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# Author: Stuart D. Gathman <stuart@bmsi.com>
# Copyright 2001 Business Management Systems, Inc.
# This code is under the GNU General Public License. See COPYING for details.
import os
from time import sleep
class PLock(object):
"A simple /etc/passwd style lock,update,rename protocol for updating files."
def __init__(self,basename):
self.basename = basename
self.fp = None
def lock(self,lockname=None,mode=0o660,strict_perms=False):
"Start an update transaction. Return FILE to write new version."
self.unlock()
if not lockname:
lockname = self.basename + '.lock'
self.lockname = lockname
try:
st = os.stat(self.basename)
mode |= st.st_mode
except OSError: pass
u = os.umask(0o2)
try:
fd = os.open(lockname,os.O_WRONLY+os.O_CREAT+os.O_EXCL,mode)
finally:
os.umask(u)
self.fp = os.fdopen(fd,'w')
try:
os.chown(self.lockname,-1,st.st_gid)
except:
if strict_perms:
self.unlock()
raise
return self.fp
def wlock(self,lockname=None):
"Wait until lock is free, then start an update transaction."
while True:
try:
return self.lock(lockname)
except OSError:
sleep(2)
def commit(self,backname=None):
"Commit update transaction with optional backup file."
if not self.fp:
raise IOError("File not locked")
self.fp.close()
self.fp = None
if backname:
try:
os.remove(backname)
except OSError: pass
os.link(self.basename,backname)
os.rename(self.lockname,self.basename)
def unlock(self):
"Cancel update transaction."
if self.fp:
try:
self.fp.close()
except: pass
self.fp = None
os.remove(self.lockname)
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