[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). _______________________________________________ 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