[ZODB-Dev] Re: Savepoints are invalidated once they are used

[Jim Fulton]

Tim Peters wrote:
> [Christian Heimes, suggests changing Transaction.commit() to start with
> 
>         if subtransaction:
>             # TODO deprecate subtransactions
>             self._subtransaction_savepoint = self.savepoint(1)
>             return
> 
>         if self._savepoint2index:
>             self._invalidate_all_savepoints()
> 
> i.e. swapping the first two blocks]
> 
> [Jim Fulton]
> 
>>subtransactions != savepoints
>>
>>There is *no* promise of nestability with subtransactions. Committing a
>>subtransaction invalidates prior savepoints by design.  This is necessary
>>to maintain backward compatibility.
> 
> 
> I'm not sure I follow this:  old code could not be using savepoints
> directly, so what would break in code that stuck solely to subtxn commits?
...
 > So I don't see how backward compatibility would be injured.  BTW, I tried
 > it, and all the ZODB tests pass with this change.


When you so commit(1), the promise is that, if the top-level transaction
commits, then any work done up to the commit(1) is committed.  Consider
this scenario:

- new code: s = transaction.savepoint()
- old code: foo.x = 42
- old code: transaction.commit(1)
- new code: s.rollback()
- old code: transaction.commit()

Now, the old code expected that foo.x was 42.  It did a commit(1).
But, in fact, if we can roll back s, then in fact, the old code
can't count on the semantics of the commit(1).  This would be
a change in behavior for subtransactions.

Subtransactions only provide one level of nesting.  They are
not the same as savepoints.

Now, I don't know for sure that anyone depends on the non-nestedness
of subtransactions.  Mostly, subtransactions are used to checkpoint
incomplete changes to disk. So perhaps the backward compatibility
risk is small.

Jim

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