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// TODO: Eventually to be replaced with tower_util::Oneshot.
use std::marker::Unpin;
use std::mem;
use tower_service::Service;
use crate::common::{task, Future, Pin, Poll};
pub(crate) fn oneshot<S, Req>(svc: S, req: Req) -> Oneshot<S, Req>
where
S: Service<Req>,
{
Oneshot {
state: State::NotReady(svc, req),
}
}
// A `Future` consuming a `Service` and request, waiting until the `Service`
// is ready, and then calling `Service::call` with the request, and
// waiting for that `Future`.
#[allow(missing_debug_implementations)]
pub struct Oneshot<S: Service<Req>, Req> {
state: State<S, Req>,
}
enum State<S: Service<Req>, Req> {
NotReady(S, Req),
Called(S::Future),
Tmp,
}
// Unpin is projected to S::Future, but never S.
impl<S, Req> Unpin for Oneshot<S, Req>
where
S: Service<Req>,
S::Future: Unpin,
{
}
impl<S, Req> Future for Oneshot<S, Req>
where
S: Service<Req>,
{
type Output = Result<S::Response, S::Error>;
fn poll(self: Pin<&mut Self>, cx: &mut task::Context<'_>) -> Poll<Self::Output> {
// Safety: The service's future is never moved once we get one.
let mut me = unsafe { Pin::get_unchecked_mut(self) };
loop {
match me.state {
State::NotReady(ref mut svc, _) => {
ready!(svc.poll_ready(cx))?;
// fallthrough out of the match's borrow
}
State::Called(ref mut fut) => {
return unsafe { Pin::new_unchecked(fut) }.poll(cx);
}
State::Tmp => unreachable!(),
}
match mem::replace(&mut me.state, State::Tmp) {
State::NotReady(mut svc, req) => {
me.state = State::Called(svc.call(req));
}
_ => unreachable!(),
}
}
}
}
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