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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".
:param async_creator: A callable. If specified, this callable will be
passed the mutex as an argument and is responsible for releasing the mutex
after it finishes some asynchronous value creation. The intent is for
this to be used to defer invocation of the creator callable until some
later time.
"""
def __init__(self,
mutex,
creator,
value_and_created_fn,
expiretime,
async_creator=None,
):
self.mutex = mutex
self.creator = creator
self.value_and_created_fn = value_and_created_fn
self.expiretime = expiretime
self.async_creator = async_creator
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
async = False
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
elif self.async_creator:
log.debug("Passing creation lock to async runner")
self.async_creator(self.mutex)
async = True
return value, createdtime
log.debug("Calling creation function")
created = self.creator()
return created
finally:
if not async:
self.mutex.release()
log.debug("Released creation lock")
def __enter__(self):
return self._enter()
def __exit__(self, type, value, traceback):
pass
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