tokio_util/task/

join_map.rs

use hashbrown::hash_table::Entry;
use hashbrown::{HashMap, HashTable};
use std::borrow::Borrow;
use std::collections::hash_map::RandomState;
use std::fmt;
use std::future::Future;
use std::hash::{BuildHasher, Hash, Hasher};
use std::marker::PhantomData;
use tokio::runtime::Handle;
use tokio::task::{AbortHandle, Id, JoinError, JoinSet, LocalSet};

/// A collection of tasks spawned on a Tokio runtime, associated with hash map
/// keys.
///
/// This type is very similar to the [`JoinSet`] type in `tokio::task`, with the
/// addition of a  set of keys associated with each task. These keys allow
/// [cancelling a task][abort] or [multiple tasks][abort_matching] in the
/// `JoinMap` based on   their keys, or [test whether a task corresponding to a
/// given key exists][contains] in the `JoinMap`.
///
/// In addition, when tasks in the `JoinMap` complete, they will return the
/// associated key along with the value returned by the task, if any.
///
/// A `JoinMap` can be used to await the completion of some or all of the tasks
/// in the map. The map is not ordered, and the tasks will be returned in the
/// order they complete.
///
/// All of the tasks must have the same return type `V`.
///
/// When the `JoinMap` is dropped, all tasks in the `JoinMap` are immediately aborted.
///
/// # Examples
///
/// Spawn multiple tasks and wait for them:
///
/// ```
/// use tokio_util::task::JoinMap;
///
/// #[tokio::main]
/// async fn main() {
///     let mut map = JoinMap::new();
///
///     for i in 0..10 {
///         // Spawn a task on the `JoinMap` with `i` as its key.
///         map.spawn(i, async move { /* ... */ });
///     }
///
///     let mut seen = [false; 10];
///
///     // When a task completes, `join_next` returns the task's key along
///     // with its output.
///     while let Some((key, res)) = map.join_next().await {
///         seen[key] = true;
///         assert!(res.is_ok(), "task {} completed successfully!", key);
///     }
///
///     for i in 0..10 {
///         assert!(seen[i]);
///     }
/// }
/// ```
///
/// Cancel tasks based on their keys:
///
/// ```
/// use tokio_util::task::JoinMap;
///
/// #[tokio::main]
/// async fn main() {
///     let mut map = JoinMap::new();
///
///     map.spawn("hello world", async move { /* ... */ });
///     map.spawn("goodbye world", async move { /* ... */});
///
///     // Look up the "goodbye world" task in the map and abort it.
///     let aborted = map.abort("goodbye world");
///
///     // `JoinMap::abort` returns `true` if a task existed for the
///     // provided key.
///     assert!(aborted);
///
///     while let Some((key, res)) = map.join_next().await {
///         if key == "goodbye world" {
///             // The aborted task should complete with a cancelled `JoinError`.
///             assert!(res.unwrap_err().is_cancelled());
///         } else {
///             // Other tasks should complete normally.
///             assert!(res.is_ok());
///         }
///     }
/// }
/// ```
///
/// [`JoinSet`]: tokio::task::JoinSet
/// [abort]: fn@Self::abort
/// [abort_matching]: fn@Self::abort_matching
/// [contains]: fn@Self::contains_key
pub struct JoinMap<K, V, S = RandomState> {
    /// A map of the [`AbortHandle`]s of the tasks spawned on this `JoinMap`,
    /// indexed by their keys.
    tasks_by_key: HashTable<(K, AbortHandle)>,

    /// A map from task IDs to the hash of the key associated with that task.
    ///
    /// This map is used to perform reverse lookups of tasks in the
    /// `tasks_by_key` map based on their task IDs. When a task terminates, the
    /// ID is provided to us by the `JoinSet`, so we can look up the hash value
    /// of that task's key, and then remove it from the `tasks_by_key` map using
    /// the raw hash code, resolving collisions by comparing task IDs.
    hashes_by_task: HashMap<Id, u64, S>,

    /// The [`JoinSet`] that awaits the completion of tasks spawned on this
    /// `JoinMap`.
    tasks: JoinSet<V>,
}

impl<K, V> JoinMap<K, V> {
    /// Creates a new empty `JoinMap`.
    ///
    /// The `JoinMap` is initially created with a capacity of 0, so it will not
    /// allocate until a task is first spawned on it.
    ///
    /// # Examples
    ///
    /// ```
    /// use tokio_util::task::JoinMap;
    /// let map: JoinMap<&str, i32> = JoinMap::new();
    /// ```
    #[inline]
    #[must_use]
    pub fn new() -> Self {
        Self::with_hasher(RandomState::new())
    }

