[tarantool-patches] Re: [PATCH 3/3] alter: fix rollback in case DDL and DML are used in the same transaction

[Vladimir Davydov]

On Thu, Jul 25, 2019 at 11:51:17AM +0300, Vladimir Davydov wrote:
> On Thu, Jul 25, 2019 at 01:32:55AM +0300, Konstantin Osipov wrote:
> > * Vladimir Davydov <vdavydov.dev at gmail.com> [19/07/23 18:49]:
> > > A txn_stmt keeps a reference to the space it modifies. Memtx uses this
> > > space reference to revert the statement on error or voluntary rollback
> > > so the space must stay valid throughout the whole transaction.
> > > 
> > > The problem is DML statements may be mixed in a DDL transaction, in
> > > which case we always roll back DML statements first and only then run
> > > rollback triggers to revert changes done to the schema by the DDL
> > > statements. This means every DDL statement modifying the space cache
> > > must be implemented in such a way that it leaves both the old space
> > > and the new space valid until commit or rollback.
> > 
> > How are we going to enforce this invariant by code structure going
> > forward other than patching it hastily when it breaks?
> 
> No way, I guess. We'll need to rethink the whole ALTER infrastructure
> if things go south. Just like we always do.

An alternative approach to this issue is attaching DDL triggers to txn
statements and running rollback triggers "synchronously" while rolling
back statements. This would allow us not to impose any strict
requirements on how DDL triggers are implemented.