242 lines
7.8 KiB
Python
242 lines
7.8 KiB
Python
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# -*- test-case-name: twisted.test.test_lockfile -*-
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# Copyright (c) 2005 Divmod, Inc.
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# Copyright (c) Twisted Matrix Laboratories.
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# See LICENSE for details.
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"""
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Filesystem-based interprocess mutex.
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"""
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import errno
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import os
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from time import time as _uniquefloat
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from twisted.python.runtime import platform
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def unique():
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return str(int(_uniquefloat() * 1000))
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from os import rename
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if not platform.isWindows():
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from os import kill, readlink, remove as rmlink, symlink
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_windows = False
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else:
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_windows = True
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# On UNIX, a symlink can be made to a nonexistent location, and
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# FilesystemLock uses this by making the target of the symlink an
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# imaginary, non-existing file named that of the PID of the process with
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# the lock. This has some benefits on UNIX -- making and removing this
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# symlink is atomic. However, because Windows doesn't support symlinks (at
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# least as how we know them), we have to fake this and actually write a
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# file with the PID of the process holding the lock instead.
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# These functions below perform that unenviable, probably-fraught-with-
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# race-conditions duty. - hawkie
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try:
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import pywintypes
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from win32api import OpenProcess
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except ImportError:
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kill = None # type: ignore[assignment]
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else:
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ERROR_ACCESS_DENIED = 5
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ERROR_INVALID_PARAMETER = 87
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# typing ignored due to:
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# https://github.com/python/typeshed/issues/4249
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def kill(pid, signal): # type: ignore[misc]
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try:
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OpenProcess(0, 0, pid)
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except pywintypes.error as e:
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if e.args[0] == ERROR_ACCESS_DENIED:
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return
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elif e.args[0] == ERROR_INVALID_PARAMETER:
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raise OSError(errno.ESRCH, None)
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raise
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else:
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raise RuntimeError("OpenProcess is required to fail.")
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# For monkeypatching in tests
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_open = open
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# typing ignored due to:
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# https://github.com/python/typeshed/issues/4249
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def symlink(value, filename): # type: ignore[misc]
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"""
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Write a file at C{filename} with the contents of C{value}. See the
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above comment block as to why this is needed.
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"""
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# XXX Implement an atomic thingamajig for win32
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newlinkname = filename + "." + unique() + ".newlink"
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newvalname = os.path.join(newlinkname, "symlink")
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os.mkdir(newlinkname)
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# Python 3 does not support the 'commit' flag of fopen in the MSVCRT
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# (http://msdn.microsoft.com/en-us/library/yeby3zcb%28VS.71%29.aspx)
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mode = "w"
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with _open(newvalname, mode) as f:
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f.write(value)
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f.flush()
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try:
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rename(newlinkname, filename)
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except BaseException:
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os.remove(newvalname)
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os.rmdir(newlinkname)
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raise
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# typing ignored due to:
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# https://github.com/python/typeshed/issues/4249
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def readlink(filename): # type: ignore[misc]
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"""
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Read the contents of C{filename}. See the above comment block as to why
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this is needed.
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"""
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try:
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fObj = _open(os.path.join(filename, "symlink"), "r")
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except OSError as e:
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if e.errno == errno.ENOENT or e.errno == errno.EIO:
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raise OSError(e.errno, None)
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raise
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else:
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with fObj:
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result = fObj.read()
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return result
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# typing ignored due to:
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# https://github.com/python/typeshed/issues/4249
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def rmlink(filename): # type: ignore[misc]
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os.remove(os.path.join(filename, "symlink"))
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os.rmdir(filename)
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class FilesystemLock:
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"""
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A mutex.
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This relies on the filesystem property that creating
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a symlink is an atomic operation and that it will
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fail if the symlink already exists. Deleting the
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symlink will release the lock.
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@ivar name: The name of the file associated with this lock.
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@ivar clean: Indicates whether this lock was released cleanly by its
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last owner. Only meaningful after C{lock} has been called and
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returns True.
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@ivar locked: Indicates whether the lock is currently held by this
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object.
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"""
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clean = None
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locked = False
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def __init__(self, name):
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self.name = name
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def lock(self):
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"""
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Acquire this lock.
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@rtype: C{bool}
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@return: True if the lock is acquired, false otherwise.
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@raise OSError: Any exception L{os.symlink()} may raise,
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other than L{errno.EEXIST}.
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"""
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clean = True
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while True:
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try:
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symlink(str(os.getpid()), self.name)
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except OSError as e:
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if _windows and e.errno in (errno.EACCES, errno.EIO):
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# The lock is in the middle of being deleted because we're
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# on Windows where lock removal isn't atomic. Give up, we
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# don't know how long this is going to take.
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return False
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if e.errno == errno.EEXIST:
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try:
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pid = readlink(self.name)
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except OSError as e:
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if e.errno == errno.ENOENT:
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# The lock has vanished, try to claim it in the
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# next iteration through the loop.
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continue
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elif _windows and e.errno == errno.EACCES:
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# The lock is in the middle of being
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# deleted because we're on Windows where
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# lock removal isn't atomic. Give up, we
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# don't know how long this is going to
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# take.
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return False
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raise
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try:
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if kill is not None:
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kill(int(pid), 0)
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except OSError as e:
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if e.errno == errno.ESRCH:
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# The owner has vanished, try to claim it in the
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# next iteration through the loop.
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try:
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rmlink(self.name)
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except OSError as e:
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if e.errno == errno.ENOENT:
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# Another process cleaned up the lock.
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# Race them to acquire it in the next
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# iteration through the loop.
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continue
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raise
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clean = False
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continue
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raise
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return False
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raise
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self.locked = True
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self.clean = clean
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return True
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def unlock(self):
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"""
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Release this lock.
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This deletes the directory with the given name.
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@raise OSError: Any exception L{os.readlink()} may raise.
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@raise ValueError: If the lock is not owned by this process.
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"""
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pid = readlink(self.name)
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if int(pid) != os.getpid():
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raise ValueError(f"Lock {self.name!r} not owned by this process")
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rmlink(self.name)
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self.locked = False
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def isLocked(name):
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"""
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Determine if the lock of the given name is held or not.
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@type name: C{str}
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@param name: The filesystem path to the lock to test
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@rtype: C{bool}
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@return: True if the lock is held, False otherwise.
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"""
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l = FilesystemLock(name)
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result = None
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try:
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result = l.lock()
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finally:
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if result:
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l.unlock()
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return not result
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__all__ = ["FilesystemLock", "isLocked"]
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