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|
#!/usr/bin/env python3
from dataclasses import dataclass
import datetime
import json
import logging
import os
import signal
import sys
from typing import Optional
import decorator_utils
logger = logging.getLogger(__name__)
class LockFileException(Exception):
pass
@dataclass
class LockFileContents:
pid: int
commandline: str
expiration_timestamp: float
class LockFile(object):
"""A file locking mechanism that has context-manager support so you
can use it in a with statement.
"""
def __init__(
self,
lockfile_path: str,
*,
do_signal_cleanup: bool = True,
expiration_timestamp: Optional[float] = None,
) -> None:
self.is_locked = False
self.lockfile = lockfile_path
if do_signal_cleanup:
signal.signal(signal.SIGINT, self._signal)
signal.signal(signal.SIGTERM, self._signal)
self.expiration_timestamp = expiration_timestamp
def locked(self):
return self.is_locked
def available(self):
return not os.path.exists(self.lockfile)
def try_acquire_lock_once(self) -> bool:
logger.debug(f"Trying to acquire {self.lockfile}.")
try:
# Attempt to create the lockfile. These flags cause
# os.open to raise an OSError if the file already
# exists.
fd = os.open(self.lockfile, os.O_CREAT | os.O_EXCL | os.O_RDWR)
with os.fdopen(fd, "a") as f:
contents = self._get_lockfile_contents()
logger.debug(contents)
f.write(contents)
logger.debug(f'Success; I own {self.lockfile}.')
self.is_locked = True
return True
except OSError:
pass
logger.debug(f'Failed; I could not acquire {self.lockfile}.')
return False
def acquire_with_retries(
self,
*,
initial_delay: float = 1.0,
backoff_factor: float = 2.0,
max_attempts = 5
) -> bool:
@decorator_utils.retry_if_false(tries = max_attempts,
delay_sec = initial_delay,
backoff = backoff_factor)
def _try_acquire_lock_with_retries() -> bool:
success = self.try_acquire_lock_once()
if not success and os.path.exists(self.lockfile):
self._detect_stale_lockfile()
return success
if os.path.exists(self.lockfile):
self._detect_stale_lockfile()
return _try_acquire_lock_with_retries()
def release(self):
try:
os.unlink(self.lockfile)
except Exception as e:
logger.exception(e)
self.is_locked = False
def __enter__(self):
if self.acquire_with_retries():
return self
msg = f"Couldn't acquire {self.lockfile}; giving up."
logger.warning(msg)
raise LockFileException(msg)
def __exit__(self, type, value, traceback):
self.release()
def __del__(self):
if self.is_locked:
self.release()
def _signal(self, *args):
if self.is_locked:
self.release()
def _get_lockfile_contents(self) -> str:
contents = LockFileContents(
pid = os.getpid(),
commandline = ' '.join(sys.argv),
expiration_timestamp = self.expiration_timestamp
)
return json.dumps(contents.__dict__)
def _detect_stale_lockfile(self) -> None:
try:
with open(self.lockfile, 'r') as rf:
lines = rf.readlines()
if len(lines) == 1:
line = lines[0]
line_dict = json.loads(line)
contents = LockFileContents(**line_dict)
logger.debug(f'Blocking lock contents="{contents}"')
# Does the PID exist still?
try:
os.kill(contents.pid, 0)
except OSError:
logger.debug('The pid seems stale; killing the lock.')
self.release()
# Has the lock expiration expired?
if contents.expiration_timestamp is not None:
now = datetime.datetime.now().timestamp()
if now > contents.expiration_datetime:
logger.debug('The expiration time has passed; ' +
'killing the lock')
self.release()
except Exception:
pass
|
