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v0.8.1
v0.8.1
https://github.com/python/cpython
Tip revision: 1da43e5e916949c8e849e656d9d05fa4b9d6836c authored by Benjamin Peterson on 26 June 2009, 13:21:52 UTC
rearrange the sections of the README, so they'll hopefully be more in the order people will interested in
rearrange the sections of the README, so they'll hopefully be more in the order people will interested in
Tip revision: 1da43e5
cvslock.py
"""CVS locking algorithm.
CVS locking strategy
====================
As reverse engineered from the CVS 1.3 sources (file lock.c):
- Locking is done on a per repository basis (but a process can hold
write locks for multiple directories); all lock files are placed in
the repository and have names beginning with "#cvs.".
- Before even attempting to lock, a file "#cvs.tfl.<pid>" is created
(and removed again), to test that we can write the repository. [The
algorithm can still be fooled (1) if the repository's mode is changed
while attempting to lock; (2) if this file exists and is writable but
the directory is not.]
- While creating the actual read/write lock files (which may exist for
a long time), a "meta-lock" is held. The meta-lock is a directory
named "#cvs.lock" in the repository. The meta-lock is also held while
a write lock is held.
- To set a read lock:
- acquire the meta-lock
- create the file "#cvs.rfl.<pid>"
- release the meta-lock
- To set a write lock:
- acquire the meta-lock
- check that there are no files called "#cvs.rfl.*"
- if there are, release the meta-lock, sleep, try again
- create the file "#cvs.wfl.<pid>"
- To release a write lock:
- remove the file "#cvs.wfl.<pid>"
- rmdir the meta-lock
- To release a read lock:
- remove the file "#cvs.rfl.<pid>"
Additional notes
----------------
- A process should read-lock at most one repository at a time.
- A process may write-lock as many repositories as it wishes (to avoid
deadlocks, I presume it should always lock them top-down in the
directory hierarchy).
- A process should make sure it removes all its lock files and
directories when it crashes.
- Limitation: one user id should not be committing files into the same
repository at the same time.
Turn this into Python code
--------------------------
rl = ReadLock(repository, waittime)
wl = WriteLock(repository, waittime)
list = MultipleWriteLock([repository1, repository2, ...], waittime)
"""
import os
import time
import stat
import pwd
# Default wait time
DELAY = 10
# XXX This should be the same on all Unix versions
EEXIST = 17
# Files used for locking (must match cvs.h in the CVS sources)
CVSLCK = "#cvs.lck"
CVSRFL = "#cvs.rfl."
CVSWFL = "#cvs.wfl."
class Error:
def __init__(self, msg):
self.msg = msg
def __repr__(self):
return repr(self.msg)
def __str__(self):
return str(self.msg)
class Locked(Error):
pass
class Lock:
def __init__(self, repository = ".", delay = DELAY):
self.repository = repository
self.delay = delay
self.lockdir = None
self.lockfile = None
pid = repr(os.getpid())
self.cvslck = self.join(CVSLCK)
self.cvsrfl = self.join(CVSRFL + pid)
self.cvswfl = self.join(CVSWFL + pid)
def __del__(self):
print("__del__")
self.unlock()
def setlockdir(self):
while 1:
try:
self.lockdir = self.cvslck
os.mkdir(self.cvslck, 0o777)
return
except os.error as msg:
self.lockdir = None
if msg.args[0] == EEXIST:
try:
st = os.stat(self.cvslck)
except os.error:
continue
self.sleep(st)
continue
raise Error("failed to lock %s: %s" % (
self.repository, msg))
def unlock(self):
self.unlockfile()
self.unlockdir()
def unlockfile(self):
if self.lockfile:
print("unlink", self.lockfile)
try:
os.unlink(self.lockfile)
except os.error:
pass
self.lockfile = None
def unlockdir(self):
if self.lockdir:
print("rmdir", self.lockdir)
try:
os.rmdir(self.lockdir)
except os.error:
pass
self.lockdir = None
def sleep(self, st):
sleep(st, self.repository, self.delay)
def join(self, name):
return os.path.join(self.repository, name)
def sleep(st, repository, delay):
if delay <= 0:
raise Locked(st)
uid = st[stat.ST_UID]
try:
pwent = pwd.getpwuid(uid)
user = pwent[0]
except KeyError:
user = "uid %d" % uid
print("[%s]" % time.ctime(time.time())[11:19], end=' ')
print("Waiting for %s's lock in" % user, repository)
time.sleep(delay)
class ReadLock(Lock):
def __init__(self, repository, delay = DELAY):
Lock.__init__(self, repository, delay)
ok = 0
try:
self.setlockdir()
self.lockfile = self.cvsrfl
fp = open(self.lockfile, 'w')
fp.close()
ok = 1
finally:
if not ok:
self.unlockfile()
self.unlockdir()
class WriteLock(Lock):
def __init__(self, repository, delay = DELAY):
Lock.__init__(self, repository, delay)
self.setlockdir()
while 1:
uid = self.readers_exist()
if not uid:
break
self.unlockdir()
self.sleep(uid)
self.lockfile = self.cvswfl
fp = open(self.lockfile, 'w')
fp.close()
def readers_exist(self):
n = len(CVSRFL)
for name in os.listdir(self.repository):
if name[:n] == CVSRFL:
try:
st = os.stat(self.join(name))
except os.error:
continue
return st
return None
def MultipleWriteLock(repositories, delay = DELAY):
while 1:
locks = []
for r in repositories:
try:
locks.append(WriteLock(r, 0))
except Locked as instance:
del locks
break
else:
break
sleep(instance.msg, r, delay)
return list
def test():
import sys
if sys.argv[1:]:
repository = sys.argv[1]
else:
repository = "."
rl = None
wl = None
try:
print("attempting write lock ...")
wl = WriteLock(repository)
print("got it.")
wl.unlock()
print("attempting read lock ...")
rl = ReadLock(repository)
print("got it.")
rl.unlock()
finally:
print([1])
print([2])
if rl:
rl.unlock()
print([3])
if wl:
wl.unlock()
print([4])
rl = None
print([5])
wl = None
print([6])
if __name__ == '__main__':
test()