[ZODB-Dev] Undo differences between Z2 and Z3

[David Pratt]

Jim Fulton wrote:
> David Pratt wrote:
>> There seems to be enough of a difference between Z2 and Z3 undo to 
>> have an effect on a backend like PGStorage from working on Z3. I 
>> believe the objective in the storage api is that it should not matter 
>> to the backend whether you are connected to Z2 or Z3. Can someone more 
>> familiar with the history of undo in z2 and z3 explain the current 
>> situation?
> 
> Undo has a fairly complicated API, and I can easily believe that Z2 and
> Z3 call that API in very different ways.
> 
> BTW, I strongly discourage use of Undo except in emergencies.
> Sadly, except when undoing the last (non-undo) transactions in
> a database can lead to inconsistency.  Undo should be used with
> caution and should generally not be exposed to non-expert users.

Undo has been a part of CMF and Plone for non-expert users for quite 
some time by exposing only the last non-undo transaction. This should be 
fine for Z3 I believe. In any case, my interest is initially to give Z3 
what it is asking for without crapping out so contents view will work 
for a principal through z3 zmi.

>> In Z3, before bringing up the contents view, there is a check of the 
>> undo transactions for the principal. It uses z3's undo manager to 
>> getPrincipalTransactions which eventually results in a lookup in the 
>> undoLog in the external storage where it dies. Just a brief look at 
>> the undo code in z3 gives me the feeling the format
> 
> Format of what?

I am referring to is this which is the route this is taking to the undoLog :

     def _getUndoInfo(self, context, principal, first, last):
         specification = {}

         if context is not None:
             locatable = IPhysicallyLocatable(context, None)
             if locatable is not None:
                 location = Prefix(locatable.getPath())
                 specification.update({'location': location})

         if principal is not None:
             # TODO: The 'user' in the transactions is a concatenation
             # of 'path' and 'user' (id).  'path' used to be the path
             # of the user folder in Zope 2.  ZopePublication currently
             # does not set a path, so ZODB lets the path default to
             # '/'.  We should change ZODB3 to set no default path at
             # some point
             path = '/' # default for now
             specification.update({'user_name': path + ' ' + principal.id})

         entries = self.__db.undoInfo(first, last, specification)

> 
> It's possible that there is a bug in PGStorage in handling
> extension data.  I can easily believe that Z2 doesn't use extension
> data and perhaps z3 does.

Thanks. I have reported this to Shane but am also experimenting with 
storages so have an interest in understanding what is happening.

Regards,
David