ordered_stream/multi.rs
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use crate::*;
use core::ops::DerefMut;
use core::pin::Pin;
use core::task::{Context, Poll};
fn poll_multiple_step<I, P, S>(
streams: I,
cx: &mut Context<'_>,
before: Option<&S::Ordering>,
mut retry: Option<&mut Option<S::Ordering>>,
) -> Poll<PollResult<S::Ordering, S::Data>>
where
I: IntoIterator<Item = Pin<P>>,
P: DerefMut<Target = Peekable<S>>,
S: OrderedStream,
S::Ordering: Clone,
{
// The stream with the earliest item that is actually before the given point
let mut best: Option<Pin<P>> = None;
// true if we have a stream that has not terminated
let mut has_data = false;
let mut has_pending = false;
let mut skip_retry = false;
for mut stream in streams {
let best_before = best.as_ref().and_then(|p| p.item().map(|i| &i.0));
let current_bound = match (before, best_before) {
(Some(given), Some(best)) if given <= best => Some(given),
(_, Some(best)) => Some(best),
(given, None) => given,
};
// improved is true if have improved the `before` bound from the initial value
match stream.as_mut().poll_peek_before(cx, current_bound) {
Poll::Pending => {
has_pending = true;
skip_retry = true;
}
Poll::Ready(PollResult::Terminated) => continue,
Poll::Ready(PollResult::NoneBefore) => {
has_data = true;
}
Poll::Ready(PollResult::Item { ordering, .. }) => {
has_data = true;
match current_bound {
Some(max) if max < ordering => continue,
_ => {}
}
match (&mut retry, before, has_pending) {
(Some(retry), Some(initial_bound), true) if ordering < initial_bound => {
// We have just improved the initial bound, so the streams that
// previously returned Pending might be able to return NoneBefore in a
// retry. This is only useful if there are no later Pending returns, so
// those will set skip_retry.
**retry = Some(ordering.clone());
skip_retry = false;
}
(Some(retry), None, true) => {
**retry = Some(ordering.clone());
skip_retry = false;
}
_ => {}
}
best = Some(stream);
}
}
}
if skip_retry {
retry.map(|r| *r = None);
}
match best {
_ if has_pending => Poll::Pending,
None if has_data => Poll::Ready(PollResult::NoneBefore),
None => Poll::Ready(PollResult::Terminated),
// This is guaranteed to return PollResult::Item
Some(mut stream) => stream.as_mut().poll_next_before(cx, before),
}
}
/// Join a collection of [`OrderedStream`]s.
///
/// This is similar to repeatedly using [`join()`] on all the streams in the contained collection.
/// It is not optimized to avoid polling streams that are not ready, so it works best if the number
/// of streams is relatively small.
//
// Unlike `FutureUnordered` or `SelectAll`, the ordering properties that this struct provides can
// easily require that all items in the collection be consulted before returning any item. An
// example of such a situation is a series of streams that all generate timestamps (locally) for
// their items and only return `NoneBefore` for past timestamps. If only one stream produces an
// item for each call to `JoinMultiple::poll_next_before`, that timestamp must be checked against
// every other stream, and no amount of preparatory work or hints will help this.
//
// On the other hand, if all streams provide a position hint that matches their next item, it is
// possible to build a priority queue to sort the streams and reduce the cost of a single poll from
// `n` to `log(n)`. This does require maintaining a snapshot of the hints (so S::Ordering: Clone),
// and will significantly increase the worst-case workload, so it should be a distinct type.
#[derive(Debug, Default, Clone)]
pub struct JoinMultiple<C>(pub C);
impl<C> Unpin for JoinMultiple<C> {}
impl<C, S> OrderedStream for JoinMultiple<C>
where
for<'a> &'a mut C: IntoIterator<Item = &'a mut Peekable<S>>,
S: OrderedStream + Unpin,
S::Ordering: Clone,
{
type Ordering = S::Ordering;
type Data = S::Data;
fn poll_next_before(
mut self: Pin<&mut Self>,
cx: &mut Context<'_>,
before: Option<&S::Ordering>,
) -> Poll<PollResult<S::Ordering, S::Data>> {
let mut retry = None;
let rv = poll_multiple_step(
self.as_mut().get_mut().0.into_iter().map(Pin::new),
cx,
before,
Some(&mut retry),
);
if rv.is_pending() && retry.is_some() {
poll_multiple_step(
self.get_mut().0.into_iter().map(Pin::new),
cx,
retry.as_ref(),
None,
)
} else {
rv
}
}
}
impl<C, S> FusedOrderedStream for JoinMultiple<C>
where
for<'a> &'a mut C: IntoIterator<Item = &'a mut Peekable<S>>,
for<'a> &'a C: IntoIterator<Item = &'a Peekable<S>>,
S: OrderedStream + Unpin,
S::Ordering: Clone,
{
fn is_terminated(&self) -> bool {
self.0.into_iter().all(|peekable| peekable.is_terminated())
}
}
pin_project_lite::pin_project! {
/// Join a collection of pinned [`OrderedStream`]s.
///
/// This is identical to [`JoinMultiple`], but accepts [`OrderedStream`]s that are not [`Unpin`] by
/// requiring that the collection provide a pinned [`IntoIterator`] implementation.
///
/// This is not a feature available in most `std` collections. If you wish to use them, you
/// should use `Box::pin` to make the stream [`Unpin`] before inserting it in the collection,
/// and then use [`JoinMultiple`] on the resulting collection.
