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use super::{current, park, Builder, JoinInner, Result, Thread};
use crate::fmt;
use crate::io;
use crate::marker::PhantomData;
use crate::panic::{catch_unwind, resume_unwind, AssertUnwindSafe};
use crate::sync::atomic::{AtomicBool, AtomicUsize, Ordering};
use crate::sync::Arc;
/// A scope to spawn scoped threads in.
///
/// See [`scope`] for details.
#[stable(feature = "scoped_threads", since = "1.63.0")]
pub struct Scope<'scope, 'env: 'scope> {
data: Arc<ScopeData>,
/// Invariance over 'scope, to make sure 'scope cannot shrink,
/// which is necessary for soundness.
///
/// Without invariance, this would compile fine but be unsound:
///
/// ```compile_fail,E0373
/// std::thread::scope(|s| {
/// s.spawn(|| {
/// let a = String::from("abcd");
/// s.spawn(|| println!("{a:?}")); // might run after `a` is dropped
/// });
/// });
/// ```
scope: PhantomData<&'scope mut &'scope ()>,
env: PhantomData<&'env mut &'env ()>,
}
/// An owned permission to join on a scoped thread (block on its termination).
///
/// See [`Scope::spawn`] for details.
#[stable(feature = "scoped_threads", since = "1.63.0")]
pub struct ScopedJoinHandle<'scope, T>(JoinInner<'scope, T>);
pub(super) struct ScopeData {
num_running_threads: AtomicUsize,
a_thread_panicked: AtomicBool,
main_thread: Thread,
}
impl ScopeData {
pub(super) fn increment_num_running_threads(&self) {
// We check for 'overflow' with usize::MAX / 2, to make sure there's no
// chance it overflows to 0, which would result in unsoundness.
if self.num_running_threads.fetch_add(1, Ordering::Relaxed) > usize::MAX / 2 {
// This can only reasonably happen by mem::forget()'ing a lot of ScopedJoinHandles.
self.decrement_num_running_threads(false);
panic!("too many running threads in thread scope");
}
}
pub(super) fn decrement_num_running_threads(&self, panic: bool) {
if panic {
self.a_thread_panicked.store(true, Ordering::Relaxed);
}
if self.num_running_threads.fetch_sub(1, Ordering::Release) == 1 {
self.main_thread.unpark();
}
}
}
/// Create a scope for spawning scoped threads.
///
/// The function passed to `scope` will be provided a [`Scope`] object,
/// through which scoped threads can be [spawned][`Scope::spawn`].
///
/// Unlike non-scoped threads, scoped threads can borrow non-`'static` data,
/// as the scope guarantees all threads will be joined at the end of the scope.
///
/// All threads spawned within the scope that haven't been manually joined
/// will be automatically joined before this function returns.
///
/// # Panics
///
/// If any of the automatically joined threads panicked, this function will panic.
///
/// If you want to handle panics from spawned threads,
/// [`join`][ScopedJoinHandle::join] them before the end of the scope.
///
/// # Example
///
/// ```
/// use std::thread;
///
/// let mut a = vec![1, 2, 3];
/// let mut x = 0;
///
/// thread::scope(|s| {
/// s.spawn(|| {
/// println!("hello from the first scoped thread");
/// // We can borrow `a` here.
/// dbg!(&a);
/// });
/// s.spawn(|| {
/// println!("hello from the second scoped thread");
/// // We can even mutably borrow `x` here,
/// // because no other threads are using it.
/// x += a[0] + a[2];
/// });
/// println!("hello from the main thread");
/// });
///
/// // After the scope, we can modify and access our variables again:
/// a.push(4);
/// assert_eq!(x, a.len());
/// ```
///
/// # Lifetimes
///
/// Scoped threads involve two lifetimes: `'scope` and `'env`.
///
/// The `'scope` lifetime represents the lifetime of the scope itself.
/// That is: the time during which new scoped threads may be spawned,
/// and also the time during which they might still be running.
