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</pre><pre class="rust ">
<span class="comment">// Copyright 2014 The Rust Project Developers. See the COPYRIGHT</span>
<span class="comment">// file at the top-level directory of this distribution and at</span>
<span class="comment">// http://rust-lang.org/COPYRIGHT.</span>
<span class="comment">//</span>
<span class="comment">// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or</span>
<span class="comment">// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license</span>
<span class="comment">// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your</span>
<span class="comment">// option. This file may not be copied, modified, or distributed</span>
<span class="comment">// except according to those terms.</span>
<span class="doccomment">//! A "once initialization" primitive</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! This primitive is meant to be used to run one-time initialization. An</span>
<span class="doccomment">//! example use case would be for initializing an FFI library.</span>
<span class="comment">// A "once" is a relatively simple primitive, and it's also typically provided</span>
<span class="comment">// by the OS as well (see `pthread_once` or `InitOnceExecuteOnce`). The OS</span>
<span class="comment">// primitives, however, tend to have surprising restrictions, such as the Unix</span>
<span class="comment">// one doesn't allow an argument to be passed to the function.</span>
<span class="comment">//</span>
<span class="comment">// As a result, we end up implementing it ourselves in the standard library.</span>
<span class="comment">// This also gives us the opportunity to optimize the implementation a bit which</span>
<span class="comment">// should help the fast path on call sites. Consequently, let's explain how this</span>
<span class="comment">// primitive works now!</span>
<span class="comment">//</span>
<span class="comment">// So to recap, the guarantees of a Once are that it will call the</span>
<span class="comment">// initialization closure at most once, and it will never return until the one</span>
<span class="comment">// that's running has finished running. This means that we need some form of</span>
<span class="comment">// blocking here while the custom callback is running at the very least.</span>
<span class="comment">// Additionally, we add on the restriction of **poisoning**. Whenever an</span>
<span class="comment">// initialization closure panics, the Once enters a "poisoned" state which means</span>
<span class="comment">// that all future calls will immediately panic as well.</span>
<span class="comment">//</span>
<span class="comment">// So to implement this, one might first reach for a `StaticMutex`, but those</span>
<span class="comment">// unfortunately need to be deallocated (e.g. call `destroy()`) to free memory</span>
<span class="comment">// on all OSes (some of the BSDs allocate memory for mutexes). It also gets a</span>
<span class="comment">// lot harder with poisoning to figure out when the mutex needs to be</span>
<span class="comment">// deallocated because it's not after the closure finishes, but after the first</span>
<span class="comment">// successful closure finishes.</span>
<span class="comment">//</span>
<span class="comment">// All in all, this is instead implemented with atomics and lock-free</span>
<span class="comment">// operations! Whee! Each `Once` has one word of atomic state, and this state is</span>
<span class="comment">// CAS'd on to determine what to do. There are four possible state of a `Once`:</span>
<span class="comment">//</span>
<span class="comment">// * Incomplete - no initialization has run yet, and no thread is currently</span>
<span class="comment">// using the Once.</span>
<span class="comment">// * Poisoned - some thread has previously attempted to initialize the Once, but</span>
<span class="comment">// it panicked, so the Once is now poisoned. There are no other</span>
<span class="comment">// threads currently accessing this Once.</span>
<span class="comment">// * Running - some thread is currently attempting to run initialization. It may</span>
<span class="comment">// succeed, so all future threads need to wait for it to finish.</span>
<span class="comment">// Note that this state is accompanied with a payload, described</span>
<span class="comment">// below.</span>
<span class="comment">// * Complete - initialization has completed and all future calls should finish</span>
<span class="comment">// immediately.</span>
<span class="comment">//</span>
<span class="comment">// With 4 states we need 2 bits to encode this, and we use the remaining bits</span>
<span class="comment">// in the word we have allocated as a queue of threads waiting for the thread</span>
<span class="comment">// responsible for entering the RUNNING state. This queue is just a linked list</span>
<span class="comment">// of Waiter nodes which is monotonically increasing in size. Each node is</span>
<span class="comment">// allocated on the stack, and whenever the running closure finishes it will</span>
<span class="comment">// consume the entire queue and notify all waiters they should try again.</span>
<span class="comment">//</span>
<span class="comment">// You'll find a few more details in the implementation, but that's the gist of</span>
<span class="comment">// it!</span>
<span class="kw">use</span> <span class="ident">fmt</span>;
<span class="kw">use</span> <span class="ident">marker</span>;
<span class="kw">use</span> <span class="ident">ptr</span>;
