[Epic]: Support returning values from client invocations

[rafikiassumani-msft]

Support server to clients acks so that reliable messaging can be implemented more easily. This would only be when using Clients.Client(). I think we should go back to the SendAsync (fire and forget) InvokeAsync (wait for ack) naming pattern. That's the one sticking point.

EDIT by @anurse: For clarity, this issue is tracking all work related to allowing values to be returned from client invocations. It also covers allowing the server to wait for a void-returning (or Task-returning) client side method to complete.

Work left for .NET 7

• his was already an issue with Task returning .On methods, but client results likely makes it more likely to block on the client side
• Hub methods can soft-lock the connection #41997
  • Today we detect if you allow parallel hub invocations and throw if you don't when trying to use the feature. This doesn't work if you use IHubContext in the Hub, or if you have multiple waiting results for the same connections Hubs.
  • This is also especially bad in OnConnectedAsync because that's a special method that runs before the receive loop starts, we need to throw/unblock/warn etc. for this
• Analyzer to warn about strongly-typed hubs and using InvokeAsync with .All, .Group, etc.
• InvokeAsync void result? Scenario, acks without needing a value
• [Scaleout] ServerA requests client result from connection on ServerB, ServerB goes down after receiving request, ServerA needs to know somehow so it can error the client result
• Look at performance
  • The biggest performance issue I can think of right now is that RawResult allocates and copy the bytes which can be expensive
• Flow cancellation from server to client
  • Inject CancellationToken into .On methods and send CancelInvocation hub messages to clients

Thanks for contacting us.

We're moving this issue to the .NET 8 Planning milestone for future evaluation / consideration. We would like to keep this around to collect more feedback, which can help us with prioritizing this work. We will re-evaluate this issue, during our next planning meeting(s).
If we later determine, that the issue has no community involvement, or it's very rare and low-impact issue, we will close it - so that the team can focus on more important and high impact issues.
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