use std::{
future::Future,
pin::Pin,
sync::{Arc, Condvar, Mutex},
task::{Context, Poll, Wake, Waker},
};
enum SignalState {
Empty,
Waiting,
Notified,
}
struct Signal {
state: Mutex<SignalState>,
cond: Condvar,
}
impl Signal {
fn new() -> Self {
Self {
state: Mutex::new(SignalState::Empty),
cond: Condvar::new(),
}
}
fn wait(&self) {
let mut state = self.state.lock().unwrap();
match *state {
SignalState::Notified => *state = SignalState::Empty,
SignalState::Waiting => unreachable!(),
SignalState::Empty => {
*state = SignalState::Waiting;
while let SignalState::Waiting = *state {
state = self.cond.wait(state).unwrap();
}
}
}
}
fn notify(&self) {
let mut state = self.state.lock().unwrap();
match *state {
SignalState::Notified => {}
SignalState::Empty => *state = SignalState::Notified,
SignalState::Waiting => {
*state = SignalState::Empty;
self.cond.notify_one();
}
}
}
}
impl Wake for Signal {
fn wake(self: Arc<Self>) {
self.notify();
}
}
pub fn block_on<F: Future>(mut fut: F) -> F::Output {
let mut fut = unsafe { Pin::new_unchecked(&mut fut) };
let signal = Arc::new(Signal::new());
let waker = Waker::from(Arc::clone(&signal));
let mut context = Context::from_waker(&waker);
loop {
match fut.as_mut().poll(&mut context) {
Poll::Pending => signal.wait(),
Poll::Ready(item) => break item,
}
}
}