Skip to content
Find file
Fetching contributors…
Cannot retrieve contributors at this time
116 lines (95 sloc) 3.59 KB
###############################################################################
#
# Copyright (c) 2012 Ruslan Spivak
#
# Permission is hereby granted, free of charge, to any person obtaining a copy
# of this software and associated documentation files (the "Software"), to deal
# in the Software without restriction, including without limitation the rights
# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
# copies of the Software, and to permit persons to whom the Software is
# furnished to do so, subject to the following conditions:
#
# The above copyright notice and this permission notice shall be included in
# all copies or substantial portions of the Software.
#
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
# THE SOFTWARE.
#
###############################################################################
"""
Demonstrates SELF-PIPE Trick that is used to
avoid race condtions when waiting for signals and calling select() on
a set of descriptors.
"""
__author__ = 'Ruslan Spivak <ruslan.spivak@gmail.com>'
import os
import sys
import fcntl
import time
import errno
import select
import signal
# read, write pipe descriptors
RD, WD = None, None
def handler(signum, frame):
try:
os.write(WD, 'x')
except OSError as e:
if e.errno != errno.EAGAIN:
print 'Signal handler write error'
os._exit(1)
def main():
print 'PID: %s' % os.getpid()
# 1. Create a pipe and make both ends nonblocking
global RD, WD
RD, WD = os.pipe()
flags = fcntl.fcntl(RD, fcntl.F_GETFL)
fcntl.fcntl(RD, fcntl.F_SETFL, flags | os.O_NONBLOCK)
flags = fcntl.fcntl(WD, fcntl.F_GETFL)
fcntl.fcntl(WD, fcntl.F_SETFL, flags | os.O_NONBLOCK)
# 2. Add read end of the pipe to the read list of descriptors
# read, write, exception lists with descriptors to poll
rlist, wlist, elist = [RD], [], []
# 3. Install signal handler for USR1 signal
signal.signal(signal.SIGUSR1, handler)
# A racing can happen at this point if the signal arrives before
# the call to 'select' - but we're ready for it, bring it on!
# call 'select' in loop - this way we determine the signal arrival
# through examinination of the read end of the pipe (RD descriptor)
while True:
try:
readables, writables, exceptions = select.select(
rlist, wlist, elist)
except select.error as e:
code, msg = e.args
# restart if interrupted by a signal
if code == errno.EINTR:
continue
else:
raise
else:
# we have result - break the loop
break
# 4. Check if our pipe's read end descriptor is in the result set
if RD in readables:
print 'Got signal'
# 5. Read all the bytes, because multiple
# signal may have arrived
while True:
try:
os.read(RD, 1)
except OSError as e:
if e.errno == errno.EAGAIN:
# the pipe is drained - exit
break
else:
raise
else:
print 'Something unexpected happened'
if __name__ == '__main__':
main()
Something went wrong with that request. Please try again.