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<!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 `/home/runner/.cargo/registry/src/github.com-1ecc6299db9ec823/regex-1.5.6/src/pool.rs`."><meta name="keywords" content="rust, rustlang, rust-lang"><title>pool.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" type="text/css" href="../../normalize.css"><link rel="stylesheet" type="text/css" href="../../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" type="text/css" href="../../ayu.css" disabled><link rel="stylesheet" type="text/css" href="../../dark.css" disabled><link rel="stylesheet" type="text/css" 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="mobile-topbar"><button class="sidebar-menu-toggle">☰</button><a class="sidebar-logo" href="../../regex/index.html"><div class="logo-container"><img class="rust-logo" src="../../rust-logo.svg" alt="logo"></div>
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</pre><pre class="rust"><code><span class="comment">// This module provides a relatively simple thread-safe pool of reusable</span>
<span class="comment">// objects. For the most part, it's implemented by a stack represented by a</span>
<span class="comment">// Mutex<Vec<T>>. It has one small trick: because unlocking a mutex is somewhat</span>
<span class="comment">// costly, in the case where a pool is accessed by the first thread that tried</span>
<span class="comment">// to get a value, we bypass the mutex. Here are some benchmarks showing the</span>
<span class="comment">// difference.</span>
<span class="comment">//</span>
<span class="comment">// 1) misc::anchored_literal_long_non_match 21 (18571 MB/s)</span>
<span class="comment">// 2) misc::anchored_literal_long_non_match 107 (3644 MB/s)</span>
<span class="comment">// 3) misc::anchored_literal_long_non_match 45 (8666 MB/s)</span>
<span class="comment">// 4) misc::anchored_literal_long_non_match 19 (20526 MB/s)</span>
<span class="comment">//</span>
<span class="comment">// (1) represents our baseline: the master branch at the time of writing when</span>
<span class="comment">// using the 'thread_local' crate to implement the pool below.</span>
<span class="comment">//</span>
<span class="comment">// (2) represents a naive pool implemented completely via Mutex<Vec<T>>. There</span>
<span class="comment">// is no special trick for bypassing the mutex.</span>
<span class="comment">//</span>
<span class="comment">// (3) is the same as (2), except it uses Mutex<Vec<Box<T>>>. It is twice as</span>
<span class="comment">// fast because a Box<T> is much smaller than the T we use with a Pool in this</span>
<span class="comment">// crate. So pushing and popping a Box<T> from a Vec is quite a bit faster</span>
<span class="comment">// than for T.</span>
<span class="comment">//</span>
<span class="comment">// (4) is the same as (3), but with the trick for bypassing the mutex in the</span>
<span class="comment">// case of the first-to-get thread.</span>
<span class="comment">//</span>
<span class="comment">// Why move off of thread_local? Even though (4) is a hair faster than (1)</span>
<span class="comment">// above, this was not the main goal. The main goal was to move off of</span>
<span class="comment">// thread_local and find a way to *simply* re-capture some of its speed for</span>
<span class="comment">// regex's specific case. So again, why move off of it? The *primary* reason is</span>
<span class="comment">// because of memory leaks. See https://github.com/rust-lang/regex/issues/362</span>
<span class="comment">// for example. (Why do I want it to be simple? Well, I suppose what I mean is,</span>
<span class="comment">// "use as much safe code as possible to minimize risk and be as sure as I can</span>
<span class="comment">// be that it is correct.")</span>
<span class="comment">//</span>
<span class="comment">// My guess is that the thread_local design is probably not appropriate for</span>
<span class="comment">// regex since its memory usage scales to the number of active threads that</span>
<span class="comment">// have used a regex, where as the pool below scales to the number of threads</span>
<span class="comment">// that simultaneously use a regex. While neither case permits contraction,</span>
<span class="comment">// since we own the pool data structure below, we can add contraction if a</span>
<span class="comment">// clear use case pops up in the wild. More pressingly though, it seems that</span>
<span class="comment">// there are at least some use case patterns where one might have many threads</span>
<span class="comment">// sitting around that might have used a regex at one point. While thread_local</span>
<span class="comment">// does try to reuse space previously used by a thread that has since stopped,</span>
