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flatten_unordered.rs
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flatten_unordered.rs
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use alloc::sync::Arc;
use core::{
cell::UnsafeCell,
convert::identity,
fmt,
num::NonZeroUsize,
pin::Pin,
sync::atomic::{AtomicU8, Ordering},
};
use pin_project_lite::pin_project;
use futures_core::{
future::Future,
ready,
stream::{FusedStream, Stream},
task::{Context, Poll, Waker},
};
#[cfg(feature = "sink")]
use futures_sink::Sink;
use futures_task::{waker, ArcWake};
use crate::stream::FuturesUnordered;
/// There is nothing to poll and stream isn't being polled/waking/woken at the moment.
const NONE: u8 = 0;
/// Inner streams need to be polled.
const NEED_TO_POLL_INNER_STREAMS: u8 = 1;
/// The base stream needs to be polled.
const NEED_TO_POLL_STREAM: u8 = 0b10;
/// Both base stream and inner streams need to be polled.
const NEED_TO_POLL_ALL: u8 = NEED_TO_POLL_INNER_STREAMS | NEED_TO_POLL_STREAM;
/// The current stream is being polled at the moment.
const POLLING: u8 = 0b100;
/// Stream is being woken at the moment.
const WAKING: u8 = 0b1000;
/// The stream was waked and will be polled.
const WOKEN: u8 = 0b10000;
/// Internal polling state of the stream.
#[derive(Clone, Debug)]
struct SharedPollState {
state: Arc<AtomicU8>,
}
impl SharedPollState {
/// Constructs new `SharedPollState` with the given state.
fn new(value: u8) -> SharedPollState {
SharedPollState { state: Arc::new(AtomicU8::new(value)) }
}
/// Attempts to start polling, returning stored state in case of success.
/// Returns `None` if either waker is waking at the moment or state is empty.
fn start_polling(
&self,
) -> Option<(u8, PollStateBomb<'_, impl FnOnce(&SharedPollState) -> u8>)> {
let value = self
.state
.fetch_update(Ordering::SeqCst, Ordering::SeqCst, |value| {
if value & WAKING == NONE && value & NEED_TO_POLL_ALL != NONE {
Some(POLLING)
} else {
None
}
})
.ok()?;
let bomb = PollStateBomb::new(self, SharedPollState::reset);
Some((value, bomb))
}
/// Attempts to start the waking process and performs bitwise or with the given value.
///
/// If some waker is already in progress or stream is already woken/being polled, waking process won't start, however
/// state will be disjuncted with the given value.
fn start_waking(
&self,
to_poll: u8,
) -> Option<(u8, PollStateBomb<'_, impl FnOnce(&SharedPollState) -> u8>)> {
let value = self
.state
.fetch_update(Ordering::SeqCst, Ordering::SeqCst, |value| {
let mut next_value = value | to_poll;
if value & (WOKEN | POLLING) == NONE {
next_value |= WAKING;
}
if next_value != value {
Some(next_value)
} else {
None
}
})
.ok()?;
debug_assert!(value & WAKING == NONE);
// Only start the waking process if we're not in the polling phase and the stream isn't woken already
if value & (WOKEN | POLLING) == NONE {
let bomb = PollStateBomb::new(self, SharedPollState::stop_waking);
Some((value, bomb))
} else {
None
}
}
/// Sets current state to
/// - `!POLLING` allowing to use wakers
/// - `WOKEN` if the state was changed during `POLLING` phase as waker will be called,
/// or `will_be_woken` flag supplied
/// - `!WAKING` as
/// * Wakers called during the `POLLING` phase won't propagate their calls
/// * `POLLING` phase can't start if some of the wakers are active
/// So no wrapped waker can touch the inner waker's cell, it's safe to poll again.
fn stop_polling(&self, to_poll: u8, will_be_woken: bool) -> u8 {
self.state
.fetch_update(Ordering::SeqCst, Ordering::SeqCst, |mut value| {
let mut next_value = to_poll;
value &= NEED_TO_POLL_ALL;
if value != NONE || will_be_woken {
next_value |= WOKEN;
}
next_value |= value;
Some(next_value & !POLLING & !WAKING)
})
.unwrap()
}
/// Toggles state to non-waking, allowing to start polling.
fn stop_waking(&self) -> u8 {
let value = self
.state
.fetch_update(Ordering::SeqCst, Ordering::SeqCst, |value| {
let next_value = value & !WAKING | WOKEN;
if next_value != value {
Some(next_value)
} else {
None
}
})
.unwrap_or_else(identity);
debug_assert!(value & (WOKEN | POLLING | WAKING) == WAKING);
value
}
/// Resets current state allowing to poll the stream and wake up wakers.
fn reset(&self) -> u8 {
self.state.swap(NEED_TO_POLL_ALL, Ordering::AcqRel)
}
}
/// Used to execute some function on the given state when dropped.
struct PollStateBomb<'a, F: FnOnce(&SharedPollState) -> u8> {
state: &'a SharedPollState,
drop: Option<F>,
}
impl<'a, F: FnOnce(&SharedPollState) -> u8> PollStateBomb<'a, F> {
/// Constructs new bomb with the given state.