    /// Creates an empty `JoinMap` with the specified capacity.
    ///
    /// The `JoinMap` will be able to hold at least `capacity` tasks without
    /// reallocating.
    ///
    /// # Examples
    ///
    /// ```
    /// use tokio_util::task::JoinMap;
    /// let map: JoinMap<&str, i32> = JoinMap::with_capacity(10);
    /// ```
    #[inline]
    #[must_use]
    pub fn with_capacity(capacity: usize) -> Self {
        JoinMap::with_capacity_and_hasher(capacity, Default::default())
    }
}

impl<K, V, S> JoinMap<K, V, S> {
    /// Creates an empty `JoinMap` which will use the given hash builder to hash
    /// keys.
    ///
    /// The created map has the default initial capacity.
    ///
    /// Warning: `hash_builder` is normally randomly generated, and
    /// is designed to allow `JoinMap` to be resistant to attacks that
    /// cause many collisions and very poor performance. Setting it
    /// manually using this function can expose a DoS attack vector.
    ///
    /// The `hash_builder` passed should implement the [`BuildHasher`] trait for
    /// the `JoinMap` to be useful, see its documentation for details.
    #[inline]
    #[must_use]
    pub fn with_hasher(hash_builder: S) -> Self {
        Self::with_capacity_and_hasher(0, hash_builder)
    }

    /// Creates an empty `JoinMap` with the specified capacity, using `hash_builder`
    /// to hash the keys.
    ///
    /// The `JoinMap` will be able to hold at least `capacity` elements without
    /// reallocating. If `capacity` is 0, the `JoinMap` will not allocate.
    ///
    /// Warning: `hash_builder` is normally randomly generated, and
    /// is designed to allow HashMaps to be resistant to attacks that
    /// cause many collisions and very poor performance. Setting it
    /// manually using this function can expose a DoS attack vector.
    ///
    /// The `hash_builder` passed should implement the [`BuildHasher`] trait for
    /// the `JoinMap`to be useful, see its documentation for details.
    ///
    /// # Examples
    ///
    /// ```
    /// # #[tokio::main]
    /// # async fn main() {
    /// use tokio_util::task::JoinMap;
    /// use std::collections::hash_map::RandomState;
    ///
    /// let s = RandomState::new();
    /// let mut map = JoinMap::with_capacity_and_hasher(10, s);
    /// map.spawn(1, async move { "hello world!" });
    /// # }
    /// ```
    #[inline]
    #[must_use]
    pub fn with_capacity_and_hasher(capacity: usize, hash_builder: S) -> Self {
        Self {
            tasks_by_key: HashTable::with_capacity(capacity),
            hashes_by_task: HashMap::with_capacity_and_hasher(capacity, hash_builder),
            tasks: JoinSet::new(),
        }
    }

    /// Returns the number of tasks currently in the `JoinMap`.
    pub fn len(&self) -> usize {
        let len = self.tasks_by_key.len();
        debug_assert_eq!(len, self.hashes_by_task.len());
        len
    }

    /// Returns whether the `JoinMap` is empty.
    pub fn is_empty(&self) -> bool {
        let empty = self.tasks_by_key.is_empty();
        debug_assert_eq!(empty, self.hashes_by_task.is_empty());
        empty
    }

    /// Returns the number of tasks the map can hold without reallocating.
    ///
    /// This number is a lower bound; the `JoinMap` might be able to hold
    /// more, but is guaranteed to be able to hold at least this many.
    ///
    /// # Examples
    ///
    /// ```
    /// use tokio_util::task::JoinMap;
    ///
    /// let map: JoinMap<i32, i32> = JoinMap::with_capacity(100);
    /// assert!(map.capacity() >= 100);
    /// ```
    #[inline]
    pub fn capacity(&self) -> usize {
        let capacity = self.tasks_by_key.capacity();
        debug_assert_eq!(capacity, self.hashes_by_task.capacity());
        capacity
    }
}

