Ensure timers always yield at least once.

This prevents a task that's constantly running late from monopolizing the CPU.
Add executor_fairness_test example showcasing it.

embassy/src/executor/timer.rs

@@ -7,16 +7,21 @@ use crate::time::{Duration, Instant};
 
 pub struct Timer {
     expires_at: Instant,
+    yielded_once: bool,
 }
 
 impl Timer {
     pub fn at(expires_at: Instant) -> Self {
-        Self { expires_at }
+        Self {
+            expires_at,
+            yielded_once: false,
+        }
     }
 
     pub fn after(duration: Duration) -> Self {
         Self {
             expires_at: Instant::now() + duration,
+            yielded_once: false,
         }
     }
 }
@@ -25,11 +30,12 @@ impl Unpin for Timer {}
 
 impl Future for Timer {
     type Output = ();
-    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
-        if self.expires_at <= Instant::now() {
+        if self.yielded_once && self.expires_at <= Instant::now() {
             Poll::Ready(())
         } else {
             unsafe { super::register_timer(self.expires_at, cx.waker()) };
+            self.yielded_once = true;
             Poll::Pending
         }
     }

examples/src/bin/executor_fairness_test.rs

@@ -0,0 +1,74 @@
+#![no_std]
+#![no_main]
+#![feature(type_alias_impl_trait)]
+
+#[path = "../example_common.rs"]
+mod example_common;
+use example_common::*;
+
+use core::task::Poll;
+use cortex_m_rt::entry;
+use defmt::panic;
+use embassy::executor::{task, Executor};
+use embassy::time::{Duration, Instant, Timer};
+use embassy::util::Forever;
+use embassy_nrf::pac;
+use embassy_nrf::{interrupt, rtc};
+use nrf52840_hal::clocks;
+
+#[task]
+async fn run1() {
+    loop {
+        info!("DING DONG");
+        Timer::after(Duration::from_ticks(16000)).await;
+    }
+}
+
+#[task]
+async fn run2() {
+    loop {
+        Timer::at(Instant::from_ticks(0)).await;
+    }
+}
+
+#[task]
+async fn run3() {
+    futures::future::poll_fn(|cx| {
+        cx.waker().wake_by_ref();
+        Poll::<()>::Pending
+    })
+    .await;
+}
+
+static RTC: Forever<rtc::RTC<pac::RTC1>> = Forever::new();
+static ALARM: Forever<rtc::Alarm<pac::RTC1>> = Forever::new();
+static EXECUTOR: Forever<Executor> = Forever::new();
+
+#[entry]
+fn main() -> ! {
+    info!("Hello World!");
+
+    let p = unwrap!(embassy_nrf::pac::Peripherals::take());
+
+    clocks::Clocks::new(p.CLOCK)
+        .enable_ext_hfosc()
+        .set_lfclk_src_external(clocks::LfOscConfiguration::NoExternalNoBypass)
+        .start_lfclk();
+
+    let rtc = RTC.put(rtc::RTC::new(p.RTC1, interrupt::take!(RTC1)));
+    rtc.start();
+
+    unsafe { embassy::time::set_clock(rtc) };
+
+    let alarm = ALARM.put(rtc.alarm0());
+    let executor = EXECUTOR.put(Executor::new_with_alarm(alarm, cortex_m::asm::sev));
+
+    unwrap!(executor.spawn(run1()));
+    unwrap!(executor.spawn(run2()));
+    unwrap!(executor.spawn(run3()));
+
+    loop {
+        executor.run();
+        cortex_m::asm::wfe();
+    }
+}