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import time
import logging
log = logging.getLogger(__name__)
class NeedRegenerationException(Exception):
"""An exception that when raised in the 'with' block,
forces the 'has_value' flag to False and incurs a
regeneration of the value.
"""
NOT_REGENERATED = object()
class Lock(object):
"""Dogpile lock class.
Provides an interface around an arbitrary mutex
that allows one thread/process to be elected as
the creator of a new value, while other threads/processes
continue to return the previous version
of that value.
.. versionadded:: 0.4.0
The :class:`.Lock` class was added as a single-use object
representing the dogpile API without dependence on
any shared state between multiple instances.
:param mutex: A mutex object that provides ``acquire()``
and ``release()`` methods.
:param creator: Callable which returns a tuple of the form
(new_value, creation_time). "new_value" should be a newly
generated value representing completed state. "creation_time"
should be a floating point time value which is relative
to Python's ``time.time()`` call, representing the time
at which the value was created. This time value should
be associated with the created value.
:param value_and_created_fn: Callable which returns
a tuple of the form (existing_value, creation_time). This
basically should return what the last local call to the ``creator()``
callable has returned, i.e. the value and the creation time,
which would be assumed here to be from a cache. If the
value is not available, the :class:`.NeedRegenerationException`
exception should be thrown.
:param expiretime: Expiration time in seconds. Set to
``None`` for never expires. This timestamp is compared
to the creation_time result and ``time.time()`` to determine if
the value returned by value_and_created_fn is "expired".
"""
def __init__(self,
mutex,
creator,
value_and_created_fn,
expiretime
):
self.mutex = mutex
self.creator = creator
self.value_and_created_fn = value_and_created_fn
self.expiretime = expiretime
def _is_expired(self, createdtime):
"""Return true if the expiration time is reached, or no
value is available."""
return not self._has_value(createdtime) or \
(
self.expiretime is not None and
time.time() - createdtime > self.expiretime
)
def _has_value(self, createdtime):
"""Return true if the creation function has proceeded
at least once."""
return createdtime > 0
def _enter(self):
value_fn = self.value_and_created_fn
try:
value = value_fn()
value, createdtime = value
except NeedRegenerationException:
log.debug("NeedRegenerationException")
value = NOT_REGENERATED
createdtime = -1
generated = self._enter_create(createdtime)
if generated is not NOT_REGENERATED:
generated, createdtime = generated
return generated
elif value is NOT_REGENERATED:
try:
value, createdtime = value_fn()
return value
except NeedRegenerationException:
raise Exception("Generation function should "
"have just been called by a concurrent "
"thread.")
else:
return value
def _enter_create(self, createdtime):
if not self._is_expired(createdtime):
return NOT_REGENERATED
if self._has_value(createdtime):
if not self.mutex.acquire(False):
log.debug("creation function in progress "
"elsewhere, returning")
return NOT_REGENERATED
else:
log.debug("no value, waiting for create lock")
self.mutex.acquire()
try:
log.debug("value creation lock %r acquired" % self.mutex)
# see if someone created the value already
try:
value, createdtime = self.value_and_created_fn()
except NeedRegenerationException:
pass
else:
if not self._is_expired(createdtime):
log.debug("value already present")
return value, createdtime
log.debug("Calling creation function")
created = self.creator()
return created
finally:
self.mutex.release()
log.debug("Released creation lock")
def __enter__(self):
return self._enter()
def __exit__(self, type, value, traceback):
pass
|