impl<K, V, S> JoinMap<K, V, S>
where
    K: Hash + Eq,
    V: 'static,
    S: BuildHasher,
{
    /// Spawn the provided task and store it in this `JoinMap` with the provided
    /// key.
    ///
    /// If a task previously existed in the `JoinMap` for this key, that task
    /// will be cancelled and replaced with the new one. The previous task will
    /// be removed from the `JoinMap`; a subsequent call to [`join_next`] will
    /// *not* return a cancelled [`JoinError`] for that task.
    ///
    /// # Panics
    ///
    /// This method panics if called outside of a Tokio runtime.
    ///
    /// [`join_next`]: Self::join_next
    #[track_caller]
    pub fn spawn<F>(&mut self, key: K, task: F)
    where
        F: Future<Output = V>,
        F: Send + 'static,
        V: Send,
    {
        let task = self.tasks.spawn(task);
        self.insert(key, task)
    }

    /// Spawn the provided task on the provided runtime and store it in this
    /// `JoinMap` with the provided key.
    ///
    /// If a task previously existed in the `JoinMap` for this key, that task
    /// will be cancelled and replaced with the new one. The previous task will
    /// be removed from the `JoinMap`; a subsequent call to [`join_next`] will
    /// *not* return a cancelled [`JoinError`] for that task.
    ///
    /// [`join_next`]: Self::join_next
    #[track_caller]
    pub fn spawn_on<F>(&mut self, key: K, task: F, handle: &Handle)
    where
        F: Future<Output = V>,
        F: Send + 'static,
        V: Send,
    {
        let task = self.tasks.spawn_on(task, handle);
        self.insert(key, task);
    }

    /// Spawn the blocking code on the blocking threadpool and store it in this `JoinMap` with the provided
    /// key.
    ///
    /// If a task previously existed in the `JoinMap` for this key, that task
    /// will be cancelled and replaced with the new one. The previous task will
    /// be removed from the `JoinMap`; a subsequent call to [`join_next`] will
    /// *not* return a cancelled [`JoinError`] for that task.
    ///
    /// Note that blocking tasks cannot be cancelled after execution starts.
    /// Replaced blocking tasks will still run to completion if the task has begun
    /// to execute when it is replaced. A blocking task which is replaced before
    /// it has been scheduled on a blocking worker thread will be cancelled.
    ///
    /// # Panics
    ///
    /// This method panics if called outside of a Tokio runtime.
    ///
    /// [`join_next`]: Self::join_next
    #[track_caller]
    pub fn spawn_blocking<F>(&mut self, key: K, f: F)
    where
        F: FnOnce() -> V,
        F: Send + 'static,
        V: Send,
    {
        let task = self.tasks.spawn_blocking(f);
        self.insert(key, task)
    }

    /// Spawn the blocking code on the blocking threadpool of the provided runtime and store it in this
    /// `JoinMap` with the provided key.
    ///
    /// If a task previously existed in the `JoinMap` for this key, that task
    /// will be cancelled and replaced with the new one. The previous task will
    /// be removed from the `JoinMap`; a subsequent call to [`join_next`] will
    /// *not* return a cancelled [`JoinError`] for that task.
    ///
    /// Note that blocking tasks cannot be cancelled after execution starts.
    /// Replaced blocking tasks will still run to completion if the task has begun
    /// to execute when it is replaced. A blocking task which is replaced before
    /// it has been scheduled on a blocking worker thread will be cancelled.
    ///
    /// [`join_next`]: Self::join_next
    #[track_caller]
    pub fn spawn_blocking_on<F>(&mut self, key: K, f: F, handle: &Handle)
    where
        F: FnOnce() -> V,
        F: Send + 'static,
        V: Send,
    {
        let task = self.tasks.spawn_blocking_on(f, handle);
        self.insert(key, task);
    }

    /// Spawn the provided task on the current [`LocalSet`] and store it in this
    /// `JoinMap` with the provided key.
    ///
    /// If a task previously existed in the `JoinMap` for this key, that task
    /// will be cancelled and replaced with the new one. The previous task will
    /// be removed from the `JoinMap`; a subsequent call to [`join_next`] will
    /// *not* return a cancelled [`JoinError`] for that task.
    ///
    /// # Panics
    ///
    /// This method panics if it is called outside of a `LocalSet`.
    ///
    /// [`LocalSet`]: tokio::task::LocalSet
    /// [`join_next`]: Self::join_next
    #[track_caller]
    pub fn spawn_local<F>(&mut self, key: K, task: F)
    where
        F: Future<Output = V>,
        F: 'static,
    {
        let task = self.tasks.spawn_local(task);
        self.insert(key, task);
    }

