codekingpro/portable-devtools

1"""Provides a mechanism for not competing with other processes interacting with an MSAL cache."""2import os3import sys4import errno5import time6import logging7 8import portalocker9from packaging.version import Version10 11 12logger = logging.getLogger(__name__)13 14 15class CrossPlatLock(object):16    """Offers a mechanism for waiting until another process is finished interacting with a shared17    resource. This is specifically written to interact with a class of the same name in the .NET18    extensions library.19    """20    def __init__(self, lockfile_path):21        self._lockpath = lockfile_path22        # Support for passing through arguments to the open syscall was added in v1.4.023        open_kwargs = ({'buffering': 0}24            if Version(portalocker.__version__) >= Version("1.4.0") else {})25        self._lock = portalocker.Lock(26            lockfile_path,27            mode='wb+',28            # In posix systems, we HAVE to use LOCK_EX(exclusive lock) bitwise ORed29            # with LOCK_NB(non-blocking) to avoid blocking on lock acquisition.30            # More information here:31            # https://docs.python.org/3/library/fcntl.html#fcntl.lockf32            flags=portalocker.LOCK_EX | portalocker.LOCK_NB,33            **open_kwargs)34 35    def _try_to_create_lock_file(self):36        timeout = 537        check_interval = 0.2538        current_time = getattr(time, "monotonic", time.time)39        timeout_end = current_time() + timeout40        pid = os.getpid()41        while timeout_end > current_time():42            try:43                with open(self._lockpath, 'x'):  # pylint: disable=unspecified-encoding44                    return True45            except ValueError:  # This needs to be the first clause, for Python 2 to hit it46                logger.warning("Python 2 does not support atomic creation of file")47                return False48            except FileExistsError:  # Only Python 3 will reach this clause49                logger.debug(50                    "Process %d found existing lock file, will retry after %f second",51                    pid, check_interval)52                time.sleep(check_interval)53        return False54 55    def __enter__(self):56        pid = os.getpid()57        if not self._try_to_create_lock_file():58            logger.warning("Process %d failed to create lock file", pid)59        file_handle = self._lock.__enter__()60        file_handle.write('{} {}'.format(pid, sys.argv[0]).encode('utf-8'))  # pylint: disable=consider-using-f-string61        return file_handle62 63    def __exit__(self, *args):64        self._lock.__exit__(*args)65        try:66            # Attempt to delete the lockfile. In either of the failure cases enumerated below, it is67            # likely that another process has raced this one and ended up clearing or locking the68            # file for itself.69            os.remove(self._lockpath)70        except OSError as ex:  # pylint: disable=invalid-name71            if ex.errno not in (errno.ENOENT, errno.EACCES):72                raise73