Add PubSub system test for fix in #4174

[jeffkpayne]

@lukesneeringer Here's my attempt at a system test for the fix in #4174

Let me know how I can improve it, etc. Thanks.

[jeffkpayne]

Not sure why, but this is showing the commit from my previous PR. I fetched and merged upstream/master, but maybe I should have rebased on master? Sorry for any inconvenience!

[jeffkpayne]

@dhermes This includes a change to the unit test that broke on #4174 in the Python 2.7 environment. Please note that the test in question passed on my machine before this change. Maybe due to the difference of environments between my Mac and CircleCI's machines?

[jeffkpayne]

Thought about factoring out:

 submit_calls = [m for m in executor.method_calls if m[0] == 'submit']

and

 add_done_callback_calls = [
m for m in future.method_calls if m[0] == 'add_done_callback']

into a function, but wasn't sure if that was overkill or where to put it...

[jeffkpayne]

Gah, coverage. Fixing now...

[jeffkpayne]

OK, so nox -s cover locally from the pubsub directory gives me:

(nox-python-3.6)
nox > Running session cover
nox > /Users/jpayne/miniconda3/envs/nox-python-3.6/bin/python -m virtualenv /Users/jpayne/repos/bombora-google-cloud-python/pubsub/.nox/cover -p python3.6
nox > chdir /Users/jpayne/repos/bombora-google-cloud-python/pubsub
nox > pip install --upgrade coverage pytest-cov
nox > coverage report --show-missing --fail-under=100
Name Stmts Miss Branch BrPart Cover Missing
--------------------------------------------------------------------------------------------------
google/cloud/pubsub.py 5 0 0 0 100%
google/cloud/pubsub_v1/__init__.py 5 0 0 0 100%
google/cloud/pubsub_v1/_gapic.py 26 0 16 0 100%
google/cloud/pubsub_v1/publisher/__init__.py 3 0 0 0 100%
google/cloud/pubsub_v1/publisher/batch/__init__.py 0 0 0 0 100%
google/cloud/pubsub_v1/publisher/batch/base.py 27 0 6 0 100%
google/cloud/pubsub_v1/publisher/batch/thread.py 66 0 14 0 100%
google/cloud/pubsub_v1/publisher/client.py 41 0 14 0 100%
google/cloud/pubsub_v1/publisher/exceptions.py 6 0 0 0 100%
google/cloud/pubsub_v1/publisher/futures.py 47 0 14 0 100%
google/cloud/pubsub_v1/subscriber/__init__.py 3 0 0 0 100%
google/cloud/pubsub_v1/subscriber/_consumer.py 56 0 8 0 100%
google/cloud/pubsub_v1/subscriber/_helper_threads.py 46 0 8 0 100%
google/cloud/pubsub_v1/subscriber/_histogram.py 37 0 16 0 100%
google/cloud/pubsub_v1/subscriber/client.py 20 0 4 0 100%
google/cloud/pubsub_v1/subscriber/message.py 30 0 0 0 100%
google/cloud/pubsub_v1/subscriber/policy/__init__.py 0 0 0 0 100%
google/cloud/pubsub_v1/subscriber/policy/base.py 94 0 24 0 100%
google/cloud/pubsub_v1/subscriber/policy/thread.py 52 0 11 0 100%
google/cloud/pubsub_v1/types.py 18 0 2 0 100%
--------------------------------------------------------------------------------------------------
TOTAL 582 0 137 0 100%
nox > coverage erase
nox > Session cover was successful.

showing 100% for everything, but CircleCI shows 98% for google/cloud/pubsub_v1/subscriber/policy/thread.py. Am I missing something?

[jeffkpayne]

Gah, now I see it locally. Adding a test for thread._callback_completed()...

[tseaver]

Thank you!