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| 1 | +"""A cross-process lock based on exclusive creation of a given file name""" |
| 2 | +import os |
| 3 | +import sys |
| 4 | +import errno |
| 5 | +import time |
| 6 | +import logging |
| 7 | + |
| 8 | + |
| 9 | +logger = logging.getLogger(__name__) |
| 10 | + |
| 11 | + |
| 12 | +class LockError(RuntimeError): |
| 13 | + """It will be raised when unable to obtain a lock""" |
| 14 | + |
| 15 | + |
| 16 | +class CrossPlatLock(object): |
| 17 | + """This implementation relies only on ``open(..., 'x')``""" |
| 18 | + def __init__(self, lockfile_path): |
| 19 | + self._lockpath = lockfile_path |
| 20 | + |
| 21 | + def __enter__(self): |
| 22 | + self._create_lock_file('{} {}'.format( |
| 23 | + os.getpid(), |
| 24 | + sys.argv[0], |
| 25 | + ).encode('utf-8')) # pylint: disable=consider-using-f-string |
| 26 | + return self |
| 27 | + |
| 28 | + def _create_lock_file(self, content): |
| 29 | + timeout = 5 |
| 30 | + check_interval = 0.25 |
| 31 | + current_time = getattr(time, "monotonic", time.time) |
| 32 | + timeout_end = current_time() + timeout |
| 33 | + while timeout_end > current_time(): |
| 34 | + try: |
| 35 | + with open(self._lockpath, 'xb') as lock_file: # pylint: disable=unspecified-encoding |
| 36 | + lock_file.write(content) |
| 37 | + return None # Happy path |
| 38 | + except ValueError: # This needs to be the first clause, for Python 2 to hit it |
| 39 | + raise LockError("Python 2 does not support atomic creation of file") |
| 40 | + except FileExistsError: # Only Python 3 will reach this clause |
| 41 | + logger.debug( |
| 42 | + "Process %d found existing lock file, will retry after %f second", |
| 43 | + os.getpid(), check_interval) |
| 44 | + time.sleep(check_interval) |
| 45 | + raise LockError( |
| 46 | + "Unable to obtain lock, despite trying for {} second(s). " |
| 47 | + "You may want to manually remove the stale lock file {}".format( |
| 48 | + timeout, |
| 49 | + self._lockpath, |
| 50 | + )) |
| 51 | + |
| 52 | + def __exit__(self, *args): |
| 53 | + try: |
| 54 | + os.remove(self._lockpath) |
| 55 | + except OSError as ex: # pylint: disable=invalid-name |
| 56 | + if ex.errno in (errno.ENOENT, errno.EACCES): |
| 57 | + # Probably another process has raced this one |
| 58 | + # and ended up clearing or locking the file for itself. |
| 59 | + logger.debug("Unable to remove lock file") |
| 60 | + else: |
| 61 | + raise |
| 62 | + |
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