| <!DOCTYPE html><html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><meta name="generator" content="rustdoc"><meta name="description" content="Source of the Rust file `/root/.cargo/registry/src/github.com-1ecc6299db9ec823/futures-util-0.3.28/src/stream/futures_unordered/mod.rs`."><meta name="keywords" content="rust, rustlang, rust-lang"><title>mod.rs - source</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../../SourceSerif4-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../../FiraSans-Regular.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../../FiraSans-Medium.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../../SourceCodePro-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../../SourceSerif4-Bold.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../../SourceCodePro-Semibold.ttf.woff2"><link rel="stylesheet" href="../../../../normalize.css"><link rel="stylesheet" href="../../../../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" href="../../../../ayu.css" disabled><link rel="stylesheet" href="../../../../dark.css" disabled><link rel="stylesheet" href="../../../../light.css" id="themeStyle"><script id="default-settings" ></script><script src="../../../../storage.js"></script><script defer src="../../../../source-script.js"></script><script defer src="../../../../source-files.js"></script><script defer src="../../../../main.js"></script><noscript><link rel="stylesheet" href="../../../../noscript.css"></noscript><link rel="alternate icon" type="image/png" href="../../../../favicon-16x16.png"><link rel="alternate icon" type="image/png" href="../../../../favicon-32x32.png"><link rel="icon" type="image/svg+xml" href="../../../../favicon.svg"></head><body class="rustdoc source"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><a class="sidebar-logo" href="../../../../futures_util/index.html"><div class="logo-container"><img class="rust-logo" src="../../../../rust-logo.svg" alt="logo"></div></a></nav><main><div class="width-limiter"><nav class="sub"><a class="sub-logo-container" href="../../../../futures_util/index.html"><img class="rust-logo" src="../../../../rust-logo.svg" alt="logo"></a><form class="search-form"><div class="search-container"><span></span><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><div id="help-button" title="help" tabindex="-1"><a href="../../../../help.html">?</a></div><div id="settings-menu" tabindex="-1"><a href="../../../../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../../../../wheel.svg"></a></div></div></form></nav><section id="main-content" class="content"><div class="example-wrap"><pre class="src-line-numbers"><span id="1">1</span> |
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| </pre><pre class="rust"><code><span class="doccomment">//! An unbounded set of futures. |
| //! |
| //! This module is only available when the `std` or `alloc` feature of this |
| //! library is activated, and it is activated by default. |
| |
| </span><span class="kw">use </span><span class="kw">crate</span>::task::AtomicWaker; |
| <span class="kw">use </span>alloc::sync::{Arc, Weak}; |
| <span class="kw">use </span>core::cell::UnsafeCell; |
| <span class="kw">use </span>core::fmt::{<span class="self">self</span>, Debug}; |
| <span class="kw">use </span>core::iter::FromIterator; |
| <span class="kw">use </span>core::marker::PhantomData; |
| <span class="kw">use </span>core::mem; |
| <span class="kw">use </span>core::pin::Pin; |
| <span class="kw">use </span>core::ptr; |
| <span class="kw">use </span>core::sync::atomic::Ordering::{AcqRel, Acquire, Relaxed, Release, SeqCst}; |
| <span class="kw">use </span>core::sync::atomic::{AtomicBool, AtomicPtr}; |
| <span class="kw">use </span>futures_core::future::Future; |
| <span class="kw">use </span>futures_core::stream::{FusedStream, Stream}; |
| <span class="kw">use </span>futures_core::task::{Context, Poll}; |
| <span class="kw">use </span>futures_task::{FutureObj, LocalFutureObj, LocalSpawn, Spawn, SpawnError}; |
| |
| <span class="kw">mod </span>abort; |
| |
| <span class="kw">mod </span>iter; |