<span class="kw">use</span> <span class="ident">sync</span>::<span class="ident">atomic</span>::{<span class="ident">AtomicUsize</span>, <span class="ident">AtomicBool</span>, <span class="ident">Ordering</span>};
<span class="kw">use</span> <span class="ident">thread</span>::{<span class="self">self</span>, <span class="ident">Thread</span>};
<span class="doccomment">/// A synchronization primitive which can be used to run a one-time global</span>
<span class="doccomment">/// initialization. Useful for one-time initialization for FFI or related</span>
<span class="doccomment">/// functionality. This type can only be constructed with the [`ONCE_INIT`]</span>
<span class="doccomment">/// value.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`ONCE_INIT`]: constant.ONCE_INIT.html</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// use std::sync::{Once, ONCE_INIT};</span>
<span class="doccomment">///</span>
<span class="doccomment">/// static START: Once = ONCE_INIT;</span>
<span class="doccomment">///</span>
<span class="doccomment">/// START.call_once(|| {</span>
<span class="doccomment">/// // run initialization here</span>
<span class="doccomment">/// });</span>
<span class="doccomment">/// ```</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="kw">pub</span> <span class="kw">struct</span> <span class="ident">Once</span> {
<span class="comment">// This `state` word is actually an encoded version of just a pointer to a</span>
<span class="comment">// `Waiter`, so we add the `PhantomData` appropriately.</span>
<span class="ident">state</span>: <span class="ident">AtomicUsize</span>,
<span class="ident">_marker</span>: <span class="ident">marker</span>::<span class="ident">PhantomData</span><span class="op"><</span><span class="kw-2">*</span><span class="kw-2">mut</span> <span class="ident">Waiter</span><span class="op">></span>,
}
<span class="comment">// The `PhantomData` of a raw pointer removes these two auto traits, but we</span>
<span class="comment">// enforce both below in the implementation so this should be safe to add.</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="kw">unsafe</span> <span class="kw">impl</span> <span class="ident">Sync</span> <span class="kw">for</span> <span class="ident">Once</span> {}
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="kw">unsafe</span> <span class="kw">impl</span> <span class="ident">Send</span> <span class="kw">for</span> <span class="ident">Once</span> {}
<span class="doccomment">/// State yielded to [`call_once_force`]’s closure parameter. The state can be</span>
<span class="doccomment">/// used to query the poison status of the [`Once`].</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`call_once_force`]: struct.Once.html#method.call_once_force</span>
<span class="doccomment">/// [`Once`]: struct.Once.html</span>
<span class="attribute">#[<span class="ident">unstable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"once_poison"</span>, <span class="ident">issue</span> <span class="op">=</span> <span class="string">"33577"</span>)]</span>
<span class="attribute">#[<span class="ident">derive</span>(<span class="ident">Debug</span>)]</span>
<span class="kw">pub</span> <span class="kw">struct</span> <span class="ident">OnceState</span> {
<span class="ident">poisoned</span>: <span class="ident">bool</span>,
}
<span class="doccomment">/// Initialization value for static [`Once`] values.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`Once`]: struct.Once.html</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// use std::sync::{Once, ONCE_INIT};</span>
<span class="doccomment">///</span>
<span class="doccomment">/// static START: Once = ONCE_INIT;</span>
<span class="doccomment">/// ```</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="kw">pub</span> <span class="kw">const</span> <span class="ident">ONCE_INIT</span>: <span class="ident">Once</span> <span class="op">=</span> <span class="ident">Once</span>::<span class="ident">new</span>();
<span class="comment">// Four states that a Once can be in, encoded into the lower bits of `state` in</span>
<span class="comment">// the Once structure.</span>
<span class="kw">const</span> <span class="ident">INCOMPLETE</span>: <span class="ident">usize</span> <span class="op">=</span> <span class="number">0x0</span>;
<span class="kw">const</span> <span class="ident">POISONED</span>: <span class="ident">usize</span> <span class="op">=</span> <span class="number">0x1</span>;
<span class="kw">const</span> <span class="ident">RUNNING</span>: <span class="ident">usize</span> <span class="op">=</span> <span class="number">0x2</span>;
<span class="kw">const</span> <span class="ident">COMPLETE</span>: <span class="ident">usize</span> <span class="op">=</span> <span class="number">0x3</span>;
<span class="comment">// Mask to learn about the state. All other bits are the queue of waiters if</span>
<span class="comment">// this is in the RUNNING state.</span>
<span class="kw">const</span> <span class="ident">STATE_MASK</span>: <span class="ident">usize</span> <span class="op">=</span> <span class="number">0x3</span>;
<span class="comment">// Representation of a node in the linked list of waiters in the RUNNING state.</span>
<span class="kw">struct</span> <span class="ident">Waiter</span> {
<span class="ident">thread</span>: <span class="prelude-ty">Option</span><span class="op"><</span><span class="ident">Thread</span><span class="op">></span>,
<span class="ident">signaled</span>: <span class="ident">AtomicBool</span>,
<span class="ident">next</span>: <span class="kw-2">*</span><span class="kw-2">mut</span> <span class="ident">Waiter</span>,
}
<span class="comment">// Helper struct used to clean up after a closure call with a `Drop`</span>
