NDB: non_transactional tasklets do not respect the non_transactional context

[ahazeemi]

Environment details

1. OS type and version: MacOS 10.15.7
2. Python version and virtual environment information: 2.7
3. google-cloud-ndb version: 1.4.2

Steps to reproduce

1. Create a tasklet.
2. Add ndb.non_transactional decorator on it.
3. Perform a fetch operation on more than 25 entity groups
4. The error 400 operating on too many entity groups in a single transaction shows up.

Also reproducible on AppEngine.

Code example

@ndb.tasklet
@ndb.non_transactional()
def non_transactional_tasklet(keys):
      result = yield ndb.get_multi_async(keys).  # keys belong to 25 different entity groups
      return result

Stack trace

File "/Users/abdulhameedazeemi/Desktop/project/packages/backend/thirdparty/google/cloud/ndb/_transaction.py", line 215, in transactional_inner_wrapper
    propagation=propagation,
  File "/Users/abdulhameedazeemi/Desktop/project/packages/backend/thirdparty/google/cloud/ndb/_transaction.py", line 72, in transaction
    return future.result()
  File "/Users/abdulhameedazeemi/Desktop/project/packages/backend/thirdparty/google/cloud/ndb/tasklets.py", line 193, in result
    self.check_success()
  File "/Users/abdulhameedazeemi/Desktop/project/packages/backend/thirdparty/google/cloud/ndb/tasklets.py", line 140, in check_success
    raise self._exception
InvalidArgument: 400 operating on too many entity groups in a single transaction.

[ShahzadUmair]

None of the following works:

# swapping ❌

@ndb.non_transactional()
@ndb.tasklet
def non_transactional_tasklet(keys):
      result = yield ndb.get_multi_async(keys).  # keys belong to 25 different entity groups
      return result

# inline ❌

@ndb.tasklet()
def non_transactional_tasklet(keys):
      result = yield ndb.non_transactional()(lambda: ndb.get_multi_async(keys))()
      return result

Though it seems to work fine with fetch_page_async. (must be some issue with get_multi_async, I believe)

# ✅

@ndb.non_transactional()
@ndb.tasklet
def non_transactional_tasklet():
    yield User.query().fetch_page_async(page_size=30)  # this works fine

cc: @chrisrossi