Avoid potential async deadlock in GetTopicClient

[jr01]

So we have a (legacy) ASP.net client that uses Rebus. We noticed that sometimes a web api call doesn't return.

I did a memory dump and analysis with windbg and the dumpasync extension revealed that about 20 async http request tasks are stuck for days at GetTopicClient:

000002378c1afea0 <0> Rebus.AzureServiceBus.AzureServiceBusTransport+<GetTopicClient>d__83
.000002378c1b0348 <0> Rebus.AzureServiceBus.AzureServiceBusTransport+<>c__DisplayClass32_0+<<GetOutgoingMessages>b__3>d
..000002378c1b08a0 <0> Rebus.AzureServiceBus.AzureServiceBusTransport+<>c__DisplayClass32_1+<<GetOutgoingMessages>b__1>d
..........

Rebus is awaiting on a Lazy in GetTopicClient .

I'm far from an async guru and did some googling and found the following:

• https://stackoverflow.com/questions/57517656/lazytaskt-with-asynchronous-initialization
• https://github.com/StephenCleary/AsyncEx/issues/9#issuecomment-69678728
• https://github.com/StephenCleary/AsyncEx/blob/db32fd5db0d1051e867b36ae039ea13d2c36eb91/src/Nito.AsyncEx.Coordination/AsyncLazy.cs
• https://devblogs.microsoft.com/pfxteam/asynclazyt/

And my conclusion is that an async lazy is a tricky to get right and that a Task.Run should fix it by making sure the code runs on a threadpool thread.

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Rebus is MIT-licensed. The code submitted in this pull request needs to carry the MIT license too. By leaving this text in, I hereby acknowledge that the code submitted in the pull request has the MIT license and can be merged with the Rebus codebase.

[mookid8000]

Hi @jr01 , sorry about the slow reply – I've been skiing all week ⛷ putting off work 😅

This is really good information! Good catch, and great that you fixed it. This combination of Lazy and Task is a new thing I learned recently, but I will have to go through all of my usage of this idiom, so I can avoid deadlocks.

Thanks for providing a fix. I'll merge it right away.

[mookid8000]

Your fix is out in Rebus.AzureServiceBus 7.0.0-a19 on NuGet.org in a few minutes.

Thanks for fixing this!!