<span class="comment">// implementation to also run on panic.</span>
<span class="kw">struct</span> <span class="ident">Finish</span> {
<span class="ident">panicked</span>: <span class="ident">bool</span>,
<span class="ident">me</span>: <span class="kw-2">&</span><span class="lifetime">'static</span> <span class="ident">Once</span>,
}
<span class="kw">impl</span> <span class="ident">Once</span> {
<span class="doccomment">/// Creates a new `Once` value.</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"once_new"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.2.0"</span>)]</span>
<span class="kw">pub</span> <span class="kw">const</span> <span class="kw">fn</span> <span class="ident">new</span>() <span class="op">-></span> <span class="ident">Once</span> {
<span class="ident">Once</span> {
<span class="ident">state</span>: <span class="ident">AtomicUsize</span>::<span class="ident">new</span>(<span class="ident">INCOMPLETE</span>),
<span class="ident">_marker</span>: <span class="ident">marker</span>::<span class="ident">PhantomData</span>,
}
}
<span class="doccomment">/// Performs an initialization routine once and only once. The given closure</span>
<span class="doccomment">/// will be executed if this is the first time `call_once` has been called,</span>
<span class="doccomment">/// and otherwise the routine will *not* be invoked.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This method will block the calling thread if another initialization</span>
<span class="doccomment">/// routine is currently running.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// When this function returns, it is guaranteed that some initialization</span>
<span class="doccomment">/// has run and completed (it may not be the closure specified). It is also</span>
<span class="doccomment">/// guaranteed that any memory writes performed by the executed closure can</span>
<span class="doccomment">/// be reliably observed by other threads at this point (there is a</span>
<span class="doccomment">/// happens-before relation between the closure and code executing after the</span>
<span class="doccomment">/// return).</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// use std::sync::{Once, ONCE_INIT};</span>
<span class="doccomment">///</span>
<span class="doccomment">/// static mut VAL: usize = 0;</span>
<span class="doccomment">/// static INIT: Once = ONCE_INIT;</span>
<span class="doccomment">///</span>
<span class="doccomment">/// // Accessing a `static mut` is unsafe much of the time, but if we do so</span>
<span class="doccomment">/// // in a synchronized fashion (e.g. write once or read all) then we're</span>
<span class="doccomment">/// // good to go!</span>
<span class="doccomment">/// //</span>
<span class="doccomment">/// // This function will only call `expensive_computation` once, and will</span>
<span class="doccomment">/// // otherwise always return the value returned from the first invocation.</span>
<span class="doccomment">/// fn get_cached_val() -> usize {</span>
<span class="doccomment">/// unsafe {</span>
<span class="doccomment">/// INIT.call_once(|| {</span>
<span class="doccomment">/// VAL = expensive_computation();</span>
<span class="doccomment">/// });</span>
<span class="doccomment">/// VAL</span>
<span class="doccomment">/// }</span>
<span class="doccomment">/// }</span>
<span class="doccomment">///</span>
<span class="doccomment">/// fn expensive_computation() -> usize {</span>
<span class="doccomment">/// // ...</span>
<span class="doccomment">/// # 2</span>
<span class="doccomment">/// }</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Panics</span>
<span class="doccomment">///</span>
<span class="doccomment">/// The closure `f` will only be executed once if this is called</span>
<span class="doccomment">/// concurrently amongst many threads. If that closure panics, however, then</span>
<span class="doccomment">/// it will *poison* this `Once` instance, causing all future invocations of</span>
<span class="doccomment">/// `call_once` to also panic.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This is similar to [poisoning with mutexes][poison].</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [poison]: struct.Mutex.html#poisoning</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">call_once</span><span class="op"><</span><span class="ident">F</span><span class="op">></span>(<span class="kw-2">&</span><span class="lifetime">'static</span> <span class="self">self</span>, <span class="ident">f</span>: <span class="ident">F</span>) <span class="kw">where</span> <span class="ident">F</span>: <span class="ident">FnOnce</span>() {
<span class="comment">// Fast path, just see if we've completed initialization.</span>
<span class="kw">if</span> <span class="self">self</span>.<span class="ident">state</span>.<span class="ident">load</span>(<span class="ident">Ordering</span>::<span class="ident">SeqCst</span>) <span class="op">==</span> <span class="ident">COMPLETE</span> {
<span class="kw">return</span>
}
<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">f</span> <span class="op">=</span> <span class="prelude-val">Some</span>(<span class="ident">f</span>);
<span class="self">self</span>.<span class="ident">call_inner</span>(<span class="bool-val">false</span>, <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="op">|</span>_<span class="op">|</span> <span class="ident">f</span>.<span class="ident">take</span>().<span class="ident">unwrap</span>()());
}
<span class="doccomment">/// Performs the same function as [`call_once`] except ignores poisoning.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Unlike [`call_once`], if this `Once` has been poisoned (i.e. a previous</span>
