tokio/macros/

join.rs

macro_rules! doc {
    ($join:item) => {
        /// Waits on multiple concurrent branches, returning when **all** branches
        /// complete.
        ///
        /// The `join!` macro must be used inside of async functions, closures, and
        /// blocks.
        ///
        /// The `join!` macro takes a list of async expressions and evaluates them
        /// concurrently on the same task. Each async expression evaluates to a future
        /// and the futures from each expression are multiplexed on the current task.
        ///
        /// When working with async expressions returning `Result`, `join!` will wait
        /// for **all** branches complete regardless if any complete with `Err`. Use
        /// [`try_join!`] to return early when `Err` is encountered.
        ///
        /// [`try_join!`]: crate::try_join
        ///
        /// # Notes
        ///
        /// The supplied futures are stored inline and do not require allocating a
        /// `Vec`.
        ///
        /// ## Runtime characteristics
        ///
        /// By running all async expressions on the current task, the expressions are
        /// able to run **concurrently** but not in **parallel**. This means all
        /// expressions are run on the same thread and if one branch blocks the thread,
        /// all other expressions will be unable to continue. If parallelism is
        /// required, spawn each async expression using [`tokio::spawn`] and pass the
        /// join handle to `join!`.
        ///
        /// [`tokio::spawn`]: crate::spawn
        ///
        /// ## Fairness
        ///
        /// By default, `join!`'s generated future rotates which contained
        /// future is polled first whenever it is woken.
        ///
        /// This behavior can be overridden by adding `biased;` to the beginning of the
        /// macro usage. See the examples for details. This will cause `join` to poll
        /// the futures in the order they appear from top to bottom.
        ///
        /// You may want this if your futures may interact in a way where known polling order is significant.
        ///
        /// But there is an important caveat to this mode. It becomes your responsibility
        /// to ensure that the polling order of your futures is fair. If for example you
        /// are joining a stream and a shutdown future, and the stream has a
        /// huge volume of messages that takes a long time to finish processing per poll, you should
        /// place the shutdown future earlier in the `join!` list to ensure that it is
        /// always polled, and will not be delayed due to the stream future taking a long time to return
        /// `Poll::Pending`.
        ///
        /// # Examples
        ///
        /// Basic join with two branches
        ///
        /// ```
        /// async fn do_stuff_async() {
        ///     // async work
        /// }
        ///
        /// async fn more_async_work() {
        ///     // more here
        /// }
        ///
        /// # #[tokio::main(flavor = "current_thread")]
        /// # async fn main() {
        /// let (first, second) = tokio::join!(
        ///     do_stuff_async(),
        ///     more_async_work());
        ///
        /// // do something with the values
        /// # }
        /// ```
        ///
        /// Using the `biased;` mode to control polling order.
        ///
        /// ```
        /// # #[cfg(not(target_family = "wasm"))]
        /// # {
        /// async fn do_stuff_async() {
        ///     // async work
        /// }
        ///
        /// async fn more_async_work() {
        ///     // more here
        /// }
        ///
        /// # #[tokio::main(flavor = "current_thread")]
        /// # async fn main() {
        /// let (first, second) = tokio::join!(
        ///     biased;
        ///     do_stuff_async(),
        ///     more_async_work()
        /// );
        ///
        /// // do something with the values
        /// # }
        /// # }
        /// ```

        #[macro_export]
        #[cfg_attr(docsrs, doc(cfg(feature = "macros")))]
        $join
    };
}

#[cfg(doc)]
doc! {macro_rules! join {
    ($(biased;)? $($future:expr),*) => { unimplemented!() }
}}

#[cfg(not(doc))]
doc! {macro_rules! join {
    (@ {
        // Type of rotator that controls which inner future to start with
        // when polling our output future.
        rotator_select=$rotator_select:ty;

        // One `_` for each branch in the `join!` macro. This is not used once
        // normalization is complete.
        ( $($count:tt)* )

        // The expression `0+1+1+ ... +1` equal to the number of branches.
        ( $($total:tt)* )

        // Normalized join! branches
        $( ( $($skip:tt)* ) $e:expr, )*

    }) => {{
        use $crate::macros::support::{maybe_done, poll_fn, Future, Pin, RotatorSelect};
        use $crate::macros::support::Poll::{Ready, Pending};

        // Safety: nothing must be moved out of `futures`. This is to satisfy
        // the requirement of `Pin::new_unchecked` called below.
        //
        // We can't use the `pin!` macro for this because `futures` is a tuple
        // and the standard library provides no way to pin-project to the fields
        // of a tuple.
        let mut futures = ( $( maybe_done($e), )* );

