Function forest_filecoin::libp2p_bitswap::task::spawn
source · pub fn spawn<F>(future: F) -> JoinHandle<<F as Future>::Output>
Expand description
Spawns a new asynchronous task, returning a
JoinHandle
for it.
The provided future will start running in the background immediately
when spawn
is called, even if you don’t await the returned
JoinHandle
.
Spawning a task enables the task to execute concurrently to other tasks. The
spawned task may execute on the current thread, or it may be sent to a
different thread to be executed. The specifics depend on the current
Runtime
configuration.
It is guaranteed that spawn will not synchronously poll the task being spawned. This means that calling spawn while holding a lock does not pose a risk of deadlocking with the spawned task.
There is no guarantee that a spawned task will execute to completion. When a runtime is shutdown, all outstanding tasks are dropped, regardless of the lifecycle of that task.
This function must be called from the context of a Tokio runtime. Tasks running on
the Tokio runtime are always inside its context, but you can also enter the context
using the Runtime::enter
method.
§Examples
In this example, a server is started and spawn
is used to start a new task
that processes each received connection.
use tokio::net::{TcpListener, TcpStream};
use std::io;
async fn process(socket: TcpStream) {
// ...
}
#[tokio::main]
async fn main() -> io::Result<()> {
let listener = TcpListener::bind("127.0.0.1:8080").await?;
loop {
let (socket, _) = listener.accept().await?;
tokio::spawn(async move {
// Process each socket concurrently.
process(socket).await
});
}
}
To run multiple tasks in parallel and receive their results, join handles can be stored in a vector.
async fn my_background_op(id: i32) -> String {
let s = format!("Starting background task {}.", id);
println!("{}", s);
s
}
let ops = vec![1, 2, 3];
let mut tasks = Vec::with_capacity(ops.len());
for op in ops {
// This call will make them start running in the background
// immediately.
tasks.push(tokio::spawn(my_background_op(op)));
}
let mut outputs = Vec::with_capacity(tasks.len());
for task in tasks {
outputs.push(task.await.unwrap());
}
println!("{:?}", outputs);
This example pushes the tasks to outputs
in the order they were
started in. If you do not care about the ordering of the outputs, then
you can also use a JoinSet
.
§Panics
Panics if called from outside of the Tokio runtime.
§Using !Send
values from a task
The task supplied to spawn
must implement Send
. However, it is
possible to use !Send
values from the task as long as they only
exist between calls to .await
.
For example, this will work:
use tokio::task;
use std::rc::Rc;
fn use_rc(rc: Rc<()>) {
// Do stuff w/ rc
}
#[tokio::main]
async fn main() {
tokio::spawn(async {
// Force the `Rc` to stay in a scope with no `.await`
{
let rc = Rc::new(());
use_rc(rc.clone());
}
task::yield_now().await;
}).await.unwrap();
}
This will not work:
use tokio::task;
use std::rc::Rc;
fn use_rc(rc: Rc<()>) {
// Do stuff w/ rc
}
#[tokio::main]
async fn main() {
tokio::spawn(async {
let rc = Rc::new(());
task::yield_now().await;
use_rc(rc.clone());
}).await.unwrap();
}
Holding on to a !Send
value across calls to .await
will result in
an unfriendly compile error message similar to:
`[... some type ...]` cannot be sent between threads safely
or:
error[E0391]: cycle detected when processing `main`