<span class="doccomment">/// call to `call_once` or `call_once_force` caused a panic), calling</span>
<span class="doccomment">/// `call_once_force` will still invoke the closure `f` and will _not_</span>
<span class="doccomment">/// result in an immediate panic. If `f` panics, the `Once` will remain</span>
<span class="doccomment">/// in a poison state. If `f` does _not_ panic, the `Once` will no</span>
<span class="doccomment">/// longer be in a poison state and all future calls to `call_once` or</span>
<span class="doccomment">/// `call_one_force` will no-op.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// The closure `f` is yielded a [`OnceState`] structure which can be used</span>
<span class="doccomment">/// to query the poison status of the `Once`.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`call_once`]: struct.Once.html#method.call_once</span>
<span class="doccomment">/// [`OnceState`]: struct.OnceState.html</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// #![feature(once_poison)]</span>
<span class="doccomment">///</span>
<span class="doccomment">/// use std::sync::{Once, ONCE_INIT};</span>
<span class="doccomment">/// use std::thread;</span>
<span class="doccomment">///</span>
<span class="doccomment">/// static INIT: Once = ONCE_INIT;</span>
<span class="doccomment">///</span>
<span class="doccomment">/// // poison the once</span>
<span class="doccomment">/// let handle = thread::spawn(|| {</span>
<span class="doccomment">/// INIT.call_once(|| panic!());</span>
<span class="doccomment">/// });</span>
<span class="doccomment">/// assert!(handle.join().is_err());</span>
<span class="doccomment">///</span>
<span class="doccomment">/// // poisoning propagates</span>
<span class="doccomment">/// let handle = thread::spawn(|| {</span>
<span class="doccomment">/// INIT.call_once(|| {});</span>
<span class="doccomment">/// });</span>
<span class="doccomment">/// assert!(handle.join().is_err());</span>
<span class="doccomment">///</span>
<span class="doccomment">/// // call_once_force will still run and reset the poisoned state</span>
<span class="doccomment">/// INIT.call_once_force(|state| {</span>
<span class="doccomment">/// assert!(state.poisoned());</span>
<span class="doccomment">/// });</span>
<span class="doccomment">///</span>
<span class="doccomment">/// // once any success happens, we stop propagating the poison</span>
<span class="doccomment">/// INIT.call_once(|| {});</span>
<span class="doccomment">/// ```</span>
<span class="attribute">#[<span class="ident">unstable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"once_poison"</span>, <span class="ident">issue</span> <span class="op">=</span> <span class="string">"33577"</span>)]</span>
<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">call_once_force</span><span class="op"><</span><span class="ident">F</span><span class="op">></span>(<span class="kw-2">&</span><span class="lifetime">'static</span> <span class="self">self</span>, <span class="ident">f</span>: <span class="ident">F</span>) <span class="kw">where</span> <span class="ident">F</span>: <span class="ident">FnOnce</span>(<span class="kw-2">&</span><span class="ident">OnceState</span>) {
<span class="comment">// same as above, just with a different parameter to `call_inner`.</span>
<span class="kw">if</span> <span class="self">self</span>.<span class="ident">state</span>.<span class="ident">load</span>(<span class="ident">Ordering</span>::<span class="ident">SeqCst</span>) <span class="op">==</span> <span class="ident">COMPLETE</span> {
<span class="kw">return</span>
}
<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">f</span> <span class="op">=</span> <span class="prelude-val">Some</span>(<span class="ident">f</span>);
<span class="self">self</span>.<span class="ident">call_inner</span>(<span class="bool-val">true</span>, <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="op">|</span><span class="ident">p</span><span class="op">|</span> {
<span class="ident">f</span>.<span class="ident">take</span>().<span class="ident">unwrap</span>()(<span class="kw-2">&</span><span class="ident">OnceState</span> { <span class="ident">poisoned</span>: <span class="ident">p</span> })
});
}
<span class="comment">// This is a non-generic function to reduce the monomorphization cost of</span>
<span class="comment">// using `call_once` (this isn't exactly a trivial or small implementation).</span>
<span class="comment">//</span>
<span class="comment">// Additionally, this is tagged with `#[cold]` as it should indeed be cold</span>
<span class="comment">// and it helps let LLVM know that calls to this function should be off the</span>
<span class="comment">// fast path. Essentially, this should help generate more straight line code</span>
<span class="comment">// in LLVM.</span>
<span class="comment">//</span>
<span class="comment">// Finally, this takes an `FnMut` instead of a `FnOnce` because there's</span>
<span class="comment">// currently no way to take an `FnOnce` and call it via virtual dispatch</span>
<span class="comment">// without some allocation overhead.</span>
<span class="attribute">#[<span class="ident">cold</span>]</span>
<span class="kw">fn</span> <span class="ident">call_inner</span>(<span class="kw-2">&</span><span class="lifetime">'static</span> <span class="self">self</span>,
<span class="ident">ignore_poisoning</span>: <span class="ident">bool</span>,
<span class="ident">init</span>: <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">FnMut</span>(<span class="ident">bool</span>)) {
