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bazarr/libs/dogpile/cache/backends/file.py

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13 KiB

"""
File Backends
------------------
Provides backends that deal with local filesystem access.
"""
from contextlib import contextmanager
import dbm
import os
import threading
from ..api import BytesBackend
from ..api import NO_VALUE
from ... import util
__all__ = ["DBMBackend", "FileLock", "AbstractFileLock"]
class DBMBackend(BytesBackend):
"""A file-backend using a dbm file to store keys.
Basic usage::
from dogpile.cache import make_region
region = make_region().configure(
'dogpile.cache.dbm',
expiration_time = 3600,
arguments = {
"filename":"/path/to/cachefile.dbm"
}
)
DBM access is provided using the Python ``anydbm`` module,
which selects a platform-specific dbm module to use.
This may be made to be more configurable in a future
release.
Note that different dbm modules have different behaviors.
Some dbm implementations handle their own locking, while
others don't. The :class:`.DBMBackend` uses a read/write
lockfile by default, which is compatible even with those
DBM implementations for which this is unnecessary,
though the behavior can be disabled.
The DBM backend by default makes use of two lockfiles.
One is in order to protect the DBM file itself from
concurrent writes, the other is to coordinate
value creation (i.e. the dogpile lock). By default,
these lockfiles use the ``flock()`` system call
for locking; this is **only available on Unix
platforms**. An alternative lock implementation, such as one
which is based on threads or uses a third-party system
such as `portalocker <https://pypi.python.org/pypi/portalocker>`_,
can be dropped in using the ``lock_factory`` argument
in conjunction with the :class:`.AbstractFileLock` base class.
Currently, the dogpile lock is against the entire
DBM file, not per key. This means there can
only be one "creator" job running at a time
per dbm file.
A future improvement might be to have the dogpile lock
using a filename that's based on a modulus of the key.
Locking on a filename that uniquely corresponds to the
key is problematic, since it's not generally safe to
delete lockfiles as the application runs, implying an
unlimited number of key-based files would need to be
created and never deleted.
Parameters to the ``arguments`` dictionary are
below.
:param filename: path of the filename in which to
create the DBM file. Note that some dbm backends
will change this name to have additional suffixes.
:param rw_lockfile: the name of the file to use for
read/write locking. If omitted, a default name
is used by appending the suffix ".rw.lock" to the
DBM filename. If False, then no lock is used.
:param dogpile_lockfile: the name of the file to use
for value creation, i.e. the dogpile lock. If
omitted, a default name is used by appending the
suffix ".dogpile.lock" to the DBM filename. If
False, then dogpile.cache uses the default dogpile
lock, a plain thread-based mutex.
:param lock_factory: a function or class which provides
for a read/write lock. Defaults to :class:`.FileLock`.
Custom implementations need to implement context-manager
based ``read()`` and ``write()`` functions - the
:class:`.AbstractFileLock` class is provided as a base class
which provides these methods based on individual read/write lock
functions. E.g. to replace the lock with the dogpile.core
:class:`.ReadWriteMutex`::
from dogpile.core.readwrite_lock import ReadWriteMutex
from dogpile.cache.backends.file import AbstractFileLock
class MutexLock(AbstractFileLock):
def __init__(self, filename):
self.mutex = ReadWriteMutex()
def acquire_read_lock(self, wait):
ret = self.mutex.acquire_read_lock(wait)
return wait or ret
def acquire_write_lock(self, wait):
ret = self.mutex.acquire_write_lock(wait)
return wait or ret
def release_read_lock(self):
return self.mutex.release_read_lock()
def release_write_lock(self):
return self.mutex.release_write_lock()
from dogpile.cache import make_region
region = make_region().configure(
"dogpile.cache.dbm",
expiration_time=300,
arguments={
"filename": "file.dbm",
"lock_factory": MutexLock
}
)
While the included :class:`.FileLock` uses ``os.flock()``, a
windows-compatible implementation can be built using a library
such as `portalocker <https://pypi.python.org/pypi/portalocker>`_.
.. versionadded:: 0.5.2
"""
def __init__(self, arguments):
self.filename = os.path.abspath(
os.path.normpath(arguments["filename"])
)
dir_, filename = os.path.split(self.filename)
self.lock_factory = arguments.get("lock_factory", FileLock)
self._rw_lock = self._init_lock(
arguments.get("rw_lockfile"), ".rw.lock", dir_, filename
)
self._dogpile_lock = self._init_lock(
arguments.get("dogpile_lockfile"),
".dogpile.lock",
dir_,
filename,
util.KeyReentrantMutex.factory,
)
self._init_dbm_file()
def _init_lock(self, argument, suffix, basedir, basefile, wrapper=None):
if argument is None:
lock = self.lock_factory(os.path.join(basedir, basefile + suffix))
elif argument is not False:
lock = self.lock_factory(
os.path.abspath(os.path.normpath(argument))
)
else:
return None
if wrapper:
lock = wrapper(lock)
return lock
def _init_dbm_file(self):
exists = os.access(self.filename, os.F_OK)
if not exists:
for ext in ("db", "dat", "pag", "dir"):
if os.access(self.filename + os.extsep + ext, os.F_OK):
exists = True
break
if not exists:
fh = dbm.open(self.filename, "c")
fh.close()
def get_mutex(self, key):
# using one dogpile for the whole file. Other ways
# to do this might be using a set of files keyed to a
# hash/modulus of the key. the issue is it's never
# really safe to delete a lockfile as this can
# break other processes trying to get at the file
# at the same time - so handling unlimited keys
# can't imply unlimited filenames
if self._dogpile_lock:
return self._dogpile_lock(key)
else:
return None
@contextmanager
def _use_rw_lock(self, write):
if self._rw_lock is None:
yield
elif write:
with self._rw_lock.write():
yield
else:
with self._rw_lock.read():
yield
@contextmanager
def _dbm_file(self, write):
with self._use_rw_lock(write):
with dbm.open(self.filename, "w" if write else "r") as dbm_obj:
yield dbm_obj
def get_serialized(self, key):
with self._dbm_file(False) as dbm_obj:
if hasattr(dbm_obj, "get"):
value = dbm_obj.get(key, NO_VALUE)
else:
# gdbm objects lack a .get method
try:
value = dbm_obj[key]
except KeyError:
value = NO_VALUE
return value
def get_serialized_multi(self, keys):
return [self.get_serialized(key) for key in keys]
def set_serialized(self, key, value):
with self._dbm_file(True) as dbm_obj:
dbm_obj[key] = value
def set_serialized_multi(self, mapping):
with self._dbm_file(True) as dbm_obj:
for key, value in mapping.items():
dbm_obj[key] = value
def delete(self, key):
with self._dbm_file(True) as dbm_obj:
try:
del dbm_obj[key]
except KeyError:
pass
def delete_multi(self, keys):
with self._dbm_file(True) as dbm_obj:
for key in keys:
try:
del dbm_obj[key]
except KeyError:
pass
class AbstractFileLock:
"""Coordinate read/write access to a file.
