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</pre><pre class="rust ">
<span class="doccomment">//! A "mutex" which only supports `try_lock`</span>
<span class="doccomment">//!</span>
<span class="doccomment">//! As a futures library the eventual call to an event loop should be the only</span>
<span class="doccomment">//! thing that ever blocks, so this is assisted with a fast user-space</span>
<span class="doccomment">//! implementation of a lock that can only have a `try_lock` operation.</span>
<span class="kw">extern</span> <span class="kw">crate</span> <span class="ident">core</span>;
<span class="kw">use</span> <span class="self">self</span>::<span class="ident">core</span>::<span class="ident">cell</span>::<span class="ident">UnsafeCell</span>;
<span class="kw">use</span> <span class="self">self</span>::<span class="ident">core</span>::<span class="ident">ops</span>::{<span class="ident">Deref</span>, <span class="ident">DerefMut</span>};
<span class="kw">use</span> <span class="self">self</span>::<span class="ident">core</span>::<span class="ident">sync</span>::<span class="ident">atomic</span>::<span class="ident">Ordering</span>::<span class="ident">SeqCst</span>;
<span class="kw">use</span> <span class="self">self</span>::<span class="ident">core</span>::<span class="ident">sync</span>::<span class="ident">atomic</span>::<span class="ident">AtomicBool</span>;
<span class="doccomment">/// A "mutex" around a value, similar to `std::sync::Mutex<T>`.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This lock only supports the `try_lock` operation, however, and does not</span>
<span class="doccomment">/// implement poisoning.</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">Lock</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
<span class="ident">locked</span>: <span class="ident">AtomicBool</span>,
<span class="ident">data</span>: <span class="ident">UnsafeCell</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>,
}
<span class="doccomment">/// Sentinel representing an acquired lock through which the data can be</span>
<span class="doccomment">/// accessed.</span>
<span class="kw">pub</span> <span class="kw">struct</span> <span class="ident">TryLock</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">T</span>: <span class="lifetime">'a</span><span class="op">></span> {
<span class="ident">__ptr</span>: <span class="kw-2">&</span><span class="lifetime">'a</span> <span class="ident">Lock</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>,
}
<span class="comment">// The `Lock` structure is basically just a `Mutex<T>`, and these two impls are</span>
<span class="comment">// intended to mirror the standard library's corresponding impls for `Mutex<T>`.</span>
<span class="comment">//</span>
<span class="comment">// If a `T` is sendable across threads, so is the lock, and `T` must be sendable</span>
<span class="comment">// across threads to be `Sync` because it allows mutable access from multiple</span>
<span class="comment">// threads.</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">Send</span> <span class="kw">for</span> <span class="ident">Lock</span><span class="op"><</span><span class="ident">T</span><span class="op">></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">Lock</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="op">></span> <span class="ident">Lock</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
<span class="doccomment">/// Creates a new lock around the given value.</span>
<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">new</span>(<span class="ident">t</span>: <span class="ident">T</span>) <span class="op">-></span> <span class="ident">Lock</span><span class="op"><</span><span class="ident">T</span><span class="op">></span> {
<span class="ident">Lock</span> {
<span class="ident">locked</span>: <span class="ident">AtomicBool</span>::<span class="ident">new</span>(<span class="bool-val">false</span>),
<span class="ident">data</span>: <span class="ident">UnsafeCell</span>::<span class="ident">new</span>(<span class="ident">t</span>),
}
}
<span class="doccomment">/// Attempts to acquire this lock, returning whether the lock was acquired or</span>
<span class="doccomment">/// not.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// If `Some` is returned then the data this lock protects can be accessed</span>
<span class="doccomment">/// through the sentinel. This sentinel allows both mutable and immutable</span>
<span class="doccomment">/// access.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// If `None` is returned then the lock is already locked, either elsewhere</span>
<span class="doccomment">/// on this thread or on another thread.</span>
