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Rollup merge of #92555 - m-ou-se:scoped-threads, r=Amanieu

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Implement RFC 3151: Scoped threads.

This implements https://github.com/rust-lang/rfcs/pull/3151

r? `@Amanieu`
This commit is contained in:
Matthias Krüger 2022-01-23 20:13:02 +01:00 committed by GitHub
commit bb260e8950
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GPG Key ID: 4AEE18F83AFDEB23
2 changed files with 386 additions and 31 deletions
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101
library/std/src/thread/mod.rs
View File

@ -180,6 +180,12 @@
#[macro_use]
mod local;
#[unstable(feature = "scoped_threads", issue = "93203")]
mod scoped;
#[unstable(feature = "scoped_threads", issue = "93203")]
pub use scoped::{scope, Scope, ScopedJoinHandle};
#[stable(feature = "rust1", since = "1.0.0")]
pub use self::local::{AccessError, LocalKey};
@ -446,6 +452,20 @@ pub unsafe fn spawn_unchecked<'a, F, T>(self, f: F) -> io::Result<JoinHandle<T>>
F: FnOnce() -> T,
F: Send + 'a,
T: Send + 'a,
{
Ok(JoinHandle(unsafe { self.spawn_unchecked_(f, None) }?))
}
unsafe fn spawn_unchecked_<'a, 'scope, F, T>(
self,
f: F,
scope_data: Option<&'scope scoped::ScopeData>,
) -> io::Result<JoinInner<'scope, T>>
where
F: FnOnce() -> T,
F: Send + 'a,
T: Send + 'a,
'scope: 'a,
{
let Builder { name, stack_size } = self;
@ -456,7 +476,8 @@ pub unsafe fn spawn_unchecked<'a, F, T>(self, f: F) -> io::Result<JoinHandle<T>>
}));
let their_thread = my_thread.clone();
let my_packet: Arc<UnsafeCell<Option<Result<T>>>> = Arc::new(UnsafeCell::new(None));
let my_packet: Arc<Packet<'scope, T>> =
Arc::new(Packet { scope: scope_data, result: UnsafeCell::new(None) });
let their_packet = my_packet.clone();
let output_capture = crate::io::set_output_capture(None);
@ -480,10 +501,14 @@ pub unsafe fn spawn_unchecked<'a, F, T>(self, f: F) -> io::Result<JoinHandle<T>>
// closure (it is an Arc<...>) and `my_packet` will be stored in the
// same `JoinInner` as this closure meaning the mutation will be
// safe (not modify it and affect a value far away).
unsafe { *their_packet.get() = Some(try_result) };
unsafe { *their_packet.result.get() = Some(try_result) };
};
Ok(JoinHandle(JoinInner {
if let Some(scope_data) = scope_data {
scope_data.increment_num_running_threads();
}
Ok(JoinInner {
// SAFETY:
//
// `imp::Thread::new` takes a closure with a `'static` lifetime, since it's passed
@ -506,8 +531,8 @@ pub unsafe fn spawn_unchecked<'a, F, T>(self, f: F) -> io::Result<JoinHandle<T>>
)?
},
thread: my_thread,
packet: Packet(my_packet),
}))
packet: my_packet,
})
}
}
@ -1242,34 +1267,48 @@ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
#[stable(feature = "rust1", since = "1.0.0")]
pub type Result<T> = crate::result::Result<T, Box<dyn Any + Send + 'static>>;
// This packet is used to communicate the return value between the spawned thread
// and the rest of the program. Memory is shared through the `Arc` within and there's
// no need for a mutex here because synchronization happens with `join()` (the
// caller will never read this packet until the thread has exited).
// This packet is used to communicate the return value between the spawned
// thread and the rest of the program. It is shared through an `Arc` and
// there's no need for a mutex here because synchronization happens with `join()`
// (the caller will never read this packet until the thread has exited).
//
// This packet itself is then stored into a `JoinInner` which in turns is placed
// in `JoinHandle` and `JoinGuard`. Due to the usage of `UnsafeCell` we need to
// manually worry about impls like Send and Sync. The type `T` should
// already always be Send (otherwise the thread could not have been created) and
// this type is inherently Sync because no methods take &self. Regardless,
// however, we add inheriting impls for Send/Sync to this type to ensure it's
// Send/Sync and that future modifications will still appropriately classify it.
struct Packet<T>(Arc<UnsafeCell<Option<Result<T>>>>);
unsafe impl<T: Send> Send for Packet<T> {}
unsafe impl<T: Sync> Sync for Packet<T> {}
/// Inner representation for JoinHandle
struct JoinInner<T> {
native: imp::Thread,
thread: Thread,
packet: Packet<T>,
// An Arc to the packet is stored into a `JoinInner` which in turns is placed
// in `JoinHandle`.
struct Packet<'scope, T> {
scope: Option<&'scope scoped::ScopeData>,
result: UnsafeCell<Option<Result<T>>>,
}
impl<T> JoinInner<T> {
// Due to the usage of `UnsafeCell` we need to manually implement Sync.
// The type `T` should already always be Send (otherwise the thread could not
// have been created) and the Packet is Sync because all access to the
// `UnsafeCell` synchronized (by the `join()` boundary), and `ScopeData` is Sync.
unsafe impl<'scope, T: Sync> Sync for Packet<'scope, T> {}
impl<'scope, T> Drop for Packet<'scope, T> {
fn drop(&mut self) {
// Book-keeping so the scope knows when it's done.
if let Some(scope) = self.scope {
// If this packet was for a thread that ran in a scope, the thread
// panicked, and nobody consumed the panic payload, we make sure
// the scope function will panic.
let unhandled_panic = matches!(self.result.get_mut(), Some(Err(_)));
scope.decrement_num_running_threads(unhandled_panic);
}
}
}
/// Inner representation for JoinHandle
struct JoinInner<'scope, T> {
native: imp::Thread,
thread: Thread,
packet: Arc<Packet<'scope, T>>,
}
impl<'scope, T> JoinInner<'scope, T> {
fn join(mut self) -> Result<T> {
self.native.join();
Arc::get_mut(&mut self.packet.0).unwrap().get_mut().take().unwrap()
Arc::get_mut(&mut self.packet).unwrap().result.get_mut().take().unwrap()
}
}
@ -1336,7 +1375,7 @@ fn join(mut self) -> Result<T> {
/// [`thread::Builder::spawn`]: Builder::spawn
/// [`thread::spawn`]: spawn
#[stable(feature = "rust1", since = "1.0.0")]
pub struct JoinHandle<T>(JoinInner<T>);
pub struct JoinHandle<T>(JoinInner<'static, T>);
#[stable(feature = "joinhandle_impl_send_sync", since = "1.29.0")]
unsafe impl<T> Send for JoinHandle<T> {}
@ -1404,13 +1443,13 @@ pub fn join(self) -> Result<T> {
self.0.join()
}
/// Checks if the the associated thread is still running its main function.
/// Checks if the associated thread is still running its main function.
///
/// This might return `false` for a brief moment after the thread's main
/// function has returned, but before the thread itself has stopped running.
#[unstable(feature = "thread_is_running", issue = "90470")]
pub fn is_running(&self) -> bool {
Arc::strong_count(&self.0.packet.0) > 1
Arc::strong_count(&self.0.packet) > 1
}
}

