Thanks, that explains everything. The code I was using to test was copied from Twisted's daemonizing function: http://github.com/powdahound/twisted/blob/master/twisted/scripts/_twistd_uni... It should probably be updated before more users upgrade to the new kernel and run into this issue. Ticket #823 ( http://twistedmatrix.com/trac/ticket/823) seems very related to this. On Thu, Sep 30, 2010 at 8:52 AM, Žiga Seilnacht <ziga.seilnacht@gmail.com>wrote:
Hello,
Garret Heaton wrote:
I've simplified this issue down and am able to reproduce it without Twisted: http://gist.github.com/603154
Still not sure what the cause is, so if anyone has any ideas I'd love to hear them. Thanks!
The behavior that you are seeing seems to be related to the change in the 2.6.32 kernel, where they changed the child-runs-first scheduler parameter to false. Setting that parameter to 1 with:
$ sudo sysctl -w kernel.sched_child_runs_first=1
and rebooting the computer restores the behavior that you saw on the old kernel for me. See http://lwn.net/Articles/352863/ for more details.
Parent (which is the controlling process when ran with -t) exiting before the child starts causes the child to receive SIGHUP signal immediately as it starts running, before it has time to disassociate itself from the parent's process group. It seems to me that this might be an actual bug in twistd, it should block the SIGHUP signal across the fork() calls.
Your test program has an additional assumption that the child will run before the parent; the child in your program tries to write to stdout, i.e. the controlling terminal, which gets closed once the parent exits.
The modified test program below works for me regardless of the setting of the kernel.sched_child_runs_first parameter.
Hope this helps, Ziga
import os import signal
f = open("test_fork.out", "w")
def daemonize(): # See http://www.faqs.org/faqs/unix-faq/programmer/faq/ - Section 1.7 print >> f, '--- %s: daemonizing' % os.getpid()
signal.signal(signal.SIGHUP, signal.SIG_IGN) if os.fork(): # launch child and... print >> f, '--- %s: kill parent 1' % os.getpid() os._exit(0) # kill off parent print >> f, '--- %s: old sid: %r' % (os.getpid(), os.getsid(os.getpid())) os.setsid() print >> f, '--- %s: new sid: %r' % (os.getpid(), os.getsid(os.getpid())) if os.fork(): # launch child and... print >> f, '--- %s: kill parent 2' % os.getpid() os._exit(0) # kill off parent again.
signal.signal(signal.SIGHUP, signal.SIG_DFL) print >> f, '--- %s: daemonizing done' % os.getpid()
if __name__ == "__main__": print >> f, 'starting as %d' % os.getpid() daemonize() print >> f, 'stopping as %s' % os.getpid()
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