| <!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 `/root/.cargo/registry/src/github.com-1ecc6299db9ec823/tokio-util-0.7.8/src/sync/cancellation_token/tree_node.rs`."><meta name="keywords" content="rust, rustlang, rust-lang"><title>tree_node.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" href="../../../../normalize.css"><link rel="stylesheet" href="../../../../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" href="../../../../ayu.css" disabled><link rel="stylesheet" href="../../../../dark.css" disabled><link rel="stylesheet" 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="sidebar"><a class="sidebar-logo" href="../../../../tokio_util/index.html"><div class="logo-container"><img class="rust-logo" src="../../../../rust-logo.svg" alt="logo"></div></a></nav><main><div class="width-limiter"><nav class="sub"><a class="sub-logo-container" href="../../../../tokio_util/index.html"><img class="rust-logo" src="../../../../rust-logo.svg" alt="logo"></a><form class="search-form"><div class="search-container"><span></span><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><div id="help-button" title="help" tabindex="-1"><a href="../../../../help.html">?</a></div><div id="settings-menu" tabindex="-1"><a href="../../../../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../../../../wheel.svg"></a></div></div></form></nav><section id="main-content" class="content"><div class="example-wrap"><pre class="src-line-numbers"><span id="1">1</span> |
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| </pre><pre class="rust"><code><span class="doccomment">//! This mod provides the logic for the inner tree structure of the CancellationToken. |
| //! |
| //! CancellationTokens are only light handles with references to TreeNode. |
| //! All the logic is actually implemented in the TreeNode. |
| //! |
| //! A TreeNode is part of the cancellation tree and may have one parent and an arbitrary number of |
| //! children. |
| //! |
| //! A TreeNode can receive the request to perform a cancellation through a CancellationToken. |
| //! This cancellation request will cancel the node and all of its descendants. |
| //! |
| //! As soon as a node cannot get cancelled any more (because it was already cancelled or it has no |
| //! more CancellationTokens pointing to it any more), it gets removed from the tree, to keep the |
| //! tree as small as possible. |
| //! |
| //! # Invariants |
| //! |
| //! Those invariants shall be true at any time. |
| //! |
| //! 1. A node that has no parents and no handles can no longer be cancelled. |
| //! This is important during both cancellation and refcounting. |
| //! |
| //! 2. If node B *is* or *was* a child of node A, then node B was created *after* node A. |
| //! This is important for deadlock safety, as it is used for lock order. |
| //! Node B can only become the child of node A in two ways: |
| //! - being created with `child_node()`, in which case it is trivially true that |
| //! node A already existed when node B was created |
| //! - being moved A->C->B to A->B because node C was removed in `decrease_handle_refcount()` |
| //! or `cancel()`. In this case the invariant still holds, as B was younger than C, and C |
| //! was younger than A, therefore B is also younger than A. |
| //! |
| //! 3. If two nodes are both unlocked and node A is the parent of node B, then node B is a child of |
| //! node A. It is important to always restore that invariant before dropping the lock of a node. |
| //! |
| //! # Deadlock safety |
| //! |
| //! We always lock in the order of creation time. We can prove this through invariant #2. |
| //! Specifically, through invariant #2, we know that we always have to lock a parent |
| //! before its child. |
| //! |
| </span><span class="kw">use </span><span class="kw">crate</span>::loom::sync::{Arc, Mutex, MutexGuard}; |
| |
| <span class="doccomment">/// A node of the cancellation tree structure |
| /// |
| /// The actual data it holds is wrapped inside a mutex for synchronization. |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">struct </span>TreeNode { |
