1use super::{Builder, JoinInner, Result, Thread, current_or_unnamed};
2use crate::marker::PhantomData;
3use crate::panic::{AssertUnwindSafe, catch_unwind, resume_unwind};
4use crate::sync::Arc;
5use crate::sync::atomic::{Atomic, AtomicBool, AtomicUsize, Ordering};
6use crate::{fmt, io};
7
8/// A scope to spawn scoped threads in.
9///
10/// See [`scope`] for details.
11#[stable(feature = "scoped_threads", since = "1.63.0")]
12pub struct Scope<'scope, 'env: 'scope> {
13 data: Arc<ScopeData>,
14 /// Invariance over 'scope, to make sure 'scope cannot shrink,
15 /// which is necessary for soundness.
16 ///
17 /// Without invariance, this would compile fine but be unsound:
18 ///
19 /// ```compile_fail,E0373
20 /// std::thread::scope(|s| {
21 /// s.spawn(|| {
22 /// let a = String::from("abcd");
23 /// s.spawn(|| println!("{a:?}")); // might run after `a` is dropped
24 /// });
25 /// });
26 /// ```
27 scope: PhantomData<&'scope mut &'scope ()>,
28 env: PhantomData<&'env mut &'env ()>,
29}
30
31/// An owned permission to join on a scoped thread (block on its termination).
32///
33/// See [`Scope::spawn`] for details.
34#[stable(feature = "scoped_threads", since = "1.63.0")]
35pub struct ScopedJoinHandle<'scope, T>(JoinInner<'scope, T>);
36
37pub(super) struct ScopeData {
38 num_running_threads: Atomic<usize>,
39 a_thread_panicked: Atomic<bool>,
40 main_thread: Thread,
41}
42
43impl ScopeData {
44 pub(super) fn increment_num_running_threads(&self) {
45 // We check for 'overflow' with usize::MAX / 2, to make sure there's no
46 // chance it overflows to 0, which would result in unsoundness.
47 if self.num_running_threads.fetch_add(1, Ordering::Relaxed) > usize::MAX / 2 {
48 // This can only reasonably happen by mem::forget()'ing a lot of ScopedJoinHandles.
49 self.overflow();
50 }
51 }
52
53 #[cold]
54 fn overflow(&self) {
55 self.decrement_num_running_threads(false);
56 panic!("too many running threads in thread scope");
57 }
58
59 pub(super) fn decrement_num_running_threads(&self, panic: bool) {
60 if panic {
61 self.a_thread_panicked.store(true, Ordering::Relaxed);
62 }
63 if self.num_running_threads.fetch_sub(1, Ordering::Release) == 1 {
64 self.main_thread.unpark();
65 }
66 }
67}
68
69/// Creates a scope for spawning scoped threads.
70///
71/// The function passed to `scope` will be provided a [`Scope`] object,
72/// through which scoped threads can be [spawned][`Scope::spawn`].
73///
74/// Unlike non-scoped threads, scoped threads can borrow non-`'static` data,
75/// as the scope guarantees all threads will be joined at the end of the scope.
76///
77/// All threads spawned within the scope that haven't been manually joined
78/// will be automatically joined before this function returns.
79///
80/// # Panics
81///
82/// If any of the automatically joined threads panicked, this function will panic.
83///
84/// If you want to handle panics from spawned threads,
85/// [`join`][ScopedJoinHandle::join] them before the end of the scope.
86///
87/// # Example
88///
89/// ```
90/// use std::thread;
91///
92/// let mut a = vec![1, 2, 3];
93/// let mut x = 0;
94///
95/// thread::scope(|s| {
96/// s.spawn(|| {
97/// println!("hello from the first scoped thread");
98/// // We can borrow `a` here.
99/// dbg!(&a);
100/// });
101/// s.spawn(|| {
102/// println!("hello from the second scoped thread");
103/// // We can even mutably borrow `x` here,
104/// // because no other threads are using it.
105/// x += a[0] + a[2];
106/// });
107/// println!("hello from the main thread");
108/// });
109///
110/// // After the scope, we can modify and access our variables again:
111/// a.push(4);
112/// assert_eq!(x, a.len());
113/// ```
114///
115/// # Lifetimes
116///
117/// Scoped threads involve two lifetimes: `'scope` and `'env`.
118///
119/// The `'scope` lifetime represents the lifetime of the scope itself.
120/// That is: the time during which new scoped threads may be spawned,
121/// and also the time during which they might still be running.
122/// Once this lifetime ends, all scoped threads are joined.
