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from contextlib import contextmanager
import logging
from threading import Lock
from typing import Dict
log = logging.getLogger(__name__)
# singleton design pattern taken from http://www.aleax.it/5ep.html
class GlobalLock(object):
"""
Global lock to serialize operations in mgr/volumes. This lock
is currently held when accessing (opening) a volume to perform
group/subvolume operations. Since this is a big lock, it's rather
inefficient -- but right now it's ok since mgr/volumes does not
expect concurrent operations via its APIs.
As and when features get added (such as clone, where mgr/volumes
would maintain subvolume states in the filesystem), there might
be a need to allow concurrent operations. In that case it would
be nice to implement an efficient path based locking mechanism.
See: https://people.eecs.berkeley.edu/~kubitron/courses/cs262a-F14/projects/reports/project6_report.pdf
"""
_shared_state = {
'lock' : Lock(),
'init' : False
} # type: Dict
def __init__(self):
with self._shared_state['lock']:
if not self._shared_state['init']:
self._shared_state['init'] = True
# share this state among all instances
self.__dict__ = self._shared_state
@contextmanager
def lock_op(self):
log.debug("entering global lock")
with self._shared_state['lock']:
log.debug("acquired global lock")
yield
log.debug("exited global lock")
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