| inner: Mutex<Inner>, |
| waker: tokio::sync::Notify, |
| } |
| <span class="kw">impl </span>TreeNode { |
| <span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>new() -> <span class="self">Self </span>{ |
| <span class="self">Self </span>{ |
| inner: Mutex::new(Inner { |
| parent: <span class="prelude-val">None</span>, |
| parent_idx: <span class="number">0</span>, |
| children: <span class="macro">vec!</span>[], |
| is_cancelled: <span class="bool-val">false</span>, |
| num_handles: <span class="number">1</span>, |
| }), |
| waker: tokio::sync::Notify::new(), |
| } |
| } |
| |
| <span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>notified(<span class="kw-2">&</span><span class="self">self</span>) -> tokio::sync::futures::Notified<<span class="lifetime">'_</span>> { |
| <span class="self">self</span>.waker.notified() |
| } |
| } |
| |
| <span class="doccomment">/// The data contained inside a TreeNode. |
| /// |
| /// This struct exists so that the data of the node can be wrapped |
| /// in a Mutex. |
| </span><span class="kw">struct </span>Inner { |
| parent: <span class="prelude-ty">Option</span><Arc<TreeNode>>, |
| parent_idx: usize, |
| children: Vec<Arc<TreeNode>>, |
| is_cancelled: bool, |
| num_handles: usize, |
| } |
| |
| <span class="doccomment">/// Returns whether or not the node is cancelled |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>is_cancelled(node: <span class="kw-2">&</span>Arc<TreeNode>) -> bool { |
| node.inner.lock().unwrap().is_cancelled |
| } |
| |
| <span class="doccomment">/// Creates a child node |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>child_node(parent: <span class="kw-2">&</span>Arc<TreeNode>) -> Arc<TreeNode> { |
| <span class="kw">let </span><span class="kw-2">mut </span>locked_parent = parent.inner.lock().unwrap(); |
| |
| <span class="comment">// Do not register as child if we are already cancelled. |
| // Cancelled trees can never be uncancelled and therefore |
| // need no connection to parents or children any more. |
| </span><span class="kw">if </span>locked_parent.is_cancelled { |
| <span class="kw">return </span>Arc::new(TreeNode { |
| inner: Mutex::new(Inner { |
| parent: <span class="prelude-val">None</span>, |
| parent_idx: <span class="number">0</span>, |
| children: <span class="macro">vec!</span>[], |
| is_cancelled: <span class="bool-val">true</span>, |
| num_handles: <span class="number">1</span>, |
| }), |
| waker: tokio::sync::Notify::new(), |
| }); |
| } |
| |
| <span class="kw">let </span>child = Arc::new(TreeNode { |
| inner: Mutex::new(Inner { |
| parent: <span class="prelude-val">Some</span>(parent.clone()), |
| parent_idx: locked_parent.children.len(), |
| children: <span class="macro">vec!</span>[], |
| is_cancelled: <span class="bool-val">false</span>, |
| num_handles: <span class="number">1</span>, |
| }), |
| waker: tokio::sync::Notify::new(), |
| }); |
| |
| locked_parent.children.push(child.clone()); |
| |
| child |
| } |
| |
| <span class="doccomment">/// Disconnects the given parent from all of its children. |
| /// |
| /// Takes a reference to [Inner] to make sure the parent is already locked. |
| </span><span class="kw">fn </span>disconnect_children(node: <span class="kw-2">&mut </span>Inner) { |
| <span class="kw">for </span>child <span class="kw">in </span>std::mem::take(<span class="kw-2">&mut </span>node.children) { |
| <span class="kw">let </span><span class="kw-2">mut </span>locked_child = child.inner.lock().unwrap(); |
| locked_child.parent_idx = <span class="number">0</span>; |
| locked_child.parent = <span class="prelude-val">None</span>; |
| } |
| } |
| |
| <span class="doccomment">/// Figures out the parent of the node and locks the node and its parent atomically. |
| /// |
| /// The basic principle of preventing deadlocks in the tree is |
| /// that we always lock the parent first, and then the child. |
| /// For more info look at *deadlock safety* and *invariant #2*. |
| /// |
| /// Sadly, it's impossible to figure out the parent of a node without |
| /// locking it. To then achieve locking order consistency, the node |
