Concurrency issues can cause orphaned events on GCal

[dhermes]

If two users have the same item id at the same time, the ensuing race condition can cause orphaned events on GCal, since the item ID will be used as the datastore ID.

Should wrap Event writes into transactions and make sure that gcal_edit doesn't change after being set the first time. Sadly, this will likely cause an extra round trip for every write.

NOTE: Migrated from Google Code Hosting

[dhermes]

THIS BAD BOY SHOULD BE A TRANSACTION

    event = ndb.Key(cls, uid).get()
    if event is not None:
      changed = False
      for attr, value in event_data.iteritems():
        if getattr(event, attr) != value:
          setattr(event, attr, value)
          logging.info('{attr} changed for {uid}'.format(attr=attr, uid=uid))
          changed = True

      if current_user not in event.attendees:  # pylint:disable-msg=E1103
        event.attendees.append(current_user)  # pylint:disable-msg=E1103
        logging.info('attendees changed for {uid}'.format(uid=uid))
        changed = True

      success = True
      if changed:
        # pylint:disable-msg=E1103
        success = event.update(credentials=credentials)
      return event, not success
    else:
      # pylint:disable-msg=W0142
      event = cls(key=ndb.Key(cls, uid), attendees=[current_user], **event_data)
      success = event.insert(credentials=credentials)
      return event, not success