/// Once this lifetime ends, all scoped threads are joined.
/// This lifetime starts within the `scope` function, before `f` (the argument to `scope`) starts.
/// It ends after `f` returns and all scoped threads have been joined, but before `scope` returns.
///
/// The `'env` lifetime represents the lifetime of whatever is borrowed by the scoped threads.
/// This lifetime must outlast the call to `scope`, and thus cannot be smaller than `'scope`.
/// It can be as small as the call to `scope`, meaning that anything that outlives this call,
/// such as local variables defined right before the scope, can be borrowed by the scoped threads.
///
/// The `'env: 'scope` bound is part of the definition of the `Scope` type.
#[track_caller]
#[stable(feature = "scoped_threads", since = "1.63.0")]
pub fn scope<'env, F, T>(f: F) -> T
where
F: for<'scope> FnOnce(&'scope Scope<'scope, 'env>) -> T,
{
// We put the `ScopeData` into an `Arc` so that other threads can finish their
// `decrement_num_running_threads` even after this function returns.
let scope = Scope {
data: Arc::new(ScopeData {
num_running_threads: AtomicUsize::new(0),
main_thread: current(),
a_thread_panicked: AtomicBool::new(false),
}),
env: PhantomData,
scope: PhantomData,
};
// Run `f`, but catch panics so we can make sure to wait for all the threads to join.
let result = catch_unwind(AssertUnwindSafe(|| f(&scope)));
// Wait until all the threads are finished.
while scope.data.num_running_threads.load(Ordering::Acquire) != 0 {
park();
}
// Throw any panic from `f`, or the return value of `f` if no thread panicked.
match result {
Err(e) => resume_unwind(e),
Ok(_) if scope.data.a_thread_panicked.load(Ordering::Relaxed) => {
panic!("a scoped thread panicked")
}
Ok(result) => result,
}
}
impl<'scope, 'env> Scope<'scope, 'env> {
/// Spawns a new thread within a scope, returning a [`ScopedJoinHandle`] for it.
///
/// Unlike non-scoped threads, threads spawned with this function may
/// borrow non-`'static` data from the outside the scope. See [`scope`] for
/// details.
///
/// The join handle provides a [`join`] method that can be used to join the spawned
/// thread. If the spawned thread panics, [`join`] will return an [`Err`] containing
/// the panic payload.
///
/// If the join handle is dropped, the spawned thread will implicitly joined at the
/// end of the scope. In that case, if the spawned thread panics, [`scope`] will
/// panic after all threads are joined.
///
/// This call will create a thread using default parameters of [`Builder`].
/// If you want to specify the stack size or the name of the thread, use
/// [`Builder::spawn_scoped`] instead.
///
/// # Panics
///
/// Panics if the OS fails to create a thread; use [`Builder::spawn_scoped`]
/// to recover from such errors.
///
/// [`join`]: ScopedJoinHandle::join
#[stable(feature = "scoped_threads", since = "1.63.0")]
pub fn spawn<F, T>(&'scope self, f: F) -> ScopedJoinHandle<'scope, T>
where
F: FnOnce() -> T + Send + 'scope,
T: Send + 'scope,
{
Builder::new().spawn_scoped(self, f).expect("failed to spawn thread")
}
}
impl Builder {
/// Spawns a new scoped thread using the settings set through this `Builder`.
///
/// Unlike [`Scope::spawn`], this method yields an [`io::Result`] to
/// capture any failure to create the thread at the OS level.
///
/// [`io::Result`]: crate::io::Result
///
/// # Panics
///
/// Panics if a thread name was set and it contained null bytes.
///
/// # Example
///
/// ```
/// use std::thread;
///
/// let mut a = vec![1, 2, 3];
/// let mut x = 0;
///
/// thread::scope(|s| {
/// thread::Builder::new()
/// .name("first".to_string())
/// .spawn_scoped(s, ||
/// {
/// println!("hello from the {:?} scoped thread", thread::current().name());
/// // We can borrow `a` here.