| <span class="attribute">#[allow(unreachable_pub)] </span><span class="comment">// https://github.com/rust-lang/rust/issues/102352 |
| </span><span class="kw">pub use </span><span class="self">self</span>::iter::{IntoIter, Iter, IterMut, IterPinMut, IterPinRef}; |
| |
| <span class="kw">mod </span>task; |
| <span class="kw">use </span><span class="self">self</span>::task::Task; |
| |
| <span class="kw">mod </span>ready_to_run_queue; |
| <span class="kw">use </span><span class="self">self</span>::ready_to_run_queue::{Dequeue, ReadyToRunQueue}; |
| |
| <span class="doccomment">/// A set of futures which may complete in any order. |
| /// |
| /// See [`FuturesOrdered`](crate::stream::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 [`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 |
| /// [`collect`](Iterator::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. |
| </span><span class="attribute">#[must_use = <span class="string">"streams do nothing unless polled"</span>] |
| </span><span class="kw">pub struct </span>FuturesUnordered<Fut> { |
| ready_to_run_queue: Arc<ReadyToRunQueue<Fut>>, |
| head_all: AtomicPtr<Task<Fut>>, |
| is_terminated: AtomicBool, |
| } |
| |
| <span class="kw">unsafe impl</span><Fut: Send> Send <span class="kw">for </span>FuturesUnordered<Fut> {} |
| <span class="kw">unsafe impl</span><Fut: Sync> Sync <span class="kw">for </span>FuturesUnordered<Fut> {} |
| <span class="kw">impl</span><Fut> Unpin <span class="kw">for </span>FuturesUnordered<Fut> {} |
| |
| <span class="kw">impl </span>Spawn <span class="kw">for </span>FuturesUnordered<FutureObj<<span class="lifetime">'_</span>, ()>> { |
| <span class="kw">fn </span>spawn_obj(<span class="kw-2">&</span><span class="self">self</span>, future_obj: FutureObj<<span class="lifetime">'static</span>, ()>) -> <span class="prelude-ty">Result</span><(), SpawnError> { |
| <span class="self">self</span>.push(future_obj); |
| <span class="prelude-val">Ok</span>(()) |
| } |
| } |
| |
| <span class="kw">impl </span>LocalSpawn <span class="kw">for </span>FuturesUnordered<LocalFutureObj<<span class="lifetime">'_</span>, ()>> { |
| <span class="kw">fn </span>spawn_local_obj(<span class="kw-2">&</span><span class="self">self</span>, future_obj: LocalFutureObj<<span class="lifetime">'static</span>, ()>) -> <span class="prelude-ty">Result</span><(), SpawnError> { |
| <span class="self">self</span>.push(future_obj); |
| <span class="prelude-val">Ok</span>(()) |
| } |
| } |
| |
| <span class="comment">// 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 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 |
| // `FuturesUnordered` value for any other operations. The second linked list is |
| // an implementation of the intrusive MPSC queue algorithm described by |
| // 1024cores.net. |
| // |
| // 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 |
| // notification is received, the task will only be inserted into the ready to |
| // run queue if it isn't inserted already. |
| |
| </span><span class="kw">impl</span><Fut> Default <span class="kw">for </span>FuturesUnordered<Fut> { |
| <span class="kw">fn </span>default() -> <span class="self">Self </span>{ |
| <span class="self">Self</span>::new() |
| } |
| } |
| |
| <span class="kw">impl</span><Fut> FuturesUnordered<Fut> { |
| <span class="doccomment">/// 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). |
| </span><span class="kw">pub fn </span>new() -> <span class="self">Self </span>{ |
| <span class="kw">let </span>stub = Arc::new(Task { |
| future: UnsafeCell::new(<span class="prelude-val">None</span>), |
| next_all: AtomicPtr::new(ptr::null_mut()), |
| prev_all: UnsafeCell::new(ptr::null()), |
| len_all: UnsafeCell::new(<span class="number">0</span>), |
| next_ready_to_run: AtomicPtr::new(ptr::null_mut()), |
| queued: AtomicBool::new(<span class="bool-val">true</span>), |
| ready_to_run_queue: Weak::new(), |
| woken: AtomicBool::new(<span class="bool-val">false</span>), |
| }); |
| <span class="kw">let </span>stub_ptr = Arc::as_ptr(<span class="kw-2">&</span>stub); |
| <span class="kw">let </span>ready_to_run_queue = Arc::new(ReadyToRunQueue { |
| waker: AtomicWaker::new(), |