#[derive(Debug,Default,Clone)]
pub struct JoinMultiplePin<C> {
#[pin]
pub streams: C,
}
}
impl<C> JoinMultiplePin<C> {
pub fn as_pin_mut(self: Pin<&mut Self>) -> Pin<&mut C> {
self.project().streams
}
}
impl<C, S> OrderedStream for JoinMultiplePin<C>
where
for<'a> Pin<&'a mut C>: IntoIterator<Item = Pin<&'a mut Peekable<S>>>,
S: OrderedStream,
S::Ordering: Clone,
{
type Ordering = S::Ordering;
type Data = S::Data;
fn poll_next_before(
mut self: Pin<&mut Self>,
cx: &mut Context<'_>,
before: Option<&S::Ordering>,
) -> Poll<PollResult<S::Ordering, S::Data>> {
let mut retry = None;
let rv = poll_multiple_step(self.as_mut().as_pin_mut(), cx, before, Some(&mut retry));
if rv.is_pending() && retry.is_some() {
poll_multiple_step(self.as_pin_mut(), cx, retry.as_ref(), None)
} else {
rv
}
}
}
#[cfg(test)]
mod test {
extern crate alloc;
use crate::{FromStream, JoinMultiple, OrderedStream, OrderedStreamExt, PollResult};
use alloc::{boxed::Box, rc::Rc, vec, vec::Vec};
use core::{cell::Cell, pin::Pin, task::Context, task::Poll};
use futures_core::Stream;
use futures_util::{pin_mut, stream::iter};
#[derive(Debug, PartialEq)]
pub struct Message {
serial: u32,
}
#[test]
fn join_mutiple() {
futures_executor::block_on(async {
pub struct RemoteLogSource {
stream: Pin<Box<dyn Stream<Item = Message>>>,
}
let mut logs = [
RemoteLogSource {
stream: Box::pin(iter([
Message { serial: 1 },
Message { serial: 4 },
Message { serial: 5 },
])),
},
RemoteLogSource {
stream: Box::pin(iter([
Message { serial: 2 },
Message { serial: 3 },
Message { serial: 6 },
])),
},
];
let streams: Vec<_> = logs
.iter_mut()
.map(|s| FromStream::with_ordering(&mut s.stream, |m| m.serial).peekable())
.collect();
let mut joined = JoinMultiple(streams);
for i in 0..6 {
let msg = joined.next().await.unwrap();
assert_eq!(msg.serial, i as u32 + 1);
}
});
}
#[test]
fn join_one_slow() {
futures_executor::block_on(async {
pub struct DelayStream(Rc<Cell<u8>>);
impl OrderedStream for DelayStream {
type Ordering = u32;
type Data = Message;
fn poll_next_before(
self: Pin<&mut Self>,
_: &mut Context<'_>,
before: Option<&Self::Ordering>,
) -> Poll<PollResult<Self::Ordering, Self::Data>> {
match self.0.get() {
0 => Poll::Pending,
1 if matches!(before, Some(&1)) => Poll::Ready(PollResult::NoneBefore),
1 => Poll::Pending,
2 => {
self.0.set(3);
Poll::Ready(PollResult::Item {
data: Message { serial: 4 },
ordering: 4,
})
}
_ => Poll::Ready(PollResult::Terminated),
}
}
}
let stream1 = iter([
Message { serial: 1 },
Message { serial: 3 },
Message { serial: 5 },
]);
let stream1 = FromStream::with_ordering(stream1, |m| m.serial);
let go = Rc::new(Cell::new(0));
let stream2 = DelayStream(go.clone());
let stream1: Pin<Box<dyn OrderedStream<Ordering = u32, Data = Message>>> =
Box::pin(stream1);
let stream2: Pin<Box<dyn OrderedStream<Ordering = u32, Data = Message>>> =
Box::pin(stream2);
let streams = vec![stream1.peekable(), stream2.peekable()];
let join = JoinMultiple(streams);
let waker = futures_util::task::noop_waker();
let mut ctx = core::task::Context::from_waker(&waker);
pin_mut!(join);
// When the DelayStream has no information about what it contains, join returns Pending
// (since there could be a serial-0 message output of DelayStream)
assert_eq!(
join.as_mut().poll_next_before(&mut ctx, None),
Poll::Pending
);
go.set(1);
// Now the DelayStream will return NoneBefore on serial 1
assert_eq!(
join.as_mut().poll_next_before(&mut ctx, None),
Poll::Ready(PollResult::Item {
data: Message { serial: 1 },
ordering: 1,
})
);
// however, it does not (yet) do so for serial 3
assert_eq!(
join.as_mut().poll_next_before(&mut ctx, None),
Poll::Pending
);
go.set(2);
assert_eq!(
join.as_mut().poll_next_before(&mut ctx, None),
Poll::Ready(PollResult::Item {
data: Message { serial: 3 },
ordering: 3,
})
);
assert_eq!(
join.as_mut().poll_next_before(&mut ctx, None),
Poll::Ready(PollResult::Item {
data: Message { serial: 4 },
ordering: 4,
})
);
assert_eq!(
join.as_mut().poll_next_before(&mut ctx, None),
Poll::Ready(PollResult::Item {
data: Message { serial: 5 },
ordering: 5,
})
);
assert_eq!(
join.as_mut().poll_next_before(&mut ctx, None),
Poll::Ready(PollResult::Terminated)
);
});
}
}