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"""Provides a mechanism for not competing with other processes interacting with an MSAL cache."""
import os
import sys
import errno
import time
import logging
import portalocker # pylint: disable=import-error
logger = logging.getLogger(__name__)
LockError = portalocker.exceptions.LockException
class CrossPlatLock(object):
"""Offers a mechanism for waiting until another process is finished interacting with a shared
resource. This is specifically written to interact with a class of the same name in the .NET
extensions library.
"""
def __init__(self, lockfile_path):
self._lockpath = lockfile_path
self._lock = portalocker.Lock(
lockfile_path,
mode='wb+',
# In posix systems, we HAVE to use LOCK_EX(exclusive lock) bitwise ORed
# with LOCK_NB(non-blocking) to avoid blocking on lock acquisition.
# More information here:
# https://docs.python.org/3/library/fcntl.html#fcntl.lockf
flags=portalocker.LOCK_EX | portalocker.LOCK_NB,
# Support for passing through arguments to the open syscall
# was added in Portalocker v1.4.0 (2019-02-11).
buffering=0,
)
def _try_to_create_lock_file(self):
timeout = 5
check_interval = 0.25
current_time = getattr(time, "monotonic", time.time)
timeout_end = current_time() + timeout
pid = os.getpid()
while timeout_end > current_time():
try:
with open(self._lockpath, 'x'): # pylint: disable=unspecified-encoding
return True
except ValueError: # This needs to be the first clause, for Python 2 to hit it
logger.warning("Python 2 does not support atomic creation of file")
return False
except FileExistsError: # Only Python 3 will reach this clause
logger.debug(
"Process %d found existing lock file, will retry after %f second",
pid, check_interval)
time.sleep(check_interval)
return False
def __enter__(self):
pid = os.getpid()
if not self._try_to_create_lock_file():
logger.warning("Process %d failed to create lock file", pid)
file_handle = self._lock.__enter__()
file_handle.write('{} {}'.format(pid, sys.argv[0]).encode('utf-8')) # pylint: disable=consider-using-f-string
return file_handle
def __exit__(self, *args):
self._lock.__exit__(*args)
try:
# Attempt to delete the lockfile. In either of the failure cases enumerated below, it is
# likely that another process has raced this one and ended up clearing or locking the
# file for itself.
os.remove(self._lockpath)
except OSError as ex: # pylint: disable=invalid-name
if ex.errno not in (errno.ENOENT, errno.EACCES):
raise
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