31b4023e24
std: allow after-main use of synchronization primitives By creating an unnamed thread handle when the actual one has already been destroyed, synchronization primitives using thread parking can be used even outside the Rust runtime. This also fixes an inefficiency in the queue-based `RwLock`: if `thread::current` was not initialized yet, it will create a new handle on every parking attempt without initializing `thread::current`. The private `current_or_unnamed` function introduced here fixes this.
350 lines
12 KiB
Rust
350 lines
12 KiB
Rust
use super::{Builder, JoinInner, Result, Thread, current_or_unnamed};
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use crate::marker::PhantomData;
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use crate::panic::{AssertUnwindSafe, catch_unwind, resume_unwind};
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use crate::sync::Arc;
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use crate::sync::atomic::{AtomicBool, AtomicUsize, Ordering};
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use crate::{fmt, io};
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/// A scope to spawn scoped threads in.
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///
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/// See [`scope`] for details.
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#[stable(feature = "scoped_threads", since = "1.63.0")]
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pub struct Scope<'scope, 'env: 'scope> {
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data: Arc<ScopeData>,
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/// Invariance over 'scope, to make sure 'scope cannot shrink,
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/// which is necessary for soundness.
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///
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/// Without invariance, this would compile fine but be unsound:
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///
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/// ```compile_fail,E0373
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/// std::thread::scope(|s| {
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/// s.spawn(|| {
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/// let a = String::from("abcd");
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/// s.spawn(|| println!("{a:?}")); // might run after `a` is dropped
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/// });
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/// });
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/// ```
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scope: PhantomData<&'scope mut &'scope ()>,
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env: PhantomData<&'env mut &'env ()>,
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}
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/// An owned permission to join on a scoped thread (block on its termination).
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///
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/// See [`Scope::spawn`] for details.
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#[stable(feature = "scoped_threads", since = "1.63.0")]
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pub struct ScopedJoinHandle<'scope, T>(JoinInner<'scope, T>);
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pub(super) struct ScopeData {
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num_running_threads: AtomicUsize,
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a_thread_panicked: AtomicBool,
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main_thread: Thread,
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}
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impl ScopeData {
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pub(super) fn increment_num_running_threads(&self) {
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// We check for 'overflow' with usize::MAX / 2, to make sure there's no
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// chance it overflows to 0, which would result in unsoundness.
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if self.num_running_threads.fetch_add(1, Ordering::Relaxed) > usize::MAX / 2 {
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// This can only reasonably happen by mem::forget()'ing a lot of ScopedJoinHandles.
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self.overflow();
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}
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}
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#[cold]
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fn overflow(&self) {
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self.decrement_num_running_threads(false);
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panic!("too many running threads in thread scope");
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}
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pub(super) fn decrement_num_running_threads(&self, panic: bool) {
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if panic {
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self.a_thread_panicked.store(true, Ordering::Relaxed);
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}
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if self.num_running_threads.fetch_sub(1, Ordering::Release) == 1 {
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self.main_thread.unpark();
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}
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}
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}
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/// Creates a scope for spawning scoped threads.
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///
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/// The function passed to `scope` will be provided a [`Scope`] object,
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/// through which scoped threads can be [spawned][`Scope::spawn`].
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///
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/// Unlike non-scoped threads, scoped threads can borrow non-`'static` data,
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/// as the scope guarantees all threads will be joined at the end of the scope.
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///
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/// All threads spawned within the scope that haven't been manually joined
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/// will be automatically joined before this function returns.
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///
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/// # Panics
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///
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/// If any of the automatically joined threads panicked, this function will panic.
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///
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/// If you want to handle panics from spawned threads,
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/// [`join`][ScopedJoinHandle::join] them before the end of the scope.
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///
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/// # Example
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///
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/// ```
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/// use std::thread;
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///
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/// let mut a = vec![1, 2, 3];
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/// let mut x = 0;
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///
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/// thread::scope(|s| {
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/// s.spawn(|| {
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/// println!("hello from the first scoped thread");
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/// // We can borrow `a` here.
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/// dbg!(&a);
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/// });
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/// s.spawn(|| {
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/// println!("hello from the second scoped thread");
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/// // We can even mutably borrow `x` here,
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/// // because no other threads are using it.
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/// x += a[0] + a[2];
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/// });
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/// println!("hello from the main thread");
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/// });
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///
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/// // After the scope, we can modify and access our variables again:
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/// a.push(4);
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/// assert_eq!(x, a.len());
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/// ```
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///
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/// # Lifetimes
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///
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/// Scoped threads involve two lifetimes: `'scope` and `'env`.
