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//! A stream that efficiently multiplexes multiple streams. //! //! This "combinator" provides the ability to maintain and drive a set of streams to completion, //! while also providing access to each stream as it yields new elements. //! //! Streams are inserted into this set and their realized values are yielded as they are produced. //! This structure is optimized to manage a large number of streams. Streams managed by //! `StreamUnordered` will only be polled when they generate notifications. This reduces the //! required amount of work needed to coordinate large numbers of streams. //! //! When a `StreamUnordered` is first created, it does not contain any streams. Calling `poll` in //! this state will result in `Poll::Ready((None)` to be returned. Streams are submitted to the //! set using `insert`; however, the stream will **not** be polled at this point. `StreamUnordered` //! will only poll managed streams when `StreamUnordered::poll` is called. As such, it is important //! to call `poll` after inserting new streams. //! //! If `StreamUnordered::poll` returns `Poll::Ready(None)` this means that the set is //! currently not managing any streams. A stream may be submitted to the set at a later time. At //! that point, a call to `StreamUnordered::poll` will either return the stream's resolved value //! **or** `Poll::Pending` if the stream has not yet completed. //! //! Whenever a value is yielded, the yielding stream's index is also included. A reference to the //! stream that originated the value is obtained by using [`StreamUnordered::get`], //! [`StreamUnordered::get_mut`], or [`StreamUnordered::get_pin_mut`]. //! //! In normal operation, `poll` will yield a `StreamYield::Item` when it completes successfully. //! This value indicates that an underlying stream (the one indicated by the included index) //! produced an item. If an underlying stream yields `Poll::Ready(None)` to indicate termination, //! a `StreamYield::Finished` is returned instead. Note that as soon as a stream returns //! `StreamYield::Finished`, its token may be reused for new streams that are added. #![deny(missing_docs)] #![warn(rust_2018_idioms, broken_intra_doc_links)] // This is mainly FuturesUnordered from futures_util, but adapted to operate over Streams rather // than Futures. extern crate alloc; use alloc::sync::{Arc, Weak}; use core::cell::UnsafeCell; use core::fmt::{self, Debug}; use core::iter::FromIterator; use core::marker::PhantomData; use core::mem; use core::ops::{Index, IndexMut}; use core::pin::Pin; use core::ptr; use core::sync::atomic::Ordering::{AcqRel, Acquire, Relaxed, Release, SeqCst}; use core::sync::atomic::{AtomicBool, AtomicPtr}; use futures_core::stream::{FusedStream, Stream}; use futures_core::task::{Context, Poll}; use futures_util::task::{ArcWake, AtomicWaker}; mod abort; mod iter; pub use self::iter::{IterMut, IterPinMut}; mod task; use self::task::Task; mod ready_to_run_queue; use self::ready_to_run_queue::{Dequeue, ReadyToRunQueue}; /// Constant used for a `StreamUnordered` to determine how many times it is /// allowed to poll underlying futures without yielding. /// /// A single call to `poll_next` may potentially do a lot of work before /// yielding. This happens in particular if the underlying futures are awoken /// frequently but continue to return `Pending`. This is problematic if other /// tasks are waiting on the executor, since they do not get to run. This value /// caps the number of calls to `poll` on underlying streams a single call to /// `poll_next` is allowed to make. /// /// The value itself is chosen somewhat arbitrarily. It needs to be high enough /// that amortize wakeup and scheduling costs, but low enough that we do not /// starve other tasks for long. /// /// See also https://github.com/rust-lang/futures-rs/issues/2047. const YIELD_EVERY: usize = 32; /// A set of streams which may yield items in any order. /// /// This structure is optimized to manage a large number of streams. /// Streams