<span class="comment">// its maximal memory usage still scales with the total number of active</span>
<span class="comment">// threads. In contrast, the pool below scales with the total number of threads</span>
<span class="comment">// *simultaneously* using the pool. The hope is that this uses less memory</span>
<span class="comment">// overall. And if it doesn't, we can hopefully tune it somehow.</span>
<span class="comment">//</span>
<span class="comment">// It seems that these sort of conditions happen frequently</span>
<span class="comment">// in FFI inside of other more "managed" languages. This was</span>
<span class="comment">// mentioned in the issue linked above, and also mentioned here:</span>
<span class="comment">// https://github.com/BurntSushi/rure-go/issues/3. And in particular, users</span>
<span class="comment">// confirm that disabling the use of thread_local resolves the leak.</span>
<span class="comment">//</span>
<span class="comment">// There were other weaker reasons for moving off of thread_local as well.</span>
<span class="comment">// Namely, at the time, I was looking to reduce dependencies. And for something</span>
<span class="comment">// like regex, maintenance can be simpler when we own the full dependency tree.</span>
<span class="kw">use</span> <span class="ident">std::panic</span>::{<span class="ident">RefUnwindSafe</span>, <span class="ident">UnwindSafe</span>};
<span class="kw">use</span> <span class="ident">std::sync::atomic</span>::{<span class="ident">AtomicUsize</span>, <span class="ident">Ordering</span>};
<span class="kw">use</span> <span class="ident">std::sync::Mutex</span>;
<span class="doccomment">/// An atomic counter used to allocate thread IDs.</span>
<span class="kw">static</span> <span class="ident">COUNTER</span>: <span class="ident">AtomicUsize</span> <span class="op">=</span> <span class="ident">AtomicUsize::new</span>(<span class="number">1</span>);
<span class="macro">thread_local!</span>(
<span class="doccomment">/// A thread local used to assign an ID to a thread.</span>
<span class="kw">static</span> <span class="ident">THREAD_ID</span>: <span class="ident">usize</span> <span class="op">=</span> {
<span class="kw">let</span> <span class="ident">next</span> <span class="op">=</span> <span class="ident">COUNTER</span>.<span class="ident">fetch_add</span>(<span class="number">1</span>, <span class="ident">Ordering::Relaxed</span>);
<span class="comment">// SAFETY: We cannot permit the reuse of thread IDs since reusing a</span>
<span class="comment">// thread ID might result in more than one thread "owning" a pool,</span>
<span class="comment">// and thus, permit accessing a mutable value from multiple threads</span>
<span class="comment">// simultaneously without synchronization. The intent of this panic is</span>
<span class="comment">// to be a sanity check. It is not expected that the thread ID space</span>
<span class="comment">// will actually be exhausted in practice.</span>
<span class="comment">//</span>
<span class="comment">// This checks that the counter never wraps around, since atomic</span>
<span class="comment">// addition wraps around on overflow.</span>
<span class="kw">if</span> <span class="ident">next</span> <span class="op">==</span> <span class="number">0</span> {
<span class="macro">panic!</span>(<span class="string">"regex: thread ID allocation space exhausted"</span>);
}
<span class="ident">next</span>
};
);
<span class="doccomment">/// The type of the function used to create values in a pool when the pool is</span>
<span class="doccomment">/// empty and the caller requests one.</span>
<span class="kw">type</span> <span class="ident">CreateFn</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> <span class="op">=</span>
<span class="ident">Box</span><span class="op"><</span><span class="kw">dyn</span> <span class="ident">Fn</span>() -> <span class="ident">T</span> <span class="op">+</span> <span class="ident">Send</span> <span class="op">+</span> <span class="ident">Sync</span> <span class="op">+</span> <span class="ident">UnwindSafe</span> <span class="op">+</span> <span class="ident">RefUnwindSafe</span> <span class="op">+</span> <span class="lifetime">'static</span><span class="op">></span>;
<span class="doccomment">/// A simple thread safe pool for reusing values.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Getting a value out comes with a guard. When that guard is dropped, the</span>
<span class="doccomment">/// value is automatically put back in the pool.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// A Pool<T> impls Sync when T is Send (even if it's not Sync). This means</span>
<span class="doccomment">/// that T can use interior mutability. This is possible because a pool is</span>
