1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529
//! Threads that can borrow variables from the stack. //! //! Create a scope when spawned threads need to access variables on the stack: //! //! ``` //! use crossbeam_utils::thread; //! //! let people = vec![ //! "Alice".to_string(), //! "Bob".to_string(), //! "Carol".to_string(), //! ]; //! //! thread::scope(|s| { //! for person in &people { //! s.spawn(move |_| { //! println!("Hello, {}!", person); //! }); //! } //! }).unwrap(); //! ``` //! //! # Why scoped threads? //! //! Suppose we wanted to re-write the previous example using plain threads: //! //! ```ignore //! use std::thread; //! //! let people = vec![ //! "Alice".to_string(), //! "Bob".to_string(), //! "Carol".to_string(), //! ]; //! //! let mut threads = Vec::new(); //! //! for person in &people { //! threads.push(thread::spawn(move |_| { //! println!("Hello, {}!", person); //! })); //! } //! //! for thread in threads { //! thread.join().unwrap(); //! } //! ``` //! //! This doesn't work because the borrow checker complains about `people` not living long enough: //! //! ```text //! error[E0597]: `people` does not live long enough //! --> src/main.rs:12:20 //! | //! 12 | for person in &people { //! | ^^^^^^ borrowed value does not live long enough //! ... //! 21 | } //! | - borrowed value only lives until here //! | //! = note: borrowed value must be valid for the static lifetime... //! ``` //! //! The problem here is that spawned threads are not allowed to borrow variables on stack because //! the compiler cannot prove they will be joined before `people` is destroyed. //! //! Scoped threads are a mechanism to guarantee to the compiler that spawned threads will be joined //! before the scope ends. //! //! # How scoped threads work //! //! If a variable is borrowed by a thread, the thread must complete before the variable is //! destroyed. Threads spawned using [`std::thread::spawn`] can only borrow variables with the //! `'static` lifetime because the borrow checker cannot be sure when the thread will complete. //! //! A scope creates a clear boundary between variables outside the scope and threads inside the //! scope. Whenever a s.spawns a thread, it promises to join the thread before the scope ends. //! This way we guarantee to the borrow checker that scoped threads only live within the scope and //! can safely access variables outside it. //! //! # Nesting scoped threads //! //! Sometimes scoped threads need to spawn more threads within the same scope. This is a little //! tricky because argument `s` lives *inside* the invocation of `thread::scope()` and as such //! cannot be borrowed by scoped threads: //! //! ```ignore //! use crossbeam_utils::thread; //! //! thread::scope(|s| { //! s.spawn(|_| { //! // Not going to compile because we're trying to borrow `s`, //! // which lives *inside* the scope! :( //! s.spawn(|_| println!("nested thread")); //! }}); //! }); //! ``` //! //! Fortunately, there is a solution. Every scoped thread is passed a reference to its scope as an //! argument, which can be used for spawning nested threads: //! //! ```ignore //! use crossbeam_utils::thread; //! //! thread::scope(|s| { //! // Note the `|s|` here. //! s.spawn(|s| { //! // Yay, this works because we're using a fresh argument `s`! :) //! s.spawn(|_| println!("nested thread")); //! }}); //! }); //! ``` //! //! [`std::thread::spawn`]: https://doc.rust-lang.org/std/thread/fn.spawn.html use std::fmt; use std::io; use std::marker::PhantomData; use std::mem; use std::panic; use std::sync::{Arc, Mutex}; use std::thread; use sync::WaitGroup; type SharedVec<T> = Arc<Mutex<Vec<T>>>; type SharedOption<T> = Arc<Mutex<Option<T>>>; /// Creates a new scope for spawning threads. /// /// All child threads that haven't been manually joined will be automatically joined just before /// this function invocation ends. If all joined threads have successfully completed, `Ok` is /// returned with the return value of `f`. If any of the joined threads has panicked, an `Err` is /// returned containing errors from panicked threads. /// /// # Examples /// /// ``` /// use crossbeam_utils::thread; /// /// let var = vec![1, 2, 3]; /// /// thread::scope(|s| { /// s.spawn(|_| { /// println!("A child thread borrowing `var`: {:?