fix: make sure reads happen in transaction if there is a transaction

[chrisrossi]

Fixes a bug where reads during a transaction wouldn't necessarily happen
in the context of the transaction.

Fixes #394

[cguardia]

Great, thanks.

[zyzniewski]

Thanks @chrisrossi but it still does not work. Please change in your unittest scenario:

yield [
    update(id, 100, 0.01),
    update(id, 100, 0.00),
]

here comes the professional illustration of the problem:

• READ1
• READ2
• WRITE2
• WRITE1

WRITE1 should fail and the whole transaction replayed

Thanks!

[chrisrossi]

I am looking at it again on master, have moved the delay to the first update call and even made it longer, and I am seeing the expected behavior, exactly as you say it should be. Are you sure you're testing with the fix?

[zyzniewski]

pip install git+https://github.com/googleapis/python-ndb.git@release-v1.2.1 --upgrade
ndb.__version__ == 1.2.1

[zyzniewski]

here's the full example I am using

from google.cloud import ndb

class Simple(ndb.Model):
    foo = ndb.IntegerProperty()

@ndb.transactional_tasklet()
def update(add, delay):

    entity = yield Simple.get_by_id_async('one')

    foo = entity.foo
    foo += add

    yield ndb.sleep(delay)

    entity.foo = foo

    yield entity.put_async()

client = ndb.Client()

@ndb.tasklet
def concurrent_tasks():
    yield [
        update(100, 0.01),
        update(100, 0.00),
    ]

with client.context():

    Simple(id='one', foo=42).put()

    concurrent_tasks().get_result()

    entity = Simple.get_by_id('one', use_cache=False)

    assert entity.foo == 242

[zyzniewski]

Fired it couple times and it's not deterministic

[chrisrossi]

I think we've hit a problem with caching....

[chrisrossi]

I'm going to open back up the original ticket. This PR does fix a problem that prevented this from working, but there is also interaction with the cache which can prevent it from working as well, which should be dealt with as well, but separately from this fix.