<span class="doccomment">/// guaranteed to provide a value to exactly one thread at any time.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Currently, a pool never contracts in size. Its size is proportional to the</span>
<span class="doccomment">/// number of simultaneous uses.</span>
<span class="kw">pub</span> <span class="kw">struct</span> <span class="ident">Pool</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
<span class="doccomment">/// A stack of T values to hand out. These are used when a Pool is</span>
<span class="doccomment">/// accessed by a thread that didn't create it.</span>
<span class="ident">stack</span>: <span class="ident">Mutex</span><span class="op"><</span><span class="ident">Vec</span><span class="op"><</span><span class="ident">Box</span><span class="op"><</span><span class="ident">T</span><span class="op">></span><span class="op">></span><span class="op">></span>,
<span class="doccomment">/// A function to create more T values when stack is empty and a caller</span>
<span class="doccomment">/// has requested a T.</span>
<span class="ident">create</span>: <span class="ident">CreateFn</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>,
<span class="doccomment">/// The ID of the thread that owns this pool. The owner is the thread</span>
<span class="doccomment">/// that makes the first call to 'get'. When the owner calls 'get', it</span>
<span class="doccomment">/// gets 'owner_val' directly instead of returning a T from 'stack'.</span>
<span class="doccomment">/// See comments elsewhere for details, but this is intended to be an</span>
<span class="doccomment">/// optimization for the common case that makes getting a T faster.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// It is initialized to a value of zero (an impossible thread ID) as a</span>
<span class="doccomment">/// sentinel to indicate that it is unowned.</span>
<span class="ident">owner</span>: <span class="ident">AtomicUsize</span>,
<span class="doccomment">/// A value to return when the caller is in the same thread that created</span>
<span class="doccomment">/// the Pool.</span>
<span class="ident">owner_val</span>: <span class="ident">T</span>,
}
<span class="comment">// SAFETY: Since we want to use a Pool from multiple threads simultaneously</span>
<span class="comment">// behind an Arc, we need for it to be Sync. In cases where T is sync, Pool<T></span>
<span class="comment">// would be Sync. However, since we use a Pool to store mutable scratch space,</span>
<span class="comment">// we wind up using a T that has interior mutability and is thus itself not</span>
<span class="comment">// Sync. So what we *really* want is for our Pool<T> to by Sync even when T is</span>
<span class="comment">// not Sync (but is at least Send).</span>
<span class="comment">//</span>
<span class="comment">// The only non-sync aspect of a Pool is its 'owner_val' field, which is used</span>
<span class="comment">// to implement faster access to a pool value in the common case of a pool</span>
<span class="comment">// being accessed in the same thread in which it was created. The 'stack' field</span>
<span class="comment">// is also shared, but a Mutex<T> where T: Send is already Sync. So we only</span>
<span class="comment">// need to worry about 'owner_val'.</span>
<span class="comment">//</span>
<span class="comment">// The key is to guarantee that 'owner_val' can only ever be accessed from one</span>
<span class="comment">// thread. In our implementation below, we guarantee this by only returning the</span>
<span class="comment">// 'owner_val' when the ID of the current thread matches the ID of the thread</span>
<span class="comment">// that created the Pool. Since this can only ever be one thread, it follows</span>
<span class="comment">// that only one thread can access 'owner_val' at any point in time. Thus, it</span>
<span class="comment">// is safe to declare that Pool<T> is Sync when T is Send.</span>
<span class="comment">//</span>
<span class="comment">// NOTE: It would also be possible to make the owning thread be the *first*</span>
<span class="comment">// thread that tries to get a value out of a Pool. However, the current</span>
<span class="comment">// implementation is a little simpler and it's not clear if making the first</span>
<span class="comment">// thread (rather than the creating thread) is meaningfully better.</span>
<span class="comment">//</span>
<span class="comment">// If there is a way to achieve our performance goals using safe code, then</span>
<span class="comment">// I would very much welcome a patch. As it stands, the implementation below</span>
<span class="comment">// tries to balance safety with performance. The case where a Regex is used</span>
<span class="comment">// from multiple threads simultaneously will suffer a bit since getting a cache</span>
