Graceful shutdown (message draining)

[sidkri]

Azure functions will support draining in-flight messages before shutdown of the runtime. The current extension implementation closes the connection to Service Bus during shutdown before allowing executing function invocations to complete processing and prevents these messages from being completed or abandoned. Additionally, new messages start processing up until the connection to Service Bus is closed.

This PR implements graceful shutdown in the extension by leveraging a new set of "Unregister" methods in the Service Bus SDK : Azure/azure-sdk-for-net#14021 and published in 4.2.0 and 5.0.0 of the Service Bus SDK used by this library.

This extension supports single message as well as batch processing and the message draining implementation is different for each path.

Batch processing:
The ServiceBusListener when created in batch mode, polls Service Bus for messages in a loop while checking to ensure that it is still in started state and no cancellation has been requested. The new Functions runtime "/drain" API will be invoked when the runtime needs to shutdown and sufficient time will be given for any functions that have already been invoked to complete. The invocation of "/drain" calls StopAsync() on all listeners and with this change, the connections to Service Bus will remain open until the runtime is shutdown and Dispose() is called.

Single message processing:
The new QueueClient.UnregisterSessionHandlerAsync, SubscriptionClient.UnregisterSessionHandlerAsync and MessageReceiver.UnregisterMessageHandlerAsync methods are called when the extension is stopped. These calls block until all in-flight messages complete processing (drain) and new messages no longer get dispatched. As the amount of time given to functions to execute depends on the SKU, a MaxValue timespan is specified when draining as the host will shut down regardless after the max function execution time.

Other changes:

1. Addressed Connection is closed immediately when StopAsync is called on IListener  #95
2. Support for logging to test output via ITestOutputHelper in both Integration test classes.
3. All test settings are exported to environment variables as WebJobs relies solely on environment variables. This obviates the need to set environment variables at the operating system level when debugging tests locally in Visual Studio.
4. Stopping and disposing the host object in each test consistently.

[soninaren]

Just a nit comment. Looks good otherwise.

[soninaren]

nit: last 2 if statements can be merged

[sidkri]

Did you mean lines 140 and 142? If so, I don't want lines 148 and 149 executed on a null _clientEntity so can't check in a single if statement. Would definitely like to compact this if chain!

[soninaren]

I see. You probably can make it easier to read by checking for negation instead. Something like and so on.

if (!_singleDispatch)
{
 return:
}

[sidkri]

have to set the _started flag to false in all cases. Would have to duplicate that logic in two places with the short circuit of (!_singleDispatch) so not sure if that's a better option as more common steps may be added at a later time and those would have to be duplicated.