    /// Spawn the provided task on the provided [`LocalSet`] and store it in
    /// this `JoinMap` with the provided key.
    ///
    /// If a task previously existed in the `JoinMap` for this key, that task
    /// will be cancelled and replaced with the new one. The previous task will
    /// be removed from the `JoinMap`; a subsequent call to [`join_next`] will
    /// *not* return a cancelled [`JoinError`] for that task.
    ///
    /// [`LocalSet`]: tokio::task::LocalSet
    /// [`join_next`]: Self::join_next
    #[track_caller]
    pub fn spawn_local_on<F>(&mut self, key: K, task: F, local_set: &LocalSet)
    where
        F: Future<Output = V>,
        F: 'static,
    {
        let task = self.tasks.spawn_local_on(task, local_set);
        self.insert(key, task)
    }

    fn insert(&mut self, mut key: K, mut abort: AbortHandle) {
        let hash_builder = self.hashes_by_task.hasher();
        let hash = hash_one(hash_builder, &key);
        let id = abort.id();

        // Insert the new key into the map of tasks by keys.
        let entry =
            self.tasks_by_key
                .entry(hash, |(k, _)| *k == key, |(k, _)| hash_one(hash_builder, k));
        match entry {
            Entry::Occupied(occ) => {
                // There was a previous task spawned with the same key! Cancel
                // that task, and remove its ID from the map of hashes by task IDs.
                (key, abort) = std::mem::replace(occ.into_mut(), (key, abort));

                // Remove the old task ID.
                let _prev_hash = self.hashes_by_task.remove(&abort.id());
                debug_assert_eq!(Some(hash), _prev_hash);

                // Associate the key's hash with the new task's ID, for looking up tasks by ID.
                let _prev = self.hashes_by_task.insert(id, hash);
                debug_assert!(_prev.is_none(), "no prior task should have had the same ID");

                // Note: it's important to drop `key` and abort the task here.
                // This defends against any panics during drop handling for causing inconsistent state.
                abort.abort();
                drop(key);
            }
            Entry::Vacant(vac) => {
                vac.insert((key, abort));

                // Associate the key's hash with this task's ID, for looking up tasks by ID.
                let _prev = self.hashes_by_task.insert(id, hash);
                debug_assert!(_prev.is_none(), "no prior task should have had the same ID");
            }
        };
    }

    /// Waits until one of the tasks in the map completes and returns its
    /// output, along with the key corresponding to that task.
    ///
    /// Returns `None` if the map is empty.
    ///
    /// # Cancel Safety
    ///
    /// This method is cancel safe. If `join_next` is used as the event in a [`tokio::select!`]
    /// statement and some other branch completes first, it is guaranteed that no tasks were
    /// removed from this `JoinMap`.
    ///
    /// # Returns
    ///
    /// This function returns:
    ///
    ///  * `Some((key, Ok(value)))` if one of the tasks in this `JoinMap` has
    ///    completed. The `value` is the return value of that ask, and `key` is
    ///    the key associated with the task.
    ///  * `Some((key, Err(err))` if one of the tasks in this `JoinMap` has
    ///    panicked or been aborted. `key` is the key associated  with the task
    ///    that panicked or was aborted.
    ///  * `None` if the `JoinMap` is empty.
    ///
    /// [`tokio::select!`]: tokio::select
    pub async fn join_next(&mut self) -> Option<(K, Result<V, JoinError>)> {
        loop {
            let (res, id) = match self.tasks.join_next_with_id().await {
                Some(Ok((id, output))) => (Ok(output), id),
                Some(Err(e)) => {
                    let id = e.id();
                    (Err(e), id)
                }
                None => return None,
            };
            if let Some(key) = self.remove_by_id(id) {
                break Some((key, res));
            }
        }
    }

    /// Aborts all tasks and waits for them to finish shutting down.
    ///
    /// Calling this method is equivalent to calling [`abort_all`] and then calling [`join_next`] in
    /// a loop until it returns `None`.
    ///
    /// This method ignores any panics in the tasks shutting down. When this call returns, the
    /// `JoinMap` will be empty.
    ///
    /// [`abort_all`]: fn@Self::abort_all
    /// [`join_next`]: fn@Self::join_next
    pub async fn shutdown(&mut self) {
        self.abort_all();
        while self.join_next().await.is_some() {}
    }

