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# Copyright (C) 1998 by the Free Software Foundation, Inc.
#
# This program is free software; you can redistribute it and/or
# modify it under the terms of the GNU General Public License
# as published by the Free Software Foundation; either version 2
# of the License, or (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program; if not, write to the Free Software
# Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
#
# flock.py: Portable file locking. John Viega, Jun 13, 1998
"""Portable (?) file locking with timeouts.
This code should work with all versions of NFS.
The algorithm was suggested by the GNU/Linux open() man page. Make
sure no malicious people have access to link() to the lock file.
"""
# Potential change: let the locker insert a field saying when he promises
# to be done with the lock, so if he needs more time than the other
# processes think he needs, he can say so.
import socket, os, time
import string
#from stat import ST_NLINK
ST_NLINK = 3 # faster
DEFAULT_HUNG_TIMEOUT = 15
DEFAULT_SLEEP_INTERVAL = .25
AlreadyCalledLockError = "AlreadyCalledLockError"
NotLockedError = "NotLockedError"
TimeOutError = "TimeOutError"
class FileLock:
def __init__(self, lockfile, hung_timeout = DEFAULT_HUNG_TIMEOUT,
sleep_interval = DEFAULT_SLEEP_INTERVAL):
self.lockfile = lockfile
self.hung_timeout = hung_timeout
self.sleep_interval = sleep_interval
self.tmpfname = "%s.%s.%d" % (lockfile, socket.gethostname(),
os.getpid())
self.__kickstart()
def __del__(self):
if self.locked():
self.unlock()
def __kickstart(self, force=0):
# forcing means to remove the original lockfile, and create a new one.
# this might be necessary if the file contains bogus locker
# information such that the owner of the lock can't be determined
if force:
try:
os.unlink(self.lockfile)
except IOError:
pass
if not os.path.exists(self.lockfile):
try:
# make sure it's group writable
oldmask = os.umask(002)
try:
file = open(self.lockfile, 'w+')
file.close()
finally:
os.umask(oldmask)
except IOError:
pass
def __write(self):
# make sure it's group writable
oldmask = os.umask(002)
try:
fp = open(self.tmpfname, 'w')
fp.write('%d %s\n' % (os.getpid(), self.tmpfname))
fp.close()
finally:
os.umask(oldmask)
def __read(self):
# can raise ValueError in two situations:
#
# either first element wasn't an integer (a valid pid), or we didn't
# get a 2-list from the string.split. Either way, the data in the
# file is bogus, but this is caught higher up
fp = open(self.tmpfname, 'r')
try:
pid, winner = string.split(string.strip(fp.read()))
finally:
fp.close()
return int(pid), winner
# Note that no one new can grab the lock once we've opened our tmpfile
# until we close it, even if we don't have the lock. So checking the PID
# and stealing the lock are guaranteed to be atomic.
def lock(self, timeout = 0):
"""Blocks until the lock can be obtained.
Raises a TimeOutError exception if a positive timeout value is given
and that time elapses before the lock is obtained.
"""
if timeout > 0:
timeout_time = time.time() + timeout
last_pid = -1
if self.locked():
raise AlreadyCalledLockError
stolen = 0
while 1:
# create the hard link and test for exactly 2 links to the file
os.link(self.lockfile, self.tmpfname)
if os.stat(self.tmpfname)[ST_NLINK] == 2:
# we have the lock (since there are no other links to the lock
# file), so we can piss on the hydrant
self.__write()
break
if timeout and timeout_time < time.time():
os.unlink(self.tmpfname)
raise TimeOutError
# someone else must have gotten the lock. let's find out who it
# is. if there is some bogosity in the lock file's data then we
# will steal the lock.
try:
pid, winner = self.__read()
except ValueError:
os.unlink(self.tmpfname)
self.__kickstart(force=1)
continue
# If we've gotten to here, we should be the winner, because
# otherwise, an AlreadyCalledLockError should have been raised
# above, and we should have never gotten into this loop. However,
# the following scenario can occur, and this is what the stolen
# flag takes care of:
#
# Say that processes A and B are already laying claim to the lock
# by creating link files, and say A actually has the lock (i.e., A
# is the winner). We are process C and we lay claim by creating a
# link file. All is cool, and we'll trip the pid <> last_pid
# test, unlink our claim, sleep and try again. Second time
# through our loop, we again determine that A is the winner but
# because it and B are swapped out, we trip our hung_timeout test
# and figure we need to steal the lock. So we piss on the hydrant
# (write our info into the lock file), unlink A's link file and go
# around the loop again. However, because B is still laying
# claim, and we never knew it (since it wasn't the winner), we
# again have 3 links to the lock file the next time through this
# loop, and the assert will trip.
#
# The stolen flag alerts us that this has happened, but I still
# worry that our logic might be flawed here.
assert stolen or winner <> self.tmpfname
# record the previous winner and the current time
if pid <> last_pid:
last_pid = pid
stime = time.time()
# here's where we potentially steal the lock. if the pid in the
# lockfile hasn't changed in hung_timeout seconds, then we assume
# that the locker crashed
elif stime + self.hung_timeout < time.time():
self.__write() # steal
stolen = 1
try:
os.unlink(winner)
except os.error:
# winner lockfile could be missing
pass
os.unlink(self.tmpfname)
continue
# okay, someone else has the lock, we didn't steal it, and it
# hasn't timed out yet. So let's wait for the owner of the lock
# to give it up. Unlink our claim to the lock and sleep for a
# while, then try again
os.unlink(self.tmpfname)
time.sleep(self.sleep_interval)
# This could error if the lock is stolen. You must catch it.
def unlock(self):
if not self.locked():
raise NotLockedError
os.unlink(self.tmpfname)
def locked(self):
if not os.path.exists(self.tmpfname):
return 0
pid, winner = self.__read()
return pid == os.getpid()
# use with caution!!!
def steal(self):
self.__write()
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