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