Add simple blocking async executor (#38)

crates/common/src/executor.rs

@@ -0,0 +1,34 @@
+//! This module contains utilities for handling async functions in the no_std environment. This allows for usage of
+//! async/await syntax for futures in a single thread.
+
+use alloc::boxed::Box;
+use core::{
+    future::Future,
+    task::{Context, Poll, RawWaker, RawWakerVTable, Waker},
+};
+
+/// This function busy waits on a future until it is ready. It uses a no-op waker to poll the future in a
+/// thread-blocking loop.
+pub fn block_on<T>(f: impl Future<Output = T>) -> T {
+    let mut f = Box::pin(f);
+
+    // Construct a no-op waker.
+    fn noop_clone(_: *const ()) -> RawWaker {
+        noop_raw_waker()
+    }
+    fn noop(_: *const ()) {}
+    fn noop_raw_waker() -> RawWaker {
+        let vtable = &RawWakerVTable::new(noop_clone, noop, noop, noop);
+        RawWaker::new(core::ptr::null(), vtable)
+    }
+    let waker = unsafe { Waker::from_raw(noop_raw_waker()) };
+    let mut context = Context::from_waker(&waker);
+
+    loop {
+        // Safety: This is safe because we only poll the future once per loop iteration,
+        // and we do not move the future after pinning it.
+        if let Poll::Ready(v) = f.as_mut().poll(&mut context) {
+            return v;
+        }
+    }
+}

crates/common/src/lib.rs

@@ -10,6 +10,8 @@
 #![cfg_attr(target_arch = "mips", feature(asm_experimental_arch))]
 #![no_std]
 
+extern crate alloc;
+
 pub mod io;
 
 pub mod malloc;
@@ -20,6 +22,9 @@ pub use traits::BasicKernelInterface;
 mod types;
 pub use types::{FileDescriptor, RegisterSize};
 
+mod executor;
+pub use executor::block_on;
+
 #[cfg(target_arch = "mips")]
 pub(crate) mod cannon;