8.0 Proposal (internal): Use IValueTaskSource for RPC continuations awaits

[bollhals]

In order to prepare for the async interface, one part of the whole library is the RpcContinuationQueue. With the current implementation we can not support async waiting on completion.
=> I propose to use a IValueTaskSource and create a class that handles this.

This way we could set the reply with one method and have another which you could use to wait.

void SetReply(in IncomingCommand cmd)
ValueTask<IncomingCommand> GetReplyAsync()

instead of the current (used my latest branch to experiment)

IncomingCommand GetReply()
void HandleCommand(in IncomingCommand cmd)

With this implementation we can avoid a lot of allocations for an RPC await.

I have a very basic working prototype up and running locally.
If the extended functionality (timeout, shutdown) is fully implmented and working, I'll set up a PR for the current sync API with GetReplyAsync().AsTask.GetAwaiter().GetResult(). in every public API. => Async under the hood, so we'd ready to switch.

[stebet]

I already have a working type in the async branch, which uses/reuses ValueTasks from a pooled resource. You can use that as a reference as well. It also uses locking to guarantee that only one operation is outstanding per model. See here: https://github.com/rabbitmq/rabbitmq-dotnet-client/blob/full-async/projects/RabbitMQ.Client/client/impl/RpcContinuationQueue.cs

[michaelklishin]

Moving to 8.0 so that we can ship a 7.0 with a smaller set of breaking API changes if necessary.

[lukebakken]

I believe that the AsyncRpcContinuation class and subclasses effectively implement this suggestion.

[stebet]

I believe that the AsyncRpcContinuation class and subclasses effectively implement this suggestion.

They are TaskCompletionSource based which does allocate where IValueTaskSource doesn't. It's not a huge issue though and something I can look at later.