"""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()
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