[ZODB-Dev] Re: [ZCM] [ZC] 875/ 6 Comment "FileStorage packing bug: backpointer not resolved"

[Jeremy Hylton]

On Wed, 7 May 2003 14:12:54 +0100
 Toby Dickenson <tdickenson at geminidataloggers.com> wrote:
> On Tuesday 06 May 2003 8:12 pm, Collector: Zope Bugs,
> Features, and Patches 
> ... wrote:
> 
> > A question about the patch.  It looks like the change
> means that back
> > pointers are always resolved during pack.  Is that
> right?  If so, it means
> > that when we're done there are no backpointers in the
> storage; every
> > backpointer has been replaced by a copy of the actual
> data.
> >
> > We need backpointers to get undo semantics right.  They
> also save space.
> 
> 
> This problem is only triggered if an undo is performed
> during pack, right?

No.  The problem occurs when a set of two undo ops cause an
object to be uncreated and recreated.  If the pack time
falls between the two undo times, then the data record
referred to be the second undo backpointer will be lost.

If that's still unclear, I can reply in detail tomorrow
when I have more bandwidth.

Jeremy