futures: add block_on

embassy-futures/src/block_on.rs

@@ -0,0 +1,31 @@
+use core::future::Future;
+use core::pin::Pin;
+use core::ptr;
+use core::task::{Context, Poll, RawWaker, RawWakerVTable, Waker};
+
+static VTABLE: RawWakerVTable = RawWakerVTable::new(|_| RawWaker::new(ptr::null(), &VTABLE), |_| {}, |_| {}, |_| {});
+
+/// Run a future to completion using a busy loop.
+///
+/// This calls `.poll()` on the future in a busy loop, which blocks
+/// the current thread at 100% cpu usage until the future is done. The
+/// future's `Waker` mechanism is not used.
+///
+/// It's suitable for systems with no or limited concurrency and without
+/// strict requirements around power consumption. For more complex use
+/// cases, prefer using a "real" executor like `embassy-executor`, which
+/// supports multiple tasks, and putting the core to sleep when no task
+/// needs to do work.
+pub fn block_on<F: Future>(mut fut: F) -> F::Output {
+    // safety: we don't move the future after this line.
+    let mut fut = unsafe { Pin::new_unchecked(&mut fut) };
+
+    let raw_waker = RawWaker::new(ptr::null(), &VTABLE);
+    let waker = unsafe { Waker::from_raw(raw_waker) };
+    let mut cx = Context::from_waker(&waker);
+    loop {
+        if let Poll::Ready(res) = fut.as_mut().poll(&mut cx) {
+            return res;
+        }
+    }
+}

embassy-futures/src/lib.rs

@@ -5,8 +5,10 @@
 // This mod MUST go first, so that the others see its macros.
 pub(crate) mod fmt;
 
+mod block_on;
 mod select;
 mod yield_now;
 
+pub use block_on::*;
 pub use select::*;
 pub use yield_now::*;