}", var); /// }); /// }).unwrap(); /// ``` pub fn scope<'env, F, R>(f: F) -> thread::Result<R> where F: FnOnce(&Scope<'env>) -> R, { let wg = WaitGroup::new(); let scope = Scope::<'env> { handles: SharedVec::default(), wait_group: wg.clone(), _marker: PhantomData, }; // Execute the scoped function, but catch any panics. let result = panic::catch_unwind(panic::AssertUnwindSafe(|| f(&scope))); // Wait until all nested scopes are dropped. drop(scope.wait_group); wg.wait(); // Join all remaining spawned threads. let panics: Vec<_> = { let mut handles = scope.handles.lock().unwrap(); // Filter handles that haven't been joined, join them, and collect errors. let panics = handles .drain(..) .filter_map(|handle| handle.lock().unwrap().take()) .filter_map(|handle| handle.join().err()) .collect(); panics }; // If `f` has panicked, resume unwinding. // If any of the child threads have panicked, return the panic errors. // Otherwise, everything is OK and return the result of `f`. match result { Err(err) => panic::resume_unwind(err), Ok(res) => { if panics.is_empty() { Ok(res) } else { Err(Box::new(panics)) } } } } /// A scope for spawning threads. pub struct Scope<'env> { /// The list of the thread join handles. handles: SharedVec<SharedOption<thread::JoinHandle<()>>>, /// Used to wait until all subscopes all dropped. wait_group: WaitGroup, /// Borrows data with invariant lifetime `'env`. _marker: PhantomData<&'env mut &'env ()>, } unsafe impl<'env> Sync for Scope<'env> {} impl<'env> Scope<'env> { /// Spawns a scoped thread. /// /// This method is similar to the [`spawn`] function in Rust's standard library. The difference /// is that this thread is scoped, meaning it's guaranteed to terminate before the scope exits, /// allowing it to reference variables outside the scope. /// /// The scoped thread is passed a reference to this scope as an argument, which can be used for /// spawning nested threads. /// /// The returned handle can be used to manually join the thread before the scope exits. /// /// [`spawn`]: https://doc.rust-lang.org/std/thread/fn.spawn.html /// /// # Examples /// /// ``` /// use crossbeam_utils::thread; /// /// thread::scope(|s| { /// let handle = s.spawn(|_| { /// println!("A child thread is running"); /// 42 /// }); /// /// // Join the thread and retrieve its result. /// let res = handle.join().unwrap(); /// assert_eq!(res, 42); /// }).unwrap(); /// ``` pub fn spawn<'scope, F, T>(&'scope self, f: F) -> ScopedJoinHandle<'scope, T> where F: FnOnce(&Scope<'env>) -> T, F: Send + 'env, T: Send + 'env, { self.builder().spawn(f).unwrap() } /// Creates a builder that can configure a thread before spawning. /// /// # Examples /// /// ``` /// use crossbeam_utils::thread; /// use std::thread::current; /// /// thread::scope(|s| { /// s.builder() /// .spawn(|_| println!("A child thread is running")) /// .unwrap(); /// }).unwrap(); /// ``` pub fn builder<'scope>(&'scope self) -> ScopedThreadBuilder<'scope, 'env> { ScopedThreadBuilder { scope: self, builder: thread::Builder::new(), } } } impl<'env> fmt::Debug for Scope<'env> { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { f.pad("Scope { .. }") } } /// Configures the properties of a new thread. /// /// The two configurable properties are: /// /// - [`name`]: Specifies an [associated name for the thread][naming-threads]. /// - [`stack_size`]: Specifies the [desired stack size for the thread][stack-size]. /// /// The [`spawn`] method will take ownership of the builder and return an [`io::Result`] of the /// thread handle with the given configuration. /// /// The [`Scope::spawn`] method uses a builder with default configuration and unwraps its return /// value. You may want to use this builder when you want to recover from a failure to launch a /// thread. /// /// # Examples /// /// ``` /// use crossbeam_utils::thread; /// /// thread::scope(|s| { /// s.builder() /// .spawn(|_| println!("Running a child thread")) /// .unwrap(); /// }).unwrap(); /// ``` /// /// [`name`]: struct.ScopedThreadBuilder.html#method.name /// [`stack_size`]: struct.ScopedThreadBuilder.html#method.stack_size /// [`spawn`]: struct.ScopedThreadBuilder.html#method.spawn /// [`Scope::spawn`]: struct.Scope.html#method.spawn /// [`io::Result`]: https://doc.rust-lang.org/std/io/type.Result.html /// [naming-threads]: https://doc.rust-lang.org/std/thread/index.html#naming-threads /// [stack-size]: https://doc.rust-lang.org/std/thread/index.html#stack-size #[derive(Debug)] pub struct ScopedThreadBuilder<'scope, 'env: 'scope> { scope: &'scope Scope<'env>, builder: thread::Builder, } impl<'scope, 'env> ScopedThreadBuilder<'scope, 'env> { /// Sets the name for the new thread. /// /// The name must not contain null bytes. For more information about named threads, see /// [here][naming-threads]. /// /// # Examples /// /// ``` /// use crossbeam_utils::thread; /// use std::thread::current; /// /// thread::scope(|s| { /// s.builder() /// .name("my thread".to_string()) /// .spawn(|_| assert_eq!(current().name(), Some("my thread"))) /// .unwrap(); /// }).unwrap(); /// ``` /// /// [naming-threads]: https://doc.rust-lang.org/std/thread/index.html#naming-threads pub fn name(mut self, name: String) -> ScopedThreadBuilder<'scope, 'env> { self.builder = self.builder.name(name); self } /// Sets the size of the stack for the new thread. /// /// The stack size is measured in bytes. /// /// # Examples /// /// ``` /// use crossbeam_utils::thread; /// /// thread::scope(|s| { /// s.builder() /// .stack_size(32 * 1024) /// .spawn(|_| println!