| head: AtomicPtr::new(stub_ptr <span class="kw">as </span><span class="kw-2">*mut </span><span class="kw">_</span>), |
| tail: UnsafeCell::new(stub_ptr), |
| stub, |
| }); |
| |
| <span class="self">Self </span>{ |
| head_all: AtomicPtr::new(ptr::null_mut()), |
| ready_to_run_queue, |
| is_terminated: AtomicBool::new(<span class="bool-val">false</span>), |
| } |
| } |
| |
| <span class="doccomment">/// Returns the number of futures contained in the set. |
| /// |
| /// This represents the total number of in-flight futures. |
| </span><span class="kw">pub fn </span>len(<span class="kw-2">&</span><span class="self">self</span>) -> usize { |
| <span class="kw">let </span>(<span class="kw">_</span>, len) = <span class="self">self</span>.atomic_load_head_and_len_all(); |
| len |
| } |
| |
| <span class="doccomment">/// Returns `true` if the set contains no futures. |
| </span><span class="kw">pub fn </span>is_empty(<span class="kw-2">&</span><span class="self">self</span>) -> bool { |
| <span class="comment">// Relaxed ordering can be used here since we don't need to read from |
| // the head pointer, only check whether it is null. |
| </span><span class="self">self</span>.head_all.load(Relaxed).is_null() |
| } |
| |
| <span class="doccomment">/// Push a future into the set. |
| /// |
| /// This method adds the given future to the set. This method will not |
| /// call [`poll`](core::future::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. |
| </span><span class="kw">pub fn </span>push(<span class="kw-2">&</span><span class="self">self</span>, future: Fut) { |
| <span class="kw">let </span>task = Arc::new(Task { |
| future: UnsafeCell::new(<span class="prelude-val">Some</span>(future)), |
| next_all: AtomicPtr::new(<span class="self">self</span>.pending_next_all()), |
| prev_all: UnsafeCell::new(ptr::null_mut()), |
| len_all: UnsafeCell::new(<span class="number">0</span>), |
| next_ready_to_run: AtomicPtr::new(ptr::null_mut()), |
| queued: AtomicBool::new(<span class="bool-val">true</span>), |
| ready_to_run_queue: Arc::downgrade(<span class="kw-2">&</span><span class="self">self</span>.ready_to_run_queue), |
| woken: AtomicBool::new(<span class="bool-val">false</span>), |
| }); |
| |
| <span class="comment">// Reset the `is_terminated` flag if we've previously marked ourselves |
| // as terminated. |
| </span><span class="self">self</span>.is_terminated.store(<span class="bool-val">false</span>, Relaxed); |
| |
| <span class="comment">// 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. |
| </span><span class="kw">let </span>ptr = <span class="self">self</span>.link(task); |
| |
| <span class="comment">// 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. |
| </span><span class="self">self</span>.ready_to_run_queue.enqueue(ptr); |
| } |
| |
| <span class="doccomment">/// Returns an iterator that allows inspecting each future in the set. |
| </span><span class="kw">pub fn </span>iter(<span class="kw-2">&</span><span class="self">self</span>) -> Iter<<span class="lifetime">'_</span>, Fut> |
| <span class="kw">where |
| </span>Fut: Unpin, |
| { |
| Iter(Pin::new(<span class="self">self</span>).iter_pin_ref()) |
| } |
| |
| <span class="doccomment">/// Returns an iterator that allows inspecting each future in the set. |
| </span><span class="kw">pub fn </span>iter_pin_ref(<span class="self">self</span>: Pin<<span class="kw-2">&</span><span class="self">Self</span>>) -> IterPinRef<<span class="lifetime">'_</span>, Fut> { |
| <span class="kw">let </span>(task, len) = <span class="self">self</span>.atomic_load_head_and_len_all(); |
| <span class="kw">let </span>pending_next_all = <span class="self">self</span>.pending_next_all(); |
| |
| IterPinRef { task, len, pending_next_all, _marker: PhantomData } |
| } |
| |
| <span class="doccomment">/// Returns an iterator that allows modifying each future in the set. |
| </span><span class="kw">pub fn </span>iter_mut(<span class="kw-2">&mut </span><span class="self">self</span>) -> IterMut<<span class="lifetime">'_</span>, Fut> |
| <span class="kw">where |
| </span>Fut: Unpin, |
| { |
