import fcntl
import shelve

class FileLock(object):
	def __init__(self, fname = None):
		self.f = open(fname, 'w+')
		self.islocked = False

	def close(self):
		self.f.close()
		self.islocked = None

	def exclusivelock(self):
		fcntl.flock(self.f.fileno(), fcntl.LOCK_EX)
		self.islocked = True

	def sharedlock(self):
		fcntl.flock(self.f.fileno(), fcntl.LOCK_SH)
		self.islocked = True

	def unlock(self):
		fcntl.flock(self.f.fileno(), fcntl.LOCK_UN)
		self.islocked = False

class LockShelve(FileLock, shelve.DbfilenameShelf):
	def __init__(self, fname, *args, **kwargs):
		FileLock.__init__(self, fname + ".lock")
		self.exclusivelock()
		try:
			shelve.DbfilenameShelf.__init__(self, fname, *args, **kwargs)
		except:
			self.close()
			raise