"""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 holdwrite locks for multiple directories); all lock files are placed inthe 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.  [Thealgorithm can still be fooled (1) if the repository's mode is changedwhile attempting to lock; (2) if this file exists and is writable butthe directory is not.] - While creating the actual read/write lock files (which may exist fora long time), a "meta-lock" is held.  The meta-lock is a directorynamed "#cvs.lock" in the repository.  The meta-lock is also held whilea 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 avoiddeadlocks, I presume it should always lock them top-down in thedirectory hierarchy). - A process should make sure it removes all its lock files anddirectories when it crashes. - Limitation: one user id should not be committing files into the samerepository at the same time.  Turn this into Python code-------------------------- rl = ReadLock(repository, waittime) wl = WriteLock(repository, waittime) list = MultipleWriteLock([repository1, repository2, ...], waittime) """  import osimport timeimport statimport pwd  # Default wait timeDELAY = 10  # XXX This should be the same on all Unix versionsEEXIST = 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, 0777)                return            except os.error, msg:                self.lockdir = None                if msg[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],    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, 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]        sys.exc_traceback = None        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()