"""A cross-process lock based on exclusive creation of a given file name"""
import os
import sys
import errno
import time
import logging
logger = logging.getLogger(__name__)
class LockError(RuntimeError):
"""It will be raised when unable to obtain a lock"""
class CrossPlatLock(object):
"""This implementation relies only on ``open(..., 'x')``"""
def __init__(self, lockfile_path):
self._lockpath = lockfile_path
def __enter__(self):
self._create_lock_file('{} {}'.format(
os.getpid(),
sys.argv[0],
).encode('utf-8')) # pylint: disable=consider-using-f-string
return self
def _create_lock_file(self, content):
timeout = 5
check_interval = 0.25
current_time = getattr(time, "monotonic", time.time)
timeout_end = current_time() + timeout
while timeout_end > current_time():
try:
with open(self._lockpath, 'xb') as lock_file: # pylint: disable=unspecified-encoding
lock_file.write(content)
return None # Happy path
except ValueError: # This needs to be the first clause, for Python 2 to hit it
raise LockError("Python 2 does not support atomic creation of file")
except FileExistsError: # Only Python 3 will reach this clause
logger.debug(
"Process %d found existing lock file, will retry after %f second",
os.getpid(), check_interval)
time.sleep(check_interval)
raise LockError(
"Unable to obtain lock, despite trying for {} second(s). "
"You may want to manually remove the stale lock file {}".format(
timeout,
self._lockpath,
))
def __exit__(self, *args):
try:
os.remove(self._lockpath)
except OSError as ex: # pylint: disable=invalid-name
if ex.errno in (errno.ENOENT, errno.EACCES):
# Probably another process has raced this one
# and ended up clearing or locking the file for itself.
logger.debug("Unable to remove lock file")
else:
raise