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</pre><pre class="rust ">
<span class="attribute">#<span class="op">!</span>[<span class="ident">cfg</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"with-deprecated"</span>)]</span>
<span class="attribute">#<span class="op">!</span>[<span class="ident">allow</span>(<span class="ident">deprecated</span>)]</span>
<span class="attribute">#<span class="op">!</span>[<span class="ident">deprecated</span>(<span class="ident">since</span> <span class="op">=</span> <span class="string">"0.1.4"</span>,
<span class="ident">note</span> <span class="op">=</span> <span class="string">"replaced with `BiLock` in many cases, otherwise slated \
for removal due to confusion"</span>)]</span>
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">prelude</span>::<span class="ident">v1</span>::<span class="kw-2">*</span>;
<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">sync</span>::<span class="ident">Arc</span>;
<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">task_impl</span>;
<span class="comment">// One critical piece of this module's contents are the `TaskRc<A>` handles.</span>
<span class="comment">// The purpose of this is to conceptually be able to store data in a task,</span>
<span class="comment">// allowing it to be accessed within multiple futures at once. For example if</span>
<span class="comment">// you have some concurrent futures working, they may all want mutable access to</span>
<span class="comment">// some data. We already know that when the futures are being poll'ed that we're</span>
<span class="comment">// entirely synchronized (aka `&mut Task`), so you shouldn't require an</span>
<span class="comment">// `Arc<Mutex<T>>` to share as the synchronization isn't necessary!</span>
<span class="comment">//</span>
<span class="comment">// So the idea here is that you insert data into a task via `Task::insert`, and</span>
<span class="comment">// a handle to that data is then returned to you. That handle can later get</span>
<span class="comment">// presented to the task itself to actually retrieve the underlying data. The</span>
<span class="comment">// invariant is that the data can only ever be accessed with the task present,</span>
<span class="comment">// and the lifetime of the actual data returned is connected to the lifetime of</span>
<span class="comment">// the task itself.</span>
<span class="comment">//</span>
<span class="comment">// Conceptually I at least like to think of this as "dynamically adding more</span>
<span class="comment">// struct fields to a `Task`". Each call to insert creates a new "name" for the</span>
<span class="comment">// struct field, a `TaskRc<A>`, and then you can access the fields of a struct</span>
<span class="comment">// with the struct itself (`Task`) as well as the name of the field</span>
<span class="comment">// (`TaskRc<A>`). If that analogy doesn't make sense then oh well, it at least</span>
<span class="comment">// helped me!</span>
<span class="comment">//</span>
<span class="comment">// So anyway, we do some interesting trickery here to actually get it to work.</span>
<span class="comment">// Each `TaskRc<A>` handle stores `Arc<UnsafeCell<A>>`. So it turns out, we're</span>
<span class="comment">// not even adding data to the `Task`! Each `TaskRc<A>` contains a reference</span>
<span class="comment">// to this `Arc`, and `TaskRc` handles can be cloned which just bumps the</span>
<span class="comment">// reference count on the `Arc` itself.</span>
<span class="comment">//</span>
<span class="comment">// As before, though, you can present the `Arc` to a `Task` and if they</span>
<span class="comment">// originated from the same place you're allowed safe access to the internals.</span>
<span class="comment">// We allow but shared and mutable access without the `Sync` bound on the data,</span>
<span class="comment">// crucially noting that a `Task` itself is not `Sync`.</span>
<span class="comment">//</span>
<span class="comment">// So hopefully I've convinced you of this point that the `get` and `get_mut`</span>
<span class="comment">// methods below are indeed safe. The data is always valid as it's stored in an</span>
<span class="comment">// `Arc`, and access is only allowed with the proof of the associated `Task`.</span>
<span class="comment">// One thing you might be asking yourself though is what exactly is this "proof</span>
<span class="comment">// of a task"? Right now it's a `usize` corresponding to the `Task`'s</span>
<span class="comment">// `TaskHandle` arc allocation.</span>
<span class="comment">//</span>
<span class="comment">// Wait a minute, isn't that the ABA problem! That is, we create a task A, add</span>
<span class="comment">// some data to it, destroy task A, do some work, create a task B, and then ask</span>
<span class="comment">// to get the data from task B. In this case though the point of the</span>
<span class="comment">// `task_inner` "proof" field is simply that there's some non-`Sync` token</span>
<span class="comment">// proving that you can get access to the data. So while weird, this case should</span>
<span class="comment">// still be safe, as the data's not stored in the task itself.</span>
<span class="doccomment">/// A reference to a piece of data that's accessible only within a specific</span>
<span class="doccomment">/// `Task`.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This data is `Send` even when `A` is not `Sync`, because the data stored</span>
<span class="doccomment">/// within is accessed in a single-threaded way. The thread accessing it may</span>
<span class="doccomment">/// change over time, if the task migrates, so `A` must be `Send`.</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">TaskRc</span><span class="op"><</span><span class="ident">A</span><span class="op">></span> {
