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//! An unbounded set of futures.
use crate::task::AtomicWaker;
use futures_core::future::Future;
use futures_core::stream::Stream;
use futures_core::task::{self as core_task, Poll};
use std::cell::UnsafeCell;
use std::fmt::{self, Debug};
use std::iter::FromIterator;
use std::marker::{PhantomData, Unpin};
use std::mem;
use std::pin::PinMut;
use std::ptr;
use std::sync::atomic::Ordering::SeqCst;
use std::sync::atomic::{AtomicPtr, AtomicBool};
use std::sync::{Arc, Weak};
use std::usize;
mod abort;
mod iter;
use self::iter::{IterMut, IterPinMut};
mod task;
use self::task::Task;
mod ready_to_run_queue;
use self::ready_to_run_queue::{ReadyToRunQueue, Dequeue};
/// A set of futures which may complete in any order.
/// This structure is optimized to manage a large number of futures.
/// Futures managed by [`FuturesUnordered`] will only be polled when they
/// generate wake-up notifications. This reduces the required amount of work
/// needed to poll large numbers of futures.
/// [`FuturesUnordered`] can be filled by [`collect`](Iterator::collect)ing an
/// iterator of futures into a [`FuturesUnordered`], or by
/// [`push`](FuturesUnordered::push)ing futures onto an existing
/// [`FuturesUnordered`]. When new futures are added,
/// [`poll_next`](Stream::poll_next) must be called in order to begin receiving
/// wake-ups for new futures.
/// Note that you can create a ready-made [`FuturesUnordered`] via the
/// [`futures_unordered`](futures_unordered()) function, or you can start with
/// an empty set with the [`FuturesUnordered::new`] constructor.
#[must_use = "streams do nothing unless polled"]
pub struct FuturesUnordered<Fut> {
ready_to_run_queue: Arc<ReadyToRunQueue<Fut>>,
len: usize,
head_all: *const Task<Fut>,
unsafe impl<Fut: Send> Send for FuturesUnordered<Fut> {}
unsafe impl<Fut: Sync> Sync for FuturesUnordered<Fut> {}
impl<Fut> Unpin for FuturesUnordered<Fut> {}
// FuturesUnordered is implemented using two linked lists. One which links all
// futures managed by a `FuturesUnordered` and one that tracks futures that have
// been scheduled for polling. The first linked list is not thread safe and is
// only accessed by the thread that owns the `FuturesUnordered` value. The
// second linked list is an implementation of the intrusive MPSC queue algorithm
// described by
// When a future is submitted to the set, a task is allocated and inserted in
// both linked lists. The next call to `poll_next` will (eventually) see this
// task and call `poll` on the future.
// Before a managed future is polled, the current context's waker is replaced
// with one that is aware of the specific future being run. This ensures that
// wake-up notifications generated by that specific future are visible to
// `FuturesUnordered`. When a wake-up notification is received, the task is
// inserted into the ready to run queue, so that its future can be polled later.
// Each task is wrapped in an `Arc` and thereby atomically reference counted.
// Also, each task contains an `AtomicBool` which acts as a flag that indicates
// whether the task is currently inserted in the atomic queue. When a wake-up
// notifiaction is received, the task will only be inserted into the ready to
// run queue if it isn't inserted already.
impl<Fut: Future> FuturesUnordered<Fut> {
/// Constructs a new, empty [`FuturesUnordered`].
/// The returned [`FuturesUnordered`] does not contain any futures.
/// In this state, [`FuturesUnordered::poll_next`](Stream::poll_next) will
/// return [`Poll::Ready(None)`](Poll::Ready).
pub fn new() -> FuturesUnordered<Fut> {
let stub = Arc::new(Task {
future: UnsafeCell::new(None),
next_all: UnsafeCell::new(ptr::null()),
prev_all: UnsafeCell::new(ptr::null()),
next_ready_to_run: AtomicPtr::new(ptr::null_mut()),
queued: AtomicBool::new(true),
ready_to_run_queue: Weak::new(),
let stub_ptr = &*stub as *const Task<Fut>;
let ready_to_run_queue = Arc::new(ReadyToRunQueue {
waker: AtomicWaker::new(),
head: AtomicPtr::new(stub_ptr as *mut _),
tail: UnsafeCell::new(stub_ptr),
FuturesUnordered {
len: 0,
head_all: ptr::null_mut(),
impl<Fut: Future> Default for FuturesUnordered<Fut> {
fn default() -> FuturesUnordered<Fut> {
impl<Fut> FuturesUnordered<Fut> {
/// Returns the number of futures contained in the set.
/// This represents the total number of in-flight futures.
pub fn len(&self) -> usize {
/// Returns `true` if the set contains no futures.
pub fn is_empty(&self) -> bool {
self.len == 0
/// Push a future into the set.