| IterMut(Pin::new(<span class="self">self</span>).iter_pin_mut()) |
| } |
| |
| <span class="doccomment">/// Returns an iterator that allows modifying each future in the set. |
| </span><span class="kw">pub fn </span>iter_pin_mut(<span class="kw-2">mut </span><span class="self">self</span>: Pin<<span class="kw-2">&mut </span><span class="self">Self</span>>) -> IterPinMut<<span class="lifetime">'_</span>, Fut> { |
| <span class="comment">// `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. |
| </span><span class="kw">let </span>task = <span class="kw-2">*</span><span class="self">self</span>.head_all.get_mut(); |
| <span class="kw">let </span>len = <span class="kw">if </span>task.is_null() { <span class="number">0 </span>} <span class="kw">else </span>{ <span class="kw">unsafe </span>{ <span class="kw-2">*</span>(<span class="kw-2">*</span>task).len_all.get() } }; |
| |
| IterPinMut { task, len, _marker: PhantomData } |
| } |
| |
| <span class="doccomment">/// 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). |
| </span><span class="kw">fn </span>atomic_load_head_and_len_all(<span class="kw-2">&</span><span class="self">self</span>) -> (<span class="kw-2">*const </span>Task<Fut>, usize) { |
| <span class="kw">let </span>task = <span class="self">self</span>.head_all.load(Acquire); |
| <span class="kw">let </span>len = <span class="kw">if </span>task.is_null() { |
| <span class="number">0 |
| </span>} <span class="kw">else </span>{ |
| <span class="kw">unsafe </span>{ |
| (<span class="kw-2">*</span>task).spin_next_all(<span class="self">self</span>.pending_next_all(), Acquire); |
| <span class="kw-2">*</span>(<span class="kw-2">*</span>task).len_all.get() |
| } |
| }; |
| |
| (task, len) |
| } |
| |
| <span class="doccomment">/// 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. |
| </span><span class="kw">fn </span>release_task(<span class="kw-2">&mut </span><span class="self">self</span>, task: Arc<Task<Fut>>) { |
| <span class="comment">// `release_task` must only be called on unlinked tasks |
| </span><span class="macro">debug_assert_eq!</span>(task.next_all.load(Relaxed), <span class="self">self</span>.pending_next_all()); |
| <span class="kw">unsafe </span>{ |
| <span class="macro">debug_assert!</span>((<span class="kw-2">*</span>task.prev_all.get()).is_null()); |
| } |
| |
| <span class="comment">// The future is done, try to reset the queued flag. This will prevent |
| // `wake` from doing any work in the future |
| </span><span class="kw">let </span>prev = task.queued.swap(<span class="bool-val">true</span>, SeqCst); |
| |
| <span class="comment">// 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. |
| </span><span class="kw">unsafe </span>{ |
| <span class="comment">// Set to `None` rather than `take()`ing to prevent moving the |
| // future. |
| </span><span class="kw-2">*</span>task.future.get() = <span class="prelude-val">None</span>; |
| } |
| |
| <span class="comment">// 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. |
| </span><span class="kw">if </span>prev { |
| mem::forget(task); |
| } |
| } |
| |
| <span class="doccomment">/// Insert a new task into the internal linked list. |
| </span><span class="kw">fn </span>link(<span class="kw-2">&</span><span class="self">self</span>, task: Arc<Task<Fut>>) -> <span class="kw-2">*const </span>Task<Fut> { |
| <span class="comment">// `next_all` should already be reset to the pending state before this |
| // function is called. |
| </span><span class="macro">debug_assert_eq!</span>(task.next_all.load(Relaxed), <span class="self">self</span>.pending_next_all()); |
| <span class="kw">let </span>ptr = Arc::into_raw(task); |
| |
| <span class="comment">// Atomically swap out the old head node to get the node that should be |
| // assigned to `next_all`. |
| </span><span class="kw">let </span>next = <span class="self">self</span>.head_all.swap(ptr <span class="kw">as </span><span class="kw-2">*mut </span><span class="kw">_</span>, AcqRel); |
| |
| <span class="kw">unsafe </span>{ |
| <span class="comment">// Store the new list length in the new node. |
