Type of issue

• Bug
• Enhancement
• Compliance
• Question
• Help wanted

Current Behavior

I am running into thread pool starvation issues when publishing to a couple (~50) of MQTT topics.

Expected Behavior

Publishing to a larger amount of topics works without thread pool starvation / hiccups.

Steps To Reproduce

Here is a fork of the starter kit that reproduces the issue:
https://github.com/jnsjll/UA-IIoT-StarterKit

Environment

- OS: Windows 10
- Environment:
- Runtime: .NET 6
- Nuget Version:
- Component: PubSub
- Server:
- Client:

Anything else?

The problem seems to be that the IntervalRunner used for each WriterGroupDataType / UaPublisher calls the PublishMessages method on a thread pool thread which will block on a lock in MqttPubSubConnection.PublishNetworkMessage:

Making the methods async and awaiting rather than blocking can resolve the issue.