316
library/std/src/thread/scoped.rs Normal file
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@ -0,0 +1,316 @@
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.
pub struct Scope<'env> {
data: ScopeData,
/// Invariance over 'env, to make sure 'env cannot shrink,
/// which is necessary for soundness.
///
/// Without invariance, this would compile fine but be unsound:
///
/// ```compile_fail
/// #![feature(scoped_threads)]
///
/// std::thread::scope(|s| {
/// s.spawn(|s| {
/// let a = String::from("abcd");
/// s.spawn(|_| println!("{:?}", a)); // might run after `a` is dropped
/// });
/// });
/// ```
env: PhantomData<&'env mut &'env ()>,
}
/// An owned permission to join on a scoped thread (block on its termination).
///
/// See [`Scope::spawn`] for details.
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 many many 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
///
/// ```
/// #![feature(scoped_threads)]
/// 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());
/// ```
#[track_caller]
pub fn scope<'env, F, T>(f: F) -> T
where
F: FnOnce(&Scope<'env>) -> T,
{
let scope = Scope {
data: ScopeData {
num_running_threads: AtomicUsize::new(0),
main_thread: current(),
a_thread_panicked: AtomicBool::new(false),
},
env: 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<'env> 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
pub fn spawn<'scope, F, T>(&'scope self, f: F) -> ScopedJoinHandle<'scope, T>
where
F: FnOnce(&Scope<'env>) -> T + Send + 'env,
T: Send + 'env,
{
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
///
/// ```
/// #![feature(scoped_threads)]
/// 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());
/// ```
pub fn spawn_scoped<'scope, 'env, F, T>(
self,
scope: &'scope Scope<'env>,
f: F,
) -> io::Result<ScopedJoinHandle<'scope, T>>
where
F: FnOnce(&Scope<'env>) -> T + Send + 'env,
T: Send + 'env,
{
Ok(ScopedJoinHandle(unsafe { self.spawn_unchecked_(|| f(scope), Some(&scope.data)) }?))
}
}
impl<'scope, T> ScopedJoinHandle<'scope, T> {
/// Extracts a handle to the underlying thread.
///
/// # Examples
///
/// ```
/// #![feature(scoped_threads)]
/// #![feature(thread_is_running)]
///
/// use std::thread;
///
/// thread::scope(|s| {
/// let t = s.spawn(|_| {
/// println!("hello");
/// });
/// println!("thread id: {:?}", t.thread().id());
/// });
/// ```
#[must_use]
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
///
/// ```
/// #![feature(scoped_threads)]
/// #![feature(thread_is_running)]
///
/// use std::thread;
///
/// thread::scope(|s| {
/// let t = s.spawn(|_| {
/// panic!("oh no");
/// });
/// assert!(t.join().is_err());
/// });
/// ```
pub fn join(self) -> Result<T> {
self.0.join()
}
/// Checks if the associated thread is still running its main function.
///
/// This might return `false` for a brief moment after the thread's main
/// function has returned, but before the thread itself has stopped running.
#[unstable(feature = "thread_is_running", issue = "90470")]
pub fn is_running(&self) -> bool {
Arc::strong_count(&self.0.packet) > 1
}
}
impl<'env> fmt::Debug for Scope<'env> {
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()
}
}
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()
}
}