JezK
Edit File: cookie.py
# -*- coding: utf-8 -*- """Persistent dict to remember state between invocations. Cookies are used to remember file positions, counters and the like between plugin invocations. It is not intended for substantial amounts of data. Cookies are serialized into JSON and saved to a state file. We prefer a plain text format to allow administrators to inspect and edit its content. See :class:`~nagiosplugin.logtail.LogTail` for an application of cookies to get only new lines of a continuously growing file. Cookies are locked exclusively so that at most one process at a time has access to it. Changes to the dict are not reflected in the file until :meth:`Cookie.commit` is called. It is recommended to use Cookie as context manager to get it opened and committed automatically. """ import codecs import json import os from .compat import UserDict, TemporaryFile from .platform import flock_exclusive class Cookie(UserDict, object): def __init__(self, statefile=None): """Creates a persistent dict to keep state. After creation, a cookie behaves like a normal dict. :param statefile: file name to save the dict's contents .. note:: If `statefile` is empty or None, the Cookie will be oblivous, i.e., it will forget its contents on garbage collection. This makes it possible to explicitely throw away state between plugin runs (for example by a command line argument). """ super(Cookie, self).__init__() self.path = statefile self.fobj = None def __enter__(self): """Allows Cookie to be used as context manager. Opens the file and passes a dict-like object into the subordinate context. See :meth:`open` for details about opening semantics. When the context is left in the regular way (no exception raised), the cookie is committed to disk. :yields: open cookie """ self.open() return self def __exit__(self, exc_type, exc_val, exc_tb): if not exc_type: self.commit() self.close() def open(self): """Reads/creates the state file and initializes the dict. If the state file does not exist, it is touched into existence. An exclusive lock is acquired to ensure serialized access. If :meth:`open` fails to parse file contents, it truncates the file before raising an exception. This guarantees that plugins will not fail repeatedly when their state files get damaged. :returns: Cookie object (self) :raises ValueError: if the state file is corrupted or does not deserialize into a dict """ self.fobj = self._create_fobj() flock_exclusive(self.fobj) if os.fstat(self.fobj.fileno()).st_size: try: self.data = self._load() except ValueError: self.fobj.truncate(0) raise return self def _create_fobj(self): if not self.path: return TemporaryFile('w+', encoding='ascii', prefix='oblivious_cookie_') # mode='a+' has problems with mixed R/W operation on Mac OS X try: return codecs.open(self.path, 'r+', encoding='ascii') except IOError: return codecs.open(self.path, 'w+', encoding='ascii') def _load(self): self.fobj.seek(0) data = json.load(self.fobj) if not isinstance(data, dict): raise ValueError('format error: cookie does not contain dict', self.path, data) return data def close(self): """Closes a cookie and its underlying state file. This method has no effect if the cookie is already closed. Once the cookie is closed, any operation (like :meth:`commit`) will raise an exception. """ if not self.fobj: return self.fobj.close() self.fobj = None def commit(self): """Persists the cookie's dict items in the state file. The cookies content is serialized as JSON string and saved to the state file. The buffers are flushed to ensure that the new content is saved in a durable way. """ if not self.fobj: raise IOError('cannot commit closed cookie', self.path) self.fobj.seek(0) self.fobj.truncate() json.dump(self.data, self.fobj) self.fobj.write('\n') self.fobj.flush() os.fsync(self.fobj)