<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">try_lock</span>(<span class="kw-2">&</span><span class="self">self</span>) <span class="op">-></span> <span class="prelude-ty">Option</span><span class="op"><</span><span class="ident">TryLock</span><span class="op"><</span><span class="ident">T</span><span class="op">>></span> {
<span class="kw">if</span> <span class="op">!</span><span class="self">self</span>.<span class="ident">locked</span>.<span class="ident">swap</span>(<span class="bool-val">true</span>, <span class="ident">SeqCst</span>) {
<span class="prelude-val">Some</span>(<span class="ident">TryLock</span> { <span class="ident">__ptr</span>: <span class="self">self</span> })
} <span class="kw">else</span> {
<span class="prelude-val">None</span>
}
}
}
<span class="kw">impl</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">T</span><span class="op">></span> <span class="ident">Deref</span> <span class="kw">for</span> <span class="ident">TryLock</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">T</span><span class="op">></span> {
<span class="kw">type</span> <span class="ident">Target</span> <span class="op">=</span> <span class="ident">T</span>;
<span class="kw">fn</span> <span class="ident">deref</span>(<span class="kw-2">&</span><span class="self">self</span>) <span class="op">-></span> <span class="kw-2">&</span><span class="ident">T</span> {
<span class="comment">// The existence of `TryLock` represents that we own the lock, so we</span>
<span class="comment">// can safely access the data here.</span>
<span class="kw">unsafe</span> { <span class="kw-2">&</span><span class="kw-2">*</span><span class="self">self</span>.<span class="ident">__ptr</span>.<span class="ident">data</span>.<span class="ident">get</span>() }
}
}
<span class="kw">impl</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">T</span><span class="op">></span> <span class="ident">DerefMut</span> <span class="kw">for</span> <span class="ident">TryLock</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">T</span><span class="op">></span> {
<span class="kw">fn</span> <span class="ident">deref_mut</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="self">self</span>) <span class="op">-></span> <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">T</span> {
<span class="comment">// The existence of `TryLock` represents that we own the lock, so we</span>
<span class="comment">// can safely access the data here.</span>
<span class="comment">//</span>
<span class="comment">// Additionally, we're the *only* `TryLock` in existence so mutable</span>
<span class="comment">// access should be ok.</span>
<span class="kw">unsafe</span> { <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="kw-2">*</span><span class="self">self</span>.<span class="ident">__ptr</span>.<span class="ident">data</span>.<span class="ident">get</span>() }
}
}
<span class="kw">impl</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">T</span><span class="op">></span> <span class="ident">Drop</span> <span class="kw">for</span> <span class="ident">TryLock</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">T</span><span class="op">></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="self">self</span>.<span class="ident">__ptr</span>.<span class="ident">locked</span>.<span class="ident">store</span>(<span class="bool-val">false</span>, <span class="ident">SeqCst</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="kw">super</span>::<span class="ident">Lock</span>;
<span class="attribute">#[<span class="ident">test</span>]</span>
<span class="kw">fn</span> <span class="ident">smoke</span>() {
<span class="kw">let</span> <span class="ident">a</span> <span class="op">=</span> <span class="ident">Lock</span>::<span class="ident">new</span>(<span class="number">1</span>);
<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">a1</span> <span class="op">=</span> <span class="ident">a</span>.<span class="ident">try_lock</span>().<span class="ident">unwrap</span>();
<span class="macro">assert</span><span class="macro">!</span>(<span class="ident">a</span>.<span class="ident">try_lock</span>().<span class="ident">is_none</span>());
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="kw-2">*</span><span class="ident">a1</span>, <span class="number">1</span>);
<span class="kw-2">*</span><span class="ident">a1</span> <span class="op">=</span> <span class="number">2</span>;
<span class="ident">drop</span>(<span class="ident">a1</span>);
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="kw-2">*</span><span class="ident">a</span>.<span class="ident">try_lock</span>().<span class="ident">unwrap</span>(), <span class="number">2</span>);
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="kw-2">*</span><span class="ident">a</span>.<span class="ident">try_lock</span>().<span class="ident">unwrap</span>(), <span class="number">2</span>);
}
}
</pre>
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