08a1846396
Instead of having a nominally useful single-threaded storage unit that uses a dummy r/w lock and a multi-threaded storage unit just have the storage unit by default protect itself from multi-threading calls being used on it (with the appropriate reader/writer locks being activated to make this work correctly). Change-Id: Ib6879edb465156a8e54fd5b4002550d1cec49137
468 lines
17 KiB
Python
468 lines
17 KiB
Python
# Copyright 2011 OpenStack Foundation.
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# All Rights Reserved.
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#
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# Licensed under the Apache License, Version 2.0 (the "License"); you may
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# not use this file except in compliance with the License. You may obtain
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# a copy of the License at
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#
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# http://www.apache.org/licenses/LICENSE-2.0
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#
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# Unless required by applicable law or agreed to in writing, software
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# distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
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# WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
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# License for the specific language governing permissions and limitations
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# under the License.
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# This is a modified version of what was in oslo-incubator lockutils.py from
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# commit 5039a610355e5265fb9fbd1f4023e8160750f32e but this one does not depend
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# on oslo.cfg or the very large oslo-incubator oslo logging module (which also
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# pulls in oslo.cfg) and is reduced to only what taskflow currently wants to
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# use from that code.
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import collections
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import contextlib
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import errno
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import os
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import threading
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import time
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import six
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from taskflow import logging
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from taskflow.utils import misc
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from taskflow.utils import threading_utils as tu
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LOG = logging.getLogger(__name__)
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@contextlib.contextmanager
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def try_lock(lock):
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"""Attempts to acquire a lock, and auto releases if acquired (on exit)."""
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# NOTE(harlowja): the keyword argument for 'blocking' does not work
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# in py2.x and only is fixed in py3.x (this adjustment is documented
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# and/or debated in http://bugs.python.org/issue10789); so we'll just
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# stick to the format that works in both (oddly the keyword argument
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# works in py2.x but only with reentrant locks).
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was_locked = lock.acquire(False)
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try:
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yield was_locked
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finally:
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if was_locked:
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lock.release()
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def locked(*args, **kwargs):
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"""A locking decorator.
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It will look for a provided attribute (typically a lock or a list
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of locks) on the first argument of the function decorated (typically this
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is the 'self' object) and before executing the decorated function it
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activates the given lock or list of locks as a context manager,
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automatically releasing that lock on exit.
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NOTE(harlowja): if no attribute is provided then by default the attribute
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named '_lock' is looked for in the instance object this decorator is
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attached to.
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NOTE(harlowja): when we get the wrapt module approved we can address the
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correctness of this decorator with regards to classmethods, to keep sanity
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and correctness it is recommended to avoid using this on classmethods, once
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https://review.openstack.org/#/c/94754/ is merged this will be refactored
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and that use-case can be provided in a correct manner.
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"""
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def decorator(f):
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attr_name = kwargs.get('lock', '_lock')
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@six.wraps(f)
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def wrapper(self, *args, **kwargs):
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lock = getattr(self, attr_name)
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if isinstance(lock, (tuple, list)):
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lock = MultiLock(locks=list(lock))
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with lock:
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return f(self, *args, **kwargs)
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return wrapper
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# This is needed to handle when the decorator has args or the decorator
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# doesn't have args, python is rather weird here...
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if kwargs or not args:
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return decorator
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else:
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if len(args) == 1:
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return decorator(args[0])
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else:
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return decorator
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class ReaderWriterLock(object):
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"""A reader/writer lock.
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This lock allows for simultaneous readers to exist but only one writer
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to exist for use-cases where it is useful to have such types of locks.
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Currently a reader can not escalate its read lock to a write lock and
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a writer can not acquire a read lock while it owns or is waiting on
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the write lock.
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In the future these restrictions may be relaxed.
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This can be eventually removed if http://bugs.python.org/issue8800 ever
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gets accepted into the python standard threading library...
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"""
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WRITER = 'w'
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READER = 'r'
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def __init__(self):
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self._writer = None
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self._pending_writers = collections.deque()
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self._readers = collections.deque()
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self._cond = threading.Condition()
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@property
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def has_pending_writers(self):
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"""Returns if there are writers waiting to become the *one* writer."""
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self._cond.acquire()
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try:
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return bool(self._pending_writers)
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finally:
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self._cond.release()
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def is_writer(self, check_pending=True):
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"""Returns if the caller is the active writer or a pending writer."""
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self._cond.acquire()
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try:
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me = tu.get_ident()
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if self._writer is not None and self._writer == me:
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return True
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if check_pending:
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return me in self._pending_writers
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else:
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return False
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finally:
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self._cond.release()
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@property
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def owner(self):
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"""Returns whether the lock is locked by a writer or reader."""