typically is a file-based lock but doesn't necessarily have to be.
The default implementation here is :class:`.FileLock`.
Implementations should provide the following methods::
* __init__()
* acquire_read_lock()
* acquire_write_lock()
* release_read_lock()
* release_write_lock()
The ``__init__()`` method accepts a single argument "filename", which
may be used as the "lock file", for those implementations that use a lock
file.
Note that multithreaded environments must provide a thread-safe
version of this lock. The recommended approach for file-
descriptor-based locks is to use a Python ``threading.local()`` so
that a unique file descriptor is held per thread. See the source
code of :class:`.FileLock` for an implementation example.
"""
def __init__(self, filename):
"""Constructor, is given the filename of a potential lockfile.
The usage of this filename is optional and no file is
created by default.
Raises ``NotImplementedError`` by default, must be
implemented by subclasses.
"""
raise NotImplementedError()
def acquire(self, wait=True):
"""Acquire the "write" lock.
This is a direct call to :meth:`.AbstractFileLock.acquire_write_lock`.
"""
return self.acquire_write_lock(wait)
def release(self):
"""Release the "write" lock.
This is a direct call to :meth:`.AbstractFileLock.release_write_lock`.
"""
self.release_write_lock()
@contextmanager
def read(self):
"""Provide a context manager for the "read" lock.
This method makes use of :meth:`.AbstractFileLock.acquire_read_lock`
and :meth:`.AbstractFileLock.release_read_lock`
"""
self.acquire_read_lock(True)
try:
yield
finally:
self.release_read_lock()
@contextmanager
def write(self):
"""Provide a context manager for the "write" lock.
This method makes use of :meth:`.AbstractFileLock.acquire_write_lock`
and :meth:`.AbstractFileLock.release_write_lock`
"""
self.acquire_write_lock(True)
try:
yield
finally:
self.release_write_lock()
@property
def is_open(self):
"""optional method."""
raise NotImplementedError()
def acquire_read_lock(self, wait):
"""Acquire a 'reader' lock.
Raises ``NotImplementedError`` by default, must be
implemented by subclasses.
"""
raise NotImplementedError()
def acquire_write_lock(self, wait):
"""Acquire a 'write' lock.
Raises ``NotImplementedError`` by default, must be
implemented by subclasses.
"""
raise NotImplementedError()
def release_read_lock(self):
"""Release a 'reader' lock.
Raises ``NotImplementedError`` by default, must be
implemented by subclasses.
"""
raise NotImplementedError()
def release_write_lock(self):
"""Release a 'writer' lock.
Raises ``NotImplementedError`` by default, must be
implemented by subclasses.
"""
raise NotImplementedError()
class FileLock(AbstractFileLock):
"""Use lockfiles to coordinate read/write access to a file.
Only works on Unix systems, using
`fcntl.flock() <http://docs.python.org/library/fcntl.html>`_.
"""
def __init__(self, filename):
self._filedescriptor = threading.local()
self.filename = filename
@util.memoized_property
def _module(self):
import fcntl
return fcntl
@property
def is_open(self):
return hasattr(self._filedescriptor, "fileno")
def acquire_read_lock(self, wait):
return self._acquire(wait, os.O_RDONLY, self._module.LOCK_SH)
def acquire_write_lock(self, wait):
return self._acquire(wait, os.O_WRONLY, self._module.LOCK_EX)
def release_read_lock(self):
self._release()
def release_write_lock(self):
self._release()
def _acquire(self, wait, wrflag, lockflag):
wrflag |= os.O_CREAT
fileno = os.open(self.filename, wrflag)
try:
if not wait:
lockflag |= self._module.LOCK_NB
self._module.flock(fileno, lockflag)
except IOError:
os.close(fileno)
if not wait:
# this is typically
# "[Errno 35] Resource temporarily unavailable",
# because of LOCK_NB
return False
else:
raise
else:
self._filedescriptor.fileno = fileno
return True
def _release(self):
try:
fileno = self._filedescriptor.fileno
except AttributeError:
return
else:
self._module.flock(fileno, self._module.LOCK_UN)
os.close(fileno)
del self._filedescriptor.fileno