("Running a child thread")) /// .unwrap(); /// }).unwrap(); /// ``` pub fn stack_size(mut self, size: usize) -> ScopedThreadBuilder<'scope, 'env> { self.builder = self.builder.stack_size(size); self } /// Spawns a scoped thread with this configuration. /// /// The scoped thread is passed a reference to this scope as an argument, which can be used for /// spawning nested threads. /// /// The returned handle can be used to manually join the thread before the scope exits. /// /// # Examples /// /// ``` /// use crossbeam_utils::thread; /// /// thread::scope(|s| { /// let handle = s.builder() /// .spawn(|_| { /// println!("A child thread is running"); /// 42 /// }) /// .unwrap(); /// /// // Join the thread and retrieve its result. /// let res = handle.join().unwrap(); /// assert_eq!(res, 42); /// }).unwrap(); /// ``` pub fn spawn<F, T>(self, f: F) -> io::Result<ScopedJoinHandle<'scope, T>> where F: FnOnce(&Scope<'env>) -> T, F: Send + 'env, T: Send + 'env, { // The result of `f` will be stored here. let result = SharedOption::default(); // Spawn the thread and grab its join handle and thread handle. let (handle, thread) = { let result = Arc::clone(&result); // A clone of the scope that will be moved into the new thread. let scope = Scope::<'env> { handles: Arc::clone(&self.scope.handles), wait_group: self.scope.wait_group.clone(), _marker: PhantomData, }; // Spawn the thread. let handle = { let closure = move || { // Make sure the scope is inside the closure with the proper `'env` lifetime. let scope: Scope<'env> = scope; // Run the closure. let res = f(&scope); // Store the result if the closure didn't panic. *result.lock().unwrap() = Some(res); }; // Change the type of `closure` from `FnOnce() -> T` to `FnMut() -> T`. let mut closure = Some(closure); let closure = move || closure.take().unwrap()(); // Allocate `clsoure` on the heap and erase the `'env` bound. let closure: Box<FnMut() + Send + 'env> = Box::new(closure); let closure: Box<FnMut() + Send + 'static> = unsafe { mem::transmute(closure) }; // Finally, spawn the closure. let mut closure = closure; self.builder.spawn(move || closure())? }; let thread = handle.thread().clone(); let handle = Arc::new(Mutex::new(Some(handle))); (handle, thread) }; // Add the handle to the shared list of join handles. self.scope.handles.lock().unwrap().push(Arc::clone(&handle)); Ok(ScopedJoinHandle { handle, result, thread, _marker: PhantomData, }) } } unsafe impl<'scope, T> Send for ScopedJoinHandle<'scope, T> {} unsafe impl<'scope, T> Sync for ScopedJoinHandle<'scope, T> {} /// A handle that can be used to join its scoped thread. pub struct ScopedJoinHandle<'scope, T> { /// A join handle to the spawned thread. handle: SharedOption<thread::JoinHandle<()>>, /// Holds the result of the inner closure. result: SharedOption<T>, /// A handle to the the spawned thread. thread: thread::Thread, /// Borrows the parent scope with lifetime `'scope`. _marker: PhantomData<&'scope ()>, } impl<'scope, T> ScopedJoinHandle<'scope, T> { /// Waits for the thread to finish and returns its result. /// /// If the child thread panics, an error is returned. /// /// # Panics /// /// This function may panic on some platforms if a thread attempts to join itself or otherwise /// may create a deadlock with joining threads. /// /// # Examples /// /// ``` /// use crossbeam_utils::thread; /// /// thread::scope(|s| { /// let handle1 = s.spawn(|_| println!("I'm a happy thread :)")); /// let handle2 = s.spawn(|_| panic!("I'm a sad thread :(")); /// /// // Join the first thread and verify that it succeeded. /// let res = handle1.join(); /// assert!(res.is_ok()); /// /// // Join the second thread and verify that it panicked. /// let res = handle2.join(); /// assert!(res.is_err()); /// }).unwrap(); /// ``` pub fn join(self) -> thread::Result<T> { // Take out the handle. The handle will surely be available because the root scope waits // for nested scopes before joining remaining threads. let handle = self.handle.lock().unwrap().take().unwrap(); // Join the thread and then take the result out of its inner closure. handle .join() .map(|()| self.result.lock().unwrap().take().unwrap()) } /// Returns a handle to the underlying thread. /// /// # Examples /// /// ``` /// use crossbeam_utils::thread; /// /// thread::scope(|s| { /// let handle = s.spawn(|_| println!("A child thread is running")); /// println!("The child thread ID: {:?}", handle.thread().id()); /// }).unwrap(); /// ``` pub fn thread(&self) -> &thread::Thread { &self.thread } } impl<'scope, T> fmt::Debug for ScopedJoinHandle<'scope, T> { fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result { f.pad("ScopedJoinHandle { .. }") } }