<span class="comment">// will require unlocking a mutex.</span>
<span class="kw">unsafe</span> <span class="kw">impl</span><span class="op"><</span><span class="ident">T</span>: <span class="ident">Send</span><span class="op">></span> <span class="ident">Sync</span> <span class="kw">for</span> <span class="ident">Pool</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {}
<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span>: <span class="ident">::std::fmt::Debug</span><span class="op">></span> <span class="ident">::std::fmt::Debug</span> <span class="kw">for</span> <span class="ident">Pool</span><span class="op"><</span><span class="ident">T</span><span class="op">></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">&mut</span> <span class="ident">::std::fmt::Formatter</span><span class="op"><</span><span class="lifetime">'_</span><span class="op">></span>) -> <span class="ident">::std::fmt::Result</span> {
<span class="ident">f</span>.<span class="ident">debug_struct</span>(<span class="string">"Pool"</span>)
.<span class="ident">field</span>(<span class="string">"stack"</span>, <span class="kw-2">&</span><span class="self">self</span>.<span class="ident">stack</span>)
.<span class="ident">field</span>(<span class="string">"owner"</span>, <span class="kw-2">&</span><span class="self">self</span>.<span class="ident">owner</span>)
.<span class="ident">field</span>(<span class="string">"owner_val"</span>, <span class="kw-2">&</span><span class="self">self</span>.<span class="ident">owner_val</span>)
.<span class="ident">finish</span>()
}
}
<span class="doccomment">/// A guard that is returned when a caller requests a value from the pool.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// The purpose of the guard is to use RAII to automatically put the value back</span>
<span class="doccomment">/// in the pool once it's dropped.</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">PoolGuard</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">T</span>: <span class="ident">Send</span><span class="op">></span> {
<span class="doccomment">/// The pool that this guard is attached to.</span>
<span class="ident">pool</span>: <span class="kw-2">&</span><span class="lifetime">'a</span> <span class="ident">Pool</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>,
<span class="doccomment">/// This is None when the guard represents the special "owned" value. In</span>
<span class="doccomment">/// which case, the value is retrieved from 'pool.owner_val'.</span>
<span class="ident">value</span>: <span class="prelude-ty">Option</span><span class="op"><</span><span class="ident">Box</span><span class="op"><</span><span class="ident">T</span><span class="op">></span><span class="op">></span>,
}
<span class="kw">impl</span><span class="op"><</span><span class="ident">T</span>: <span class="ident">Send</span><span class="op">></span> <span class="ident">Pool</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
<span class="doccomment">/// Create a new pool. The given closure is used to create values in the</span>
<span class="doccomment">/// pool when necessary.</span>
<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">new</span>(<span class="ident">create</span>: <span class="ident">CreateFn</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>) -> <span class="ident">Pool</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
<span class="kw">let</span> <span class="ident">owner</span> <span class="op">=</span> <span class="ident">AtomicUsize::new</span>(<span class="number">0</span>);
<span class="kw">let</span> <span class="ident">owner_val</span> <span class="op">=</span> <span class="ident">create</span>();
<span class="ident">Pool</span> { <span class="ident">stack</span>: <span class="ident">Mutex::new</span>(<span class="macro">vec!</span>[]), <span class="ident">create</span>, <span class="ident">owner</span>, <span class="ident">owner_val</span> }
}
<span class="doccomment">/// Get a value from the pool. The caller is guaranteed to have exclusive</span>
<span class="doccomment">/// access to the given value.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Note that there is no guarantee provided about which value in the</span>
<span class="doccomment">/// pool is returned. That is, calling get, dropping the guard (causing</span>
<span class="doccomment">/// the value to go back into the pool) and then calling get again is NOT</span>
<span class="doccomment">/// guaranteed to return the same value received in the first get call.</span>
<span class="attribute">#[<span class="ident">cfg_attr</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"perf-inline"</span>, <span class="ident">inline</span>(<span class="ident">always</span>))]</span>
