Struct futures_util::stream::futures_unordered::FuturesUnordered

pub struct FuturesUnordered<Fut> {
    ready_to_run_queue: Arc<ReadyToRunQueue<Fut>>,
    head_all: AtomicPtr<Task<Fut>>,
    is_terminated: AtomicBool,
}

A set of futures which may complete in any order.

See FuturesOrdered for a version of this type that preserves a FIFO 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 collecting an iterator of futures into a FuturesUnordered, or by pushing futures onto an existing FuturesUnordered. When new futures are added, 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 collect method, or you can start with an empty set with the FuturesUnordered::new constructor.

This type is only available when the std or alloc feature of this library is activated, and it is activated by default.

Fields

ready_to_run_queue: Arc<ReadyToRunQueue<Fut>>

head_all: AtomicPtr<Task<Fut>>

is_terminated: AtomicBool

Implementations

impl<Fut> FuturesUnordered<Fut>

pub fn new() -> Self

Constructs a new, empty FuturesUnordered.

The returned FuturesUnordered does not contain any futures. In this state, FuturesUnordered::poll_next will return Poll::Ready(None).

pub fn len(&self) -> usize

Returns the number of futures contained in the set.

This represents the total number of in-flight futures.

pub fn is_empty(&self) -> bool

Returns true if the set contains no futures.

pub fn push(&self, future: Fut)

Push a future into the set.

This method adds the given future to the set. This method will not call poll on the submitted future. The caller must ensure that FuturesUnordered::poll_next is called in order to receive wake-up notifications for the given future.

pub fn iter(&self) -> Iter<'_, Fut>

where Fut: Unpin,

Returns an iterator that allows inspecting each future in the set.

pub fn iter_pin_ref(self: Pin<&Self>) -> IterPinRef<'_, Fut>

Returns an iterator that allows inspecting 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.

pub fn iter_pin_mut(self: Pin<&mut Self>) -> IterPinMut<'_, Fut>

Returns an iterator that allows modifying each future in the set.

fn atomic_load_head_and_len_all(&self) -> (*const Task<Fut>, usize)

Returns the current head node and number of futures in the list of all futures within a context where access is shared with other threads (mostly for use with the len and iter_pin_ref methods).

fn release_task(&mut self, task: Arc<Task<Fut>>)

Releases the task. It destroys 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 link(&self, task: Arc<Task<Fut>>) -> *const Task<Fut>

Insert a new task into the internal linked list.

unsafe fn unlink(&mut self, task: *const Task<Fut>) -> Arc<Task<Fut>>

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.

fn pending_next_all(&self) -> *mut Task<Fut>

Returns the reserved value for Task::next_all to indicate a pending assignment from the thread that inserted the task.

FuturesUnordered::link needs to update Task pointers in an order that ensures any iterators created on other threads can correctly traverse the entire Task list using the chain of next_all pointers. This could be solved with a compare-exchange loop that stores the current head_all in next_all and swaps out head_all with the new Task pointer if the head hasn’t already changed. Under heavy thread contention, this compare-exchange loop could become costly.

An alternative is to initialize next_all to a reserved pending state first, perform an atomic swap on head_all, and finally update next_all with the old head node. Iterators will then either see the pending state value or the correct next node pointer, and can reload next_all as needed until the correct value is loaded. The number of retries needed (if any) would be small and will always be finite, so this should generally perform better than the compare-exchange loop.

A valid Task pointer in the head_all list is guaranteed to never be this value, so it is safe to use as a reserved value until the correct value can be written.

impl<Fut> FuturesUnordered<Fut>

pub fn clear(&mut self)

Clears the set, removing all futures.

Trait Implementations

impl<Fut> Debug for FuturesUnordered<Fut>

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<Fut> Default for FuturesUnordered<Fut>

fn default() -> Self

Returns the “default value” for a type.

impl<Fut> Drop for FuturesUnordered<Fut>

fn drop(&mut self)

Executes the destructor for this type.

impl<Fut> Extend<Fut> for FuturesUnordered<Fut>

fn extend<I>(&mut self, iter: I)

where I: IntoIterator<Item = Fut>,

Extends a collection with the contents of an iterator.

fn extend_one(&mut self, item: A)

🔬This is a nightly-only experimental API. (extend_one)

Extends a collection with exactly one element.

fn extend_reserve(&mut self, additional: usize)

🔬This is a nightly-only experimental API. (extend_one)

Reserves capacity in a collection for the given number of additional elements.

impl<Fut> FromIterator<Fut> for FuturesUnordered<Fut>

fn from_iter<I>(iter: I) -> Self

where I: IntoIterator<Item = Fut>,

Creates a value from an iterator.

impl<Fut: Future> FusedStream for FuturesUnordered<Fut>

fn is_terminated(&self) -> bool

Returns true if the stream should no longer be polled.

impl<'a, Fut: Unpin> IntoIterator for &'a FuturesUnordered<Fut>

type Item = &'a Fut

The type of the elements being iterated over.

type IntoIter = Iter<'a, Fut>

Which kind of iterator are we turning this into?

fn into_iter(self) -> Self::IntoIter

Creates an iterator from a value.

impl<'a, Fut: Unpin> IntoIterator for &'a mut FuturesUnordered<Fut>

type Item = &'a mut Fut

The type of the elements being iterated over.

type IntoIter = IterMut<'a, Fut>

Which kind of iterator are we turning this into?

fn into_iter(self) -> Self::IntoIter

Creates an iterator from a value.

impl<Fut: Unpin> IntoIterator for FuturesUnordered<Fut>

type Item = Fut

The type of the elements being iterated over.

type IntoIter = IntoIter<Fut>

Which kind of iterator are we turning this into?

fn into_iter(self) -> Self::IntoIter

Creates an iterator from a value.

impl LocalSpawn for FuturesUnordered<LocalFutureObj<'_, ()>>

fn spawn_local_obj( &self, future_obj: LocalFutureObj<'static, ()>, ) -> Result<(), SpawnError>

Spawns a future that will be run to completion.

fn status_local(&self) -> Result<(), SpawnError>

Determines whether the executor is able to spawn new tasks.

impl Spawn for FuturesUnordered<FutureObj<'_, ()>>

fn spawn_obj( &self, future_obj: FutureObj<'static, ()>, ) -> Result<(), SpawnError>

Spawns a future that will be run to completion.

fn status(&self) -> Result<(), SpawnError>

Determines whether the executor is able to spawn new tasks.

impl<Fut: Future> Stream for FuturesUnordered<Fut>

type Item = <Fut as Future>::Output

Values yielded by the stream.

fn poll_next( self: Pin<&mut Self>, cx: &mut Context<'_>, ) -> Poll<Option<Self::Item>>

Attempt to pull out the next value of this stream, registering the current task for wakeup if the value is not yet available, and returning None if the stream is exhausted.

fn size_hint(&self) -> (usize, Option<usize>)

Returns the bounds on the remaining length of the stream.

impl<Fut: Send> Send for FuturesUnordered<Fut>

impl<Fut: Send + Sync> Sync for FuturesUnordered<Fut>

impl<Fut> Unpin for FuturesUnordered<Fut>