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
  12 import decorator_utils
  13
  14
  15 logger = logging.getLogger(__name__)
  16
  17
  18 class LockFileException(Exception):
  19     pass
  20
  21
  22 @dataclass
  23 class LockFileContents:
  24     pid: int
  25     commandline: str
  26     expiration_timestamp: float
  27
  28
  29 class LockFile(object):
  30     """A file locking mechanism that has context-manager support so you
  31     can use it in a with statement.
  32     """
  33
  34     def __init__(
  35             self,
  36             lockfile_path: str,
  37             *,
  38             do_signal_cleanup: bool = True,
  39             expiration_timestamp: Optional[float] = None,
  40             override_command: Optional[str] = None,
  41     ) -> None:
  42         self.is_locked = False
  43         self.lockfile = lockfile_path
  44         self.override_command = override_command
  45         if do_signal_cleanup:
  46             signal.signal(signal.SIGINT, self._signal)
  47             signal.signal(signal.SIGTERM, self._signal)
  48         self.expiration_timestamp = expiration_timestamp
  49
  50     def locked(self):
  51         return self.is_locked
  52
  53     def available(self):
  54         return not os.path.exists(self.lockfile)
  55
  56     def try_acquire_lock_once(self) -> bool:
  57         logger.debug(f"Trying to acquire {self.lockfile}.")
  58         try:
  59             # Attempt to create the lockfile.  These flags cause
  60             # os.open to raise an OSError if the file already
  61             # exists.
  62             fd = os.open(self.lockfile, os.O_CREAT | os.O_EXCL | os.O_RDWR)
  63             with os.fdopen(fd, "a") as f:
  64                 contents = self._get_lockfile_contents()
  65                 logger.debug(contents)
  66                 f.write(contents)
  67             logger.debug(f'Success; I own {self.lockfile}.')
  68             self.is_locked = True
  69             return True
  70         except OSError:
  71             pass
  72         logger.debug(f'Failed; I could not acquire {self.lockfile}.')
  73         return False
  74
  75     def acquire_with_retries(
  76             self,
  77             *,
  78             initial_delay: float = 1.0,
  79             backoff_factor: float = 2.0,
  80             max_attempts = 5
  81     ) -> bool:
  82
  83         @decorator_utils.retry_if_false(tries = max_attempts,
  84                                         delay_sec = initial_delay,
  85                                         backoff = backoff_factor)
  86         def _try_acquire_lock_with_retries() -> bool:
  87             success = self.try_acquire_lock_once()
  88             if not success and os.path.exists(self.lockfile):
  89                 self._detect_stale_lockfile()
  90             return success
  91
  92         if os.path.exists(self.lockfile):
  93             self._detect_stale_lockfile()
  94         return _try_acquire_lock_with_retries()
  95
  96     def release(self):
  97         try:
  98             os.unlink(self.lockfile)
  99         except Exception as e:
 100             logger.exception(e)
 101         self.is_locked = False
 102
 103     def __enter__(self):
 104         if self.acquire_with_retries():
 105             return self
 106         msg = f"Couldn't acquire {self.lockfile}; giving up."
 107         logger.warning(msg)
 108         raise LockFileException(msg)
 109
 110     def __exit__(self, type, value, traceback):
 111         self.release()
 112
 113     def __del__(self):
 114         if self.is_locked:
 115             self.release()
 116
 117     def _signal(self, *args):
 118         if self.is_locked:
 119             self.release()
 120
 121     def _get_lockfile_contents(self) -> str:
 122         if self.override_command:
 123             cmd = self.override_command
 124         else:
 125             cmd = ' '.join(sys.argv)
 126         print(cmd)
 127         contents = LockFileContents(
 128             pid = os.getpid(),
 129             commandline = cmd,
 130             expiration_timestamp = self.expiration_timestamp,
 131         )
 132         return json.dumps(contents.__dict__)
 133
 134     def _detect_stale_lockfile(self) -> None:
 135         try:
 136             with open(self.lockfile, 'r') as rf:
 137                 lines = rf.readlines()
 138                 if len(lines) == 1:
 139                     line = lines[0]
 140                     line_dict = json.loads(line)
 141                     contents = LockFileContents(**line_dict)
 142                     logger.debug(f'Blocking lock contents="{contents}"')
 143
 144                     # Does the PID exist still?
 145                     try:
 146                         os.kill(contents.pid, 0)
 147                     except OSError:
 148                         logger.debug('The pid seems stale; killing the lock.')
 149                         self.release()
 150
 151                     # Has the lock expiration expired?
 152                     if contents.expiration_timestamp is not None:
 153                         now = datetime.datetime.now().timestamp()
 154                         if now > contents.expiration_datetime:
 155                             logger.debug('The expiration time has passed; ' +
 156                                          'killing the lock')
 157                             self.release()
 158         except Exception:
 159             pass