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import logging
import threading
log = logging.getLogger(__name__)
class LockError(Exception):
pass
class ReadWriteMutex(object):
"""A mutex which allows multiple readers, single writer.
:class:`.ReadWriteMutex` uses a Python ``threading.Condition``
to provide this functionality across threads within a process.
The Beaker package also contained a file-lock based version
of this concept, so that readers/writers could be synchronized
across processes with a common filesystem. A future Dogpile
release may include this additional class at some point.
"""
def __init__(self):
# counts how many asynchronous methods are executing
self.async_ = 0
# pointer to thread that is the current sync operation
self.current_sync_operation = None
# condition object to lock on
self.condition = threading.Condition(threading.Lock())
def acquire_read_lock(self, wait=True):
"""Acquire the 'read' lock."""
self.condition.acquire()
try:
# see if a synchronous operation is waiting to start
# or is already running, in which case we wait (or just
# give up and return)
if wait:
while self.current_sync_operation is not None:
self.condition.wait()
else:
if self.current_sync_operation is not None:
return False
self.async_ += 1
log.debug("%s acquired read lock", self)
finally:
self.condition.release()
if not wait:
return True
def release_read_lock(self):
"""Release the 'read' lock."""
self.condition.acquire()
try:
self.async_ -= 1
# check if we are the last asynchronous reader thread
# out the door.
if self.async_ == 0:
# yes. so if a sync operation is waiting, notify_all to wake
# it up
if self.current_sync_operation is not None:
self.condition.notify_all()
elif self.async_ < 0:
raise LockError(
"Synchronizer error - too many "
"release_read_locks called"
)
log.debug("%s released read lock", self)
finally:
self.condition.release()
def acquire_write_lock(self, wait=True):
"""Acquire the 'write' lock."""
self.condition.acquire()
try:
# here, we are not a synchronous reader, and after returning,
# assuming waiting or immediate availability, we will be.
if wait:
# if another sync is working, wait
while self.current_sync_operation is not None:
self.condition.wait()
else:
# if another sync is working,
# we dont want to wait, so forget it
if self.current_sync_operation is not None:
return False
# establish ourselves as the current sync
# this indicates to other read/write operations
# that they should wait until this is None again
self.current_sync_operation = threading.current_thread()
# now wait again for asyncs to finish
if self.async_ > 0:
if wait:
# wait
self.condition.wait()
else:
# we dont want to wait, so forget it
self.current_sync_operation = None
return False
log.debug("%s acquired write lock", self)
finally:
self.condition.release()
if not wait:
return True
def release_write_lock(self):
"""Release the 'write' lock."""
self.condition.acquire()
try:
if self.current_sync_operation is not threading.current_thread():
raise LockError(
"Synchronizer error - current thread doesn't "
"have the write lock"
)
# reset the current sync operation so
# another can get it
self.current_sync_operation = None
# tell everyone to get ready
self.condition.notify_all()
log.debug("%s released write lock", self)
finally:
# everyone go !!
self.condition.release()
|
