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lockfiles.py
72 lines | 2.1 KiB | text/x-python | PythonLexer
import os
class LockFile(object):
"""Provides methods to obtain, check for, and release a file based lock which
should be used to handle concurrent access to the same file.
As we are a utility class to be derived from, we only use protected methods.
Locks will automatically be released on destruction"""
__slots__ = ("_file_path", "_owns_lock")
def __init__(self, file_path):
self._file_path = file_path
self._owns_lock = False
def __del__(self):
self._release_lock()
def _lock_file_path(self):
""":return: Path to lockfile"""
return "%s.lock" % (self._file_path)
def _has_lock(self):
""":return: True if we have a lock and if the lockfile still exists
:raise AssertionError: if our lock-file does not exist"""
if not self._owns_lock:
return False
return True
def _obtain_lock_or_raise(self):
"""Create a lock file as flag for other instances, mark our instance as lock-holder
:raise IOError: if a lock was already present or a lock file could not be written"""
if self._has_lock():
return
lock_file = self._lock_file_path()
if os.path.isfile(lock_file):
raise IOError("Lock for file %r did already exist, delete %r in case the lock is illegal" % (self._file_path, lock_file))
try:
fd = os.open(lock_file, os.O_WRONLY | os.O_CREAT | os.O_EXCL, 0)
os.close(fd)
except OSError,e:
raise IOError(str(e))
self._owns_lock = True
def _obtain_lock(self):
"""The default implementation will raise if a lock cannot be obtained.
Subclasses may override this method to provide a different implementation"""
return self._obtain_lock_or_raise()
def _release_lock(self):
"""Release our lock if we have one"""
if not self._has_lock():
return
# if someone removed our file beforhand, lets just flag this issue
# instead of failing, to make it more usable.
lfp = self._lock_file_path()
try:
# on bloody windows, the file needs write permissions to be removable.
# Why ...
if os.name == 'nt':
os.chmod(lfp, 0777)
# END handle win32
os.remove(lfp)
except OSError:
pass
self._owns_lock = False