managed by [`StreamUnordered`] will only be polled when they /// generate wake-up notifications. This reduces the required amount of work /// needed to poll large numbers of streams. /// /// [`StreamUnordered`] can be filled by [`collect`](Iterator::collect)ing an /// iterator of streams into a [`StreamUnordered`], or by /// [`insert`](StreamUnordered::insert)ing streams onto an existing /// [`StreamUnordered`]. When new streams are added, /// [`poll_next`](Stream::poll_next) must be called in order to begin receiving /// wake-ups for new streams. /// /// Note that you can create a ready-made [`StreamUnordered`] via the /// [`collect`](Iterator::collect) method, or you can start with an empty set /// with the [`StreamUnordered::new`] constructor. #[must_use = "streams do nothing unless polled"] pub struct StreamUnordered<S> { ready_to_run_queue: Arc<ReadyToRunQueue<S>>, head_all: AtomicPtr<Task<S>>, is_terminated: AtomicBool, by_id: slab::Slab<*const Task<S>>, } unsafe impl<S: Send> Send for StreamUnordered<S> {} unsafe impl<S: Sync> Sync for StreamUnordered<S> {} impl<S> Unpin for StreamUnordered<S> {} // StreamUnordered is implemented using two linked lists. One which links all // streams managed by a `StreamUnordered` and one that tracks streams that have // been scheduled for polling. The first linked list allows for thread safe // insertion of nodes at the head as well as forward iteration, but is otherwise // not thread safe and is only accessed by the thread that owns the // `StreamUnordered` value for any other operations. The second linked list is // an implementation of the intrusive MPSC queue algorithm described by // 1024cores.net. // // When a stream 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 stream. // // Before a managed stream is polled, the current context's waker is replaced // with one that is aware of the specific stream being run. This ensures that // wake-up notifications generated by that specific stream are visible to // `StreamUnordered`. When a wake-up notification is received, the task is // inserted into the ready to run queue, so that its stream 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. /// A handle to an vacant stream slot in a `StreamUnordered`. /// /// `StreamEntry` allows constructing streams that hold the token that they will be assigned. #[derive(Debug)] pub struct StreamEntry<'a, S> { token: usize, inserted: bool, backref: &'a mut StreamUnordered<S>, } impl<'a, S: 'a> StreamEntry<'a, S> { /// Insert a stream in the slot, and return a mutable reference to the value. /// /// To get the token associated with the stream, use key prior to calling insert. pub fn insert(mut self, stream: S) { self.inserted = true; // this is safe because we've held &mut StreamUnordered the entire time, // so the token still points to a valid task, and no-one else is // touching the .stream of it. unsafe { (*(*self.backref.by_id[self.token]).stream.get()) = Some(stream); } } /// Return the token associated with this slot. /// /// A stream stored in this slot will be associated with this token. pub fn token(&self) -> usize { self.token } } impl<'a, S: 'a> Drop for StreamEntry<'a, S> { fn drop(&mut self) { if !self.inserted { // undo the insertion let task_ptr = self.backref.by_id.remove(self.token); // we know task_ptr points to a valid task, since the StreamEntry // has held the &mut StreamUnordered the entire time. let task = unsafe { self.backref.unlink(task_ptr) }; self.backref.release_task(task); } } } impl<S: Stream> StreamUnordered<S> { /// Constructs a new, empty [`StreamUnordered`]. /// /// The returned [`StreamUnordered`] does not contain any streams. /// In this state, [`StreamUnordered::poll_next`](Stream::poll_next) will /// return [`Poll::Ready(None)`](Poll::Ready). pub fn new() -> StreamUnordered<S> { let mut slab = slab::Slab::new(); let slot = slab.vacant_entry(); let stub = Arc::new(Task { stream: UnsafeCell::new(None), is_done: UnsafeCell::new(false), next_all: AtomicPtr::new(ptr::null_mut()), prev_all: