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