        // This assignment makes sure that the `poll_fn` closure only has a
        // reference to the futures, instead of taking ownership of them. This
        // mitigates the issue described in
        // <https://internals.rust-lang.org/t/surprising-soundness-trouble-around-pollfn/17484>
        let mut futures = &mut futures;

        // Each time the future created by poll_fn is polled, if not using biased mode,
        // a different future is polled first to ensure every future passed to join!
        // can make progress even if one of the futures consumes the whole budget.
        let mut rotator = <$rotator_select as RotatorSelect>::Rotator::<{$($total)*}>::default();

        poll_fn(move |cx| {
            const COUNT: u32 = $($total)*;

            let mut is_pending = false;
            let mut to_run = COUNT;

            // The number of futures that will be skipped in the first loop iteration.
            let mut skip = rotator.num_skip();

            // This loop runs twice and the first `skip` futures
            // are not polled in the first iteration.
            loop {
            $(
                if skip == 0 {
                    if to_run == 0 {
                        // Every future has been polled
                        break;
                    }
                    to_run -= 1;

                    // Extract the future for this branch from the tuple.
                    let ( $($skip,)* fut, .. ) = &mut *futures;

                    // Safety: future is stored on the stack above
                    // and never moved.
                    let mut fut = unsafe { Pin::new_unchecked(fut) };

                    // Try polling
                    if fut.poll(cx).is_pending() {
                        is_pending = true;
                    }
                } else {
                    // Future skipped, one less future to skip in the next iteration
                    skip -= 1;
                }
            )*
            }

            if is_pending {
                Pending
            } else {
                Ready(($({
                    // Extract the future for this branch from the tuple.
                    let ( $($skip,)* fut, .. ) = &mut futures;

                    // Safety: future is stored on the stack above
                    // and never moved.
                    let mut fut = unsafe { Pin::new_unchecked(fut) };

                    fut.take_output().expect("expected completed future")
                },)*))
            }
        }).await
    }};

    // ===== Normalize =====

    (@ { rotator_select=$rotator_select:ty; ( $($s:tt)* ) ( $($n:tt)* ) $($t:tt)* } $e:expr, $($r:tt)* ) => {
        $crate::join!(@{ rotator_select=$rotator_select; ($($s)* _) ($($n)* + 1) $($t)* ($($s)*) $e, } $($r)*)
    };

    // ===== Entry point =====
    ( biased; $($e:expr),+ $(,)?) => {
        $crate::join!(@{ rotator_select=$crate::macros::support::SelectBiased; () (0) } $($e,)*)
    };

    ( $($e:expr),+ $(,)?) => {
        $crate::join!(@{ rotator_select=$crate::macros::support::SelectNormal; () (0) } $($e,)*)
    };

    (biased;) => { async {}.await };

    () => { async {}.await }
}}

/// Helper trait to select which type of `Rotator` to use.
// We need this to allow specifying a const generic without
// colliding with caller const names due to macro hygiene.
pub trait RotatorSelect {
    type Rotator<const COUNT: u32>: Default;
}

/// Marker type indicating that the starting branch should
/// rotate each poll.
#[derive(Debug)]
pub struct SelectNormal;
/// Marker type indicating that the starting branch should
/// be the first declared branch each poll.
#[derive(Debug)]
pub struct SelectBiased;

impl RotatorSelect for SelectNormal {
    type Rotator<const COUNT: u32> = Rotator<COUNT>;
}

impl RotatorSelect for SelectBiased {
    type Rotator<const COUNT: u32> = BiasedRotator;
}

/// Rotates by one each [`Self::num_skip`] call up to COUNT - 1.
#[derive(Default, Debug)]
pub struct Rotator<const COUNT: u32> {
    next: u32,
}

impl<const COUNT: u32> Rotator<COUNT> {
    /// Rotates by one each [`Self::num_skip`] call up to COUNT - 1
    #[inline]
    pub fn num_skip(&mut self) -> u32 {
        let num_skip = self.next;
        self.next += 1;
        if self.next == COUNT {
            self.next = 0;
        }
        num_skip
    }
}

/// [`Self::num_skip`] always returns 0.
#[derive(Default, Debug)]
pub struct BiasedRotator {}

impl BiasedRotator {
    /// Always returns 0.
    #[inline]
    pub fn num_skip(&mut self) -> u32 {
        0
    }
}