Core Async Helpers

This crate contains helpers related to common asynchronous programming patterns used in nearcore:

• messaging: common interfaces for sending messages between components.
• test_loop: a event-loop-based test framework that can test multiple components together in a synchronous way.

Messaging

Sender<T> and AsyncSender<T> are abstractions of our Actix interfaces. When a component needs to send a message to another component, the component should keep a Sender<T> as a field and require it during construction, like:

```rust struct MyComponent { downstream_component: Sender, }

impl MyComponent { pub fn new(downstream_component: Sender) -> Self { ... } } ```

The sender can then be used to send messages: rust impl MyComponent { fn do_something(&mut self, args: ...) { self.downstream_component.send(DownstreamMessage::DataReady(...)); } }

To create a Sender<T>, we need any implementation of CanSend<T>. One way is to use an Actix address: ```rust impl Handler for DownstreamActor {...}

impl DownstreamActor { pub fn spawn(...) -> Addr {...} }

fn setupsystem() { let addr = DownstreamActor::spawn(...); let mycomponent = MyComponent::new(addr.into_sender()); } ```

In tests, the TestLoopBuilder provides the sender() function which also implements CanSend, see the examples directory under this crate.

AsyncSender<T> is similar, except that calling send_async returns a future that carries the response to the message.