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#!/usr/bin/env python3
"""File-based locking helper."""
import datetime
import json
import logging
import os
import signal
import sys
import warnings
from dataclasses import dataclass
from typing import Optional
import config
import datetime_utils
import decorator_utils
cfg = config.add_commandline_args(f'Lockfile ({__file__})', 'Args related to lockfiles')
cfg.add_argument(
'--lockfile_held_duration_warning_threshold_sec',
type=float,
default=60.0,
metavar='SECONDS',
help='If a lock is held for longer than this threshold we log a warning',
)
logger = logging.getLogger(__name__)
class LockFileException(Exception):
"""An exception related to lock files."""
pass
@dataclass
class LockFileContents:
"""The contents we'll write to each lock file."""
pid: int
commandline: str
expiration_timestamp: Optional[float]
class LockFile(object):
"""A file locking mechanism that has context-manager support so you
can use it in a with statement. e.g.
with LockFile('./foo.lock'):
# do a bunch of stuff... if the process dies we have a signal
# handler to do cleanup. Other code (in this process or another)
# that tries to take the same lockfile will block. There is also
# some logic for detecting stale locks.
"""
def __init__(
self,
lockfile_path: str,
*,
do_signal_cleanup: bool = True,
expiration_timestamp: Optional[float] = None,
override_command: Optional[str] = None,
) -> None:
self.is_locked: bool = False
self.lockfile: str = lockfile_path
self.locktime: Optional[int] = None
self.override_command: Optional[str] = override_command
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("Trying to acquire %s.", 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('Success; I own %s.', self.lockfile)
self.is_locked = True
return True
except OSError:
pass
logger.warning('Couldn\'t acquire %s.', 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():
self.locktime = datetime.datetime.now().timestamp()
return self
msg = f"Couldn't acquire {self.lockfile}; giving up."
logger.warning(msg)
raise LockFileException(msg)
def __exit__(self, _, value, traceback):
if self.locktime:
ts = datetime.datetime.now().timestamp()
duration = ts - self.locktime
if duration >= config.config['lockfile_held_duration_warning_threshold_sec']:
str_duration = datetime_utils.describe_duration_briefly(duration)
msg = f'Held {self.lockfile} for {str_duration}'
logger.warning(msg)
warnings.warn(msg, stacklevel=2)
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:
if self.override_command:
cmd = self.override_command
else:
cmd = ' '.join(sys.argv)
contents = LockFileContents(
pid=os.getpid(),
commandline=cmd,
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('Blocking lock contents="%s"', contents)
# Does the PID exist still?
try:
os.kill(contents.pid, 0)
except OSError:
msg = f'Lockfile {self.lockfile}\'s pid ({contents.pid}) is stale; force acquiring'
logger.warning(msg)
self.release()
# Has the lock expiration expired?
if contents.expiration_timestamp is not None:
now = datetime.datetime.now().timestamp()
if now > contents.expiration_timestamp:
msg = f'Lockfile {self.lockfile} expiration time has passed; force acquiring'
logger.warning(msg)
self.release()
except Exception:
pass
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