    /// Abort the task corresponding to the provided `key`.
    ///
    /// If this `JoinMap` contains a task corresponding to `key`, this method
    /// will abort that task and return `true`. Otherwise, if no task exists for
    /// `key`, this method returns `false`.
    ///
    /// # Examples
    ///
    /// Aborting a task by key:
    ///
    /// ```
    /// use tokio_util::task::JoinMap;
    ///
    /// # #[tokio::main]
    /// # async fn main() {
    /// let mut map = JoinMap::new();
    ///
    /// map.spawn("hello world", async move { /* ... */ });
    /// map.spawn("goodbye world", async move { /* ... */});
    ///
    /// // Look up the "goodbye world" task in the map and abort it.
    /// map.abort("goodbye world");
    ///
    /// while let Some((key, res)) = map.join_next().await {
    ///     if key == "goodbye world" {
    ///         // The aborted task should complete with a cancelled `JoinError`.
    ///         assert!(res.unwrap_err().is_cancelled());
    ///     } else {
    ///         // Other tasks should complete normally.
    ///         assert!(res.is_ok());
    ///     }
    /// }
    /// # }
    /// ```
    ///
    /// `abort` returns `true` if a task was aborted:
    /// ```
    /// use tokio_util::task::JoinMap;
    ///
    /// # #[tokio::main]
    /// # async fn main() {
    /// let mut map = JoinMap::new();
    ///
    /// map.spawn("hello world", async move { /* ... */ });
    /// map.spawn("goodbye world", async move { /* ... */});
    ///
    /// // A task for the key "goodbye world" should exist in the map:
    /// assert!(map.abort("goodbye world"));
    ///
    /// // Aborting a key that does not exist will return `false`:
    /// assert!(!map.abort("goodbye universe"));
    /// # }
    /// ```
    pub fn abort<Q>(&mut self, key: &Q) -> bool
    where
        Q: ?Sized + Hash + Eq,
        K: Borrow<Q>,
    {
        match self.get_by_key(key) {
            Some((_, handle)) => {
                handle.abort();
                true
            }
            None => false,
        }
    }

    /// Aborts all tasks with keys matching `predicate`.
    ///
    /// `predicate` is a function called with a reference to each key in the
    /// map. If it returns `true` for a given key, the corresponding task will
    /// be cancelled.
    ///
    /// # Examples
    /// ```
    /// use tokio_util::task::JoinMap;
    ///
    /// # // use the current thread rt so that spawned tasks don't
    /// # // complete in the background before they can be aborted.
    /// # #[tokio::main(flavor = "current_thread")]
    /// # async fn main() {
    /// let mut map = JoinMap::new();
    ///
    /// map.spawn("hello world", async move {
    ///     // ...
    ///     # tokio::task::yield_now().await; // don't complete immediately, get aborted!
    /// });
    /// map.spawn("goodbye world", async move {
    ///     // ...
    ///     # tokio::task::yield_now().await; // don't complete immediately, get aborted!
    /// });
    /// map.spawn("hello san francisco", async move {
    ///     // ...
    ///     # tokio::task::yield_now().await; // don't complete immediately, get aborted!
    /// });
    /// map.spawn("goodbye universe", async move {
    ///     // ...
    ///     # tokio::task::yield_now().await; // don't complete immediately, get aborted!
    /// });
    ///
    /// // Abort all tasks whose keys begin with "goodbye"
    /// map.abort_matching(|key| key.starts_with("goodbye"));
    ///
    /// let mut seen = 0;
    /// while let Some((key, res)) = map.join_next().await {
    ///     seen += 1;
    ///     if key.starts_with("goodbye") {
    ///         // The aborted task should complete with a cancelled `JoinError`.
    ///         assert!(res.unwrap_err().is_cancelled());
    ///     } else {
    ///         // Other tasks should complete normally.
    ///         assert!(key.starts_with("hello"));
    ///         assert!(res.is_ok());
    ///     }
    /// }
    ///
    /// // All spawned tasks should have completed.
    /// assert_eq!(seen, 4);
    /// # }
    /// ```
    pub fn abort_matching(&mut self, mut predicate: impl FnMut(&K) -> bool) {
        // Note: this method iterates over the tasks and keys *without* removing
        // any entries, so that the keys from aborted tasks can still be
        // returned when calling `join_next` in the future.
        for (key, task) in &self.tasks_by_key {
            if predicate(key) {
                task.abort();
            }
        }
    }