<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">state</span> <span class="op">=</span> <span class="self">self</span>.<span class="ident">state</span>.<span class="ident">load</span>(<span class="ident">Ordering</span>::<span class="ident">SeqCst</span>);
<span class="lifetime">'outer</span>: <span class="kw">loop</span> {
<span class="kw">match</span> <span class="ident">state</span> {
<span class="comment">// If we're complete, then there's nothing to do, we just</span>
<span class="comment">// jettison out as we shouldn't run the closure.</span>
<span class="ident">COMPLETE</span> <span class="op">=></span> <span class="kw">return</span>,
<span class="comment">// If we're poisoned and we're not in a mode to ignore</span>
<span class="comment">// poisoning, then we panic here to propagate the poison.</span>
<span class="ident">POISONED</span> <span class="kw">if</span> <span class="op">!</span><span class="ident">ignore_poisoning</span> <span class="op">=></span> {
<span class="macro">panic</span><span class="macro">!</span>(<span class="string">"Once instance has previously been poisoned"</span>);
}
<span class="comment">// Otherwise if we see a poisoned or otherwise incomplete state</span>
<span class="comment">// we will attempt to move ourselves into the RUNNING state. If</span>
<span class="comment">// we succeed, then the queue of waiters starts at null (all 0</span>
<span class="comment">// bits).</span>
<span class="ident">POISONED</span> <span class="op">|</span>
<span class="ident">INCOMPLETE</span> <span class="op">=></span> {
<span class="kw">let</span> <span class="ident">old</span> <span class="op">=</span> <span class="self">self</span>.<span class="ident">state</span>.<span class="ident">compare_and_swap</span>(<span class="ident">state</span>, <span class="ident">RUNNING</span>,
<span class="ident">Ordering</span>::<span class="ident">SeqCst</span>);
<span class="kw">if</span> <span class="ident">old</span> <span class="op">!=</span> <span class="ident">state</span> {
<span class="ident">state</span> <span class="op">=</span> <span class="ident">old</span>;
<span class="kw">continue</span>
}
<span class="comment">// Run the initialization routine, letting it know if we're</span>
<span class="comment">// poisoned or not. The `Finish` struct is then dropped, and</span>
<span class="comment">// the `Drop` implementation here is responsible for waking</span>
<span class="comment">// up other waiters both in the normal return and panicking</span>
<span class="comment">// case.</span>
<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">complete</span> <span class="op">=</span> <span class="ident">Finish</span> {
<span class="ident">panicked</span>: <span class="bool-val">true</span>,
<span class="ident">me</span>: <span class="self">self</span>,
};
<span class="ident">init</span>(<span class="ident">state</span> <span class="op">==</span> <span class="ident">POISONED</span>);
<span class="ident">complete</span>.<span class="ident">panicked</span> <span class="op">=</span> <span class="bool-val">false</span>;
<span class="kw">return</span>
}
<span class="comment">// All other values we find should correspond to the RUNNING</span>
<span class="comment">// state with an encoded waiter list in the more significant</span>
<span class="comment">// bits. We attempt to enqueue ourselves by moving us to the</span>
<span class="comment">// head of the list and bail out if we ever see a state that's</span>
<span class="comment">// not RUNNING.</span>
_ <span class="op">=></span> {
<span class="macro">assert</span><span class="macro">!</span>(<span class="ident">state</span> <span class="op">&</span> <span class="ident">STATE_MASK</span> <span class="op">==</span> <span class="ident">RUNNING</span>);
<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">node</span> <span class="op">=</span> <span class="ident">Waiter</span> {
<span class="ident">thread</span>: <span class="prelude-val">Some</span>(<span class="ident">thread</span>::<span class="ident">current</span>()),
<span class="ident">signaled</span>: <span class="ident">AtomicBool</span>::<span class="ident">new</span>(<span class="bool-val">false</span>),
<span class="ident">next</span>: <span class="ident">ptr</span>::<span class="ident">null_mut</span>(),
};
<span class="kw">let</span> <span class="ident">me</span> <span class="op">=</span> <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">node</span> <span class="kw">as</span> <span class="kw-2">*</span><span class="kw-2">mut</span> <span class="ident">Waiter</span> <span class="kw">as</span> <span class="ident">usize</span>;
<span class="macro">assert</span><span class="macro">!</span>(<span class="ident">me</span> <span class="op">&</span> <span class="ident">STATE_MASK</span> <span class="op">==</span> <span class="number">0</span>);
<span class="kw">while</span> <span class="ident">state</span> <span class="op">&</span> <span class="ident">STATE_MASK</span> <span class="op">==</span> <span class="ident">RUNNING</span> {
<span class="ident">node</span>.<span class="ident">next</span> <span class="op">=</span> (<span class="ident">state</span> <span class="op">&</span> <span class="op">!</span><span class="ident">STATE_MASK</span>) <span class="kw">as</span> <span class="kw-2">*</span><span class="kw-2">mut</span> <span class="ident">Waiter</span>;
<span class="kw">let</span> <span class="ident">old</span> <span class="op">=</span> <span class="self">self</span>.<span class="ident">state</span>.<span class="ident">compare_and_swap</span>(<span class="ident">state</span>,
<span class="ident">me</span> <span class="op">|</span> <span class="ident">RUNNING</span>,
