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 scope 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:
//!
//! ```
//! 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 { .. }")
    }
}