123/// This lifetime starts within the `scope` function, before `f` (the argument to `scope`) starts.
124/// It ends after `f` returns and all scoped threads have been joined, but before `scope` returns.
125///
126/// The `'env` lifetime represents the lifetime of whatever is borrowed by the scoped threads.
127/// This lifetime must outlast the call to `scope`, and thus cannot be smaller than `'scope`.
128/// It can be as small as the call to `scope`, meaning that anything that outlives this call,
129/// such as local variables defined right before the scope, can be borrowed by the scoped threads.
130///
131/// The `'env: 'scope` bound is part of the definition of the `Scope` type.
132#[track_caller]
133#[stable(feature = "scoped_threads", since = "1.63.0")]
134pub fn scope<'env, F, T>(f: F) -> T
135where
136 F: for<'scope> dynFnOnce(&'scope Scope<'scope, 'env>) -> T,
137{
138 // We put the `ScopeData` into an `Arc` so that other threads can finish their
139 // `decrement_num_running_threads` even after this function returns.
140 let scope = Scope {
141 data: Arc::new(ScopeData {
142 num_running_threads: AtomicUsize::new(0),
143 main_thread: current_or_unnamed(),
144 a_thread_panicked: AtomicBool::new(false),
145 }),
146 env: PhantomData,
147 scope: PhantomData,
148 };
149
150 // Run `f`, but catch panics so we can make sure to wait for all the threads to join.
151 let result = catch_unwind(AssertUnwindSafe(|| f(&scope)));
152
153 // Wait until all the threads are finished.
154 while scope.data.num_running_threads.load(Ordering::Acquire) != 0 {
155 // SAFETY: this is the main thread, the handle belongs to us.
156 unsafe { scope.data.main_thread.park() };
157 }
158
159 // Throw any panic from `f`, or the return value of `f` if no thread panicked.
160 match result {
161 Err(e) => resume_unwind(e),
162 Ok(_) if scope.data.a_thread_panicked.load(Ordering::Relaxed) => {
163 panic!("a scoped thread panicked")
164 }
165 Ok(result) => result,
166 }
167}
168
169impl<'scope, 'env> Scope<'scope, 'env> {
170 /// Spawns a new thread within a scope, returning a [`ScopedJoinHandle`] for it.
171 ///
172 /// Unlike non-scoped threads, threads spawned with this function may
173 /// borrow non-`'static` data from the outside the scope. See [`scope`] for
174 /// details.
175 ///
176 /// The join handle provides a [`join`] method that can be used to join the spawned
177 /// thread. If the spawned thread panics, [`join`] will return an [`Err`] containing
178 /// the panic payload.
179 ///
180 /// If the join handle is dropped, the spawned thread will be implicitly joined at the
181 /// end of the scope. In that case, if the spawned thread panics, [`scope`] will
182 /// panic after all threads are joined.
183 ///
184 /// This call will create a thread using default parameters of [`Builder`].
185 /// If you want to specify the stack size or the name of the thread, use
186 /// [`Builder::spawn_scoped`] instead.
187 ///
188 /// # Panics
189 ///
190 /// Panics if the OS fails to create a thread; use [`Builder::spawn_scoped`]
191 /// to recover from such errors.
192 ///
193 /// [`join`]: ScopedJoinHandle::join
194 #[stable(feature = "scoped_threads", since = "1.63.0")]
195 pub fn spawn<F, T>(&'scope self, f: F) -> ScopedJoinHandle<'scope, T>
196 where
197 F: FnOnce() -> T + Send + 'scope,
198 T: Send + 'scope,
199 {
200 Builder::new().spawn_scoped(self, f).expect("failed to spawn thread")
201 }
202}
203
204impl Builder {
205 /// Spawns a new scoped thread using the settings set through this `Builder`.
206 ///
207 /// Unlike [`Scope::spawn`], this method yields an [`io::Result`] to
208 /// capture any failure to create the thread at the OS level.
209 ///
210 /// [`io::Result`]: crate::io::Result
211 ///
212 /// # Panics
213 ///
214 /// Panics if a thread name was set and it contained null bytes.
215 ///
216 /// # Example
217 ///
218 /// ```
219 /// use std::thread;
220 ///
221 /// let mut a = vec![1, 2, 3];
222 /// let mut x = 0;
223 ///
224 /// thread::scope(|s| {
225 /// thread::Builder::new()
226 /// .name("first".to_string())
227 /// .spawn_scoped(s, ||
228 /// {
229 /// println!("hello from the {:?} scoped thread", thread::current().name());
230 /// // We can borrow `a` here.