<span class="ident">Ordering</span>::<span class="ident">SeqCst</span>);
<span class="kw">if</span> <span class="ident">old</span> <span class="op">!=</span> <span class="ident">state</span> {
<span class="ident">state</span> <span class="op">=</span> <span class="ident">old</span>;
<span class="kw">continue</span>
}
<span class="comment">// Once we've enqueued ourselves, wait in a loop.</span>
<span class="comment">// Afterwards reload the state and continue with what we</span>
<span class="comment">// were doing from before.</span>
<span class="kw">while</span> <span class="op">!</span><span class="ident">node</span>.<span class="ident">signaled</span>.<span class="ident">load</span>(<span class="ident">Ordering</span>::<span class="ident">SeqCst</span>) {
<span class="ident">thread</span>::<span class="ident">park</span>();
}
<span class="ident">state</span> <span class="op">=</span> <span class="self">self</span>.<span class="ident">state</span>.<span class="ident">load</span>(<span class="ident">Ordering</span>::<span class="ident">SeqCst</span>);
<span class="kw">continue</span> <span class="lifetime">'outer</span>
}
}
}
}
}
}
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"std_debug"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.16.0"</span>)]</span>
<span class="kw">impl</span> <span class="ident">fmt</span>::<span class="ident">Debug</span> <span class="kw">for</span> <span class="ident">Once</span> {
<span class="kw">fn</span> <span class="ident">fmt</span>(<span class="kw-2">&</span><span class="self">self</span>, <span class="ident">f</span>: <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">fmt</span>::<span class="ident">Formatter</span>) <span class="op">-></span> <span class="ident">fmt</span>::<span class="prelude-ty">Result</span> {
<span class="ident">f</span>.<span class="ident">pad</span>(<span class="string">"Once { .. }"</span>)
}
}
<span class="kw">impl</span> <span class="ident">Drop</span> <span class="kw">for</span> <span class="ident">Finish</span> {
<span class="kw">fn</span> <span class="ident">drop</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>) {
<span class="comment">// Swap out our state with however we finished. We should only ever see</span>
<span class="comment">// an old state which was RUNNING.</span>
<span class="kw">let</span> <span class="ident">queue</span> <span class="op">=</span> <span class="kw">if</span> <span class="self">self</span>.<span class="ident">panicked</span> {
<span class="self">self</span>.<span class="ident">me</span>.<span class="ident">state</span>.<span class="ident">swap</span>(<span class="ident">POISONED</span>, <span class="ident">Ordering</span>::<span class="ident">SeqCst</span>)
} <span class="kw">else</span> {
<span class="self">self</span>.<span class="ident">me</span>.<span class="ident">state</span>.<span class="ident">swap</span>(<span class="ident">COMPLETE</span>, <span class="ident">Ordering</span>::<span class="ident">SeqCst</span>)
};
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">queue</span> <span class="op">&</span> <span class="ident">STATE_MASK</span>, <span class="ident">RUNNING</span>);
<span class="comment">// Decode the RUNNING to a list of waiters, then walk that entire list</span>
<span class="comment">// and wake them up. Note that it is crucial that after we store `true`</span>
<span class="comment">// in the node it can be free'd! As a result we load the `thread` to</span>
<span class="comment">// signal ahead of time and then unpark it after the store.</span>
<span class="kw">unsafe</span> {
<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">queue</span> <span class="op">=</span> (<span class="ident">queue</span> <span class="op">&</span> <span class="op">!</span><span class="ident">STATE_MASK</span>) <span class="kw">as</span> <span class="kw-2">*</span><span class="kw-2">mut</span> <span class="ident">Waiter</span>;
<span class="kw">while</span> <span class="op">!</span><span class="ident">queue</span>.<span class="ident">is_null</span>() {
<span class="kw">let</span> <span class="ident">next</span> <span class="op">=</span> (<span class="kw-2">*</span><span class="ident">queue</span>).<span class="ident">next</span>;
<span class="kw">let</span> <span class="ident">thread</span> <span class="op">=</span> (<span class="kw-2">*</span><span class="ident">queue</span>).<span class="ident">thread</span>.<span class="ident">take</span>().<span class="ident">unwrap</span>();
(<span class="kw-2">*</span><span class="ident">queue</span>).<span class="ident">signaled</span>.<span class="ident">store</span>(<span class="bool-val">true</span>, <span class="ident">Ordering</span>::<span class="ident">SeqCst</span>);
<span class="ident">thread</span>.<span class="ident">unpark</span>();
<span class="ident">queue</span> <span class="op">=</span> <span class="ident">next</span>;
}
}
}
}
<span class="kw">impl</span> <span class="ident">OnceState</span> {
<span class="doccomment">/// Returns whether the associated [`Once`] was poisoned prior to the</span>
<span class="doccomment">/// invocation of the closure passed to [`call_once_force`].</span>
<span class="doccomment">///</span>
<span class="doccomment">/// [`call_once_force`]: struct.Once.html#method.call_once_force</span>
<span class="doccomment">/// [`Once`]: struct.Once.html</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// A poisoned `Once`:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// #![feature(once_poison)]</span>
<span class="doccomment">///</span>
<span class="doccomment">/// use std::sync::{Once, ONCE_INIT};</span>
<span class="doccomment">/// use std::thread;</span>
