On 7/12/05, Tim Peters <[EMAIL PROTECTED]> wrote:
> [Jeremy Hylton]
> > IIRC, the old implementation of savepoints kept a copy of the index at
> > the time the savepoint was taken so that you could rollback to it
> > multiple times. I don't think there's any way to avoid such a copy.
>
> Right, and the current implementation did that too. The "surprise" was that
> it wasn't enough. Sketch:
>
> 1. Modify object 0.
> 2. Make savepoint 1.
> It makes a copy of the current index, say {0: 0}, and remembers
> the TmpStore size, say 100.
> 3. Fiddle around.
> 4. Rollback to savepoint 1.
> This sets the TmpStore index to the saved {0: 0}, and truncatss
> TmpStore to size 100. So far so good -- or so it seems.
> 5. Modify object 0 again, and make savepoint 2.
> This changes the TmpStore index to {0: 100}, makes of a copy of
> {0: 100} in savepoint 2, and increases TmpStore size to 200. This
> just did something horribly wrong too, although it's subtle.
> 6. Rollback to savepoint 1 again.
> Because a copy of savepoint 1's index wasn't _also_ made in
> step #4, the index savepoint 1 is holding onto mutated to
> {0: 100} during step #5 (object sharing). This (#6) step
> leaves TmpStore with (the mutated) index {0: 100} and size 100.
> 7. Reference object 0.
> Oops. The index tells us to seek to pos 100, but TmpStore has
> been truncated to 100. We get a low-level exception from
> struct.unpack() about not enough bytes to unpack the data record
> header.
>
> You can guess that I saw that happening <wink>. Step #4 also needs to copy
> the index (from the savepoint to TmpStore) instead of sharing a reference,
> although this wasn't needed so long as a savepoint could be "used" at most
> once (then mutating the savepoint's index after a rollback had no ill
> effect, as the savepoint's index could never be referenced again).
I understand. The further invariant is that the index captured when a
savepoint created is immutable.
Jeremy
_______________________________________________
For more information about ZODB, see the ZODB Wiki:
http://www.zope.org/Wikis/ZODB/
ZODB-Dev mailing list - ZODB-Dev@zope.org
http://mail.zope.org/mailman/listinfo/zodb-dev