fn new(state: &'a SharedPollState, drop: F) -> Self {
Self { state, drop: Some(drop) }
}
/// Deactivates bomb, forces it to not call provided function when dropped.
fn deactivate(mut self) {
self.drop.take();
}
}
impl<F: FnOnce(&SharedPollState) -> u8> Drop for PollStateBomb<'_, F> {
fn drop(&mut self) {
if let Some(drop) = self.drop.take() {
(drop)(self.state);
}
}
}
/// Will update state with the provided value on `wake_by_ref` call
/// and then, if there is a need, call `inner_waker`.
struct WrappedWaker {
inner_waker: UnsafeCell<Option<Waker>>,
poll_state: SharedPollState,
need_to_poll: u8,
}
unsafe impl Send for WrappedWaker {}
unsafe impl Sync for WrappedWaker {}
impl WrappedWaker {
/// Replaces given waker's inner_waker for polling stream/futures which will
/// update poll state on `wake_by_ref` call. Use only if you need several
/// contexts.
///
/// ## Safety
///
/// This function will modify waker's `inner_waker` via `UnsafeCell`, so
/// it should be used only during `POLLING` phase by one thread at the time.
unsafe fn replace_waker(self_arc: &mut Arc<Self>, cx: &Context<'_>) -> Waker {
*self_arc.inner_waker.get() = cx.waker().clone().into();
waker(self_arc.clone())
}
/// Attempts to start the waking process for the waker with the given value.
/// If succeeded, then the stream isn't yet woken and not being polled at the moment.
fn start_waking(&self) -> Option<(u8, PollStateBomb<'_, impl FnOnce(&SharedPollState) -> u8>)> {
self.poll_state.start_waking(self.need_to_poll)
}
}
impl ArcWake for WrappedWaker {
fn wake_by_ref(self_arc: &Arc<Self>) {
if let Some((_, state_bomb)) = self_arc.start_waking() {
// Safety: now state is not `POLLING`
let waker_opt = unsafe { self_arc.inner_waker.get().as_ref().unwrap() };
if let Some(inner_waker) = waker_opt.clone() {
// Stop waking to allow polling stream
drop(state_bomb);
// Wake up inner waker
inner_waker.wake();
}
}
}
}
pin_project! {
/// Future which contains optional stream.
///
/// If it's `Some`, it will attempt to call `poll_next` on it,
/// returning `Some((item, next_item_fut))` in case of `Poll::Ready(Some(...))`
/// or `None` in case of `Poll::Ready(None)`.
///
/// If `poll_next` will return `Poll::Pending`, it will be forwarded to
/// the future and current task will be notified by waker.
#[must_use = "futures do nothing unless you `.await` or poll them"]
struct PollStreamFut<St> {
#[pin]
stream: Option<St>,
}
}
impl<St> PollStreamFut<St> {
/// Constructs new `PollStreamFut` using given `stream`.
fn new(stream: impl Into<Option<St>>) -> Self {
Self { stream: stream.into() }
}
}
impl<St: Stream + Unpin> Future for PollStreamFut<St> {
type Output = Option<(St::Item, PollStreamFut<St>)>;
fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
let mut stream = self.project().stream;
let item = if let Some(stream) = stream.as_mut().as_pin_mut() {
ready!(stream.poll_next(cx))
} else {
None
};
let next_item_fut = PollStreamFut::new(stream.get_mut().take());
let out = item.map(|item| (item, next_item_fut));
Poll::Ready(out)
}
}
pin_project! {
/// Stream for the [`flatten_unordered`](super::StreamExt::flatten_unordered)
/// method.
#[project = FlattenUnorderedProj]
#[must_use = "streams do nothing unless polled"]
pub struct FlattenUnordered<St> where St: Stream {
#[pin]
inner_streams: FuturesUnordered<PollStreamFut<St::Item>>,
#[pin]
stream: St,
poll_state: SharedPollState,
limit: Option<NonZeroUsize>,
is_stream_done: bool,
inner_streams_waker: Arc<WrappedWaker>,
stream_waker: Arc<WrappedWaker>,
}
}
impl<St> fmt::Debug for FlattenUnordered<St>
where
St: Stream + fmt::Debug,
St::Item: Stream + fmt::Debug,
{
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("FlattenUnordered")
.field("poll_state", &self.poll_state)
.field("inner_streams", &self.inner_streams)
.field("limit", &self.limit)
.field("stream", &self.stream)
.field("is_stream_done", &self.is_stream_done)
.finish()
}
}
impl<St> FlattenUnordered<St>
where
St: Stream,
St::Item: Stream + Unpin,
{
pub(super) fn new(stream: St, limit: Option<usize>) -> FlattenUnordered<St> {
let poll_state = SharedPollState::new(NEED_TO_POLL_STREAM);
FlattenUnordered {
inner_streams: FuturesUnordered::new(),
stream,
is_stream_done: false,
limit: limit.and_then(NonZeroUsize::new),
inner_streams_waker: Arc::new(WrappedWaker {
inner_waker: UnsafeCell::new(None),
poll_state: poll_state.clone(),
need_to_poll: NEED_TO_POLL_INNER_STREAMS,
}),
stream_waker: Arc::new(WrappedWaker {
inner_waker: UnsafeCell::new(None),
poll_state: poll_state.clone(),
need_to_poll: NEED_TO_POLL_STREAM,
}),
poll_state,
}
}
delegate_access_inner!(stream, St, ());
}
impl<St> FlattenUnorderedProj<'_, St>
where
St: Stream,
{
/// Checks if current `inner_streams` bucket size is greater than optional limit.
