Small persistence related bug in atreal.filestorage.common

[tiazma]

From Dieter Maurer::

A client has complained about a large increase in "FileStorage" size.
A quick analysis suggests that the "atreal.richfile.preview" previews
are responsible for this increase as previews are stored inside
the main ZODB not in blobs.

I have implemented a new "blobstore" (similar to "zodbstore") which
stores files in "BlobFile" rather than "ZodbFile". While developing
the corresponding migration script I hit the following bug:

  "atreal.filestorage.common.zodbstore.ZodbDir" derives from
  "OOBTree" but does not define its own "getstate/setstate".
  As a consequence, the content of its instances dicts is
 not persisted: after a "ZodbDir" is loaded from the ZODB,
 it no longer has attributes "name" and "parent".


In general: it is not without problems to inherit from a persistent
class with special C-level data structures (such as "OOBTree").
In order to get their private C-level data persisted, such a class must define
a special "getstate/setstate" which (usually) do not take an instance
dict into account. A derived class with instance dicts must provide
its own "getstate/setstate" implementation which ensures that both
the C-level structures of the base class as well as the instance dicts
are persisted.