[ZODB-Dev] Merge request - transaction savepoint release support

[Laurence Rowe]

Hi Jim,

I've created a new branch for my savepoint release changes following
the 1.1 release here:

  svn+ssh://svn.zope.org/repos/main/transaction/branches/elro-savepoint-release-1.1

This does seem to be a real requirement, as I've had another request
to provide this functionality for zope.sqlalchemy - when a large
number of savepoints are used, the eventual commit can lead to a
`RuntimeError: maximum recursion depth exceeded` in SQLAlchemy as it
attempts to unroll its nested substransactions.

Laurence

On 17 January 2010 15:45, Laurence Rowe <l at lrowe.co.uk> wrote:
> 2010/1/17 Jim Fulton <jim at zope.com>:
>> On Sat, Jan 16, 2010 at 1:03 PM, Laurence Rowe <l at lrowe.co.uk> wrote:
>>> I've had a request to add savepoint release support to zope.sqlalchemy
>>> as some databases seem to limit the number of savepoints in a
>>> transaction.
>>>
>>> I've added this in a branch of transaction here:
>>> svn+ssh://svn.zope.org/repos/main/transaction/branches/elro-savepoint-release
>>>
>>> >From the changelog:
>>>
>>> * Add support for savepoint.release(). Some databases only support a limited
>>>  number of savepoints or subtransactions, this provides an opportunity for a
>>>  data manager to free those resources.
>>>
>>> * Rename InvalidSavepointRollbackError to InvalidSavepointError (BBB provided.)
>>>
>>> If there are no objections, I shall merge this to trunk.
>>
>> I'll review and merge.
>
> Great, thanks!
>
>> What does it mean to "release" a savepoint? How is this different from aborting
>> a save point? I ask particularly in light of:
>>
>> On Sat, Jan 16, 2010 at 2:26 PM, Laurence Rowe <l at lrowe.co.uk> wrote:
>>> 2010/1/16 Laurence Rowe <l at lrowe.co.uk>:
>>>> I'm still not sure this will allow me to add savepoint release support
>>>> to zope.sqlalchemy, as SQLAlchemy has a concept of nested transactions
>>>> rather than savepoints.
>>>> http://groups.google.com/group/sqlalchemy/browse_thread/thread/7a4632587fd97724
>>>
>>> Michael Bayer noted on the sqlalchemy group that on RELEASE SAVEPOINT
>>> Postgresql destroys all subsequent savepoints. My branch now
>>> implements this behaviour.
>
> For zope.sqlalchemy I commit the sqlalchemy substransaction on
> savepoint.release(). This translates to a RELEASE SAVEPOINT on
> postgresql, best described by their docs here:
>
> """
> RELEASE SAVEPOINT destroys a savepoint previously defined in the
> current transaction.
>
> Destroying a savepoint makes it unavailable as a rollback point, but
> it has no other user visible behavior. It does not undo the effects of
> commands executed after the savepoint was established. (To do that,
> see ROLLBACK TO SAVEPOINT.) Destroying a savepoint when it is no
> longer needed allows the system to reclaim some resources earlier than
> transaction end.
>
> RELEASE SAVEPOINT also destroys all savepoints that were established
> after the named savepoint was established.
> """
> http://developer.postgresql.org/pgdocs/postgres/sql-release-savepoint.html
>
> Laurence
>