[Sqlalchemy-tickets] Issue #4297: Supporting Tests with Rollbacks fails (zzzeek/sqlalchemy)

[marderko <iss...@bi...>]

New issue 4297: Supporting Tests with Rollbacks fails
https://bitbucket.org/zzzeek/sqlalchemy/issues/4297/supporting-tests-with-rollbacks-fails

marderko:

Founded bug in "Transactions and Connection Management" documentation http://docs.sqlalchemy.org/en/latest/orm/session_transaction.html section "Supporting Tests with Rollbacks". There is:
```
# then each time that SAVEPOINT ends, reopen it
@event.listens_for(self.session, "after_transaction_end")
    def restart_savepoint(session, transaction):
        if transaction.nested and not transaction._parent.nested:
            # ensure that state is expired the way
            # session.commit() at the top level normally does
            # (optional step)
            session.expire_all()
            session.begin_nested()
```

I think the if condition is not correct and the right condition is:

```
if transaction.nested and not session.transaction.nested:
```

Explanation: 

There are constructed some transaction path (or tree) which have `session.transaction` in the end (tree leaf) and `session.transaction._parent._parent....._parent` in the start (tree root) and variable transaction from `restart_savepoint` is the start point (tree leaf). I think the second condition wants to restrict maximal length of path (or tree depth) to 2. But the condition is for path start and `session.begin_nested()` begins nested transaction in the end - so I change the condition also for the path end.


I attached failing example with two factories (second has first as subfactory) and simple parametrized test. It is really slow and it fails for tests number > 919 because of maximal recursion depth.
After condition change it is really quickly and no fail in any test.