| </span><span class="kw">let </span>new_len = <span class="kw">if </span>next.is_null() { |
| <span class="number">1 |
| </span>} <span class="kw">else </span>{ |
| <span class="comment">// Make sure `next_all` has been written to signal that it is |
| // safe to read `len_all`. |
| </span>(<span class="kw-2">*</span>next).spin_next_all(<span class="self">self</span>.pending_next_all(), Acquire); |
| <span class="kw-2">*</span>(<span class="kw-2">*</span>next).len_all.get() + <span class="number">1 |
| </span>}; |
| <span class="kw-2">*</span>(<span class="kw-2">*</span>ptr).len_all.get() = new_len; |
| |
| <span class="comment">// Write the old head as the next node pointer, signaling to other |
| // threads that `len_all` and `next_all` are ready to read. |
| </span>(<span class="kw-2">*</span>ptr).next_all.store(next, Release); |
| |
| <span class="comment">// `prev_all` updates don't need to be synchronized, as the field is |
| // only ever used after exclusive access has been acquired. |
| </span><span class="kw">if </span>!next.is_null() { |
| <span class="kw-2">*</span>(<span class="kw-2">*</span>next).prev_all.get() = ptr; |
| } |
| } |
| |
| ptr |
| } |
| |
| <span class="doccomment">/// 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. |
| </span><span class="kw">unsafe fn </span>unlink(<span class="kw-2">&mut </span><span class="self">self</span>, task: <span class="kw-2">*const </span>Task<Fut>) -> Arc<Task<Fut>> { |
| <span class="comment">// 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. |
| </span><span class="kw">let </span>head = <span class="kw-2">*</span><span class="self">self</span>.head_all.get_mut(); |
| <span class="macro">debug_assert!</span>(!head.is_null()); |
| <span class="kw">let </span>new_len = <span class="kw-2">*</span>(<span class="kw-2">*</span>head).len_all.get() - <span class="number">1</span>; |
| |
| <span class="kw">let </span>task = Arc::from_raw(task); |
| <span class="kw">let </span>next = task.next_all.load(Relaxed); |
| <span class="kw">let </span>prev = <span class="kw-2">*</span>task.prev_all.get(); |
| task.next_all.store(<span class="self">self</span>.pending_next_all(), Relaxed); |
| <span class="kw-2">*</span>task.prev_all.get() = ptr::null_mut(); |
| |
| <span class="kw">if </span>!next.is_null() { |
| <span class="kw-2">*</span>(<span class="kw-2">*</span>next).prev_all.get() = prev; |
| } |
| |
| <span class="kw">if </span>!prev.is_null() { |
| (<span class="kw-2">*</span>prev).next_all.store(next, Relaxed); |
| } <span class="kw">else </span>{ |
| <span class="kw-2">*</span><span class="self">self</span>.head_all.get_mut() = next; |
| } |
| |
| <span class="comment">// Store the new list length in the head node. |
| </span><span class="kw">let </span>head = <span class="kw-2">*</span><span class="self">self</span>.head_all.get_mut(); |
| <span class="kw">if </span>!head.is_null() { |
| <span class="kw-2">*</span>(<span class="kw-2">*</span>head).len_all.get() = new_len; |
| } |
| |
| task |
| } |
| |
| <span class="doccomment">/// 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. |
| </span><span class="kw">fn </span>pending_next_all(<span class="kw-2">&</span><span class="self">self</span>) -> <span class="kw-2">*mut </span>Task<Fut> { |
| <span class="comment">// The `ReadyToRunQueue` stub is never inserted into the `head_all` |
| // list, and its pointer value will remain valid for the lifetime of |
| // this `FuturesUnordered`, so we can make use of its value here. |
| </span>Arc::as_ptr(<span class="kw-2">&</span><span class="self">self</span>.ready_to_run_queue.stub) <span class="kw">as </span><span class="kw-2">*mut </span><span class="kw">_ |
| </span>} |
| } |
| |
| <span class="kw">impl</span><Fut: Future> Stream <span class="kw">for </span>FuturesUnordered<Fut> { |
| <span class="kw">type </span>Item = Fut::Output; |
| |
| <span class="kw">fn </span>poll_next(<span class="kw-2">mut </span><span class="self">self</span>: Pin<<span class="kw-2">&mut </span><span class="self">Self</span>>, cx: <span class="kw-2">&mut </span>Context<<span class="lifetime">'_</span>>) -> Poll<<span class="prelude-ty">Option</span><<span class="self">Self</span>::Item>> { |
