Blocking/Async Function Calls


* A system's API contains async functions.


Otherwise, you get the [_Callback Hell_](https://en.wiktionary.org/wiki/callback_hell)
which is JavaScript before all the async extensions.


* This pattern is based upon the _[Multi-Threaded Synchronization](multi-threading.md)_ pattern.

* An independent thread is used to run the scripting [`Engine`].

* An MPSC channel (or any other appropriate synchronization primitive) is used to send function call
  arguments, packaged as a message, to another Rust thread that will perform the actual async calls.

* Results are marshaled back to the [`Engine`] thread via another MPSC channel.


See the _[Multi-Threaded Synchronization](multi-threading.md)_ pattern.

Implementation

  1. Spawn a thread to run the scripting Engine. Usually the sync feature is NOT used for this pattern.

  2. Spawn another thread (the worker thread) that can perform the actual async calls in Rust. This thread may actually be the main thread of the program.

  3. Create a pair of MPSC channels (named command and reply below) for full-duplex communications between the two threads.

  4. Register async API function to the Engine with a closure that captures the MPSC end-points.

  5. If there are more than one async function, the receive end-point on the reply channel can simply be cloned. The send end-point on the command channel can be wrapped in an Arc<Mutex<Channel>> for shared access.

  6. In the async function, the name of the function and call arguments are serialized into JSON (or any appropriate message format) and sent to command channel, where they’ll be removed by the worker thread and the appropriate async function called.

  7. The Engine blocks on the function call, waiting for a reply message on the reply channel.

  8. When the async function call complete on the worker thread, the result is sent back to the Engine thread via the reply channel.

  9. After the result is obtained from the reply channel, the Engine returns it as the return value of the function call, ending the block and continuing evaluation.