231 /// dbg!(&a);
232 /// })
233 /// .unwrap();
234 /// thread::Builder::new()
235 /// .name("second".to_string())
236 /// .spawn_scoped(s, ||
237 /// {
238 /// println!("hello from the {:?} scoped thread", thread::current().name());
239 /// // We can even mutably borrow `x` here,
240 /// // because no other threads are using it.
241 /// x += a[0] + a[2];
242 /// })
243 /// .unwrap();
244 /// println!("hello from the main thread");
245 /// });
246 ///
247 /// // After the scope, we can modify and access our variables again:
248 /// a.push(4);
249 /// assert_eq!(x, a.len());
250 /// ```
251 #[stable(feature = "scoped_threads", since = "1.63.0")]
252 pub fn spawn_scoped<'scope, 'env, F, T>(
253 self,
254 scope: &'scope Scope<'scope, 'env>,
255 f: F,
256 ) -> io::Result<ScopedJoinHandle<'scope, T>>
257 where
258 F: FnOnce() -> T + Send + 'scope,
259 T: Send + 'scope,
260 {
261 Ok(ScopedJoinHandle(unsafe { self.spawn_unchecked_(f, Some(scope.data.clone())) }?))
262 }
263}
264
265impl<'scope, T> ScopedJoinHandle<'scope, T> {
266 /// Extracts a handle to the underlying thread.
267 ///
268 /// # Examples
269 ///
270 /// ```
271 /// use std::thread;
272 ///
273 /// thread::scope(|s| {
274 /// let t = s.spawn(|| {
275 /// println!("hello");
276 /// });
277 /// println!("thread id: {:?}", t.thread().id());
278 /// });
279 /// ```
280 #[must_use]
281 #[stable(feature = "scoped_threads", since = "1.63.0")]
282 pub fn thread(&self) -> &Thread {
283 &self.0.thread
284 }
285
286 /// Waits for the associated thread to finish.
287 ///
288 /// This function will return immediately if the associated thread has already finished.
289 ///
290 /// In terms of [atomic memory orderings], the completion of the associated
291 /// thread synchronizes with this function returning.
292 /// In other words, all operations performed by that thread
293 /// [happen before](https://doc.rust-lang.org/nomicon/atomics.html#data-accesses)
294 /// all operations that happen after `join` returns.
295 ///
296 /// If the associated thread panics, [`Err`] is returned with the panic payload.
297 ///
298 /// [atomic memory orderings]: crate::sync::atomic
299 ///
300 /// # Examples
301 ///
302 /// ```
303 /// use std::thread;
304 ///
305 /// thread::scope(|s| {
306 /// let t = s.spawn(|| {
307 /// panic!("oh no");
308 /// });
309 /// assert!(t.join().is_err());
310 /// });
311 /// ```
312 #[stable(feature = "scoped_threads", since = "1.63.0")]
313 pub fn join(self) -> Result<T> {
314 self.0.join()
315 }
316
317 /// Checks if the associated thread has finished running its main function.
318 ///
319 /// `is_finished` supports implementing a non-blocking join operation, by checking
320 /// `is_finished`, and calling `join` if it returns `true`. This function does not block. To
321 /// block while waiting on the thread to finish, use [`join`][Self::join].
322 ///
323 /// This might return `true` for a brief moment after the thread's main
324 /// function has returned, but before the thread itself has stopped running.
325 /// However, once this returns `true`, [`join`][Self::join] can be expected
326 /// to return quickly, without blocking for any significant amount of time.
327 #[stable(feature = "scoped_threads", since = "1.63.0")]
328 pub fn is_finished(&self) -> bool {
329 Arc::strong_count(&self.0.packet) == 1
330 }
331}
332
333#[stable(feature = "scoped_threads", since = "1.63.0")]
334impl fmt::Debug for Scope<'_, '_> {
335 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
336 f&mut DebugStruct<'_, '_>.debug_struct("Scope")
337 .field("num_running_threads", &self.data.num_running_threads.load(Ordering::Relaxed))
338 .field("a_thread_panicked", &self.data.a_thread_panicked.load(Ordering::Relaxed))
339 .field(name:"main_thread", &self.data.main_thread)
340 .finish_non_exhaustive()
341 }
342}
343
344#[stable(feature = "scoped_threads", since = "1.63.0")]
345impl<'scope, T> fmt::Debug for ScopedJoinHandle<'scope, T> {
346 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
347 f.debug_struct(name:"ScopedJoinHandle").finish_non_exhaustive()
348 }
349}
350