<span class="doccomment">///</span>
<span class="doccomment">/// static INIT: Once = ONCE_INIT;</span>
<span class="doccomment">///</span>
<span class="doccomment">/// // poison the once</span>
<span class="doccomment">/// let handle = thread::spawn(|| {</span>
<span class="doccomment">/// INIT.call_once(|| panic!());</span>
<span class="doccomment">/// });</span>
<span class="doccomment">/// assert!(handle.join().is_err());</span>
<span class="doccomment">///</span>
<span class="doccomment">/// INIT.call_once_force(|state| {</span>
<span class="doccomment">/// assert!(state.poisoned());</span>
<span class="doccomment">/// });</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">///</span>
<span class="doccomment">/// An unpoisoned `Once`:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// #![feature(once_poison)]</span>
<span class="doccomment">///</span>
<span class="doccomment">/// use std::sync::{Once, ONCE_INIT};</span>
<span class="doccomment">///</span>
<span class="doccomment">/// static INIT: Once = ONCE_INIT;</span>
<span class="doccomment">///</span>
<span class="doccomment">/// INIT.call_once_force(|state| {</span>
<span class="doccomment">/// assert!(!state.poisoned());</span>
<span class="doccomment">/// });</span>
<span class="attribute">#[<span class="ident">unstable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"once_poison"</span>, <span class="ident">issue</span> <span class="op">=</span> <span class="string">"33577"</span>)]</span>
<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">poisoned</span>(<span class="kw-2">&</span><span class="self">self</span>) <span class="op">-></span> <span class="ident">bool</span> {
<span class="self">self</span>.<span class="ident">poisoned</span>
}
}
<span class="attribute">#[<span class="ident">cfg</span>(<span class="ident">all</span>(<span class="ident">test</span>, <span class="ident">not</span>(<span class="ident">target_os</span> <span class="op">=</span> <span class="string">"emscripten"</span>)))]</span>
<span class="kw">mod</span> <span class="ident">tests</span> {
<span class="kw">use</span> <span class="ident">panic</span>;
<span class="kw">use</span> <span class="ident">sync</span>::<span class="ident">mpsc</span>::<span class="ident">channel</span>;
<span class="kw">use</span> <span class="ident">thread</span>;
<span class="kw">use</span> <span class="kw">super</span>::<span class="ident">Once</span>;
<span class="attribute">#[<span class="ident">test</span>]</span>
<span class="kw">fn</span> <span class="ident">smoke_once</span>() {
<span class="kw">static</span> <span class="ident">O</span>: <span class="ident">Once</span> <span class="op">=</span> <span class="ident">Once</span>::<span class="ident">new</span>();
<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">a</span> <span class="op">=</span> <span class="number">0</span>;
<span class="ident">O</span>.<span class="ident">call_once</span>(<span class="op">||</span> <span class="ident">a</span> <span class="op">+=</span> <span class="number">1</span>);
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">a</span>, <span class="number">1</span>);
<span class="ident">O</span>.<span class="ident">call_once</span>(<span class="op">||</span> <span class="ident">a</span> <span class="op">+=</span> <span class="number">1</span>);
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">a</span>, <span class="number">1</span>);
}
<span class="attribute">#[<span class="ident">test</span>]</span>
<span class="kw">fn</span> <span class="ident">stampede_once</span>() {
<span class="kw">static</span> <span class="ident">O</span>: <span class="ident">Once</span> <span class="op">=</span> <span class="ident">Once</span>::<span class="ident">new</span>();
<span class="kw">static</span> <span class="kw-2">mut</span> <span class="ident">RUN</span>: <span class="ident">bool</span> <span class="op">=</span> <span class="bool-val">false</span>;
<span class="kw">let</span> (<span class="ident">tx</span>, <span class="ident">rx</span>) <span class="op">=</span> <span class="ident">channel</span>();
<span class="kw">for</span> _ <span class="kw">in</span> <span class="number">0</span>..<span class="number">10</span> {
<span class="kw">let</span> <span class="ident">tx</span> <span class="op">=</span> <span class="ident">tx</span>.<span class="ident">clone</span>();
<span class="ident">thread</span>::<span class="ident">spawn</span>(<span class="kw">move</span><span class="op">||</span> {
<span class="kw">for</span> _ <span class="kw">in</span> <span class="number">0</span>..<span class="number">4</span> { <span class="ident">thread</span>::<span class="ident">yield_now</span>() }
<span class="kw">unsafe</span> {
<span class="ident">O</span>.<span class="ident">call_once</span>(<span class="op">||</span> {
<span class="macro">assert</span><span class="macro">!</span>(<span class="op">!</span><span class="ident">RUN</span>);
<span class="ident">RUN</span> <span class="op">=</span> <span class="bool-val">true</span>;
});
<span class="macro">assert</span><span class="macro">!</span>(<span class="ident">RUN</span>);
}
<span class="ident">tx</span>.<span class="ident">send</span>(()).<span class="ident">unwrap</span>();
});
}
<span class="kw">unsafe</span> {
<span class="ident">O</span>.<span class="ident">call_once</span>(<span class="op">||</span> {
<span class="macro">assert</span><span class="macro">!</span>(<span class="op">!</span><span class="ident">RUN</span>);
<span class="ident">RUN</span> <span class="op">=</span> <span class="bool-val">true</span>;
});
<span class="macro">assert</span><span class="macro">!</span>(<span class="ident">RUN</span>);
}