| /// has to be unlocked before the parent gets locked. |
| /// This leaves a small window where we already assume that we know the parent, |
| /// but neither the parent nor the node is locked. Therefore, the parent could change. |
| /// |
| /// To prevent that this problem leaks into the rest of the code, it is abstracted |
| /// in this function. |
| /// |
| /// The locked child and optionally its locked parent, if a parent exists, get passed |
| /// to the `func` argument via (node, None) or (node, Some(parent)). |
| </span><span class="kw">fn </span>with_locked_node_and_parent<F, Ret>(node: <span class="kw-2">&</span>Arc<TreeNode>, func: F) -> Ret |
| <span class="kw">where |
| </span>F: FnOnce(MutexGuard<<span class="lifetime">'_</span>, Inner>, <span class="prelude-ty">Option</span><MutexGuard<<span class="lifetime">'_</span>, Inner>>) -> Ret, |
| { |
| <span class="kw">use </span>std::sync::TryLockError; |
| |
| <span class="kw">let </span><span class="kw-2">mut </span>locked_node = node.inner.lock().unwrap(); |
| |
| <span class="comment">// Every time this fails, the number of ancestors of the node decreases, |
| // so the loop must succeed after a finite number of iterations. |
| </span><span class="kw">loop </span>{ |
| <span class="comment">// Look up the parent of the currently locked node. |
| </span><span class="kw">let </span>potential_parent = <span class="kw">match </span>locked_node.parent.as_ref() { |
| <span class="prelude-val">Some</span>(potential_parent) => potential_parent.clone(), |
| <span class="prelude-val">None </span>=> <span class="kw">return </span>func(locked_node, <span class="prelude-val">None</span>), |
| }; |
| |
| <span class="comment">// Lock the parent. This may require unlocking the child first. |
| </span><span class="kw">let </span>locked_parent = <span class="kw">match </span>potential_parent.inner.try_lock() { |
| <span class="prelude-val">Ok</span>(locked_parent) => locked_parent, |
| <span class="prelude-val">Err</span>(TryLockError::WouldBlock) => { |
| drop(locked_node); |
| <span class="comment">// Deadlock safety: |
| // |
| // Due to invariant #2, the potential parent must come before |
| // the child in the creation order. Therefore, we can safely |
| // lock the child while holding the parent lock. |
| </span><span class="kw">let </span>locked_parent = potential_parent.inner.lock().unwrap(); |
| locked_node = node.inner.lock().unwrap(); |
| locked_parent |
| } |
| <span class="prelude-val">Err</span>(TryLockError::Poisoned(err)) => <span class="prelude-val">Err</span>(err).unwrap(), |
| }; |
| |
| <span class="comment">// If we unlocked the child, then the parent may have changed. Check |
| // that we still have the right parent. |
| </span><span class="kw">if let </span><span class="prelude-val">Some</span>(actual_parent) = locked_node.parent.as_ref() { |
| <span class="kw">if </span>Arc::ptr_eq(actual_parent, <span class="kw-2">&</span>potential_parent) { |
| <span class="kw">return </span>func(locked_node, <span class="prelude-val">Some</span>(locked_parent)); |
| } |
| } |
| } |
| } |
| |
| <span class="doccomment">/// Moves all children from `node` to `parent`. |
| /// |
| /// `parent` MUST have been a parent of the node when they both got locked, |
| /// otherwise there is a potential for a deadlock as invariant #2 would be violated. |
| /// |
| /// To acquire the locks for node and parent, use [with_locked_node_and_parent]. |
| </span><span class="kw">fn </span>move_children_to_parent(node: <span class="kw-2">&mut </span>Inner, parent: <span class="kw-2">&mut </span>Inner) { |
| <span class="comment">// Pre-allocate in the parent, for performance |
| </span>parent.children.reserve(node.children.len()); |
| |
| <span class="kw">for </span>child <span class="kw">in </span>std::mem::take(<span class="kw-2">&mut </span>node.children) { |
| { |
| <span class="kw">let </span><span class="kw-2">mut </span>child_locked = child.inner.lock().unwrap(); |
| child_locked.parent = node.parent.clone(); |
| child_locked.parent_idx = parent.children.len(); |
| } |
| parent.children.push(child); |
