[Sqlalchemy-tickets] Issue #3309: AssertionError in rollback on failed commit. (zzzeek/sqlalchemy)

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

New issue 3309: AssertionError in rollback on failed commit.
https://bitbucket.org/zzzeek/sqlalchemy/issue/3309/assertionerror-in-rollback-on-failed

zoomorph:

My commit code...
```
            try:
                self._session.commit()
            except Exception as e:
                logger.exception("Caught exception in commit.")
                self.rollback()
```

Caught the following exception:
```
Traceback (most recent call last):
  File "/myapp/tpcommon/db/core.py", line 111, in commit
    self._session.commit()
  File "/myapp/env_pypy/site-packages/sqlalchemy/orm/session.py", line 779, in commit
    self.transaction.commit()
  File "/myapp/env_pypy/site-packages/sqlalchemy/orm/session.py", line 384, in commit
    self._prepare_impl()
  File "/myapp/env_pypy/site-packages/sqlalchemy/orm/session.py", line 364, in _prepare_impl
    self.session.flush()
  File "/myapp/env_pypy/site-packages/sqlalchemy/orm/session.py", line 1993, in flush
    self._flush(objects)
  File "/myapp/env_pypy/site-packages/sqlalchemy/orm/session.py", line 2111, in _flush
    transaction.rollback(_capture_exception=True)
  File "/myapp/env_pypy/site-packages/sqlalchemy/util/langhelpers.py", line 63, in __exit__
    compat.reraise(type_, value, traceback)
  File "/myapp/env_pypy/site-packages/sqlalchemy/orm/session.py", line 2111, in _flush
    transaction.rollback(_capture_exception=True)
  File "/myapp/env_pypy/site-packages/sqlalchemy/orm/session.py", line 426, in rollback
    self._restore_snapshot(dirty_only=self.nested)
  File "/myapp/env_pypy/site-packages/sqlalchemy/orm/session.py", line 266, in _restore_snapshot
    assert self._is_transaction_boundary
AssertionError
```

When the exception handler called rollback(), it again raised this exception. After that, my code received the following when trying to continue using the session:

```InvalidRequestError: This Session's transaction has been rolled back by a nested rollback() call.  To begin a new transaction, issue Session.rollback() first.
```

I'm using master branch code from about a week ago. I'm not sure what causes this error but it's happening infrequently.