SignalR .NET client restart failed with "The HubConnection cannot be started while StopAsync is running"

[dlyz]

Describe the bug

I'm using .NET SignalR Client. After connection breaks I invoke await connection.StopAsync() and then await connection.StartAsync(). StartAsync throws InvalidOperationException: "The HubConnection cannot be started while StopAsync is running". The super easy way to reproduce it is to create a connection, invoke await connection.StopAsync() and then await connection.StartAsync(). After that the connection can never be started.

It looks like a bug. I looked at the code and it seems that StopAsyncCore cancels the cts, but never directly recreates it.

Further technical details

• ASP.NET Core 3.1

[halter73]

Are you sure you're fully awaiting the completion of connection.StopAsync() before calling connection.StartAsync()? I just double-checked this with 3.1, and stopping and restarting a HubConnection still works for me. We also have test coverage for this.

https://github.com/aspnet/AspNetCore/blob/057ec9cfae1831f91f1a0e01397059b20431db7b/src/SignalR/clients/csharp/Client/test/UnitTests/HubConnectionTests.ConnectionLifecycle.cs#L95-L104

Can you provide a sample app that reproduces this behavior?

[BrennanConroy]

After connection breaks I invoke await connection.StopAsync() and then await connection.StartAsync()

I think the key here is that stop is called "after connection breaks".

And a quick test shows that:

var connection = builder.build();
await connection.StopAsync();
await connection.StartAsync(); // throws here

fails.

@dlyz you shouldn't need to call stop after the connection "breaks", so you can workaround the issue by removing that logic.

[dlyz]

I think the key here is that stop is called "after connection breaks".

Yes, that's the key.

@dlyz you shouldn't need to call stop after the connection "breaks", so you can workaround the issue by removing that logic.

Sometimes it's hard to distinguish this situation from other fails. For example, now in my code, if something goes wrong while working with the connection, I just restart it.

Ok, so i can check HubConnection.State and decide whether I need to call StopAsync. But I think, workaround will not work if the connection "breaks" right before the stop call. It's a small chance, but still there are edge cases.