| <span class="kw">let </span>len = <span class="self">self</span>.len(); |
| |
| <span class="comment">// Keep track of how many child futures we have polled, |
| // in case we want to forcibly yield. |
| </span><span class="kw">let </span><span class="kw-2">mut </span>polled = <span class="number">0</span>; |
| <span class="kw">let </span><span class="kw-2">mut </span>yielded = <span class="number">0</span>; |
| |
| <span class="comment">// Ensure `parent` is correctly set. |
| </span><span class="self">self</span>.ready_to_run_queue.waker.register(cx.waker()); |
| |
| <span class="kw">loop </span>{ |
| <span class="comment">// Safety: &mut self guarantees the mutual exclusion `dequeue` |
| // expects |
| </span><span class="kw">let </span>task = <span class="kw">match unsafe </span>{ <span class="self">self</span>.ready_to_run_queue.dequeue() } { |
| Dequeue::Empty => { |
| <span class="kw">if </span><span class="self">self</span>.is_empty() { |
| <span class="comment">// We can only consider ourselves terminated once we |
| // have yielded a `None` |
| </span><span class="kw-2">*</span><span class="self">self</span>.is_terminated.get_mut() = <span class="bool-val">true</span>; |
| <span class="kw">return </span>Poll::Ready(<span class="prelude-val">None</span>); |
| } <span class="kw">else </span>{ |
| <span class="kw">return </span>Poll::Pending; |
| } |
| } |
| Dequeue::Inconsistent => { |
| <span class="comment">// At this point, it may be worth yielding the thread & |
| // spinning a few times... but for now, just yield using the |
| // task system. |
| </span>cx.waker().wake_by_ref(); |
| <span class="kw">return </span>Poll::Pending; |
| } |
| Dequeue::Data(task) => task, |
| }; |
| |
| <span class="macro">debug_assert!</span>(task != <span class="self">self</span>.ready_to_run_queue.stub()); |
| |
| <span class="comment">// Safety: |
| // - `task` is a valid pointer. |
| // - We are the only thread that accesses the `UnsafeCell` that |
| // contains the future |
| </span><span class="kw">let </span>future = <span class="kw">match unsafe </span>{ <span class="kw-2">&mut *</span>(<span class="kw-2">*</span>task).future.get() } { |
| <span class="prelude-val">Some</span>(future) => future, |
| |
| <span class="comment">// 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. |
| </span><span class="prelude-val">None </span>=> { |
| <span class="comment">// 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 |
| </span><span class="kw">let </span>task = <span class="kw">unsafe </span>{ Arc::from_raw(task) }; |
| |
| <span class="comment">// Double check that the call to `release_task` really |
| // happened. Calling it required the task to be unlinked. |
| </span><span class="macro">debug_assert_eq!</span>(task.next_all.load(Relaxed), <span class="self">self</span>.pending_next_all()); |
| <span class="kw">unsafe </span>{ |
| <span class="macro">debug_assert!</span>((<span class="kw-2">*</span>task.prev_all.get()).is_null()); |
| } |
| <span class="kw">continue</span>; |
| } |
| }; |
| |
| <span class="comment">// Safety: `task` is a valid pointer |
| </span><span class="kw">let </span>task = <span class="kw">unsafe </span>{ <span class="self">self</span>.unlink(task) }; |
| |
| <span class="comment">// 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`. |
| </span><span class="kw">let </span>prev = task.queued.swap(<span class="bool-val">false</span>, SeqCst); |
| <span class="macro">assert!</span>(prev); |
| |
| <span class="comment">// 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. |
| </span><span class="kw">struct </span>Bomb<<span class="lifetime">'a</span>, Fut> { |
| queue: <span class="kw-2">&</span><span class="lifetime">'a </span><span class="kw-2">mut </span>FuturesUnordered<Fut>, |
| task: <span class="prelude-ty">Option</span><Arc<Task<Fut>>>, |
| } |
| |
| <span class="kw">impl</span><Fut> Drop <span class="kw">for </span>Bomb<<span class="lifetime">'_</span>, Fut> { |
| <span class="kw">fn </span>drop(<span class="kw-2">&mut </span><span class="self">self</span>) { |
