[−][src]Macro tokio::select
Wait on multiple concurrent branches, returning when the first branch completes, cancelling the remaining branches.
The select!
macro must be used inside of async functions, closures, and
blocks.
The select!
macro accepts one or more branches with the following pattern:
<pattern> = <async expression> (, if <precondition>)? => <handler>,
Additionally, the select!
macro may include a single, optional else
branch, which evaluates if none of the other branches match their patterns:
else <expression>
The macro aggregates all <async expression>
expressions and runs them
concurrently on the current task. Once the first expression
completes with a value that matches its <pattern>
, the select!
macro
returns the result of evaluating the completed branch's <handler>
expression.
Additionally, each branch may include an optional if
precondition. This
precondition is evaluated before the <async expression>
. If the
precondition returns false
, the branch is entirely disabled. This
capability is useful when using select!
within a loop.
The complete lifecycle of a select!
expression is as follows:
- Evaluate all provided
<precondition>
expressions. If the precondition returnsfalse
, disable the branch for the remainder of the current call toselect!
. Re-enteringselect!
due to a loop clears the "disabled" state. - Aggregate the
<async expression>
s from each branch, including the disabled ones. If the branch is disabled,<async expression>
is still evaluated, but the resulting future is not polled. - Concurrently await on the results for all remaining
<async expression>
s. - Once an
<async expression>
returns a value, attempt to apply the value to the provided<pattern>
, if the pattern matches, evaluate<handler>
and return. If the pattern does not match, disable the current branch and for the remainder of the current call toselect!
. Continue from step 3. - If all branches are disabled, evaluate the
else
expression. If none is provided, panic.
Notes
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 select!
.
Avoid racy if
preconditions
Given that if
preconditions are used to disable select!
branches, some
caution must be used to avoid missing values.
For example, here is incorrect usage of sleep
with if
. The objective
is to repeatedly run an asynchronous task for up to 50 milliseconds.
However, there is a potential for the sleep
completion to be missed.
use tokio::time::{self, Duration}; async fn some_async_work() { // do work } #[tokio::main] async fn main() { let sleep = time::sleep(Duration::from_millis(50)); tokio::pin!(sleep); while !sleep.is_elapsed() { tokio::select! { _ = &mut sleep, if !sleep.is_elapsed() => { println!("operation timed out"); } _ = some_async_work() => { println!("operation completed"); } } } }
In the above example, sleep.is_elapsed()
may return true
even if
sleep.poll()
never returned Ready
. This opens up a potential race
condition where sleep
expires between the while !sleep.is_elapsed()
check and the call to select!
resulting in the some_async_work()
call to
run uninterrupted despite the sleep having elapsed.
One way to write the above example without the race would be:
use tokio::time::{self, Duration}; async fn some_async_work() { // do work } #[tokio::main] async fn main() { let sleep = time::sleep(Duration::from_millis(50)); tokio::pin!(sleep); loop { tokio::select! { _ = &mut sleep => { println!("operation timed out"); break; } _ = some_async_work() => { println!("operation completed"); } } } }
Fairness
select!
randomly picks a branch to check first. This provides some level
of fairness when calling select!
in a loop with branches that are always
ready.
Panics
select!
panics if all branches are disabled and there is no provided
else
branch. A branch is disabled when the provided if
precondition
returns false
or when the pattern does not match the result of `
Examples
Basic select with two branches.
async fn do_stuff_async() { // async work } async fn more_async_work() { // more here } #[tokio::main] async fn main() { tokio::select! { _ = do_stuff_async() => { println!("do_stuff_async() completed first") } _ = more_async_work() => { println!("more_async_work() completed first") } }; }
Basic stream selecting.
use tokio_stream::{self as stream, StreamExt}; #[tokio::main] async fn main() { let mut stream1 = stream::iter(vec![1, 2, 3]); let mut stream2 = stream::iter(vec![4, 5, 6]); let next = tokio::select! { v = stream1.next() => v.unwrap(), v = stream2.next() => v.unwrap(), }; assert!(next == 1 || next == 4); }
Collect the contents of two streams. In this example, we rely on pattern
matching and the fact that stream::iter
is "fused", i.e. once the stream
is complete, all calls to next()
return None
.
use tokio_stream::{self as stream, StreamExt}; #[tokio::main] async fn main() { let mut stream1 = stream::iter(vec![1, 2, 3]); let mut stream2 = stream::iter(vec![4, 5, 6]); let mut values = vec![]; loop { tokio::select! { Some(v) = stream1.next() => values.push(v), Some(v) = stream2.next() => values.push(v), else => break, } } values.sort(); assert_eq!(&[1, 2, 3, 4, 5, 6], &values[..]); }
Using the same future in multiple select!
expressions can be done by passing
a reference to the future. Doing so requires the future to be Unpin
. A
future can be made Unpin
by either using Box::pin
or stack pinning.
Here, a stream is consumed for at most 1 second.
use tokio_stream::{self as stream, StreamExt}; use tokio::time::{self, Duration}; #[tokio::main] async fn main() { let mut stream = stream::iter(vec![1, 2, 3]); let sleep = time::sleep(Duration::from_secs(1)); tokio::pin!(sleep); loop { tokio::select! { maybe_v = stream.next() => { if let Some(v) = maybe_v { println!("got = {}", v); } else { break; } } _ = &mut sleep => { println!("timeout"); break; } } } }
Joining two values using select!
.
use tokio::sync::oneshot; #[tokio::main] async fn main() { let (tx1, mut rx1) = oneshot::channel(); let (tx2, mut rx2) = oneshot::channel(); tokio::spawn(async move { tx1.send("first").unwrap(); }); tokio::spawn(async move { tx2.send("second").unwrap(); }); let mut a = None; let mut b = None; while a.is_none() || b.is_none() { tokio::select! { v1 = (&mut rx1), if a.is_none() => a = Some(v1.unwrap()), v2 = (&mut rx2), if b.is_none() => b = Some(v2.unwrap()), } } let res = (a.unwrap(), b.unwrap()); assert_eq!(res.0, "first"); assert_eq!(res.1, "second"); }