93 lines
2.9 KiB
Rust
93 lines
2.9 KiB
Rust
// Copyright 2020 The Chromium OS Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file.
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// Need non-snake case so the macro can re-use type names for variables.
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#![allow(non_snake_case)]
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use std::{
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future::Future,
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pin::Pin,
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task::{Context, Poll},
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};
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use futures::future::{maybe_done, FutureExt, MaybeDone};
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pub enum SelectResult<F: Future> {
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Pending(F),
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Finished(F::Output),
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}
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// Macro-generate future combinators to allow for running different numbers of top-level futures in
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// this FutureList. Generates the implementation of `FutureList` for the select types. For an
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// explicit example this is modeled after, see `UnitFutures`.
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macro_rules! generate {
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($(
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$(#[$doc:meta])*
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($Select:ident, <$($Fut:ident),*>),
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)*) => ($(
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paste::item! {
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pub(crate) struct $Select<$($Fut: Future + Unpin),*> {
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$($Fut: MaybeDone<$Fut>,)*
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}
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}
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impl<$($Fut: Future + Unpin),*> $Select<$($Fut),*> {
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paste::item! {
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pub(crate) fn new($($Fut: $Fut),*) -> $Select<$($Fut),*> {
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$Select {
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$($Fut: maybe_done($Fut),)*
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}
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}
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}
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}
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impl<$($Fut: Future + Unpin),*> Future for $Select<$($Fut),*> {
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type Output = ($(SelectResult<$Fut>),*);
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fn poll(mut self: Pin<&mut Self>, cx: &mut Context) -> Poll<Self::Output> {
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let mut complete = false;
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$(
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let $Fut = Pin::new(&mut self.$Fut);
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// The future impls `Unpin`, use `poll_unpin` to avoid wrapping it in
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// `Pin` to call `poll`.
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complete |= self.$Fut.poll_unpin(cx).is_ready();
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)*
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if complete {
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Poll::Ready(($(
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match std::mem::replace(&mut self.$Fut, MaybeDone::Gone) {
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MaybeDone::Future(f) => SelectResult::Pending(f),
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MaybeDone::Done(o) => SelectResult::Finished(o),
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MaybeDone::Gone => unreachable!(),
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}
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), *))
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} else {
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Poll::Pending
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}
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}
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}
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)*)
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}
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generate! {
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/// _Future for the [`select2`] function.
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(Select2, <_Fut1, _Fut2>),
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/// _Future for the [`select3`] function.
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(Select3, <_Fut1, _Fut2, _Fut3>),
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/// _Future for the [`select4`] function.
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(Select4, <_Fut1, _Fut2, _Fut3, _Fut4>),
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/// _Future for the [`select5`] function.
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(Select5, <_Fut1, _Fut2, _Fut3, _Fut4, _Fut5>),
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/// _Future for the [`select6`] function.
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(Select6, <_Fut1, _Fut2, _Fut3, _Fut4, _Fut5, _Fut6>),
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/// _Future for the [`select7`] function.
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(Select7, <_Fut1, _Fut2, _Fut3, _Fut4, _Fut5, _Fut6, _Fut7>),
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}
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