65 lines
2.5 KiB
Python
65 lines
2.5 KiB
Python
# Copyright (C) 2020 Canonical Ltd.
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#
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# Author: Daniel Watkins <oddbloke@ubuntu.com>
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#
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# This file is part of cloud-init. See LICENSE file for license information.
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class CloudInitPickleMixin:
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"""Scaffolding for versioning of pickles.
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This class implements ``__getstate__`` and ``__setstate__`` to provide
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lightweight versioning of the pickles that are generated for classes which
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use it. Versioning is done at the class level.
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The current version of a class's pickle should be set in the class variable
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``_ci_pkl_version``, as an int. If not overridden, it will default to 0.
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On unpickle, the object's state will be restored and then
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``self._unpickle`` is called with the version of the stored pickle as the
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only argument: this is where classes should implement any deserialization
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fixes they require. (If the stored pickle has no version, 0 is passed.)
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"""
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_ci_pkl_version = 0
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def __getstate__(self):
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"""Persist instance state, adding a pickle version attribute.
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This adds a ``_ci_pkl_version`` attribute to ``self.__dict__`` and
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returns that for serialisation. The attribute is stripped out in
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``__setstate__`` on unpickle.
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The value of ``_ci_pkl_version`` is ``type(self)._ci_pkl_version``.
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"""
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state = self.__dict__.copy()
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state["_ci_pkl_version"] = type(self)._ci_pkl_version
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return state
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def __setstate__(self, state: dict) -> None:
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"""Restore instance state and handle missing attributes on upgrade.
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This will be called when an instance of this class is unpickled; the
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previous instance's ``__dict__`` is passed as ``state``. This method
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removes the pickle version from the stored state, restores the
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remaining state into the current instance, and then calls
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``self._unpickle`` with the version (or 0, if no version is found in
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the stored state).
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See https://docs.python.org/3/library/pickle.html#object.__setstate__
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for further background.
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"""
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version = state.pop("_ci_pkl_version", 0)
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self.__dict__.update(state)
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self._unpickle(version)
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def _unpickle(self, ci_pkl_version: int) -> None:
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"""Perform any deserialization fixes required.
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By default, this does nothing. Classes using this mixin should
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override this method if they have fixes they need to apply.
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``ci_pkl_version`` will be the version stored in the pickle for this
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object, or 0 if no version is present.
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"""
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