UnsafeCell::new(ptr::null()), len_all: UnsafeCell::new(0), next_ready_to_run: AtomicPtr::new(ptr::null_mut()), queued: AtomicBool::new(true), ready_to_run_queue: Weak::new(), id: slot.key(), }); let stub_ptr = &*stub as *const Task<S>; let _ = slab.insert(stub_ptr); let ready_to_run_queue = Arc::new(ReadyToRunQueue { waker: AtomicWaker::new(), head: AtomicPtr::new(stub_ptr as *mut _), tail: UnsafeCell::new(stub_ptr), stub, }); StreamUnordered { head_all: AtomicPtr::new(ptr::null_mut()), ready_to_run_queue, is_terminated: AtomicBool::new(false), by_id: slab, } } } impl<S: Stream> Default for StreamUnordered<S> { fn default() -> StreamUnordered<S> { StreamUnordered::new() } } impl<S> StreamUnordered<S> { /// Returns the number of streams contained in the set. /// /// This represents the total number of in-flight streams. pub fn len(&self) -> usize { let (_, len) = self.atomic_load_head_and_len_all(); len } /// Returns `true` if the set contains no streams. pub fn is_empty(&self) -> bool { // Relaxed ordering can be used here since we don't need to read from // the head pointer, only check whether it is null. self.head_all.load(Relaxed).is_null() } /// Returns a handle to a vacant stream entry allowing for further manipulation. /// /// This function is useful when creating values that must contain their stream token. The /// returned `StreamEntry` reserves an entry for the stream and is able to query the associated /// token. pub fn stream_entry<'a>(&'a mut self) -> StreamEntry<'a, S> { let next_all = self.pending_next_all(); let slot = self.by_id.vacant_entry(); let token = slot.key(); let task = Arc::new(Task { stream: UnsafeCell::new(None), is_done: UnsafeCell::new(false), next_all: AtomicPtr::new(next_all), prev_all: UnsafeCell::new(ptr::null_mut()), len_all: UnsafeCell::new(0), next_ready_to_run: AtomicPtr::new(ptr::null_mut()), queued: AtomicBool::new(true), ready_to_run_queue: Arc::downgrade(&self.ready_to_run_queue), id: token, }); let _ = slot.insert(&*task as *const _); // Reset the `is_terminated` flag if we've previously marked ourselves // as terminated. self.is_terminated.store(false, Relaxed); // 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 = self.link(task); // We'll need to get the stream "into the system" to start tracking it, // e.g. getting its wake-up notifications going to us tracking which // streams are ready. To do that we unconditionally enqueue it for // polling here. self.ready_to_run_queue.enqueue(ptr); StreamEntry { token, inserted: false, backref: self, } } /// Insert a stream into the set. /// /// A deprecated synonym for [`insert`]. #[deprecated(since = "0.5.2", note = "Prefer StreamUnordered::insert")] pub fn push(&mut self, stream: S) -> usize { self.insert(stream) } /// Insert a stream into the set. /// /// This method adds the given stream to the set. This method will not call /// [`poll_next`](futures_util::stream::Stream::poll_next) on the submitted stream. The caller /// must ensure that [`StreamUnordered::poll_next`](Stream::poll_next) is called in order to /// receive wake-up notifications for the given stream. /// /// The returned token is an identifier that uniquely identifies the given stream in the /// current set. To get a handle to the inserted stream, pass the token to /// [`StreamUnordered::get`], [`StreamUnordered::get_mut`], or [`StreamUnordered::get_pin_mut`] /// (or just index `StreamUnordered` directly). The same token will be yielded whenever an /// element is pulled from this stream. /// /// Note that the streams are not ordered, and may not be yielded back in insertion or token /// order when you iterate over them. pub fn insert(&mut self, stream: S) -> usize { let s = self.stream_entry(); let token = s.token(); s.insert(stream); token } /// Remove a stream from the set. /// /// The stream will be dropped and will no longer yield stream events. pub fn remove(mut self: Pin<&mut Self>, token: usize) -> bool { if token == 0 { return false; } let task = if let Some(task) = self.by_id.get(token) { *task } else { return