fn is_exceeded_limit(&self) -> bool {
self.limit.map_or(false, |limit| self.inner_streams.len() >= limit.get())
}
}
impl<St> FusedStream for FlattenUnordered<St>
where
St: FusedStream,
St::Item: Stream + Unpin,
{
fn is_terminated(&self) -> bool {
self.stream.is_terminated() && self.inner_streams.is_empty()
}
}
impl<St> Stream for FlattenUnordered<St>
where
St: Stream,
St::Item: Stream + Unpin,
{
type Item = <St::Item as Stream>::Item;
fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
let mut next_item = None;
let mut need_to_poll_next = NONE;
let mut this = self.as_mut().project();
let (mut poll_state_value, state_bomb) = match this.poll_state.start_polling() {
Some(value) => value,
_ => {
// Waker was called, just wait for the next poll
return Poll::Pending;
}
};
if poll_state_value & NEED_TO_POLL_STREAM != NONE {
let mut stream_waker = None;
// Here we need to poll the base stream.
//
// To improve performance, we will attempt to place as many items as we can
// to the `FuturesUnordered` bucket before polling inner streams
loop {
if this.is_exceeded_limit() || *this.is_stream_done {
// We either exceeded the limit or the stream is exhausted
if !*this.is_stream_done {
// The stream needs to be polled in the next iteration
need_to_poll_next |= NEED_TO_POLL_STREAM;
}
break;
} else {
// Initialize base stream waker if it's not yet initialized
if stream_waker.is_none() {
// Safety: now state is `POLLING`.
stream_waker
.replace(unsafe { WrappedWaker::replace_waker(this.stream_waker, cx) });
}
let mut cx = Context::from_waker(stream_waker.as_ref().unwrap());
match this.stream.as_mut().poll_next(&mut cx) {
Poll::Ready(Some(inner_stream)) => {
let next_item_fut = PollStreamFut::new(inner_stream);
// Add new stream to the inner streams bucket
this.inner_streams.as_mut().push(next_item_fut);
// Inner streams must be polled afterward
poll_state_value |= NEED_TO_POLL_INNER_STREAMS;
}
Poll::Ready(None) => {
// Mark the base stream as done
*this.is_stream_done = true;
}
Poll::Pending => {
break;
}
}
}
}
}
if poll_state_value & NEED_TO_POLL_INNER_STREAMS != NONE {
// Safety: now state is `POLLING`.
let inner_streams_waker =
unsafe { WrappedWaker::replace_waker(this.inner_streams_waker, cx) };
let mut cx = Context::from_waker(&inner_streams_waker);
match this.inner_streams.as_mut().poll_next(&mut cx) {
Poll::Ready(Some(Some((item, next_item_fut)))) => {
// Push next inner stream item future to the list of inner streams futures
this.inner_streams.as_mut().push(next_item_fut);
// Take the received item
next_item = Some(item);
// On the next iteration, inner streams must be polled again
need_to_poll_next |= NEED_TO_POLL_INNER_STREAMS;
}
Poll::Ready(Some(None)) => {
// On the next iteration, inner streams must be polled again
need_to_poll_next |= NEED_TO_POLL_INNER_STREAMS;
}
_ => {}
}
}
// We didn't have any `poll_next` panic, so it's time to deactivate the bomb
state_bomb.deactivate();
// Call the waker at the end of polling if
let mut force_wake =
// we need to poll the stream and didn't reach the limit yet
need_to_poll_next & NEED_TO_POLL_STREAM != NONE && !this.is_exceeded_limit()
// or we need to poll the inner streams again
|| need_to_poll_next & NEED_TO_POLL_INNER_STREAMS != NONE;
// Stop polling and swap the latest state
poll_state_value = this.poll_state.stop_polling(need_to_poll_next, force_wake);
// If state was changed during `POLLING` phase, we also need to manually call a waker
force_wake |= poll_state_value & NEED_TO_POLL_ALL != NONE;
let is_done = *this.is_stream_done && this.inner_streams.is_empty();
if next_item.is_some() || is_done {
Poll::Ready(next_item)
} else {
if force_wake {
cx.waker().wake_by_ref();
}
Poll::Pending
}
}
}
// Forwarding impl of Sink from the underlying stream
#[cfg(feature = "sink")]
impl<St, Item> Sink<Item> for FlattenUnordered<St>
where
St: Stream + Sink<Item>,
{
type Error = St::Error;
delegate_sink!(stream, Item);
}