lockfile.py

   1 #!/usr/bin/env python3
   2
   3 from dataclasses import dataclass
   4 import datetime
   5 import json
   6 import logging
   7 import os
   8 import signal
   9 import sys
  10 from typing import Optional
  11 import warnings
  12
  13 import config
  14 import datetime_utils
  15 import decorator_utils
  16
  17
  18 cfg = config.add_commandline_args(
  19     f'Lockfile ({__file__})',
  20     'Args related to lockfiles')
  21 cfg.add_argument(
  22     '--lockfile_held_duration_warning_threshold_sec',
  23     type=float,
  24     default=10.0,
  25     metavar='SECONDS',
  26     help='If a lock is held for longer than this threshold we log a warning'
  27 )
  28 logger = logging.getLogger(__name__)
  29
  30
  31 class LockFileException(Exception):
  32     pass
  33
  34
  35 @dataclass
  36 class LockFileContents:
  37     pid: int
  38     commandline: str
  39     expiration_timestamp: float
  40
  41
  42 class LockFile(object):
  43     """A file locking mechanism that has context-manager support so you
  44     can use it in a with statement.  e.g.
  45
  46     with LockFile('./foo.lock'):
  47         # do a bunch of stuff... if the process dies we have a signal
  48         # handler to do cleanup.  Other code (in this process or another)
  49         # that tries to take the same lockfile will block.  There is also
  50         # some logic for detecting stale locks.
  51
  52     """
  53     def __init__(
  54             self,
  55             lockfile_path: str,
  56             *,
  57             do_signal_cleanup: bool = True,
  58             expiration_timestamp: Optional[float] = None,
  59             override_command: Optional[str] = None,
  60     ) -> None:
  61         self.is_locked = False
  62         self.lockfile = lockfile_path
  63         self.override_command = override_command
  64         if do_signal_cleanup:
  65             signal.signal(signal.SIGINT, self._signal)
  66             signal.signal(signal.SIGTERM, self._signal)
  67         self.expiration_timestamp = expiration_timestamp
  68
  69     def locked(self):
  70         return self.is_locked
  71
  72     def available(self):
  73         return not os.path.exists(self.lockfile)
  74
  75     def try_acquire_lock_once(self) -> bool:
  76         logger.debug(f"Trying to acquire {self.lockfile}.")
  77         try:
  78             # Attempt to create the lockfile.  These flags cause
  79             # os.open to raise an OSError if the file already
  80             # exists.
  81             fd = os.open(self.lockfile, os.O_CREAT | os.O_EXCL | os.O_RDWR)
  82             with os.fdopen(fd, "a") as f:
  83                 contents = self._get_lockfile_contents()
  84                 logger.debug(contents)
  85                 f.write(contents)
  86             logger.debug(f'Success; I own {self.lockfile}.')
  87             self.is_locked = True
  88             return True
  89         except OSError:
  90             pass
  91         msg = f'Could not acquire {self.lockfile}.'
  92         logger.warning(msg)
  93         return False
  94
  95     def acquire_with_retries(
  96             self,
  97             *,
  98             initial_delay: float = 1.0,
  99             backoff_factor: float = 2.0,
 100             max_attempts = 5
 101     ) -> bool:
 102
 103         @decorator_utils.retry_if_false(tries = max_attempts,
 104                                         delay_sec = initial_delay,
 105                                         backoff = backoff_factor)
 106         def _try_acquire_lock_with_retries() -> bool:
 107             success = self.try_acquire_lock_once()
 108             if not success and os.path.exists(self.lockfile):
 109                 self._detect_stale_lockfile()
 110             return success
 111
 112         if os.path.exists(self.lockfile):
 113             self._detect_stale_lockfile()
 114         return _try_acquire_lock_with_retries()
 115
 116     def release(self):
 117         try:
 118             os.unlink(self.lockfile)
 119         except Exception as e:
 120             logger.exception(e)
 121         self.is_locked = False
 122
 123     def __enter__(self):
 124         if self.acquire_with_retries():
 125             self.locktime = datetime.datetime.now().timestamp()
 126             return self
 127         msg = f"Couldn't acquire {self.lockfile}; giving up."
 128         logger.warning(msg)
 129         raise LockFileException(msg)
 130
 131     def __exit__(self, type, value, traceback):
 132         if self.locktime:
 133             ts = datetime.datetime.now().timestamp()
 134             duration = ts - self.locktime
 135             if duration >= config.config['lockfile_held_duration_warning_threshold_sec']:
 136                 str_duration = datetime_utils.describe_duration_briefly(duration)
 137                 msg = f'Held {self.lockfile} for {str_duration}'
 138                 logger.warning(msg)
 139                 warnings.warn(msg, stacklevel=2)
 140         self.release()
 141
 142     def __del__(self):
 143         if self.is_locked:
 144             self.release()
 145
 146     def _signal(self, *args):
 147         if self.is_locked:
 148             self.release()
 149
 150     def _get_lockfile_contents(self) -> str:
 151         if self.override_command:
 152             cmd = self.override_command
 153         else:
 154             cmd = ' '.join(sys.argv)
 155         print(cmd)
 156         contents = LockFileContents(
 157             pid = os.getpid(),
 158             commandline = cmd,
 159             expiration_timestamp = self.expiration_timestamp,
 160         )
 161         return json.dumps(contents.__dict__)
 162
 163     def _detect_stale_lockfile(self) -> None:
 164         try:
 165             with open(self.lockfile, 'r') as rf:
 166                 lines = rf.readlines()
 167                 if len(lines) == 1:
 168                     line = lines[0]
 169                     line_dict = json.loads(line)
 170                     contents = LockFileContents(**line_dict)
 171                     logger.debug(f'Blocking lock contents="{contents}"')
 172
 173                     # Does the PID exist still?
 174                     try:
 175                         os.kill(contents.pid, 0)
 176                     except OSError:
 177                         msg = f'Lockfile {self.lockfile}\'s pid ({contents.pid}) is stale; force acquiring'
 178                         logger.warning(msg)
 179                         self.release()
 180
 181                     # Has the lock expiration expired?
 182                     if contents.expiration_timestamp is not None:
 183                         now = datetime.datetime.now().timestamp()
 184                         if now > contents.expiration_datetime:
 185                             msg = f'Lockfile {self.lockfile} expiration time has passed; force acquiring'
 186                             logger.warning(msg)
 187                             self.release()
 188         except Exception:
 189             pass