false; }; // we know that by_id only references valid tasks let task = unsafe { self.unlink(task) }; self.release_task(task); true } /// Remove and return a stream from the set. /// /// The stream will no longer be polled, and will no longer yield stream events. /// /// Note that since this method moves `S`, which we may have given out a `Pin` to, it requires /// that `S` is `Unpin`. pub fn take(mut self: Pin<&mut Self>, token: usize) -> Option<S> where S: Unpin, { if token == 0 { return None; } let task = *self.by_id.get(token)?; // we know that by_id only references valid tasks let task = unsafe { self.unlink(task) }; // This is safe because we're dropping the stream on the thread that owns // `StreamUnordered`, which correctly tracks `S`'s lifetimes and such. // The logic is the same as for why release_task is allowed to touch task.stream. // Since S: Unpin, it is okay for us to move S. let stream = unsafe { &mut *task.stream.get() }.take(); self.release_task(task); stream } /// Returns `true` if the stream with the given token has yielded `None`. pub fn is_finished(&self, token: usize) -> Option<bool> { if token == 0 { return None; } // we know that by_id only references valid tasks Some(unsafe { *(**self.by_id.get(token)?).is_done.get() }) } /// Returns a reference to the stream with the given token pub fn get<'a>(&'a self, token: usize) -> Option<&'a S> { // don't allow access to the 0th task, since it's not a stream if token == 0 { return None; } // we know that by_id only references valid tasks Some(unsafe { (*(**self.by_id.get(token)?).stream.get()).as_ref().unwrap() }) } /// Returns a reference that allows modifying the stream with the given token. pub fn get_mut<'a>(&'a mut self, token: usize) -> Option<&'a mut S> where S: Unpin, { // don't allow access to the 0th task, since it's not a stream if token == 0 { return None; } // this is safe for the same reason that IterMut::next is safe Some(unsafe { (*(**self.by_id.get_mut(token)?).stream.get()) .as_mut() .unwrap() }) } /// Returns a pinned reference that allows modifying the stream with the given token. pub fn get_pin_mut<'a>(mut self: Pin<&'a mut Self>, token: usize) -> Option<Pin<&'a mut S>> { // don't allow access to the 0th task, since it's not a stream if token == 0 { return None; } // this is safe for the same reason that IterPinMut::next is safe Some(unsafe { Pin::new_unchecked( (*(**self.by_id.get_mut(token)?).stream.get()) .as_mut() .unwrap(), ) }) } /// Returns an iterator that allows modifying each stream in the set. pub fn iter_mut(&mut self) -> IterMut<'_, S> where S: Unpin, { IterMut(Pin::new(self).iter_pin_mut()) } /// Returns an iterator that allows modifying each stream in the set. pub fn iter_pin_mut(mut self: Pin<&mut Self>) -> IterPinMut<'_, S> { // `head_all` can be accessed directly and we don't need to spin on // `Task::next_all` since we have exclusive access to the set. let task = *self.head_all.get_mut(); let len = if task.is_null() { 0 } else { unsafe { *(*task).len_all.get() } }; IterPinMut { task, len, _marker: PhantomData, } } /// Returns the current head node and number of streams in the list of all /// streams within a context where access is shared with other threads /// (mostly for use with the `len` and `iter_pin_ref` methods). fn atomic_load_head_and_len_all(&self) -> (*const Task<S>, usize) { let task = self.head_all.load(Acquire); let len = if task.is_null() { 0 } else { unsafe { (*task).spin_next_all(self.pending_next_all(), Acquire); *(*task).len_all.get() } }; (task, len) } /// Releases the task. It destorys the stream 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<S>>) { self.by_id.remove(task.id); // `release_task` must only be called on unlinked tasks debug_assert_eq!(task.next_all.load(Relaxed), self.pending_next_all()); unsafe { debug_assert!