<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">get</span>(<span class="kw-2">&</span><span class="self">self</span>) -> <span class="ident">PoolGuard</span><span class="op"><</span><span class="lifetime">'_</span>, <span class="ident">T</span><span class="op">></span> {
<span class="comment">// Our fast path checks if the caller is the thread that "owns" this</span>
<span class="comment">// pool. Or stated differently, whether it is the first thread that</span>
<span class="comment">// tried to extract a value from the pool. If it is, then we can return</span>
<span class="comment">// a T to the caller without going through a mutex.</span>
<span class="comment">//</span>
<span class="comment">// SAFETY: We must guarantee that only one thread gets access to this</span>
<span class="comment">// value. Since a thread is uniquely identified by the THREAD_ID thread</span>
<span class="comment">// local, it follows that is the caller's thread ID is equal to the</span>
<span class="comment">// owner, then only one thread may receive this value.</span>
<span class="kw">let</span> <span class="ident">caller</span> <span class="op">=</span> <span class="ident">THREAD_ID</span>.<span class="ident">with</span>(<span class="op">|</span><span class="ident">id</span><span class="op">|</span> <span class="kw-2">*</span><span class="ident">id</span>);
<span class="kw">let</span> <span class="ident">owner</span> <span class="op">=</span> <span class="self">self</span>.<span class="ident">owner</span>.<span class="ident">load</span>(<span class="ident">Ordering::Relaxed</span>);
<span class="kw">if</span> <span class="ident">caller</span> <span class="op">==</span> <span class="ident">owner</span> {
<span class="kw">return</span> <span class="self">self</span>.<span class="ident">guard_owned</span>();
}
<span class="self">self</span>.<span class="ident">get_slow</span>(<span class="ident">caller</span>, <span class="ident">owner</span>)
}
<span class="doccomment">/// This is the "slow" version that goes through a mutex to pop an</span>
<span class="doccomment">/// allocated value off a stack to return to the caller. (Or, if the stack</span>
<span class="doccomment">/// is empty, a new value is created.)</span>
<span class="doccomment">///</span>
<span class="doccomment">/// If the pool has no owner, then this will set the owner.</span>
<span class="attribute">#[<span class="ident">cold</span>]</span>
<span class="kw">fn</span> <span class="ident">get_slow</span>(<span class="kw-2">&</span><span class="self">self</span>, <span class="ident">caller</span>: <span class="ident">usize</span>, <span class="ident">owner</span>: <span class="ident">usize</span>) -> <span class="ident">PoolGuard</span><span class="op"><</span><span class="lifetime">'_</span>, <span class="ident">T</span><span class="op">></span> {
<span class="kw">use</span> <span class="ident">std::sync::atomic::Ordering::Relaxed</span>;
<span class="kw">if</span> <span class="ident">owner</span> <span class="op">==</span> <span class="number">0</span> {
<span class="comment">// The sentinel 0 value means this pool is not yet owned. We</span>
<span class="comment">// try to atomically set the owner. If we do, then this thread</span>
<span class="comment">// becomes the owner and we can return a guard that represents</span>
<span class="comment">// the special T for the owner.</span>
<span class="kw">let</span> <span class="ident">res</span> <span class="op">=</span> <span class="self">self</span>.<span class="ident">owner</span>.<span class="ident">compare_exchange</span>(<span class="number">0</span>, <span class="ident">caller</span>, <span class="ident">Relaxed</span>, <span class="ident">Relaxed</span>);
<span class="kw">if</span> <span class="ident">res</span>.<span class="ident">is_ok</span>() {
<span class="kw">return</span> <span class="self">self</span>.<span class="ident">guard_owned</span>();
}
}
<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">stack</span> <span class="op">=</span> <span class="self">self</span>.<span class="ident">stack</span>.<span class="ident">lock</span>().<span class="ident">unwrap</span>();
<span class="kw">let</span> <span class="ident">value</span> <span class="op">=</span> <span class="kw">match</span> <span class="ident">stack</span>.<span class="ident">pop</span>() {
<span class="prelude-val">None</span> => <span class="ident">Box::new</span>((<span class="self">self</span>.<span class="ident">create</span>)()),
<span class="prelude-val">Some</span>(<span class="ident">value</span>) => <span class="ident">value</span>,
};
<span class="self">self</span>.<span class="ident">guard_stack</span>(<span class="ident">value</span>)
}
<span class="doccomment">/// Puts a value back into the pool. Callers don't need to call this. Once</span>
