src/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
        ///
        /// # Examples
        ///
        /// Basic join with two branches
        ///
        /// ```
        /// async fn do_stuff_async() {
        ///     // async work
        /// }
        ///
        /// async fn more_async_work() {
        ///     // more here
        /// }
        ///
        /// #[tokio::main]
        /// async fn main() {
        ///     let (first, second) = tokio::join!(
        ///         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 {
    ($($future:expr),*) => { unimplemented!() }
}}

#[cfg(not(doc))]
doc! {macro_rules! join {
    (@ {
        // 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};
        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, a different future will be polled first
        // to ensure every future passed to join! gets a chance to make progress even if
        // one of the futures consumes the whole budget.
        //
        // This is number of futures that will be skipped in the first loop
        // iteration the next time.
        let mut skip_next_time: u32 = 0;

        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 = skip_next_time;

            skip_next_time = if skip + 1 == COUNT { 0 } else { skip + 1 };

            // 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 =====

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

    // ===== Entry point =====

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

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