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///
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/// The `'scope` lifetime represents the lifetime of the scope itself.
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/// That is: the time during which new scoped threads may be spawned,
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/// and also the time during which they might still be running.
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/// Once this lifetime ends, all scoped threads are joined.
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/// This lifetime starts within the `scope` function, before `f` (the argument to `scope`) starts.
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/// It ends after `f` returns and all scoped threads have been joined, but before `scope` returns.
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///
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/// The `'env` lifetime represents the lifetime of whatever is borrowed by the scoped threads.
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/// This lifetime must outlast the call to `scope`, and thus cannot be smaller than `'scope`.
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/// It can be as small as the call to `scope`, meaning that anything that outlives this call,
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/// such as local variables defined right before the scope, can be borrowed by the scoped threads.
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///
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/// The `'env: 'scope` bound is part of the definition of the `Scope` type.
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#[track_caller]
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#[stable(feature = "scoped_threads", since = "1.63.0")]
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pub fn scope<'env, F, T>(f: F) -> T
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where
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F: for<'scope> FnOnce(&'scope Scope<'scope, 'env>) -> T,
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{
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// We put the `ScopeData` into an `Arc` so that other threads can finish their
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// `decrement_num_running_threads` even after this function returns.
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let scope = Scope {
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data: Arc::new(ScopeData {
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num_running_threads: AtomicUsize::new(0),
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main_thread: current_or_unnamed(),
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a_thread_panicked: AtomicBool::new(false),
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}),
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env: PhantomData,
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scope: PhantomData,
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};
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// Run `f`, but catch panics so we can make sure to wait for all the threads to join.
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let result = catch_unwind(AssertUnwindSafe(|| f(&scope)));
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// Wait until all the threads are finished.
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while scope.data.num_running_threads.load(Ordering::Acquire) != 0 {
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// SAFETY: this is the main thread, the handle belongs to us.
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unsafe { scope.data.main_thread.park() };
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}
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// Throw any panic from `f`, or the return value of `f` if no thread panicked.
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match result {
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Err(e) => resume_unwind(e),
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Ok(_) if scope.data.a_thread_panicked.load(Ordering::Relaxed) => {
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panic!("a scoped thread panicked")
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}
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Ok(result) => result,
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}
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}
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impl<'scope, 'env> Scope<'scope, 'env> {
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/// Spawns a new thread within a scope, returning a [`ScopedJoinHandle`] for it.
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///
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/// Unlike non-scoped threads, threads spawned with this function may
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/// borrow non-`'static` data from the outside the scope. See [`scope`] for
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/// details.
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///
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/// The join handle provides a [`join`] method that can be used to join the spawned
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/// thread. If the spawned thread panics, [`join`] will return an [`Err`] containing
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/// the panic payload.
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///
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/// If the join handle is dropped, the spawned thread will be implicitly joined at the
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/// end of the scope. In that case, if the spawned thread panics, [`scope`] will
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/// panic after all threads are joined.
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///
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/// This call will create a thread using default parameters of [`Builder`].
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/// If you want to specify the stack size or the name of the thread, use
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/// [`Builder::spawn_scoped`] instead.
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///
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/// # Panics
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///
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/// Panics if the OS fails to create a thread; use [`Builder::spawn_scoped`]
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/// to recover from such errors.
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///
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/// [`join`]: ScopedJoinHandle::join
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#[stable(feature = "scoped_threads", since = "1.63.0")]
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pub fn spawn<F, T>(&'scope self, f: F) -> ScopedJoinHandle<'scope, T>
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where
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F: FnOnce() -> T + Send + 'scope,
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T: Send + 'scope,
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{
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Builder::new().spawn_scoped(self, f).expect("failed to spawn thread")
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}
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}
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impl Builder {
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/// Spawns a new scoped thread using the settings set through this `Builder`.
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///
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/// Unlike [`Scope::spawn`], this method yields an [`io::Result`] to
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/// capture any failure to create the thread at the OS level.
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///
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/// [`io::Result`]: crate::io::Result
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///
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/// # Panics
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///
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/// Panics if a thread name was set and it contained null bytes.
