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</pre><pre class="rust ">
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">cell</span>::<span class="ident">UnsafeCell</span>;
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">sync</span>::<span class="ident">atomic</span>::<span class="ident">AtomicUsize</span>;
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">sync</span>::<span class="ident">atomic</span>::<span class="ident">Ordering</span>::<span class="ident">SeqCst</span>;
<span class="doccomment">/// A "lock" around data `D`, which employs a *helping* strategy.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Used to ensure that concurrent `unpark` invocations lead to (1) `poll` being</span>
<span class="doccomment">/// invoked on only a single thread at a time (2) `poll` being invoked at least</span>
<span class="doccomment">/// once after each `unpark` (unless the future has completed).</span>
<span class="kw">pub</span> <span class="kw">struct</span> <span class="ident">UnparkMutex</span><span class="op"><</span><span class="ident">D</span><span class="op">></span> {
<span class="comment">// The state of task execution (state machine described below)</span>
<span class="ident">status</span>: <span class="ident">AtomicUsize</span>,
<span class="comment">// The actual task data, accessible only in the POLLING state</span>
<span class="ident">inner</span>: <span class="ident">UnsafeCell</span><span class="op"><</span><span class="prelude-ty">Option</span><span class="op"><</span><span class="ident">D</span><span class="op">>></span>,
}
<span class="comment">// `UnparkMutex<D>` functions in many ways like a `Mutex<D>`, except that on</span>
<span class="comment">// acquisition failure, the current lockholder performs the desired work --</span>
<span class="comment">// re-polling.</span>
<span class="comment">//</span>
<span class="comment">// As such, these impls mirror those for `Mutex<D>`. In particular, a reference</span>
<span class="comment">// to `UnparkMutex` can be used to gain `&mut` access to the inner data, which</span>
<span class="comment">// must therefore be `Send`.</span>
<span class="kw">unsafe</span> <span class="kw">impl</span><span class="op"><</span><span class="ident">D</span>: <span class="ident">Send</span><span class="op">></span> <span class="ident">Send</span> <span class="kw">for</span> <span class="ident">UnparkMutex</span><span class="op"><</span><span class="ident">D</span><span class="op">></span> {}
<span class="kw">unsafe</span> <span class="kw">impl</span><span class="op"><</span><span class="ident">D</span>: <span class="ident">Send</span><span class="op">></span> <span class="ident">Sync</span> <span class="kw">for</span> <span class="ident">UnparkMutex</span><span class="op"><</span><span class="ident">D</span><span class="op">></span> {}
<span class="comment">// There are four possible task states, listed below with their possible</span>
<span class="comment">// transitions:</span>
<span class="comment">// The task is blocked, waiting on an event</span>
<span class="kw">const</span> <span class="ident">WAITING</span>: <span class="ident">usize</span> <span class="op">=</span> <span class="number">0</span>; <span class="comment">// --> POLLING</span>
<span class="comment">// The task is actively being polled by a thread; arrival of additional events</span>
<span class="comment">// of interest should move it to the REPOLL state</span>
<span class="kw">const</span> <span class="ident">POLLING</span>: <span class="ident">usize</span> <span class="op">=</span> <span class="number">1</span>; <span class="comment">// --> WAITING, REPOLL, or COMPLETE</span>
<span class="comment">// The task is actively being polled, but will need to be re-polled upon</span>
<span class="comment">// completion to ensure that all events were observed.</span>
<span class="kw">const</span> <span class="ident">REPOLL</span>: <span class="ident">usize</span> <span class="op">=</span> <span class="number">2</span>; <span class="comment">// --> POLLING</span>
<span class="comment">// The task has finished executing (either successfully or with an error/panic)</span>
<span class="kw">const</span> <span class="ident">COMPLETE</span>: <span class="ident">usize</span> <span class="op">=</span> <span class="number">3</span>; <span class="comment">// No transitions out</span>
<span class="kw">impl</span><span class="op"><</span><span class="ident">D</span><span class="op">></span> <span class="ident">UnparkMutex</span><span class="op"><</span><span class="ident">D</span><span class="op">></span> {
<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">new</span>() <span class="op">-></span> <span class="ident">UnparkMutex</span><span class="op"><</span><span class="ident">D</span><span class="op">></span> {
<span class="ident">UnparkMutex</span> {
<span class="ident">status</span>: <span class="ident">AtomicUsize</span>::<span class="ident">new</span>(<span class="ident">WAITING</span>),
<span class="ident">inner</span>: <span class="ident">UnsafeCell</span>::<span class="ident">new</span>(<span class="prelude-val">None</span>),
}
}
<span class="doccomment">/// Attempt to "notify" the mutex that a poll should occur.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// An `Ok` result indicates that the `POLLING` state has been entered, and</span>
<span class="doccomment">/// the caller can proceed to poll the future. An `Err` result indicates</span>
<span class="doccomment">/// that polling is not necessary (because the task is finished or the</span>
<span class="doccomment">/// polling has been delegated).</span>
<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">notify</span>(<span class="kw-2">&</span><span class="self">self</span>) <span class="op">-></span> <span class="prelude-ty">Result</span><span class="op"><</span><span class="ident">D</span>, ()<span class="op">></span> {
<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">status</span> <span class="op">=</span> <span class="self">self</span>.<span class="ident">status</span>.<span class="ident">load</span>(<span class="ident">SeqCst</span>);
<span class="kw">loop</span> {
<span class="kw">match</span> <span class="ident">status</span> {
<span class="comment">// The task is idle, so try to run it immediately.</span>