<span class="ident">task</span>: <span class="ident">task_impl</span>::<span class="ident">Task</span>,
<span class="ident">ptr</span>: <span class="ident">Arc</span><span class="op"><</span><span class="ident">UnsafeCell</span><span class="op"><</span><span class="ident">A</span><span class="op">>></span>,
}
<span class="comment">// for safety here, see docs at the top of this module</span>
<span class="kw">unsafe</span> <span class="kw">impl</span><span class="op"><</span><span class="ident">A</span>: <span class="ident">Send</span><span class="op">></span> <span class="ident">Send</span> <span class="kw">for</span> <span class="ident">TaskRc</span><span class="op"><</span><span class="ident">A</span><span class="op">></span> {}
<span class="kw">unsafe</span> <span class="kw">impl</span><span class="op"><</span><span class="ident">A</span>: <span class="ident">Sync</span><span class="op">></span> <span class="ident">Sync</span> <span class="kw">for</span> <span class="ident">TaskRc</span><span class="op"><</span><span class="ident">A</span><span class="op">></span> {}
<span class="kw">impl</span><span class="op"><</span><span class="ident">A</span><span class="op">></span> <span class="ident">TaskRc</span><span class="op"><</span><span class="ident">A</span><span class="op">></span> {
<span class="doccomment">/// Inserts a new piece of task-local data into this task, returning a</span>
<span class="doccomment">/// reference to it.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Ownership of the data will be transferred to the task, and the data will</span>
<span class="doccomment">/// be destroyed when the task itself is destroyed. The returned value can</span>
<span class="doccomment">/// be passed to the `with` method to get a reference back to the original</span>
<span class="doccomment">/// data.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Note that the returned handle is cloneable and copyable and can be sent</span>
<span class="doccomment">/// to other futures which will be associated with the same task. All</span>
<span class="doccomment">/// futures will then have access to this data when passed the reference</span>
<span class="doccomment">/// back.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Panics</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This function will panic if a task is not currently running.</span>
<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">new</span>(<span class="ident">a</span>: <span class="ident">A</span>) <span class="op">-></span> <span class="ident">TaskRc</span><span class="op"><</span><span class="ident">A</span><span class="op">></span> {
<span class="ident">TaskRc</span> {
<span class="ident">task</span>: <span class="ident">task_impl</span>::<span class="ident">park</span>(),
<span class="ident">ptr</span>: <span class="ident">Arc</span>::<span class="ident">new</span>(<span class="ident">UnsafeCell</span>::<span class="ident">new</span>(<span class="ident">a</span>)),
}
}
<span class="doccomment">/// Operate with a reference to the underlying data.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This method should be passed a handle previously returned by</span>
<span class="doccomment">/// `Task::insert`. That handle, when passed back into this method, will</span>
<span class="doccomment">/// retrieve a reference to the original data.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Panics</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This method will panic if a task is not currently running or if `self`</span>
<span class="doccomment">/// does not belong to the task that is currently running. That is, if</span>
<span class="doccomment">/// another task generated the `data` handle passed in, this method will</span>
<span class="doccomment">/// panic.</span>
<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">with</span><span class="op"><</span><span class="ident">F</span>, <span class="ident">R</span><span class="op">></span>(<span class="kw-2">&</span><span class="self">self</span>, <span class="ident">f</span>: <span class="ident">F</span>) <span class="op">-></span> <span class="ident">R</span>
<span class="kw">where</span> <span class="ident">F</span>: <span class="ident">FnOnce</span>(<span class="kw-2">&</span><span class="ident">A</span>) <span class="op">-></span> <span class="ident">R</span>
{
<span class="kw">if</span> <span class="op">!</span><span class="self">self</span>.<span class="ident">task</span>.<span class="ident">is_current</span>() {
<span class="macro">panic</span><span class="macro">!</span>(<span class="string">"TaskRc being accessed on task it does not belong to"</span>);
}
<span class="ident">f</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">get</span>() })
}
}
<span class="kw">impl</span><span class="op"><</span><span class="ident">A</span><span class="op">></span> <span class="ident">Clone</span> <span class="kw">for</span> <span class="ident">TaskRc</span><span class="op"><</span><span class="ident">A</span><span class="op">></span> {
<span class="kw">fn</span> <span class="ident">clone</span>(<span class="kw-2">&</span><span class="self">self</span>) <span class="op">-></span> <span class="ident">TaskRc</span><span class="op"><</span><span class="ident">A</span><span class="op">></span> {
<span class="ident">TaskRc</span> {
<span class="ident">task</span>: <span class="self">self</span>.<span class="ident">task</span>.<span class="ident">clone</span>(),
<span class="ident">ptr</span>: <span class="self">self</span>.<span class="ident">ptr</span>.<span class="ident">clone</span>(),
}
}
}
</pre>
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