| } |
| } |
| |
| <span class="doccomment">/// Removes a child from the parent. |
| /// |
| /// `parent` MUST be the parent of `node`. |
| /// To acquire the locks for node and parent, use [with_locked_node_and_parent]. |
| </span><span class="kw">fn </span>remove_child(parent: <span class="kw-2">&mut </span>Inner, <span class="kw-2">mut </span>node: MutexGuard<<span class="lifetime">'_</span>, Inner>) { |
| <span class="comment">// Query the position from where to remove a node |
| </span><span class="kw">let </span>pos = node.parent_idx; |
| node.parent = <span class="prelude-val">None</span>; |
| node.parent_idx = <span class="number">0</span>; |
| |
| <span class="comment">// Unlock node, so that only one child at a time is locked. |
| // Otherwise we would violate the lock order (see 'deadlock safety') as we |
| // don't know the creation order of the child nodes |
| </span>drop(node); |
| |
| <span class="comment">// If `node` is the last element in the list, we don't need any swapping |
| </span><span class="kw">if </span>parent.children.len() == pos + <span class="number">1 </span>{ |
| parent.children.pop().unwrap(); |
| } <span class="kw">else </span>{ |
| <span class="comment">// If `node` is not the last element in the list, we need to |
| // replace it with the last element |
| </span><span class="kw">let </span>replacement_child = parent.children.pop().unwrap(); |
| replacement_child.inner.lock().unwrap().parent_idx = pos; |
| parent.children[pos] = replacement_child; |
| } |
| |
| <span class="kw">let </span>len = parent.children.len(); |
| <span class="kw">if </span><span class="number">4 </span>* len <= parent.children.capacity() { |
| parent.children.shrink_to(<span class="number">2 </span>* len); |
| } |
| } |
| |
| <span class="doccomment">/// Increases the reference count of handles. |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>increase_handle_refcount(node: <span class="kw-2">&</span>Arc<TreeNode>) { |
| <span class="kw">let </span><span class="kw-2">mut </span>locked_node = node.inner.lock().unwrap(); |
| |
| <span class="comment">// Once no handles are left over, the node gets detached from the tree. |
| // There should never be a new handle once all handles are dropped. |
| </span><span class="macro">assert!</span>(locked_node.num_handles > <span class="number">0</span>); |
| |
| locked_node.num_handles += <span class="number">1</span>; |
| } |
| |
| <span class="doccomment">/// Decreases the reference count of handles. |
| /// |
| /// Once no handle is left, we can remove the node from the |
| /// tree and connect its parent directly to its children. |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>decrease_handle_refcount(node: <span class="kw-2">&</span>Arc<TreeNode>) { |
| <span class="kw">let </span>num_handles = { |
| <span class="kw">let </span><span class="kw-2">mut </span>locked_node = node.inner.lock().unwrap(); |
| locked_node.num_handles -= <span class="number">1</span>; |
| locked_node.num_handles |
| }; |
| |
| <span class="kw">if </span>num_handles == <span class="number">0 </span>{ |
| with_locked_node_and_parent(node, |<span class="kw-2">mut </span>node, parent| { |
| <span class="comment">// Remove the node from the tree |
| </span><span class="kw">match </span>parent { |
| <span class="prelude-val">Some</span>(<span class="kw-2">mut </span>parent) => { |
| <span class="comment">// As we want to remove ourselves from the tree, |
| // we have to move the children to the parent, so that |
| // they still receive the cancellation event without us. |
| // Moving them does not violate invariant #1. |
| </span>move_children_to_parent(<span class="kw-2">&mut </span>node, <span class="kw-2">&mut </span>parent); |
| |
| <span class="comment">// Remove the node from the parent |
| </span>remove_child(<span class="kw-2">&mut </span>parent, node); |
| } |
| <span class="prelude-val">None </span>=> { |
| <span class="comment">// Due to invariant #1, we can assume that our |
| // children can no longer be cancelled through us. |
| // (as we now have neither a parent nor handles) |