<span class="doccomment">/// the guard that's returned by 'get' is dropped, it is put back into the</span>
<span class="doccomment">/// pool automatically.</span>
<span class="kw">fn</span> <span class="ident">put</span>(<span class="kw-2">&</span><span class="self">self</span>, <span class="ident">value</span>: <span class="ident">Box</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>) {
<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">stack</span> <span class="op">=</span> <span class="self">self</span>.<span class="ident">stack</span>.<span class="ident">lock</span>().<span class="ident">unwrap</span>();
<span class="ident">stack</span>.<span class="ident">push</span>(<span class="ident">value</span>);
}
<span class="doccomment">/// Create a guard that represents the special owned T.</span>
<span class="kw">fn</span> <span class="ident">guard_owned</span>(<span class="kw-2">&</span><span class="self">self</span>) -> <span class="ident">PoolGuard</span><span class="op"><</span><span class="lifetime">'_</span>, <span class="ident">T</span><span class="op">></span> {
<span class="ident">PoolGuard</span> { <span class="ident">pool</span>: <span class="self">self</span>, <span class="ident">value</span>: <span class="prelude-val">None</span> }
}
<span class="doccomment">/// Create a guard that contains a value from the pool's stack.</span>
<span class="kw">fn</span> <span class="ident">guard_stack</span>(<span class="kw-2">&</span><span class="self">self</span>, <span class="ident">value</span>: <span class="ident">Box</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>) -> <span class="ident">PoolGuard</span><span class="op"><</span><span class="lifetime">'_</span>, <span class="ident">T</span><span class="op">></span> {
<span class="ident">PoolGuard</span> { <span class="ident">pool</span>: <span class="self">self</span>, <span class="ident">value</span>: <span class="prelude-val">Some</span>(<span class="ident">value</span>) }
}
}
<span class="kw">impl</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">T</span>: <span class="ident">Send</span><span class="op">></span> <span class="ident">PoolGuard</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">T</span><span class="op">></span> {
<span class="doccomment">/// Return the underlying value.</span>
<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">value</span>(<span class="kw-2">&</span><span class="self">self</span>) -> <span class="kw-2">&</span><span class="ident">T</span> {
<span class="kw">match</span> <span class="self">self</span>.<span class="ident">value</span> {
<span class="prelude-val">None</span> => <span class="kw-2">&</span><span class="self">self</span>.<span class="ident">pool</span>.<span class="ident">owner_val</span>,
<span class="prelude-val">Some</span>(<span class="kw-2">ref</span> <span class="ident">v</span>) => <span class="kw-2">&</span><span class="kw-2">*</span><span class="kw-2">*</span><span class="ident">v</span>,
}
}
}
<span class="kw">impl</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">T</span>: <span class="ident">Send</span><span class="op">></span> <span class="ident">Drop</span> <span class="kw">for</span> <span class="ident">PoolGuard</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">T</span><span class="op">></span> {
<span class="attribute">#[<span class="ident">cfg_attr</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"perf-inline"</span>, <span class="ident">inline</span>(<span class="ident">always</span>))]</span>
<span class="kw">fn</span> <span class="ident">drop</span>(<span class="kw-2">&mut</span> <span class="self">self</span>) {
<span class="kw">if</span> <span class="kw">let</span> <span class="prelude-val">Some</span>(<span class="ident">value</span>) <span class="op">=</span> <span class="self">self</span>.<span class="ident">value</span>.<span class="ident">take</span>() {
<span class="self">self</span>.<span class="ident">pool</span>.<span class="ident">put</span>(<span class="ident">value</span>);
}
}
}
<span class="attribute">#[<span class="ident">cfg</span>(<span class="ident">test</span>)]</span>
<span class="kw">mod</span> <span class="ident">tests</span> {
<span class="kw">use</span> <span class="ident">std::panic</span>::{<span class="ident">RefUnwindSafe</span>, <span class="ident">UnwindSafe</span>};
<span class="kw">use</span> <span class="kw">super</span>::<span class="kw-2">*</span>;
<span class="attribute">#[<span class="ident">test</span>]</span>
<span class="kw">fn</span> <span class="ident">oibits</span>() {
<span class="kw">use</span> <span class="ident"><span class="kw">crate</span>::exec::ProgramCache</span>;