((*task.prev_all.get()).is_null()); } // The stream is done, try to reset the queued flag. This will prevent // `wake` from doing any work in the stream let prev = task.queued.swap(true, SeqCst); // Drop the stream, even if it hasn't finished yet. This is safe // because we're dropping the stream on the thread that owns // `StreamUnordered`, which correctly tracks `S`'s lifetimes and // such. unsafe { // Set to `None` rather than `take()`ing to prevent moving the // stream. *task.stream.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 stream 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 stream `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 { mem::forget(task); } } /// Insert a new task into the internal linked list. fn link(&self, task: Arc<Task<S>>) -> *const Task<S> { // `next_all` should already be reset to the pending state before this // function is called. debug_assert_eq!(task.next_all.load(Relaxed), self.pending_next_all()); let ptr = Arc::into_raw(task); // Atomically swap out the old head node to get the node that should be // assigned to `next_all`. let next = self.head_all.swap(ptr as *mut _, AcqRel); unsafe { // Store the new list length in the new node. let new_len = if next.is_null() { 1 } else { // Make sure `next_all` has been written to signal that it is // safe to read `len_all`. (*next).spin_next_all(self.pending_next_all(), Acquire); *(*next).len_all.get() + 1 }; *(*ptr).len_all.get() = new_len; // Write the old head as the next node pointer, signaling to other // threads that `len_all` and `next_all` are ready to read. (*ptr).next_all.store(next, Release); // `prev_all` updates don't need to be synchronized, as the field is // only ever used after exclusive access has been acquired. if !next.is_null() { *(*next).prev_all.get() = ptr; } } ptr } /// Remove the task from the linked list tracking all tasks currently /// managed by `StreamUnordered`. /// This method is unsafe because it has be guaranteed that `task` is a /// valid pointer. unsafe fn unlink(&mut self, task: *const Task<S>) -> Arc<Task<S>> { // Compute the new list length now in case we're removing the head node // and won't be able to retrieve the correct length later. let head = *self.head_all.get_mut(); debug_assert!(!head.is_null()); let new_len = *(*head).len_all.get() - 1; let task = Arc::from_raw(task); let next = task.next_all.load(Relaxed); let prev = *task.prev_all.get(); task.next_all.store(self.pending_next_all(), Relaxed); *task.prev_all.get() = ptr::null_mut(); if !next.is_null() { *(*next).prev_all.get() = prev; } if !prev.is_null() { (*prev).next_all.store(next, Relaxed); } else { *self.head_all.get_mut() = next; } // Store the new list length in the head node. let head = *self.head_all.get_mut(); if !head.is_null() { *(*head).len_all.get() = new_len; } task } /// Returns the reserved value for `Task::next_all` to indicate a pending /// assignment from the thread that inserted the task. /// /// `StreamUnordered::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. fn pending_next_all(&self) -> *mut Task<S> { // The `ReadyToRunQueue` stub is never inserted into the `head_all` // list, and its pointer value will remain valid for the lifetime of // this `StreamUnordered`, so we can make use of its value here. &*self.ready_to_run_queue.stub as *const _ as *mut _ } } impl<S> Index<usize> for StreamUnordered<S> { type Output = S; fn index(&self, stream: usize) -> &Self::Output { self.get(stream).unwrap() } } impl<S> IndexMut<usize> for StreamUnordered<S> where S: Unpin, { fn index_mut(&mut self, stream: usize) -> &mut Self::Output { self.get_mut(stream).unwrap() } } /// An event that occurred for a managed stream. pub enum StreamYield<S> where S: Stream, { /// The underlying stream produced an item. Item(S::Item), /// The underlying stream has completed. Finished(FinishedStream), } /// A stream that has yielded all the items it ever will. /// /// The underlying stream will only be dropped by explicitly removing it from the associated /// `StreamUnordered`. This method is marked as `#[must_use]` to ensure that you either remove the /// stream immediately, or you explicitly ask for it to be kept around for later use. /// /// If the `FinishedStream` is dropped, the exhausted