[ZODB-Dev] [FileStorage] Potential data loss through packing

[Dieter Maurer]

Looking at the current (not Jims new) pack algorithm to optimize
the reachability analysis, I recognized a behaviour that looks
like a potential data loss through packing.

The potential data loss can occur when an object unreachable at
pack time becomes reachable again after pack time.

The current pack supports a single use case which can cause such
an object resurrection: the use of backpointers (probably from "undo").

However, resurrection is possible by other means as well -- e.g.
by reinstating a historical version which references objects
meanwhile deleted.
Packing can cause such objects to get lost (resulting in POSKeyErrors).


Reinstating a historical version which references to meanwhile
deleted objects is probably quite a rare situation such
that the potential data loss seems not to be very critical.

But, potential data loss is nasty, even when the probablity is quite low.


-- 
Dieter