/// Waits on multiple concurrent branches, returning when **all** branches
/// complete with `Ok(_)` or on the first `Err(_)`.
///
/// The `try_join!` macro must be used inside of async functions, closures, and
/// blocks.
///
/// Similar to [`join!`], the `try_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. The `try_join!` macro returns when **all**
/// branches return with `Ok` or when the **first** branch returns with `Err`.
///
/// [`join!`]: macro@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 `try_join!`.
///
/// [`tokio::spawn`]: crate::spawn
///
/// # Examples
///
/// Basic `try_join` with two branches.
///
/// ```
/// async fn do_stuff_async() -> Result<(), &'static str> {
/// // async work
/// # Ok(())
/// }
///
/// async fn more_async_work() -> Result<(), &'static str> {
/// // more here
/// # Ok(())
/// }
///
/// #[tokio::main]
/// async fn main() {
/// let res = tokio::try_join!(
/// do_stuff_async(),
/// more_async_work());
///
/// match res {
/// Ok((first, second)) => {
/// // do something with the values
/// }
/// Err(err) => {
/// println!("processing failed; error = {}", err);
/// }
/// }
/// }
/// ```
///
/// Using `try_join!` with spawned tasks.
///
/// ```
/// use tokio::task::JoinHandle;
///
/// async fn do_stuff_async() -> Result<(), &'static str> {
/// // async work
/// # Err("failed")
/// }
///
/// async fn more_async_work() -> Result<(), &'static str> {
/// // more here
/// # Ok(())
/// }
///
/// async fn flatten<T>(handle: JoinHandle<Result<T, &'static str>>) -> Result<T, &'static str> {
/// match handle.await {
/// Ok(Ok(result)) => Ok(result),
/// Ok(Err(err)) => Err(err),
/// Err(err) => Err("handling failed"),
/// }
/// }
///
/// #[tokio::main]
/// async fn main() {
/// let handle1 = tokio::spawn(do_stuff_async());
/// let handle2 = tokio::spawn(more_async_work());
/// match tokio::try_join!(flatten(handle1), flatten(handle2)) {
/// Ok(val) => {
/// // do something with the values
/// }
/// Err(err) => {
/// println!("Failed with {}.", err);
/// # assert_eq!(err, "failed");
/// }
/// }
/// }
/// ```
#[macro_export]
#[cfg_attr(docsrs, doc(cfg(feature = "macros")))]
macro_rules! try_join {
(@ {
// One `_` for each branch in the `try_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 try_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.as_mut().poll(cx).is_pending() {
is_pending = true;
} else if fut.as_mut().output_mut().expect("expected completed future").is_err() {
return Ready(Err(fut.take_output().expect("expected completed future").err().unwrap
()))
}
} else {
// Future skipped, one less future to skip in the next iteration
skip -= 1;
}
)*
}
if is_pending {
Pending
} else {
Ready(Ok(($({
// 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")
.ok()
.expect("expected Ok(_)")
},)*)))
}
}).await
}};
// ===== Normalize =====
(@ { ( $($s:tt)* ) ( $($n:tt)* ) $($t:tt)* } $e:expr, $($r:tt)* ) => {
$crate::try_join!(@{ ($($s)* _) ($($n)* + 1) $($t)* ($($s)*) $e, } $($r)*)
};
// ===== Entry point =====
( $($e:expr),+ $(,)?) => {
$crate::try_join!(@{ () (0) } $($e,)*)
};
() => { async { Ok(()) }.await }
}