<span class="ident">WAITING</span> <span class="op">=></span> {
<span class="kw">match</span> <span class="self">self</span>.<span class="ident">status</span>.<span class="ident">compare_exchange</span>(<span class="ident">WAITING</span>, <span class="ident">POLLING</span>,
<span class="ident">SeqCst</span>, <span class="ident">SeqCst</span>) {
<span class="prelude-val">Ok</span>(_) <span class="op">=></span> {
<span class="kw">let</span> <span class="ident">data</span> <span class="op">=</span> <span class="kw">unsafe</span> {
<span class="comment">// SAFETY: we've ensured mutual exclusion via</span>
<span class="comment">// the status protocol; we are the only thread</span>
<span class="comment">// that has transitioned to the POLLING state,</span>
<span class="comment">// and we won't transition back to QUEUED until</span>
<span class="comment">// the lock is "released" by this thread. See</span>
<span class="comment">// the protocol diagram above.</span>
(<span class="kw-2">*</span><span class="self">self</span>.<span class="ident">inner</span>.<span class="ident">get</span>()).<span class="ident">take</span>().<span class="ident">unwrap</span>()
};
<span class="kw">return</span> <span class="prelude-val">Ok</span>(<span class="ident">data</span>);
}
<span class="prelude-val">Err</span>(<span class="ident">cur</span>) <span class="op">=></span> <span class="ident">status</span> <span class="op">=</span> <span class="ident">cur</span>,
}
}
<span class="comment">// The task is being polled, so we need to record that it should</span>
<span class="comment">// be *repolled* when complete.</span>
<span class="ident">POLLING</span> <span class="op">=></span> {
<span class="kw">match</span> <span class="self">self</span>.<span class="ident">status</span>.<span class="ident">compare_exchange</span>(<span class="ident">POLLING</span>, <span class="ident">REPOLL</span>,
<span class="ident">SeqCst</span>, <span class="ident">SeqCst</span>) {
<span class="prelude-val">Ok</span>(_) <span class="op">=></span> <span class="kw">return</span> <span class="prelude-val">Err</span>(()),
<span class="prelude-val">Err</span>(<span class="ident">cur</span>) <span class="op">=></span> <span class="ident">status</span> <span class="op">=</span> <span class="ident">cur</span>,
}
}
<span class="comment">// The task is already scheduled for polling, or is complete, so</span>
<span class="comment">// we've got nothing to do.</span>
_ <span class="op">=></span> <span class="kw">return</span> <span class="prelude-val">Err</span>(()),
}
}
}
<span class="doccomment">/// Alert the mutex that polling is about to begin, clearing any accumulated</span>
<span class="doccomment">/// re-poll requests.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Safety</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Callable only from the `POLLING`/`REPOLL` states, i.e. between</span>
<span class="doccomment">/// successful calls to `notify` and `wait`/`complete`.</span>
<span class="kw">pub</span> <span class="kw">unsafe</span> <span class="kw">fn</span> <span class="ident">start_poll</span>(<span class="kw-2">&</span><span class="self">self</span>) {
<span class="self">self</span>.<span class="ident">status</span>.<span class="ident">store</span>(<span class="ident">POLLING</span>, <span class="ident">SeqCst</span>);
}
<span class="doccomment">/// Alert the mutex that polling completed with NotReady.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Safety</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Callable only from the `POLLING`/`REPOLL` states, i.e. between</span>
<span class="doccomment">/// successful calls to `notify` and `wait`/`complete`.</span>
<span class="kw">pub</span> <span class="kw">unsafe</span> <span class="kw">fn</span> <span class="ident">wait</span>(<span class="kw-2">&</span><span class="self">self</span>, <span class="ident">data</span>: <span class="ident">D</span>) <span class="op">-></span> <span class="prelude-ty">Result</span><span class="op"><</span>(), <span class="ident">D</span><span class="op">></span> {
<span class="kw-2">*</span><span class="self">self</span>.<span class="ident">inner</span>.<span class="ident">get</span>() <span class="op">=</span> <span class="prelude-val">Some</span>(<span class="ident">data</span>);
<span class="kw">match</span> <span class="self">self</span>.<span class="ident">status</span>.<span class="ident">compare_exchange</span>(<span class="ident">POLLING</span>, <span class="ident">WAITING</span>, <span class="ident">SeqCst</span>, <span class="ident">SeqCst</span>) {
<span class="comment">// no unparks came in while we were running</span>
<span class="prelude-val">Ok</span>(_) <span class="op">=></span> <span class="prelude-val">Ok</span>(()),
<span class="comment">// guaranteed to be in REPOLL state; just clobber the</span>
<span class="comment">// state and run again.</span>
<span class="prelude-val">Err</span>(<span class="ident">status</span>) <span class="op">=></span> {
<span class="macro">assert_eq</span><span class="macro">!</span>(<span class="ident">status</span>, <span class="ident">REPOLL</span>);
<span class="self">self</span>.<span class="ident">status</span>.<span class="ident">store</span>(<span class="ident">POLLING</span>, <span class="ident">SeqCst</span>);
<span class="prelude-val">Err</span>((<span class="kw-2">*</span><span class="self">self</span>.<span class="ident">inner</span>.<span class="ident">get</span>()).<span class="ident">take</span>().<span class="ident">unwrap</span>())
}
}
}
<span class="doccomment">/// Alert the mutex that the task has completed execution and should not be</span>
<span class="doccomment">/// notified again.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Safety</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Callable only from the `POLLING`/`REPOLL` states, i.e. between</span>
<span class="doccomment">/// successful calls to `notify` and `wait`/`complete`.</span>
<span class="kw">pub</span> <span class="kw">unsafe</span> <span class="kw">fn</span> <span class="ident">complete</span>(<span class="kw-2">&</span><span class="self">self</span>) {
<span class="self">self</span>.<span class="ident">status</span>.<span class="ident">store</span>(<span class="ident">COMPLETE</span>, <span class="ident">SeqCst</span>);
}
}
</pre>
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