New submission from joseph.h.garvin <joseph.h.gar...@gmail.com>:
The following code will result in a traceback 99% of the time, though it may
take two runs (sometimes the first run won't trigger it, I think due to the
changing in timing from genrating the .pyc file). It spawns an instance of
/bin/echo, chosen because it's a very quick to finish program. Any program that
executes faster than subprocess.Popen can return will work as a substitute
though:
import signal
import subprocess
to_launch = None
def sig_chld_handler(signum, frame):
global to_launch
# Crashes here.
# 'NoneType' object has no attribute 'poll'
to_launch.poll()
print to_launch.returncode
signal.signal(signal.SIGCHLD, sig_chld_handler)
to_launch = subprocess.Popen("/bin/echo")
And the traceback:
Traceback (most recent call last):
File "/tmp/sigchld.py", line 15, in <module>
to_launch = subprocess.Popen("/bin/echo")
File "/usr/lib/python2.6/subprocess.py", line 623, in __init__
errread, errwrite)
File "/usr/lib/python2.6/subprocess.py", line 1130, in _execute_child
data = _eintr_retry_call(os.read, errpipe_read, 1048576)
File "/usr/lib/python2.6/subprocess.py", line 455, in _eintr_retry_call
return func(*args)
File "/tmp/sigchld.py", line 9, in sig_chld_handler
to_launch.poll()
AttributeError: 'NoneType' object has no attribute 'poll'
I believe the problem is that the process completes before Popen can return,
which means the assignment of to_launch hasn't happened yet, so it's not
defined when we get into sig_chld_handler.
I tried to work around this issue by setting preexec_fn to signal.pause and
sending the child process a signal after the assignment, but then ran into
another bug: http://bugs.python.org/issue10635
If when it caught SIGCHLD python pushed an event onto its internal event loop
to execute the handler, I think that would make sure it's deferred until after
the assignment. There might be other consequences of that, but I'm not familiar
with the interpreter internals. Alternatively it could be fixed with an API
change -- let Popen return an object before it actually launches the process,
and have a separate start() method.
----------
components: Library (Lib)
messages: 123445
nosy: joseph.h.garvin
priority: normal
severity: normal
status: open
title: subprocess module has race condition with SIGCHLD handlers
versions: Python 2.6
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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue10636>
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