The flow control settings for the Cloud Pub/Sub subscriber should bound the number of unacknowledged messages outstanding to the callback at a time. However, it seems to have no effect on the client, where the number of outstanding messages always ends up being 10.

OS type and version: Mac OS X 10.14.3, Debian Linux
Python version: Python 2.7.10 on Mac, Python 3.6.5 on Linux
google-cloud-pubsub version: 0.40.0

Steps to reproduce

1. Create a Cloud Pub/Sub topic and subscription
2. Publish many messages to the topic
3. Update the project name and subscription name in the code below.
4. Run the code.

Expected behavior: Since the flow control settings have max_messages set to 2, only two "Received Message" lines should print at a time, with two "Acking Message" lines printing out 20 seconds later. Instead, 10 messages come to the callback at a time. Changing the value of max_messages has no impact on the number of messages sent to the callback.

Code example

import time

from google.cloud import pubsub_v1

project_name=<insert project name here>
subscription_name=<insert subscription name here>
subscriber = pubsub_v1.SubscriberClient()
subscription_path = subscriber.subscription_path(project_name, subscription_name)

def callback(message):
    print ("Received Message")
    time.sleep(20)
    print ("Acking Message")
    message.ack()

flow_control = pubsub_v1.types.FlowControl(max_messages=2)
subscriber.subscribe(
    subscription_path, callback=callback, flow_control=flow_control)

print('Subscriber started for {}'.format(subscription_path))
while True:
    time.sleep(60)