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self._cond.acquire()
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try:
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if self._writer is not None:
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return self.WRITER
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if self._readers:
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return self.READER
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return None
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finally:
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self._cond.release()
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def is_reader(self):
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"""Returns if the caller is one of the readers."""
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self._cond.acquire()
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try:
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return tu.get_ident() in self._readers
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finally:
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self._cond.release()
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@contextlib.contextmanager
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def read_lock(self):
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"""Context manager that grants a read lock.
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Will wait until no active or pending writers.
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Raises a RuntimeError if an active or pending writer tries to acquire
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a read lock.
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"""
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me = tu.get_ident()
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if self.is_writer():
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raise RuntimeError("Writer %s can not acquire a read lock"
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" while holding/waiting for the write lock"
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% me)
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self._cond.acquire()
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try:
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while True:
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# No active writer; we are good to become a reader.
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if self._writer is None:
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self._readers.append(me)
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break
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# An active writer; guess we have to wait.
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self._cond.wait()
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finally:
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self._cond.release()
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try:
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yield self
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finally:
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# I am no longer a reader, remove *one* occurrence of myself.
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# If the current thread acquired two read locks, then it will
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# still have to remove that other read lock; this allows for
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# basic reentrancy to be possible.
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self._cond.acquire()
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try:
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self._readers.remove(me)
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self._cond.notify_all()
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finally:
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self._cond.release()
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@contextlib.contextmanager
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def write_lock(self):
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"""Context manager that grants a write lock.
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Will wait until no active readers. Blocks readers after acquiring.
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Raises a RuntimeError if an active reader attempts to acquire a lock.
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"""
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me = tu.get_ident()
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if self.is_reader():
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raise RuntimeError("Reader %s to writer privilege"
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" escalation not allowed" % me)
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if self.is_writer(check_pending=False):
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# Already the writer; this allows for basic reentrancy.
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yield self
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else:
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self._cond.acquire()
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try:
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self._pending_writers.append(me)
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while True:
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# No readers, and no active writer, am I next??
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if len(self._readers) == 0 and self._writer is None:
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if self._pending_writers[0] == me:
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self._writer = self._pending_writers.popleft()
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break
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self._cond.wait()
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finally:
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self._cond.release()
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try:
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yield self
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finally:
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self._cond.acquire()
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try:
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self._writer = None
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self._cond.notify_all()
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finally:
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self._cond.release()
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class MultiLock(object):
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"""A class which attempts to obtain & release many locks at once.
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It is typically useful as a context manager around many locks (instead of
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having to nest individual lock context managers, which can become pretty
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awkward looking).
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NOTE(harlowja): The locks that will be obtained will be in the order the
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locks are given in the constructor, they will be acquired in order and
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released in reverse order (so ordering matters).
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"""
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def __init__(self, locks):
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if not isinstance(locks, tuple):
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locks = tuple(locks)
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if len(locks) <= 0:
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raise ValueError("Zero locks requested")
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self._locks = locks
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self._local = threading.local()
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@property
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def _lock_stacks(self):
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# This is weird, but this is how thread locals work (in that each
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# thread will need to check if it has already created the attribute and
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# if not then create it and set it to the thread local variable...)
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#
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# This isn't done in the constructor since the constructor is only
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# activated by one of the many threads that could use this object,
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# and that means that the attribute will only exist for that one
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# thread.
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try:
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return self._local.stacks
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except AttributeError:
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self._local.stacks = []
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return self._local.stacks
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def __enter__(self):
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return self.acquire()
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@property
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def obtained(self):
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"""Returns how many locks were last acquired/obtained."""
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try:
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return self._lock_stacks[-1]
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except IndexError:
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return 0
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def __len__(self):
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return len(self._locks)
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def acquire(self):
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"""This will attempt to acquire all the locks given in the constructor.
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If all the locks can not be acquired (and say only X of Y locks could
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be acquired then this will return false to signify that not all the
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locks were able to be acquired, you can later use the :attr:`.obtained`
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property to determine how many were obtained during the last
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acquisition attempt).
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NOTE(harlowja): When not all locks were acquired it is still required
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to release since under partial acquisition the acquired locks
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must still be released. For example if 4 out of 5 locks were acquired
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this will return false, but the user **must** still release those
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other 4 to avoid causing locking issues...
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"""
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gotten = 0
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for lock in self._locks:
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try:
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acked = lock.acquire()
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except (threading.ThreadError, RuntimeError) as e:
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# If we have already gotten some set of the desired locks
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# make sure we track that and ensure that we later release them
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# instead of losing them.