    /// Returns an iterator visiting all keys in this `JoinMap` in arbitrary order.
    ///
    /// If a task has completed, but its output hasn't yet been consumed by a
    /// call to [`join_next`], this method will still return its key.
    ///
    /// [`join_next`]: fn@Self::join_next
    pub fn keys(&self) -> JoinMapKeys<'_, K, V> {
        JoinMapKeys {
            iter: self.tasks_by_key.iter(),
            _value: PhantomData,
        }
    }

    /// Returns `true` if this `JoinMap` contains a task for the provided key.
    ///
    /// If the task has completed, but its output hasn't yet been consumed by a
    /// call to [`join_next`], this method will still return `true`.
    ///
    /// [`join_next`]: fn@Self::join_next
    pub fn contains_key<Q>(&self, key: &Q) -> bool
    where
        Q: ?Sized + Hash + Eq,
        K: Borrow<Q>,
    {
        self.get_by_key(key).is_some()
    }

    /// Returns `true` if this `JoinMap` contains a task with the provided
    /// [task ID].
    ///
    /// If the task has completed, but its output hasn't yet been consumed by a
    /// call to [`join_next`], this method will still return `true`.
    ///
    /// [`join_next`]: fn@Self::join_next
    /// [task ID]: tokio::task::Id
    pub fn contains_task(&self, task: &Id) -> bool {
        self.hashes_by_task.contains_key(task)
    }

    /// Reserves capacity for at least `additional` more tasks to be spawned
    /// on this `JoinMap` without reallocating for the map of task keys. The
    /// collection may reserve more space to avoid frequent reallocations.
    ///
    /// Note that spawning a task will still cause an allocation for the task
    /// itself.
    ///
    /// # Panics
    ///
    /// Panics if the new allocation size overflows [`usize`].
    ///
    /// # Examples
    ///
    /// ```
    /// use tokio_util::task::JoinMap;
    ///
    /// let mut map: JoinMap<&str, i32> = JoinMap::new();
    /// map.reserve(10);
    /// ```
    #[inline]
    pub fn reserve(&mut self, additional: usize) {
        let hash_builder = self.hashes_by_task.hasher();
        self.tasks_by_key
            .reserve(additional, |(k, _)| hash_one(hash_builder, k));
        self.hashes_by_task.reserve(additional);
    }

    /// Shrinks the capacity of the `JoinMap` as much as possible. It will drop
    /// down as much as possible while maintaining the internal rules
    /// and possibly leaving some space in accordance with the resize policy.
    ///
    /// # Examples
    ///
    /// ```
    /// # #[tokio::main]
    /// # async fn main() {
    /// use tokio_util::task::JoinMap;
    ///
    /// let mut map: JoinMap<i32, i32> = JoinMap::with_capacity(100);
    /// map.spawn(1, async move { 2 });
    /// map.spawn(3, async move { 4 });
    /// assert!(map.capacity() >= 100);
    /// map.shrink_to_fit();
    /// assert!(map.capacity() >= 2);
    /// # }
    /// ```
    #[inline]
    pub fn shrink_to_fit(&mut self) {
        self.hashes_by_task.shrink_to_fit();
        let hash_builder = self.hashes_by_task.hasher();
        self.tasks_by_key
            .shrink_to_fit(|(k, _)| hash_one(hash_builder, k));
    }

    /// Shrinks the capacity of the map with a lower limit. It will drop
    /// down no lower than the supplied limit while maintaining the internal rules
    /// and possibly leaving some space in accordance with the resize policy.
    ///
    /// If the current capacity is less than the lower limit, this is a no-op.
    ///
    /// # Examples
    ///
    /// ```
    /// # #[tokio::main]
    /// # async fn main() {
    /// use tokio_util::task::JoinMap;
    ///
    /// let mut map: JoinMap<i32, i32> = JoinMap::with_capacity(100);
    /// map.spawn(1, async move { 2 });
    /// map.spawn(3, async move { 4 });
    /// assert!(map.capacity() >= 100);
    /// map.shrink_to(10);
    /// assert!(map.capacity() >= 10);
    /// map.shrink_to(0);
    /// assert!(map.capacity() >= 2);
    /// # }
    /// ```
    #[inline]
    pub fn shrink_to(&mut self, min_capacity: usize) {
        self.hashes_by_task.shrink_to(min_capacity);
        let hash_builder = self.hashes_by_task.hasher();
        self.tasks_by_key
            .shrink_to(min_capacity, |(k, _)| hash_one(hash_builder, k))
    }

