Don't catch exception within session

[ChangBo Guo(gcb)]

In code review, some projects are using try/except within the context
managed by session. That's wrong, need add the note in doc string,
hope others put try/except in right place.

Error will be raised in session's __exit__ handler, and any try/except
within the context will drop the ROLLBACK, need catch the error out of
the context.

Solution:

Need catch exeption out of session , except developer want to recover from failure and continue commit part of stuff of one session.

[ChangBo Guo(gcb) (glongwave)]

[root@guochbo-10-7-0-151 test]# cat ./sqlalchemy_test.py
#!/usr/bin/env python
# This is a sqlalchemy sample code
# author: guochbo ChangBo Guo

import sqlalchemy;
from sqlalchemy import create_engine
from sqlalchemy import Table, Column, Integer, String
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy.orm import sessionmaker
from sqlalchemy.exc import IntegrityError

Base = declarative_base()

# Create engine
#connect='mysql://nova:nova@localhost/nova?charset=utf8'
connect='sqlite:///:memory:'
engine = create_engine(connect,echo=True)
Session = sessionmaker(bind=engine)
session = Session()

# Create one table
class User(Base):
    __tablename__ = 'users'

    id = Column(Integer, primary_key=True)
    name = Column(String(36))

    def __init__(self, id, name):
        self.id = id
        self.name = name

Base.metadata.create_all(engine)

user1= User(1, 'ed1')
user2 = User(1, 'ed2')
# duplicated entry will rollback
try:
    with session.begin(subtransactions=True):
        try:
            session.add(user1)
            session.add(user2)
        except Exception :
            print('get exception1 in with block')
except IntegrityError:
    print('get IntegrityError2 out of with block')

# catch exception in contex won't rollback
session1= Session()
try:
    with session1.begin(subtransactions=True):
        try:
            session1.add(user1)
            dict1={}
            dict1.pop('test')
            session1.add(user2)
        except Exception :
            print('get exception3 in with block')
except IntegrityError:
    print('get IntegrityError4 out of with block')

[root@guochbo-10-7-0-151 test]# ./sqlalchemy_test.py
2013-12-17 23:52:06,547 INFO sqlalchemy.engine.base.Engine PRAGMA table_info("users")
2013-12-17 23:52:06,548 INFO sqlalchemy.engine.base.Engine ()
2013-12-17 23:52:06,548 INFO sqlalchemy.engine.base.Engine
CREATE TABLE users (
        id INTEGER NOT NULL,
        name VARCHAR(36),
        PRIMARY KEY (id)
)

2013-12-17 23:52:06,548 INFO sqlalchemy.engine.base.Engine ()
2013-12-17 23:52:06,549 INFO sqlalchemy.engine.base.Engine COMMIT
2013-12-17 23:52:06,550 INFO sqlalchemy.engine.base.Engine BEGIN (implicit)
2013-12-17 23:52:06,550 INFO sqlalchemy.engine.base.Engine INSERT INTO users (id, name) VALUES (?, ?)
2013-12-17 23:52:06,550 INFO sqlalchemy.engine.base.Engine ((1, 'ed1'), (1, 'ed2'))
2013-12-17 23:52:06,551 INFO sqlalchemy.engine.base.Engine ROLLBACK
get IntegrityError2 out of with block
get exception3 in with block
2013-12-17 23:52:06,551 INFO sqlalchemy.engine.base.Engine BEGIN (implicit)
2013-12-17 23:52:06,552 INFO sqlalchemy.engine.base.Engine INSERT INTO users (id, name) VALUES (?, ?)
2013-12-17 23:52:06,552 INFO sqlalchemy.engine.base.Engine (1, 'ed1')

[ChangBo Guo(gcb) (glongwave)]

Last commet is an example, shows incorrect method won't rollback the commit.

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/60144
Committed: https://git.openstack.org/cgit/openstack/oslo-incubator/commit/?id=9bc593e8af0b6c47383d97463de5b0980b78d106
Submitter: Jenkins
Branch: master

commit 9bc593e8af0b6c47383d97463de5b0980b78d106
Author: Chang Bo Guo <email address hidden>
Date: Wed Dec 4 19:05:37 2013 -0800

    Add docstring for exception handlers of session

    In code review, some projects are using try/except within the
    context managed by session. That's wrong, add the note in
    docstring, hope others put try/except in right place.

    Database Errors like IntegrityError will be raised in session's
    __exit__ handler, and any try/except within the context managed
    by session will not be triggered. And catching other non-database
    errors in the session will not trigger the ROLLBACK, so exception
    handlers should always be outside the session, except developer
    want to commit parts of stuff on purpose.

    Closes-Bug: #1262065

    Change-Id: I23223fbe9b6d9fa8e92e59894fb3b1fa195be159