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bazarr/libs/dogpile/lock.py
Louis Vézina 645952c61a WIP
2019-09-13 15:12:26 -04:00

189 lines
7 KiB
Python

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.
: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(value, createdtime)
if generated is not NOT_REGENERATED:
generated, createdtime = generated
return generated
elif value is NOT_REGENERATED:
# we called upon the creator, and it said that it
# didn't regenerate. this typically means another
# thread is running the creation function, and that the
# cache should still have a value. However,
# we don't have a value at all, which is unusual since we just
# checked for it, so check again (TODO: is this a real codepath?)
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, value, createdtime):
if not self._is_expired(createdtime):
return NOT_REGENERATED
_async = False
if self._has_value(createdtime):
has_value = True
if not self.mutex.acquire(False):
log.debug(
"creation function in progress "
"elsewhere, returning")
return NOT_REGENERATED
else:
has_value = False
log.debug("no value, waiting for create lock")
self.mutex.acquire()
try:
log.debug("value creation lock %r acquired" % self.mutex)
if not has_value:
# we entered without a value, or at least with "creationtime ==
# 0". Run the "getter" function again, to see if another
# thread has already generated the value while we waited on the
# mutex, or if the caller is otherwise telling us there is a
# value already which allows us to use async regeneration. (the
# latter is used by the multi-key routine).
try:
value, createdtime = self.value_and_created_fn()
except NeedRegenerationException:
# nope, nobody created the value, we're it.
# we must create it right now
pass
else:
has_value = True
# caller is telling us there is a value and that we can
# use async creation if it is expired.
if not self._is_expired(createdtime):
# it's not expired, return it
log.debug("Concurrent thread created the value")
return value, createdtime
# otherwise it's expired, call creator again
if has_value and self.async_creator:
# we have a value we can return, safe to use async_creator
log.debug("Passing creation lock to async runner")
# so...run it!
self.async_creator(self.mutex)
_async = True
# and return the expired value for now
return value, createdtime
# it's expired, and it's our turn to create it synchronously, *or*,
# there's no value at all, and we have to create it synchronously
log.debug(
"Calling creation function for %s value",
"not-yet-present" if not has_value else
"previously expired"
)
return self.creator()
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