/// dbg!(&a);
/// })
/// .unwrap();
/// thread::Builder::new()
/// .name("second".to_string())
/// .spawn_scoped(s, ||
/// {
/// println!("hello from the {:?} scoped thread", thread::current().name());
/// // We can even mutably borrow `x` here,
/// // because no other threads are using it.
/// x += a[0] + a[2];
/// })
/// .unwrap();
/// println!("hello from the main thread");
/// });
///
/// // After the scope, we can modify and access our variables again:
/// a.push(4);
/// assert_eq!(x, a.len());
/// ```
#[stable(feature = "scoped_threads", since = "1.63.0")]
pub fn spawn_scoped<'scope, 'env, F, T>(
self,
scope: &'scope Scope<'scope, 'env>,
f: F,
) -> io::Result<ScopedJoinHandle<'scope, T>>
where
F: FnOnce() -> T + Send + 'scope,
T: Send + 'scope,
{
Ok(ScopedJoinHandle(unsafe { self.spawn_unchecked_(f, Some(scope.data.clone())) }?))
}
}
impl<'scope, T> ScopedJoinHandle<'scope, T> {
/// Extracts a handle to the underlying thread.
///
/// # Examples
///
/// ```
/// use std::thread;
///
/// thread::scope(|s| {
/// let t = s.spawn(|| {
/// println!("hello");
/// });
/// println!("thread id: {:?}", t.thread().id());
/// });
/// ```
#[must_use]
#[stable(feature = "scoped_threads", since = "1.63.0")]
pub fn thread(&self) -> &Thread {
&self.0.thread
}
/// Waits for the associated thread to finish.
///
/// This function will return immediately if the associated thread has already finished.
///
/// In terms of [atomic memory orderings], the completion of the associated
/// thread synchronizes with this function returning.
/// In other words, all operations performed by that thread
/// [happen before](https://doc.rust-lang.org/nomicon/atomics.html#data-accesses)
/// all operations that happen after `join` returns.
///
/// If the associated thread panics, [`Err`] is returned with the panic payload.
///
/// [atomic memory orderings]: crate::sync::atomic
///
/// # Examples
///
/// ```
/// use std::thread;
///
/// thread::scope(|s| {
/// let t = s.spawn(|| {
/// panic!("oh no");
/// });
/// assert!(t.join().is_err());
/// });
/// ```
#[stable(feature = "scoped_threads", since = "1.63.0")]
pub fn join(self) -> Result<T> {
self.0.join()
}
/// Checks if the associated thread has finished running its main function.
///
/// `is_finished` supports implementing a non-blocking join operation, by checking
/// `is_finished`, and calling `join` if it returns `false`. This function does not block. To
/// block while waiting on the thread to finish, use [`join`][Self::join].
///
/// This might return `true` for a brief moment after the thread's main
/// function has returned, but before the thread itself has stopped running.
/// However, once this returns `true`, [`join`][Self::join] can be expected
/// to return quickly, without blocking for any significant amount of time.
#[stable(feature = "scoped_threads", since = "1.63.0")]
pub fn is_finished(&self) -> bool {
Arc::strong_count(&self.0.packet) == 1
}
}
#[stable(feature = "scoped_threads", since = "1.63.0")]
impl fmt::Debug for Scope<'_, '_> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("Scope")
.field("num_running_threads", &self.data.num_running_threads.load(Ordering::Relaxed))
.field("a_thread_panicked", &self.data.a_thread_panicked.load(Ordering::Relaxed))
.field("main_thread", &self.data.main_thread)
.finish_non_exhaustive()
}
}
#[stable(feature = "scoped_threads", since = "1.63.0")]
impl<'scope, T> fmt::Debug for ScopedJoinHandle<'scope, T> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("ScopedJoinHandle").finish_non_exhaustive()
}
}