async-usercalls 0.5.0

This crate provides an interface for performing asynchronous usercalls in SGX enclaves. The motivation behind asynchronous usercalls and ABI documentation can be found [here](https://edp.fortanix.com/docs/api/fortanix_sgx_abi/async/index.html). The API provided here is fairly low level and is not meant for general use. These APIs can be used to implement [mio] abstractions which in turn allows us to use [tokio] in SGX enclaves! The main interface is provided through `AsyncUsercallProvider` which works in tandem with `CallbackHandler`: ``` use async_usercalls::AsyncUsercallProvider; use std::{io::Result, net::TcpStream, sync::mpsc, time::Duration}; let (provider, callback_handler) = AsyncUsercallProvider::new(); let (tx, rx) = mpsc::sync_channel(1); // The closure is called when userspace sends back the result of the // usercall. let cancel_handle = provider.connect_stream("www.example.com:80", move |res| { tx.send(res).unwrap(); }); // We can cancel the connect usercall using `cancel_handle.cancel()`, but // note that we may still get a successful result. // We need to poll `callback_handler` to make progress. loop { let n = callback_handler.poll(Some(Duration::from_millis(100))); if n > 0 { break; // at least 1 callback function was executed! } } let connect_result: Result = rx.recv().unwrap(); ``` [mio]: https://docs.rs/mio/latest/mio/ [tokio]: https://docs.rs/tokio/latest/tokio/