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use crate::runtime::BOX_FUTURE_THRESHOLD;
use crate::task::JoinHandle;
use std::future::Future;
cfg_rt! {
/// Spawns a new asynchronous task, returning a
/// [`JoinHandle`](super::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`](crate::runtime::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`](crate::runtime::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.
///
/// ```no_run
/// use tokio::net::{TcpListener, TcpStream};
///
/// use std::io;
///
/// async fn process(socket: TcpStream) {
/// // ...
/// # drop(socket);
/// }
///
/// #[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.
/// ```
/// # #[tokio::main(flavor = "current_thread")] async fn main() {
/// 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`].
///
/// [`JoinSet`]: struct@crate::task::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
/// # drop(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:
///
/// ```compile_fail
/// use tokio::task;
///
/// use std::rc::Rc;
///
/// fn use_rc(rc: Rc<()>) {
/// // Do stuff w/ rc
/// # drop(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:
///
/// ```text
/// `[... some type ...]` cannot be sent between threads safely
/// ```
///
/// or:
///
/// ```text
/// error[E0391]: cycle detected when processing `main`
/// ```
#[track_caller]
pub fn spawn<F>(future: F) -> JoinHandle<F::Output>
where
F: Future + Send + 'static,
F::Output: Send + 'static,
{
// preventing stack overflows on debug mode, by quickly sending the
// task to the heap.
if cfg!(debug_assertions) && std::mem::size_of::<F>() > BOX_FUTURE_THRESHOLD {
spawn_inner(Box::pin(future), None)
} else {
spawn_inner(future, None)
}
}
#[track_caller]
pub(super) fn spawn_inner<T>(future: T, name: Option<&str>) -> JoinHandle<T::Output>
where
T: Future + Send + 'static,
T::Output: Send + 'static,
{
use crate::runtime::{context, task};
#[cfg(all(
tokio_unstable,
tokio_taskdump,
feature = "rt",
target_os = "linux",
any(
target_arch = "aarch64",
target_arch = "x86",
target_arch = "x86_64"
)
))]
let future = task::trace::Trace::root(future);
let id = task::Id::next();
let task = crate::util::trace::task(future, "task", name, id.as_u64());
match context::with_current(|handle| handle.spawn(task, id)) {
Ok(join_handle) => join_handle,
Err(e) => panic!("{}", e),
}
}
}