<span class="kw">for</span> _ <span class="kw">in</span> <span class="number">0</span>..<span class="number">10</span> {
<span class="ident">rx</span>.<span class="ident">recv</span>().<span class="ident">unwrap</span>();
}
}
<span class="attribute">#[<span class="ident">test</span>]</span>
<span class="kw">fn</span> <span class="ident">poison_bad</span>() {
<span class="kw">static</span> <span class="ident">O</span>: <span class="ident">Once</span> <span class="op">=</span> <span class="ident">Once</span>::<span class="ident">new</span>();
<span class="comment">// poison the once</span>
<span class="kw">let</span> <span class="ident">t</span> <span class="op">=</span> <span class="ident">panic</span>::<span class="ident">catch_unwind</span>(<span class="op">||</span> {
<span class="ident">O</span>.<span class="ident">call_once</span>(<span class="op">||</span> <span class="macro">panic</span><span class="macro">!</span>());
});
<span class="macro">assert</span><span class="macro">!</span>(<span class="ident">t</span>.<span class="ident">is_err</span>());
<span class="comment">// poisoning propagates</span>
<span class="kw">let</span> <span class="ident">t</span> <span class="op">=</span> <span class="ident">panic</span>::<span class="ident">catch_unwind</span>(<span class="op">||</span> {
<span class="ident">O</span>.<span class="ident">call_once</span>(<span class="op">||</span> {});
});
<span class="macro">assert</span><span class="macro">!</span>(<span class="ident">t</span>.<span class="ident">is_err</span>());
<span class="comment">// we can subvert poisoning, however</span>
<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">called</span> <span class="op">=</span> <span class="bool-val">false</span>;
<span class="ident">O</span>.<span class="ident">call_once_force</span>(<span class="op">|</span><span class="ident">p</span><span class="op">|</span> {
<span class="ident">called</span> <span class="op">=</span> <span class="bool-val">true</span>;
<span class="macro">assert</span><span class="macro">!</span>(<span class="ident">p</span>.<span class="ident">poisoned</span>())
});
<span class="macro">assert</span><span class="macro">!</span>(<span class="ident">called</span>);
<span class="comment">// once any success happens, we stop propagating the poison</span>
<span class="ident">O</span>.<span class="ident">call_once</span>(<span class="op">||</span> {});
}
<span class="attribute">#[<span class="ident">test</span>]</span>
<span class="kw">fn</span> <span class="ident">wait_for_force_to_finish</span>() {
<span class="kw">static</span> <span class="ident">O</span>: <span class="ident">Once</span> <span class="op">=</span> <span class="ident">Once</span>::<span class="ident">new</span>();
<span class="comment">// poison the once</span>
<span class="kw">let</span> <span class="ident">t</span> <span class="op">=</span> <span class="ident">panic</span>::<span class="ident">catch_unwind</span>(<span class="op">||</span> {
<span class="ident">O</span>.<span class="ident">call_once</span>(<span class="op">||</span> <span class="macro">panic</span><span class="macro">!</span>());
});
<span class="macro">assert</span><span class="macro">!</span>(<span class="ident">t</span>.<span class="ident">is_err</span>());
<span class="comment">// make sure someone's waiting inside the once via a force</span>
<span class="kw">let</span> (<span class="ident">tx1</span>, <span class="ident">rx1</span>) <span class="op">=</span> <span class="ident">channel</span>();
<span class="kw">let</span> (<span class="ident">tx2</span>, <span class="ident">rx2</span>) <span class="op">=</span> <span class="ident">channel</span>();
<span class="kw">let</span> <span class="ident">t1</span> <span class="op">=</span> <span class="ident">thread</span>::<span class="ident">spawn</span>(<span class="kw">move</span> <span class="op">||</span> {
<span class="ident">O</span>.<span class="ident">call_once_force</span>(<span class="op">|</span><span class="ident">p</span><span class="op">|</span> {
<span class="macro">assert</span><span class="macro">!</span>(<span class="ident">p</span>.<span class="ident">poisoned</span>());
<span class="ident">tx1</span>.<span class="ident">send</span>(()).<span class="ident">unwrap</span>();
<span class="ident">rx2</span>.<span class="ident">recv</span>().<span class="ident">unwrap</span>();
});
});
<span class="ident">rx1</span>.<span class="ident">recv</span>().<span class="ident">unwrap</span>();
<span class="comment">// put another waiter on the once</span>
<span class="kw">let</span> <span class="ident">t2</span> <span class="op">=</span> <span class="ident">thread</span>::<span class="ident">spawn</span>(<span class="op">||</span> {
<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">called</span> <span class="op">=</span> <span class="bool-val">false</span>;
<span class="ident">O</span>.<span class="ident">call_once</span>(<span class="op">||</span> {
<span class="ident">called</span> <span class="op">=</span> <span class="bool-val">true</span>;
});
<span class="macro">assert</span><span class="macro">!</span>(<span class="op">!</span><span class="ident">called</span>);
});
<span class="ident">tx2</span>.<span class="ident">send</span>(()).<span class="ident">unwrap</span>();
<span class="macro">assert</span><span class="macro">!</span>(<span class="ident">t1</span>.<span class="ident">join</span>().<span class="ident">is_ok</span>());
<span class="macro">assert</span><span class="macro">!</span>(<span class="ident">t2</span>.<span class="ident">join</span>().<span class="ident">is_ok</span>());
}
}
</pre>
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