stream will not be dropped until the owning /// `StreamUnordered` is. #[must_use] pub struct FinishedStream { token: usize, } impl FinishedStream { /// Remove the exhausted stream. /// /// See [`StreamUnordered::remove`]. pub fn remove<S>(self, so: Pin<&mut StreamUnordered<S>>) { so.remove(self.token); } /// Take the exhausted stream. /// /// Note that this requires `S: Unpin` since it moves the stream even though it has already /// been pinned by `StreamUnordered`. /// /// See [`StreamUnordered::take`]. pub fn take<S>(self, so: Pin<&mut StreamUnordered<S>>) -> Option<S> where S: Unpin, { so.take(self.token) } /// Leave the exhausted stream in the `StreamUnordered`. /// /// This allows you to continue to access the stream through [`StreamUnordered::get_mut`] and /// friends should you need to perform further operations on it (e.g., if it is also being used /// as a `Sink`). Note that the stream will then not be dropped until you explicitly `remove` /// or `take` it from the `StreamUnordered`. pub fn keep(self) {} /// Return the token associated with the exhausted stream. pub fn token(self) -> usize { self.token } } impl<S> Debug for StreamYield<S> where S: Stream, S::Item: Debug, { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { match self { StreamYield::Item(ref i) => f.debug_tuple("StreamYield::Item").field(i).finish(), StreamYield::Finished(_) => f.debug_tuple("StreamYield::Finished").finish(), } } } impl<S> PartialEq for StreamYield<S> where S: Stream, S::Item: PartialEq, { fn eq(&self, other: &Self) -> bool { match (self, other) { (&StreamYield::Item(ref s), &StreamYield::Item(ref o)) => s == o, _ => false, } } } impl<S: Stream> Stream for StreamUnordered<S> { type Item = (StreamYield<S>, usize); fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> { // Keep track of how many child futures we have polled, // in case we want to forcibly yield. let mut polled = 0; // Ensure `parent` is correctly set. self.ready_to_run_queue.waker.register(cx.waker()); 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() { // We can only consider ourselves terminated once we // have yielded a `None` *self.is_terminated.get_mut() = true; 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. cx.waker().wake_by_ref(); 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 stream let stream = match unsafe { &mut *(*task).stream.get() } { Some(stream) => stream, // If the stream 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. debug_assert_eq!(task.next_all.load(Relaxed), self.pending_next_all()); unsafe { debug_assert!((*task.prev_all.get()).is_null()); } continue; } }; // Safety: we only ever access is_done on the thread that owns StreamUnordered. if unsafe { *(*task).is_done.get() } { // This stream has already been polled to completion. // We're keeping it around because the user has not removed it yet. // We can ignore any wake-ups for the Stream. continue; } // 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 stream's task gets rescheduled if it sends a wake-up // notification **during** the call to `poll`. let prev = task.queued.swap(false, SeqCst); assert!(prev); // 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 steam. 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, S> { queue: &'a mut StreamUnordered<S>, task: Option<Arc<Task<S>>>, } impl<S> Drop for Bomb<'_, S> { fn drop(&mut self) { if let Some(task) = self.task.take() { self.queue.release_task(task); } } } let id = task.id; let mut bomb = Bomb { task: Some(task), queue: &mut *self, }; // Poll the underlying stream 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<S>>` and can schedule the stream for polling by // enqueuing itself in the ready to run queue. // // Critically though `Task<S>` won't actually access `S`, the // stream, while it's floating around inside of wakers. // These structs will basically just use `S` to size // the internal allocation, appropriately accessing fields and // deallocating the task if need be. let res = { let waker = Task::waker_ref(bomb.task.as_ref().unwrap()); let