<span class="kw">fn</span> <span class="ident">has_oibits</span><span class="op"><</span><span class="ident">T</span>: <span class="ident">Send</span> <span class="op">+</span> <span class="ident">Sync</span> <span class="op">+</span> <span class="ident">UnwindSafe</span> <span class="op">+</span> <span class="ident">RefUnwindSafe</span><span class="op">></span>() {}
<span class="ident">has_oibits</span>::<span class="op"><</span><span class="ident">Pool</span><span class="op"><</span><span class="ident">ProgramCache</span><span class="op">></span><span class="op">></span>();
}
<span class="comment">// Tests that Pool implements the "single owner" optimization. That is, the</span>
<span class="comment">// thread that first accesses the pool gets its own copy, while all other</span>
<span class="comment">// threads get distinct copies.</span>
<span class="attribute">#[<span class="ident">test</span>]</span>
<span class="kw">fn</span> <span class="ident">thread_owner_optimization</span>() {
<span class="kw">use</span> <span class="ident">std::cell::RefCell</span>;
<span class="kw">use</span> <span class="ident">std::sync::Arc</span>;
<span class="kw">let</span> <span class="ident">pool</span>: <span class="ident">Arc</span><span class="op"><</span><span class="ident">Pool</span><span class="op"><</span><span class="ident">RefCell</span><span class="op"><</span><span class="ident">Vec</span><span class="op"><</span><span class="ident">char</span><span class="op">></span><span class="op">></span><span class="op">></span><span class="op">></span> <span class="op">=</span>
<span class="ident">Arc::new</span>(<span class="ident">Pool::new</span>(<span class="ident">Box::new</span>(<span class="op">|</span><span class="op">|</span> <span class="ident">RefCell::new</span>(<span class="macro">vec!</span>[<span class="string">'a'</span>]))));
<span class="ident">pool</span>.<span class="ident">get</span>().<span class="ident">value</span>().<span class="ident">borrow_mut</span>().<span class="ident">push</span>(<span class="string">'x'</span>);
<span class="kw">let</span> <span class="ident">pool1</span> <span class="op">=</span> <span class="ident">pool</span>.<span class="ident">clone</span>();
<span class="kw">let</span> <span class="ident">t1</span> <span class="op">=</span> <span class="ident">std::thread::spawn</span>(<span class="kw">move</span> <span class="op">|</span><span class="op">|</span> {
<span class="kw">let</span> <span class="ident">guard</span> <span class="op">=</span> <span class="ident">pool1</span>.<span class="ident">get</span>();
<span class="kw">let</span> <span class="ident">v</span> <span class="op">=</span> <span class="ident">guard</span>.<span class="ident">value</span>();
<span class="ident">v</span>.<span class="ident">borrow_mut</span>().<span class="ident">push</span>(<span class="string">'y'</span>);
});
<span class="kw">let</span> <span class="ident">pool2</span> <span class="op">=</span> <span class="ident">pool</span>.<span class="ident">clone</span>();
<span class="kw">let</span> <span class="ident">t2</span> <span class="op">=</span> <span class="ident">std::thread::spawn</span>(<span class="kw">move</span> <span class="op">|</span><span class="op">|</span> {
<span class="kw">let</span> <span class="ident">guard</span> <span class="op">=</span> <span class="ident">pool2</span>.<span class="ident">get</span>();
<span class="kw">let</span> <span class="ident">v</span> <span class="op">=</span> <span class="ident">guard</span>.<span class="ident">value</span>();
<span class="ident">v</span>.<span class="ident">borrow_mut</span>().<span class="ident">push</span>(<span class="string">'z'</span>);
});
<span class="ident">t1</span>.<span class="ident">join</span>().<span class="ident">unwrap</span>();
<span class="ident">t2</span>.<span class="ident">join</span>().<span class="ident">unwrap</span>();
<span class="comment">// If we didn't implement the single owner optimization, then one of</span>
<span class="comment">// the threads above is likely to have mutated the [a, x] vec that</span>
<span class="comment">// we stuffed in the pool before spawning the threads. But since</span>
<span class="comment">// neither thread was first to access the pool, and because of the</span>
<span class="comment">// optimization, we should be guaranteed that neither thread mutates</span>
<span class="comment">// the special owned pool value.</span>
<span class="comment">//</span>
<span class="comment">// (Technically this is an implementation detail and not a contract of</span>
<span class="comment">// Pool's API.)</span>
<span class="macro">assert_eq!</span>(<span class="macro">vec!</span>[<span class="string">'a'</span>, <span class="string">'x'</span>], <span class="kw-2">*</span><span class="ident">pool</span>.<span class="ident">get</span>().<span class="ident">value</span>().<span class="ident">borrow</span>());
}
}
</code></pre></div>
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