| <span class="kw">if let </span><span class="prelude-val">Some</span>(task) = <span class="self">self</span>.task.take() { |
| <span class="self">self</span>.queue.release_task(task); |
| } |
| } |
| } |
| |
| <span class="kw">let </span><span class="kw-2">mut </span>bomb = Bomb { task: <span class="prelude-val">Some</span>(task), queue: <span class="kw-2">&mut *</span><span class="self">self </span>}; |
| |
| <span class="comment">// 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. |
| </span><span class="kw">let </span>res = { |
| <span class="kw">let </span>task = bomb.task.as_ref().unwrap(); |
| <span class="comment">// We are only interested in whether the future is awoken before it |
| // finishes polling, so reset the flag here. |
| </span>task.woken.store(<span class="bool-val">false</span>, Relaxed); |
| <span class="kw">let </span>waker = Task::waker_ref(task); |
| <span class="kw">let </span><span class="kw-2">mut </span>cx = Context::from_waker(<span class="kw-2">&</span>waker); |
| |
| <span class="comment">// Safety: We won't move the future ever again |
| </span><span class="kw">let </span>future = <span class="kw">unsafe </span>{ Pin::new_unchecked(future) }; |
| |
| future.poll(<span class="kw-2">&mut </span>cx) |
| }; |
| polled += <span class="number">1</span>; |
| |
| <span class="kw">match </span>res { |
| Poll::Pending => { |
| <span class="kw">let </span>task = bomb.task.take().unwrap(); |
| <span class="comment">// If the future was awoken during polling, we assume |
| // the future wanted to explicitly yield. |
| </span>yielded += task.woken.load(Relaxed) <span class="kw">as </span>usize; |
| bomb.queue.link(task); |
| |
| <span class="comment">// If a future yields, we respect it and yield here. |
| // If all futures have been polled, we also yield here to |
| // avoid starving other tasks waiting on the executor. |
| // (polling the same future twice per iteration may cause |
| // the problem: https://github.com/rust-lang/futures-rs/pull/2333) |
| </span><span class="kw">if </span>yielded >= <span class="number">2 </span>|| polled == len { |
| cx.waker().wake_by_ref(); |
| <span class="kw">return </span>Poll::Pending; |
| } |
| <span class="kw">continue</span>; |
| } |
| Poll::Ready(output) => <span class="kw">return </span>Poll::Ready(<span class="prelude-val">Some</span>(output)), |
| } |
| } |
| } |
| |
| <span class="kw">fn </span>size_hint(<span class="kw-2">&</span><span class="self">self</span>) -> (usize, <span class="prelude-ty">Option</span><usize>) { |
| <span class="kw">let </span>len = <span class="self">self</span>.len(); |
| (len, <span class="prelude-val">Some</span>(len)) |
| } |
| } |
| |
| <span class="kw">impl</span><Fut> Debug <span class="kw">for </span>FuturesUnordered<Fut> { |
| <span class="kw">fn </span>fmt(<span class="kw-2">&</span><span class="self">self</span>, f: <span class="kw-2">&mut </span>fmt::Formatter<<span class="lifetime">'_</span>>) -> fmt::Result { |
| <span class="macro">write!</span>(f, <span class="string">"FuturesUnordered {{ ... }}"</span>) |
| } |
| } |
| |
| <span class="kw">impl</span><Fut> FuturesUnordered<Fut> { |
| <span class="doccomment">/// Clears the set, removing all futures. |
| </span><span class="kw">pub fn </span>clear(<span class="kw-2">&mut </span><span class="self">self</span>) { |
| <span class="self">self</span>.clear_head_all(); |
| |
| <span class="comment">// we just cleared all the tasks, and we have &mut self, so this is safe. |
| </span><span class="kw">unsafe </span>{ <span class="self">self</span>.ready_to_run_queue.clear() }; |
| |
| <span class="self">self</span>.is_terminated.store(<span class="bool-val">false</span>, Relaxed); |
| } |
| |
| <span class="kw">fn </span>clear_head_all(<span class="kw-2">&mut </span><span class="self">self</span>) { |
| <span class="kw">while </span>!<span class="self">self</span>.head_all.get_mut().is_null() { |
| <span class="kw">let </span>head = <span class="kw-2">*</span><span class="self">self</span>.head_all.get_mut(); |
| <span class="kw">let </span>task = <span class="kw">unsafe </span>{ <span class="self">self</span>.unlink(head) }; |