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///
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/// # Example
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///
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/// ```
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/// use std::thread;
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///
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/// let mut a = vec![1, 2, 3];
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/// let mut x = 0;
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///
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/// thread::scope(|s| {
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/// thread::Builder::new()
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/// .name("first".to_string())
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/// .spawn_scoped(s, ||
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/// {
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/// println!("hello from the {:?} scoped thread", thread::current().name());
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/// // We can borrow `a` here.
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/// dbg!(&a);
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/// })
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/// .unwrap();
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/// thread::Builder::new()
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/// .name("second".to_string())
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/// .spawn_scoped(s, ||
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/// {
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/// println!("hello from the {:?} scoped thread", thread::current().name());
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/// // We can even mutably borrow `x` here,
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/// // because no other threads are using it.
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/// x += a[0] + a[2];
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/// })
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/// .unwrap();
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/// println!("hello from the main thread");
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/// });
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///
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/// // After the scope, we can modify and access our variables again:
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/// a.push(4);
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/// assert_eq!(x, a.len());
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/// ```
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#[stable(feature = "scoped_threads", since = "1.63.0")]
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pub fn spawn_scoped<'scope, 'env, F, T>(
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self,
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scope: &'scope Scope<'scope, 'env>,
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f: F,
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) -> io::Result<ScopedJoinHandle<'scope, T>>
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where
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F: FnOnce() -> T + Send + 'scope,
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T: Send + 'scope,
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{
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Ok(ScopedJoinHandle(unsafe { self.spawn_unchecked_(f, Some(scope.data.clone())) }?))
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}
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}
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impl<'scope, T> ScopedJoinHandle<'scope, T> {
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/// Extracts a handle to the underlying thread.
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///
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/// # Examples
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///
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/// ```
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/// use std::thread;
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///
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/// thread::scope(|s| {
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/// let t = s.spawn(|| {
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/// println!("hello");
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/// });
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/// println!("thread id: {:?}", t.thread().id());
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/// });
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/// ```
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#[must_use]
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#[stable(feature = "scoped_threads", since = "1.63.0")]
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pub fn thread(&self) -> &Thread {
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&self.0.thread
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}
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/// Waits for the associated thread to finish.
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///
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/// This function will return immediately if the associated thread has already finished.
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///
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/// In terms of [atomic memory orderings], the completion of the associated
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/// thread synchronizes with this function returning.
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/// In other words, all operations performed by that thread
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/// [happen before](https://doc.rust-lang.org/nomicon/atomics.html#data-accesses)
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/// all operations that happen after `join` returns.
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///
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/// If the associated thread panics, [`Err`] is returned with the panic payload.
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///
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/// [atomic memory orderings]: crate::sync::atomic
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///
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/// # Examples
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///
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/// ```
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/// use std::thread;
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///
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/// thread::scope(|s| {
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/// let t = s.spawn(|| {
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/// panic!("oh no");
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/// });
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/// assert!(t.join().is_err());
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/// });
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/// ```
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#[stable(feature = "scoped_threads", since = "1.63.0")]
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pub fn join(self) -> Result<T> {
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self.0.join()
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}
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/// Checks if the associated thread has finished running its main function.
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///
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/// `is_finished` supports implementing a non-blocking join operation, by checking
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/// `is_finished`, and calling `join` if it returns `true`. This function does not block. To
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/// block while waiting on the thread to finish, use [`join`][Self::join].
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///
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/// This might return `true` for a brief moment after the thread's main
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/// function has returned, but before the thread itself has stopped running.
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/// However, once this returns `true`, [`join`][Self::join] can be expected
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/// to return quickly, without blocking for any significant amount of time.
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#[stable(feature = "scoped_threads", since = "1.63.0")]
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pub fn is_finished(&self) -> bool {
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Arc::strong_count(&self.0.packet) == 1
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}
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}
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#[stable(feature = "scoped_threads", since = "1.63.0")]
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impl fmt::Debug for Scope<'_, '_> {
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fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
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f.debug_struct("Scope")
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.field("num_running_threads", &self.data.num_running_threads.load(Ordering::Relaxed))
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.field("a_thread_panicked", &self.data.a_thread_panicked.load(Ordering::Relaxed))
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.field("main_thread", &self.data.main_thread)
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.finish_non_exhaustive()
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}
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}
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#[stable(feature = "scoped_threads", since = "1.63.0")]
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impl<'scope, T> fmt::Debug for ScopedJoinHandle<'scope, T> {
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fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
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f.debug_struct("ScopedJoinHandle").finish_non_exhaustive()
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}
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}
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