bisect_kit/locking.py

# -*- coding: utf-8 -*-
# Copyright 2018 The Chromium OS Authors. All rights reserved.
# Use of this source code is governed by a BSD-style license that can be
# found in the LICENSE file.
"""File locks"""
from __future__ import print_function
import contextlib
import fcntl
import logging
import time
import errno

logger = logging.getLogger(__name__)

# Lock file to prevent multiple bisect instances sync code at the same time.
# Each bisection's workdir is supposed only used by one bisector and no need to
# lock. However the code mirror is shared and need lock to protect.
# Relative to mirror root.
LOCK_FILE_FOR_MIRROR_SYNC = '.bisect-kit.sync.lock'

LOCK_FILE_FOR_BUILD = '/var/lock/bisect-kit.build'


@contextlib.contextmanager
def lock_file(filename, polling_time=1.0):
  """Process level file lock.

  This is advisory file lock between processes. If the process holding a lock
  is terminated, the lock will be unlocked automatically.

  This is implemented by python's fcntl.lockf (backed by fcntl(2) syscall). Due
  to the mess and weakness of unix's file locking, do not recursively lock the
  same filename (no matter the same process, child process, or other thread).

  Args:
    filename: lock filename
    polling_time: delay between two locking trial
  """
  with open(filename, 'w') as f:
    try_count = 0
    # Loop forever until lock successfully.
    while True:
      try:
        try_count += 1
        fcntl.lockf(f, fcntl.LOCK_EX | fcntl.LOCK_NB)
        break
      except IOError as exc:
        if exc.errno != errno.EAGAIN:
          raise
        if try_count % 60 == 1:
          logger.warning('waiting for lock file %s ...', filename)
        time.sleep(polling_time)

    try:
      yield
    finally:
      fcntl.lockf(f, fcntl.LOCK_UN)