    /// Look up a task in the map by its key, returning the key and abort handle.
    fn get_by_key<'map, Q>(&'map self, key: &Q) -> Option<&'map (K, AbortHandle)>
    where
        Q: ?Sized + Hash + Eq,
        K: Borrow<Q>,
    {
        let hash_builder = self.hashes_by_task.hasher();
        let hash = hash_one(hash_builder, key);
        self.tasks_by_key.find(hash, |(k, _)| k.borrow() == key)
    }

    /// Remove a task from the map by ID, returning the key for that task.
    fn remove_by_id(&mut self, id: Id) -> Option<K> {
        // Get the hash for the given ID.
        let hash = self.hashes_by_task.remove(&id)?;

        // Remove the entry for that hash.
        let entry = self
            .tasks_by_key
            .find_entry(hash, |(_, abort)| abort.id() == id);
        let (key, _) = match entry {
            Ok(entry) => entry.remove().0,
            _ => return None,
        };
        self.hashes_by_task.remove(&id);
        Some(key)
    }
}

/// Returns the hash for a given key.
#[inline]
fn hash_one<S, Q>(hash_builder: &S, key: &Q) -> u64
where
    Q: ?Sized + Hash,
    S: BuildHasher,
{
    let mut hasher = hash_builder.build_hasher();
    key.hash(&mut hasher);
    hasher.finish()
}

impl<K, V, S> JoinMap<K, V, S>
where
    V: 'static,
{
    /// Aborts all tasks on this `JoinMap`.
    ///
    /// This does not remove the tasks from the `JoinMap`. To wait for the tasks to complete
    /// cancellation, you should call `join_next` in a loop until the `JoinMap` is empty.
    pub fn abort_all(&mut self) {
        self.tasks.abort_all()
    }

    /// Removes all tasks from this `JoinMap` without aborting them.
    ///
    /// The tasks removed by this call will continue to run in the background even if the `JoinMap`
    /// is dropped. They may still be aborted by key.
    pub fn detach_all(&mut self) {
        self.tasks.detach_all();
        self.tasks_by_key.clear();
        self.hashes_by_task.clear();
    }
}

// Hand-written `fmt::Debug` implementation in order to avoid requiring `V:
// Debug`, since no value is ever actually stored in the map.
impl<K: fmt::Debug, V, S> fmt::Debug for JoinMap<K, V, S> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        // format the task keys and abort handles a little nicer by just
        // printing the key and task ID pairs, without format the `Key` struct
        // itself or the `AbortHandle`, which would just format the task's ID
        // again.
        struct KeySet<'a, K: fmt::Debug>(&'a HashTable<(K, AbortHandle)>);
        impl<K: fmt::Debug> fmt::Debug for KeySet<'_, K> {
            fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                f.debug_map()
                    .entries(self.0.iter().map(|(key, abort)| (key, abort.id())))
                    .finish()
            }
        }

        f.debug_struct("JoinMap")
            // The `tasks_by_key` map is the only one that contains information
            // that's really worth formatting for the user, since it contains
            // the tasks' keys and IDs. The other fields are basically
            // implementation details.
            .field("tasks", &KeySet(&self.tasks_by_key))
            .finish()
    }
}

impl<K, V> Default for JoinMap<K, V> {
    fn default() -> Self {
        Self::new()
    }
}

/// An iterator over the keys of a [`JoinMap`].
#[derive(Debug, Clone)]
pub struct JoinMapKeys<'a, K, V> {
    iter: hashbrown::hash_table::Iter<'a, (K, AbortHandle)>,
    /// To make it easier to change `JoinMap` in the future, keep V as a generic
    /// parameter.
    _value: PhantomData<&'a V>,
}

impl<'a, K, V> Iterator for JoinMapKeys<'a, K, V> {
    type Item = &'a K;

    fn next(&mut self) -> Option<&'a K> {
        self.iter.next().map(|(key, _)| key)
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        self.iter.size_hint()
    }
}

impl<'a, K, V> ExactSizeIterator for JoinMapKeys<'a, K, V> {
    fn len(&self) -> usize {
        self.iter.len()
    }
}

impl<'a, K, V> std::iter::FusedIterator for JoinMapKeys<'a, K, V> {}