another lock module

[scott cotton]

i got tired of worrying about whether or not a system does
flock, of thinking that posix fcntl might not work on all
systems due to the differences in the struct flock, and
certainly not nfs, so i took the file locking locking module
from mailman (locks with link() so it _should_ work on NFS,
i think), added shared vs exclusive locking, and made the
interface something i like.

hope someone else finds it handy, and thanx josh for adding
the timeouts!

file lock.py:
----------------------------------------------------------------------
""" 
unix portable file locking, reportedly works even on NFS file
systems.
 
usage: shared and exclusive locking (read and write locks)
 
import lock
shl = lock.shlock(<resource name>)
shl.release()
 
exl = lock.exlock(<resource name>)
exl.release()

try:
    shl = lock.shlock(<resource name>, <timeout in seconds>)
except lock.TimeoutError:
    # SOL, no lock

try:
    exl = lock.exlock(<resource name>, <timeout in seconds>)
except lock.TimeoutError:
    # no lock

#
# By default, all locks will be stored in files in
# the /tmp directory.  this may not be desirable.
# the user can change the lock directory from the
# default '/tmp' using the setlockdir() function.
#
lock.setlockdir("/tmp/lock")

#
# it is recommended that users of this module
# surround code between obtaining and releasing
# a lock with a try: ... finally:.  This will
# help guarantee that an application using this
# module will never deadlock. system crashes may
# cause deadlock, but that is fairly unlikely.
#
shl = lock.shlock(<resource name>)
try:
    # do stuff with the lock
finally:
    shl.release()

"""
#
# system mods
#
import os
from stat import ST_NLINK
import string
import socket
import time

#
# how frequently to retry obtaining locks in seconds
#
retry_intv = 0.3

#
# the default directory in which locks are
# housed.
#
lockdir = "/tmp"

class TimeoutError(StandardError):
    pass

def setlockdir(newlockdir):
    global lockdir
    lockdir = newlockdir

class _lock:

    def __init__(self, resource, lockdir):

        self.resource = resource
        self.lockpath = os.path.join(lockdir,
                                     "%s.lock" % (self.resource))
        self.tmpfname = os.path.join(lockdir,
                                      "%s.lock.%s.%d" % (self.resource,
                                                         socket.gethostname(),
                                                         os.getpid()))
        if not os.path.exists(self.lockpath):
            open(self.lockpath, "a+").close()
        self.type = "unknown"

    def release(self):
        if not self.locked():
            return
        if self.type == "ex" \
           or os.stat(self.lockpath)[ST_NLINK] == 2:
            open(self.lockpath, "w").close() # remove the info in the lock file
        os.unlink(self.tmpfname)

    def __del__(self):
        self.release()

    def locked(self):
        return os.path.exists(self.tmpfname)

    #
    # can raise ValueError or IOError, ValueError is handled further up.
    #
    def _getlockdata(self):
        fp = open(self.lockpath, 'r')
        try:
            raw = fp.read()
            ltype, host, pid = string.split(string.strip(raw))
            pid = string.atoi(pid)
        finally:
            fp.close()
        return ltype, host, pid

    def _setlockdata(self):
        s = "%s %s %d\n" % (self.type, socket.gethostname(), os.getpid())
        fp = open(self.lockpath, "w")
        try:
            fp.write(s)
        finally:
            fp.close()
	

class exlock(_lock):

    def __init__(self, resource, timeout = 0):
        _lock.__init__(self, resource, lockdir)
        self.type = "ex"
        starttime = time.time()
        while 1:
            os.link(self.lockpath, self.tmpfname)
            if os.stat(self.lockpath)[ST_NLINK] == 2:
                # we have the exclusive lock
                self._setlockdata()
                break
            os.unlink(self.tmpfname)
            time.sleep(retry_intv)
            time_elapsed = time.time() - starttime
            if (timeout) and (time_elapsed > timeout):
                raise TimeoutError("Timeout limit of %d seconds"
                                   " has been reached" % timeout)
            

class shlock(_lock):

    #
    # in order to allow a single process
    # to acquire multiple shared locks
    # on a single resource, we must supplant
    # the pid part of the tmpfname with a counter.
    # this information in stored in the dict shared_ct
    # keyed by resource name and valued by counter.
    # this is not thread safe.
    shared_ct = {}

    def __init__(self, resource, timeout = 0):
        _lock.__init__(self, resource, lockdir)
        self.type = "sh"
        if not self.shared_ct.has_key(resource):
            self.shared_ct[resource] = 1
        else:
            self.shared_ct[resource] = self.shared_ct[resource] + 1
        self.tmpfname = self.tmpfname + "-%d" % (self.shared_ct[resource])
        starttime = time.time() 
        while 1:
            os.link(self.lockpath, self.tmpfname)
            nlink = os.stat(self.lockpath)[ST_NLINK]
            if nlink == 2:
                # first shared lock
                self._setlockdata()
                break
            elif nlink > 2:
                # is it shared or exclusive?
                # we have to read from the lock file
                # here, so if a read happens after
                # another process has truncated the file,
                # it'll raise ValueError, in which case we just
                # keep on tryin'
                try:
                    ltype, host, pid = self._getlockdata()
                except ValueError: 
                    # read after other lock erased lockinfo from file
                    ltype = "unknown"
                if ltype == "sh":
                    break
            os.unlink(self.tmpfname)
            time.sleep(retry_intv)
            time_elapsed = time.time() - starttime
            if (timeout) and (time_elapsed > timeout):
    		raise TimeoutError("Timeout limit of %d seconds"
                                   "has been reached" % timeout)