/// This method adds the given future to the set. This method will not
/// call [`poll`](Future::poll) on the submitted future. The caller must
/// ensure that [`FuturesUnordered::poll_next`](Stream::poll_next) is called
/// in order to receive wake-up notifications for the given future.
pub fn push(&mut self, future: Fut) {
let task = Arc::new(Task {
future: UnsafeCell::new(Some(future)),
next_all: UnsafeCell::new(ptr::null_mut()),
prev_all: UnsafeCell::new(ptr::null_mut()),
next_ready_to_run: AtomicPtr::new(ptr::null_mut()),
queued: AtomicBool::new(true),
ready_to_run_queue: Arc::downgrade(&self.ready_to_run_queue),
// Right now our task has a strong reference count of 1. We transfer
// ownership of this reference count to our internal linked list
// and we'll reclaim ownership through the `unlink` method below.
let ptr =;
// We'll need to get the future "into the system" to start tracking it,
// e.g. getting its wake-up notifications going to us tracking which
// futures are ready. To do that we unconditionally enqueue it for
// polling here.
/// Returns an iterator that allows modifying each future in the set.
pub fn iter_mut(&mut self) -> IterMut<Fut> where Fut: Unpin {
/// Returns an iterator that allows modifying each future in the set.
#[allow(clippy::needless_lifetimes)] //
pub fn iter_pin_mut<'a>(self: PinMut<'a, Self>) -> IterPinMut<'a, Fut> {
IterPinMut {
task: self.head_all,
len: self.len,
_marker: PhantomData
/// Releases the task. It destorys the future inside and either drops
/// the `Arc<Task>` or transfers ownership to the ready to run queue.
/// The task this method is called on must have been unlinked before.
fn release_task(&mut self, task: Arc<Task<Fut>>) {
// `release_task` must only be called on unlinked tasks
unsafe {
// The future is done, try to reset the queued flag. This will prevent
// `wake` from doing any work in the future
let prev = task.queued.swap(true, SeqCst);
// Drop the future, even if it hasn't finished yet. This is safe
// because we're dropping the future on the thread that owns
// `FuturesUnordered`, which correctly tracks `Fut`'s lifetimes and
// such.
unsafe {
// Set to `None` rather than `take()`ing to prevent moving the
// future.
*task.future.get() = None;
// If the queued flag was previously set, then it means that this task
// is still in our internal ready to run queue. We then transfer
// ownership of our reference count to the ready to run queue, and it'll
// come along and free it later, noticing that the future is `None`.
// If, however, the queued flag was *not* set then we're safe to
// release our reference count on the task. The queued flag was set
// above so all future `enqueue` operations will not actually
// enqueue the task, so our task will never see the ready to run queue
// again. The task itself will be deallocated once all reference counts
// have been dropped elsewhere by the various wakers that contain it.
if prev {
/// Insert a new task into the internal linked list.
fn link(&mut self, task: Arc<Task<Fut>>) -> *const Task<Fut> {
let ptr = Arc::into_raw(task);
unsafe {
*(*ptr).next_all.get() = self.head_all;
if !self.head_all.is_null() {
*(*self.head_all).prev_all.get() = ptr;
self.head_all = ptr;
self.len += 1;
/// Remove the task from the linked list tracking all tasks currently
/// managed by `FuturesUnordered`.
/// This method is unsafe because it has be guaranteed that `task` is a
/// valid pointer.
unsafe fn unlink(&mut self, task: *const Task<Fut>) -> Arc<Task<Fut>> {
let task = Arc::from_raw(task);
let next = *task.next_all.get();
let prev = *task.prev_all.get();
*task.next_all.get() = ptr::null_mut();
*task.prev_all.get() = ptr::null_mut();
if !next.is_null() {
*(*next).prev_all.get() = prev;
if !prev.is_null() {
*(*prev).next_all.get() = next;
} else {
self.head_all = next;
self.len -= 1;
impl<Fut: Future> Stream for FuturesUnordered<Fut> {
type Item = Fut::Output;
fn poll_next(mut self: PinMut<Self>, cx: &mut core_task::Context)
-> Poll<Option<Self::Item>>
// Ensure `parent` is correctly set.
loop {
// Safety: &mut self guarantees the mutual exclusion `dequeue`
// expects
let task = match unsafe { self.ready_to_run_queue.dequeue() } {
Dequeue::Empty => {
if self.is_empty() {
return Poll::Ready(None);
} else {
return Poll::Pending;
Dequeue::Inconsistent => {
// At this point, it may be worth yielding the thread &
// spinning a few times... but for now, just yield using the
// task system.
return Poll::Pending;
Dequeue::Data(task) => task,
debug_assert!(task != self.ready_to_run_queue.stub());
// Safety:
// - `task` is a valid pointer.
