
trunk version of subprocess as well.
Here is a test program:
import subprocess, sys
p1 = subprocess.Popen("cat", bufsize=0, stdin=subprocess.PIPE)
p2 = subprocess.Popen("cat", bufsize=0, stdin=subprocess.PIPE)
ret = p1.wait()
print >>sys.stderr, "Child 1 wait completed with ret", ret
ret = p2.wait()
print >>sys.stderr, "Child 2 wait completed with ret", ret, "Bye bye"
The call to p1.wait() will never return. If p2.wait is called first,
the program terminates cleanly.
p1 never terminates because p1.stdin is duplicated in the (second) child
process when the parent process forks as part of the call to p2 =
subprocess.Popen(). The line p1.stdin.close() thus has no effect.
I am not sure whether this is a bug or deliberate design, but the
behavior above seems a bit weird to me, and it took me quite some time
to figure out what happened. However, the proposed fix below of course
has side effects that may be undesirable.
wait() will not hang if the "close on exec" flag is set on the
subprocess pipes. Conveniently, the method _set_cloexec_flag already
exists to do this, so the fix amounts to calling this from
elif stdin == PIPE:
p2cread, p2cwrite = os.pipe()
The last line is the added one, similar lines must be added for stdout
Alternatively, the test program above will terminate cleanly if
"p1._set_cloexec_flag(p1.stdin)" is added before the creation of p2.
components: Library (Lib)
title: Subprocess: Popen'ed children hang due to open pipes
versions: Python 3.1
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