mut cx = Context::from_waker(&waker); // Safety: We won't move the stream ever again let stream = unsafe { Pin::new_unchecked(stream) }; stream.poll_next(&mut cx) }; polled += 1; match res { Poll::Pending => { let task = bomb.task.take().unwrap(); bomb.queue.link(task); if polled == YIELD_EVERY { // We have polled a large number of futures in a row without yielding. // To ensure we do not starve other tasks waiting on the executor, // we yield here, but immediately wake ourselves up to continue. cx.waker().wake_by_ref(); return Poll::Pending; } continue; } Poll::Ready(None) => { // The stream has completed -- let the user know. // Note that we do not remove the stream here. Instead, we let the user decide // whether to keep the stream for a bit longer, in case they still need to do // some work with it (like if it's also a Sink and they need to flush some more // stuff). // Safe as we only ever access is_done on the thread that owns StreamUnordered. let task = bomb.task.take().unwrap(); unsafe { *task.is_done.get() = true; } bomb.queue.link(task); return Poll::Ready(Some(( StreamYield::Finished(FinishedStream { token: id }), id, ))); } Poll::Ready(Some(output)) => { // We're not done with the stream just because it yielded something // We're going to need to poll it again! Task::wake_by_ref(bomb.task.as_ref().unwrap()); // And also return it to the task queue let task = bomb.task.take().unwrap(); bomb.queue.link(task); return Poll::Ready(Some((StreamYield::Item(output), id))); } } } } fn size_hint(&self) -> (usize, Option<usize>) { let len = self.len(); (len, Some(len)) } } impl<S> Debug for StreamUnordered<S> { fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result { write!(f, "StreamUnordered {{ ... }}") } } impl<S> Drop for StreamUnordered<S> { fn drop(&mut self) { // When a `StreamUnordered` is dropped we want to drop all streams // associated with it. At the same time though there may be tons of // wakers flying around which contain `Task<S>` references // inside them. We'll let those naturally get deallocated. unsafe { while !self.head_all.get_mut().is_null() { let head = *self.head_all.get_mut(); let task = self.unlink(head); self.release_task(task); } } // 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 streams // associated with them so they're safe to destroy on any thread. At // this point the `StreamUnordered` 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<S: Stream> FromIterator<S> for StreamUnordered<S> { fn from_iter<I>(iter: I) -> Self where I: IntoIterator<Item = S>, { let acc = StreamUnordered::new(); iter.into_iter().fold(acc, |mut acc, item| { acc.insert(item); acc }) } } impl<S: Stream> FusedStream for StreamUnordered<S> { fn is_terminated(&self) -> bool { self.is_terminated.load(Relaxed) } } #[cfg(test)] mod micro { use super::*; use futures_util::{stream, stream::StreamExt}; use std::pin::Pin; #[test] fn no_starvation() { let forever0 = Box::pin(stream::iter(vec![0].into_iter().cycle())); let forever1 = Box::pin(stream::iter(vec![1].into_iter().cycle())); let two = Box::pin(stream::iter(vec![2].into_iter())); let mut s = StreamUnordered::new(); let forever0 = s.insert(forever0 as Pin<Box<dyn Stream<Item = i32>>>); let forever1 = s.insert(forever1 as Pin<Box<dyn Stream<Item = i32>>>); let two = s.insert(two as Pin<Box<dyn Stream<Item = i32>>>); let rt = tokio::runtime::Builder::new_current_thread() .build() .unwrap(); let mut s = rt.block_on(s.take(100).collect::<Vec<_>>()).into_iter(); let mut got_two = false; let mut got_two_end = false; while let Some((v, si)) = s.next() { if let StreamYield::Item(v) = v { if si == two { assert_eq!(v, 2); got_two = true; } else if si == forever0 { assert_eq!(v, 0); } else if si == forever1 { assert_eq!(v, 1); } else { unreachable!("unknown stream {} yielded {}", si, v); } } else if si == two { got_two_end = true; } else { unreachable!("unexpected stream end for stream {}", si); } } assert!(got_two, "stream was starved"); assert!(got_two_end, "stream end was not announced"); } }