| // Therefore we can disconnect them. |
| </span>disconnect_children(<span class="kw-2">&mut </span>node); |
| } |
| } |
| }); |
| } |
| } |
| |
| <span class="doccomment">/// Cancels a node and its children. |
| </span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">fn </span>cancel(node: <span class="kw-2">&</span>Arc<TreeNode>) { |
| <span class="kw">let </span><span class="kw-2">mut </span>locked_node = node.inner.lock().unwrap(); |
| |
| <span class="kw">if </span>locked_node.is_cancelled { |
| <span class="kw">return</span>; |
| } |
| |
| <span class="comment">// One by one, adopt grandchildren and then cancel and detach the child |
| </span><span class="kw">while let </span><span class="prelude-val">Some</span>(child) = locked_node.children.pop() { |
| <span class="comment">// This can't deadlock because the mutex we are already |
| // holding is the parent of child. |
| </span><span class="kw">let </span><span class="kw-2">mut </span>locked_child = child.inner.lock().unwrap(); |
| |
| <span class="comment">// Detach the child from node |
| // No need to modify node.children, as the child already got removed with `.pop` |
| </span>locked_child.parent = <span class="prelude-val">None</span>; |
| locked_child.parent_idx = <span class="number">0</span>; |
| |
| <span class="comment">// If child is already cancelled, detaching is enough |
| </span><span class="kw">if </span>locked_child.is_cancelled { |
| <span class="kw">continue</span>; |
| } |
| |
| <span class="comment">// Cancel or adopt grandchildren |
| </span><span class="kw">while let </span><span class="prelude-val">Some</span>(grandchild) = locked_child.children.pop() { |
| <span class="comment">// This can't deadlock because the two mutexes we are already |
| // holding is the parent and grandparent of grandchild. |
| </span><span class="kw">let </span><span class="kw-2">mut </span>locked_grandchild = grandchild.inner.lock().unwrap(); |
| |
| <span class="comment">// Detach the grandchild |
| </span>locked_grandchild.parent = <span class="prelude-val">None</span>; |
| locked_grandchild.parent_idx = <span class="number">0</span>; |
| |
| <span class="comment">// If grandchild is already cancelled, detaching is enough |
| </span><span class="kw">if </span>locked_grandchild.is_cancelled { |
| <span class="kw">continue</span>; |
| } |
| |
| <span class="comment">// For performance reasons, only adopt grandchildren that have children. |
| // Otherwise, just cancel them right away, no need for another iteration. |
| </span><span class="kw">if </span>locked_grandchild.children.is_empty() { |
| <span class="comment">// Cancel the grandchild |
| </span>locked_grandchild.is_cancelled = <span class="bool-val">true</span>; |
| locked_grandchild.children = Vec::new(); |
| drop(locked_grandchild); |
| grandchild.waker.notify_waiters(); |
| } <span class="kw">else </span>{ |
| <span class="comment">// Otherwise, adopt grandchild |
| </span>locked_grandchild.parent = <span class="prelude-val">Some</span>(node.clone()); |
| locked_grandchild.parent_idx = locked_node.children.len(); |
| drop(locked_grandchild); |
| locked_node.children.push(grandchild); |
| } |
| } |
| |
| <span class="comment">// Cancel the child |
| </span>locked_child.is_cancelled = <span class="bool-val">true</span>; |
| locked_child.children = Vec::new(); |
| drop(locked_child); |
| child.waker.notify_waiters(); |
| |
| <span class="comment">// Now the child is cancelled and detached and all its children are adopted. |
| // Just continue until all (including adopted) children are cancelled and detached. |
| </span>} |
| |
| <span class="comment">// Cancel the node itself. |
| </span>locked_node.is_cancelled = <span class="bool-val">true</span>; |
| locked_node.children = Vec::new(); |
| drop(locked_node); |
| node.waker.notify_waiters(); |
| } |
| </code></pre></div> |
| </section></div></main><div id="rustdoc-vars" data-root-path="../../../../" data-current-crate="tokio_util" data-themes="ayu,dark,light" data-resource-suffix="" data-rustdoc-version="1.66.0-nightly (5c8bff74b 2022-10-21)" ></div></body></html> |