// - We are the only thread that accesses the `UnsafeCell` that
// contains the future
let future = match unsafe { &mut *(*task).future.get() } {
Some(future) => future,
// If the future has already gone away then we're just
// cleaning out this task. See the comment in
// `release_task` for more information, but we're basically
// just taking ownership of our reference count here.
None => {
// This case only happens when `release_task` was called
// for this task before and couldn't drop the task
// because it was already enqueued in the ready to run
// queue.
// Safety: `task` is a valid pointer
let task = unsafe { Arc::from_raw(task) };
// Double check that the call to `release_task` really
// happened. Calling it required the task to be unlinked.
unsafe {
// Safety: `task` is a valid pointer
let task = unsafe { self.unlink(task) };
// Unset queued flag: This must be done before polling to ensure
// that the future's task gets rescheduled if it sends a wake-up
// notification **during** the call to `poll`.
let prev = task.queued.swap(false, SeqCst);
// We're going to need to be very careful if the `poll`
// method below panics. We need to (a) not leak memory and
// (b) ensure that we still don't have any use-after-frees. To
// manage this we do a few things:
// * A "bomb" is created which if dropped abnormally will call
// `release_task`. That way we'll be sure the memory management
// of the `task` is managed correctly. In particular
// `release_task` will drop the future. This ensures that it is
// dropped on this thread and not accidentally on a different
// thread (bad).
// * We unlink the task from our internal queue to preemptively
// assume it'll panic, in which case we'll want to discard it
// regardless.
struct Bomb<'a, Fut: 'a> {
queue: &'a mut FuturesUnordered<Fut>,
task: Option<Arc<Task<Fut>>>,
impl<'a, Fut> Drop for Bomb<'a, Fut> {
fn drop(&mut self) {
if let Some(task) = self.task.take() {
let mut bomb = Bomb {
task: Some(task),
queue: &mut *self,
// Poll the underlying future with the appropriate waker
// implementation. This is where a large bit of the unsafety
// starts to stem from internally. The waker is basically just
// our `Arc<Task<Fut>>` and can schedule the future for polling by
// enqueuing itself in the ready to run queue.
// Critically though `Task<Fut>` won't actually access `Fut`, the
// future, while it's floating around inside of wakers.
// These structs will basically just use `Fut` to size
// the internal allocation, appropriately accessing fields and
// deallocating the task if need be.
let res = {
let local_waker = bomb.task.as_ref().unwrap().local_waker();
let mut cx = cx.with_waker(&*local_waker);
// Safety: We won't move the future ever again
let future = unsafe { PinMut::new_unchecked(future) };
future.poll(&mut cx)
match res {
Poll::Pending => {
let task = bomb.task.take().unwrap();;
Poll::Ready(output) => {
return Poll::Ready(Some(output))
impl<Fut> Debug for FuturesUnordered<Fut> {
fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
write!(fmt, "FuturesUnordered {{ ... }}")
impl<Fut> Drop for FuturesUnordered<Fut> {
fn drop(&mut self) {
// When a `FuturesUnordered` is dropped we want to drop all futures
// associated with it. At the same time though there may be tons of
// wakers flying around which contain `Task<Fut>` references
// inside them. We'll let those naturally get deallocated.
unsafe {
while !self.head_all.is_null() {
let head = self.head_all;
let task = self.unlink(head);
// Note that at this point we could still have a bunch of tasks in the
// ready to run queue. None of those tasks, however, have futures
// associated with them so they're safe to destroy on any thread. At
// this point the `FuturesUnordered` struct, the owner of the one strong
// reference to the ready to run queue will drop the strong reference.
// At that point whichever thread releases the strong refcount last (be
// it this thread or some other thread as part of an `upgrade`) will
// clear out the ready to run queue and free all remaining tasks.
// While that freeing operation isn't guaranteed to happen here, it's
// guaranteed to happen "promptly" as no more "blocking work" will
// happen while there's a strong refcount held.
impl<Fut: Future> FromIterator<Fut> for FuturesUnordered<Fut> {
fn from_iter<I>(iter: I) -> Self
I: IntoIterator<Item = Fut>,
let acc = FuturesUnordered::new();
iter.into_iter().fold(acc, |mut acc, item| { acc.push(item); acc })
/// Converts a list of futures into a [`Stream`] of outputs from the futures.
/// This function will take an list of futures (e.g. a [`Vec`], an [`Iterator`],
/// etc), and return a stream. The stream will yield items as they become
/// available on the futures internally, in the order that they become
/// available. This function is similar to
/// [`buffer_unordered`](super::StreamExt::buffer_unordered) in that it may
/// return items in a different order than in the list specified.
/// Note that the returned set can also be used to dynamically push more
/// futures into the set as they become available.
pub fn futures_unordered<I>(futures: I) -> FuturesUnordered<I::Item>
I: IntoIterator,
I::Item: Future,