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if gotten:
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self._lock_stacks.append(gotten)
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raise threading.ThreadError(
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"Unable to acquire lock %s/%s due to '%s'"
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% (gotten + 1, len(self._locks), e))
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else:
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if not acked:
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break
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else:
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gotten += 1
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if gotten:
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self._lock_stacks.append(gotten)
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return gotten == len(self._locks)
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def __exit__(self, type, value, traceback):
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self.release()
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def release(self):
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"""Releases any past acquired locks (partial or otherwise)."""
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height = len(self._lock_stacks)
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if not height:
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# Raise the same error type as the threading.Lock raises so that
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# it matches the behavior of the built-in class (it's odd though
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# that the threading.RLock raises a runtime error on this same
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# method instead...)
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raise threading.ThreadError('Release attempted on unlocked lock')
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# Cleans off one level of the stack (this is done so that if there
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# are multiple __enter__() and __exit__() pairs active that this will
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# only remove one level (the last one), and not all levels...
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leftover = self._lock_stacks[-1]
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while leftover:
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lock = self._locks[leftover - 1]
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try:
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lock.release()
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except (threading.ThreadError, RuntimeError) as e:
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# Ensure that we adjust the lock stack under failure so that
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# if release is attempted again that we do not try to release
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# the locks we already released...
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self._lock_stacks[-1] = leftover
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raise threading.ThreadError(
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"Unable to release lock %s/%s due to '%s'"
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% (leftover, len(self._locks), e))
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else:
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leftover -= 1
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# At the end only clear it off, so that under partial failure we don't
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# lose any locks...
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self._lock_stacks.pop()
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class _InterProcessLock(object):
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"""An interprocess locking implementation.
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This is a lock implementation which allows multiple locks, working around
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issues like bugs.debian.org/cgi-bin/bugreport.cgi?bug=632857 and does
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not require any cleanup. Since the lock is always held on a file
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descriptor rather than outside of the process, the lock gets dropped
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automatically if the process crashes, even if __exit__ is not executed.
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There are no guarantees regarding usage by multiple green threads in a
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single process here. This lock works only between processes.
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Note these locks are released when the descriptor is closed, so it's not
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safe to close the file descriptor while another green thread holds the
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lock. Just opening and closing the lock file can break synchronisation,
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so lock files must be accessed only using this abstraction.
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"""
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def __init__(self, name):
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self.lockfile = None
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self.fname = name
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def acquire(self):
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basedir = os.path.dirname(self.fname)
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if not os.path.exists(basedir):
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misc.ensure_tree(basedir)
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LOG.debug('Created lock path: %s', basedir)
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self.lockfile = open(self.fname, 'w')
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while True:
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try:
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# Using non-blocking locks since green threads are not
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# patched to deal with blocking locking calls.
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# Also upon reading the MSDN docs for locking(), it seems
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# to have a laughable 10 attempts "blocking" mechanism.
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self.trylock()
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LOG.debug('Got file lock "%s"', self.fname)
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return True
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except IOError as e:
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if e.errno in (errno.EACCES, errno.EAGAIN):
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# external locks synchronise things like iptables
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# updates - give it some time to prevent busy spinning
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time.sleep(0.01)
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else:
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raise threading.ThreadError("Unable to acquire lock on"
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" `%(filename)s` due to"
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" %(exception)s" %
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{
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'filename': self.fname,
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'exception': e,
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})
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def __enter__(self):
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self.acquire()
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return self
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def release(self):
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try:
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self.unlock()
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self.lockfile.close()
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LOG.debug('Released file lock "%s"', self.fname)
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except IOError:
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LOG.exception("Could not release the acquired lock `%s`",
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self.fname)
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def __exit__(self, exc_type, exc_val, exc_tb):
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self.release()
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def exists(self):
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return os.path.exists(self.fname)
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def trylock(self):
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raise NotImplementedError()
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def unlock(self):
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raise NotImplementedError()
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class _WindowsLock(_InterProcessLock):
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def trylock(self):
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msvcrt.locking(self.lockfile.fileno(), msvcrt.LK_NBLCK, 1)
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def unlock(self):
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msvcrt.locking(self.lockfile.fileno(), msvcrt.LK_UNLCK, 1)
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class _PosixLock(_InterProcessLock):
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def trylock(self):
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fcntl.lockf(self.lockfile, fcntl.LOCK_EX | fcntl.LOCK_NB)
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def unlock(self):
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fcntl.lockf(self.lockfile, fcntl.LOCK_UN)
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if os.name == 'nt':
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import msvcrt
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InterProcessLock = _WindowsLock
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else:
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import fcntl
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InterProcessLock = _PosixLock
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