| <span class="self">self</span>.release_task(task); |
| } |
| } |
| } |
| |
| <span class="kw">impl</span><Fut> Drop <span class="kw">for </span>FuturesUnordered<Fut> { |
| <span class="kw">fn </span>drop(<span class="kw-2">&mut </span><span class="self">self</span>) { |
| <span class="comment">// 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. |
| </span><span class="self">self</span>.clear_head_all(); |
| |
| <span class="comment">// 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. |
| </span>} |
| } |
| |
| <span class="kw">impl</span><<span class="lifetime">'a</span>, Fut: Unpin> IntoIterator <span class="kw">for </span><span class="kw-2">&</span><span class="lifetime">'a </span>FuturesUnordered<Fut> { |
| <span class="kw">type </span>Item = <span class="kw-2">&</span><span class="lifetime">'a </span>Fut; |
| <span class="kw">type </span>IntoIter = Iter<<span class="lifetime">'a</span>, Fut>; |
| |
| <span class="kw">fn </span>into_iter(<span class="self">self</span>) -> <span class="self">Self</span>::IntoIter { |
| <span class="self">self</span>.iter() |
| } |
| } |
| |
| <span class="kw">impl</span><<span class="lifetime">'a</span>, Fut: Unpin> IntoIterator <span class="kw">for </span><span class="kw-2">&</span><span class="lifetime">'a </span><span class="kw-2">mut </span>FuturesUnordered<Fut> { |
| <span class="kw">type </span>Item = <span class="kw-2">&</span><span class="lifetime">'a </span><span class="kw-2">mut </span>Fut; |
| <span class="kw">type </span>IntoIter = IterMut<<span class="lifetime">'a</span>, Fut>; |
| |
| <span class="kw">fn </span>into_iter(<span class="self">self</span>) -> <span class="self">Self</span>::IntoIter { |
| <span class="self">self</span>.iter_mut() |
| } |
| } |
| |
| <span class="kw">impl</span><Fut: Unpin> IntoIterator <span class="kw">for </span>FuturesUnordered<Fut> { |
| <span class="kw">type </span>Item = Fut; |
| <span class="kw">type </span>IntoIter = IntoIter<Fut>; |
| |
| <span class="kw">fn </span>into_iter(<span class="kw-2">mut </span><span class="self">self</span>) -> <span class="self">Self</span>::IntoIter { |
| <span class="comment">// `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. |
| </span><span class="kw">let </span>task = <span class="kw-2">*</span><span class="self">self</span>.head_all.get_mut(); |
| <span class="kw">let </span>len = <span class="kw">if </span>task.is_null() { <span class="number">0 </span>} <span class="kw">else </span>{ <span class="kw">unsafe </span>{ <span class="kw-2">*</span>(<span class="kw-2">*</span>task).len_all.get() } }; |
| |
| IntoIter { len, inner: <span class="self">self </span>} |
| } |
| } |
| |
| <span class="kw">impl</span><Fut> FromIterator<Fut> <span class="kw">for </span>FuturesUnordered<Fut> { |
| <span class="kw">fn </span>from_iter<I>(iter: I) -> <span class="self">Self |
| </span><span class="kw">where |
| </span>I: IntoIterator<Item = Fut>, |
| { |
| <span class="kw">let </span>acc = <span class="self">Self</span>::new(); |
| iter.into_iter().fold(acc, |acc, item| { |
| acc.push(item); |
| acc |
| }) |
| } |
| } |
| |
| <span class="kw">impl</span><Fut: Future> FusedStream <span class="kw">for </span>FuturesUnordered<Fut> { |
| <span class="kw">fn </span>is_terminated(<span class="kw-2">&</span><span class="self">self</span>) -> bool { |
| <span class="self">self</span>.is_terminated.load(Relaxed) |
| } |
| } |
| |
| <span class="kw">impl</span><Fut> Extend<Fut> <span class="kw">for </span>FuturesUnordered<Fut> { |
| <span class="kw">fn </span>extend<I>(<span class="kw-2">&mut </span><span class="self">self</span>, iter: I) |
| <span class="kw">where |
| </span>I: IntoIterator<Item = Fut>, |
| { |
| <span class="kw">for </span>item <span class="kw">in </span>iter { |
| <span class="self">self</span>.push(item); |
| } |
| } |
| } |
| </code></pre></div> |
| </section></div></main><div id="rustdoc-vars" data-root-path="../../../../" data-current-crate="futures_util" data-themes="ayu,dark,light" data-resource-suffix="" data-rustdoc-version="1.66.0-nightly (5c8bff74b 2022-10-21)" ></div></body></html> |