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
use crate::enter;
use futures_core::future::Future;
use futures_core::stream::Stream;
use futures_core::task::{Context, Poll};
use futures_task::{waker_ref, ArcWake};
use futures_task::{FutureObj, LocalFutureObj, LocalSpawn, Spawn, SpawnError};
use futures_util::pin_mut;
use futures_util::stream::FuturesUnordered;
use futures_util::stream::StreamExt;
use std::cell::RefCell;
use std::ops::{Deref, DerefMut};
use std::rc::{Rc, Weak};
use std::sync::{Arc, atomic::{AtomicBool, Ordering}};
use std::thread::{self, Thread};

/// A single-threaded task pool for polling futures to completion.
///
/// This executor allows you to multiplex any number of tasks onto a single
/// thread. It's appropriate to poll strictly I/O-bound futures that do very
/// little work in between I/O actions.
///
/// To get a handle to the pool that implements
/// [`Spawn`](futures_task::Spawn), use the
/// [`spawner()`](LocalPool::spawner) method. Because the executor is
/// single-threaded, it supports a special form of task spawning for non-`Send`
/// futures, via [`spawn_local_obj`](futures_task::LocalSpawn::spawn_local_obj).
#[derive(Debug)]
pub struct LocalPool {
    pool: FuturesUnordered<LocalFutureObj<'static, ()>>,
    incoming: Rc<Incoming>,
}

/// A handle to a [`LocalPool`](LocalPool) that implements
/// [`Spawn`](futures_task::Spawn).
#[derive(Clone, Debug)]
pub struct LocalSpawner {
    incoming: Weak<Incoming>,
}

type Incoming = RefCell<Vec<LocalFutureObj<'static, ()>>>;

pub(crate) struct ThreadNotify {
    /// The (single) executor thread.
    thread: Thread,
    /// A flag to ensure a wakeup (i.e. `unpark()`) is not "forgotten"
    /// before the next `park()`, which may otherwise happen if the code
    /// being executed as part of the future(s) being polled makes use of
    /// park / unpark calls of its own, i.e. we cannot assume that no other
    /// code uses park / unpark on the executing `thread`.
    unparked: AtomicBool,
}

thread_local! {
    static CURRENT_THREAD_NOTIFY: Arc<ThreadNotify> = Arc::new(ThreadNotify {
        thread: thread::current(),
        unparked: AtomicBool::new(false),
    });
}

impl ArcWake for ThreadNotify {
    fn wake_by_ref(arc_self: &Arc<Self>) {
        // Make sure the wakeup is remembered until the next `park()`.
        let unparked = arc_self.unparked.swap(true, Ordering::Relaxed);
        if !unparked {
            // If the thread has not been unparked yet, it must be done
            // now. If it was actually parked, it will run again,
            // otherwise the token made available by `unpark`
            // may be consumed before reaching `park()`, but `unparked`
            // ensures it is not forgotten.
            arc_self.thread.unpark();
        }
    }
}

// Set up and run a basic single-threaded spawner loop, invoking `f` on each
// turn.
fn run_executor<T, F: FnMut(&mut Context<'_>) -> Poll<T>>(mut f: F) -> T {
    let _enter = enter().expect(
        "cannot execute `LocalPool` executor from within \
         another executor",
    );

    CURRENT_THREAD_NOTIFY.with(|thread_notify| {
        let waker = waker_ref(thread_notify);
        let mut cx = Context::from_waker(&waker);
        loop {
            if let Poll::Ready(t) = f(&mut cx) {
                return t;
            }
            // Consume the wakeup that occurred while executing `f`, if any.
            let unparked = thread_notify.unparked.swap(false, Ordering::Acquire);
            if !unparked {
                // No wakeup occurred. It may occur now, right before parking,
                // but in that case the token made available by `unpark()`
                // is guaranteed to still be available and `park()` is a no-op.
                thread::park();
                // When the thread is unparked, `unparked` will have been set
                // and needs to be unset before the next call to `f` to avoid
                // a redundant loop iteration.
                thread_notify.unparked.store(false, Ordering::Release);
            }
        }
    })
}

fn poll_executor<T, F: FnMut(&mut Context<'_>) -> T>(mut f: F) -> T {
    let _enter = enter().expect(
        "cannot execute `LocalPool` executor from within \
         another executor",
    );

    CURRENT_THREAD_NOTIFY.with(|thread_notify| {
        let waker = waker_ref(thread_notify);
        let mut cx = Context::from_waker(&waker);
        f(&mut cx)
    })
}

impl LocalPool {
    /// Create a new, empty pool of tasks.
    pub fn new() -> Self {
        Self {
            pool: FuturesUnordered::new(),
            incoming: Default::default(),
        }
    }

    /// Get a clonable handle to the pool as a [`Spawn`].
    pub fn spawner(&self) -> LocalSpawner {
        LocalSpawner {
            incoming: Rc::downgrade(&self.incoming),
        }
    }

    /// Run all tasks in the pool to completion.
    ///
    /// ```
    /// use futures::executor::LocalPool;
    ///
    /// let mut pool = LocalPool::new();
    ///
    /// // ... spawn some initial tasks using `spawn.spawn()` or `spawn.spawn_local()`
    ///
    /// // run *all* tasks in the pool to completion, including any newly-spawned ones.
    /// pool.run();
    /// ```
    ///
    /// The function will block the calling thread until *all* tasks in the pool
    /// are complete, including any spawned while running existing tasks.
    pub fn run(&mut self) {
        run_executor(|cx| self.poll_pool(cx))
    }

    /// Runs all the tasks in the pool until the given future completes.
    ///
    /// ```
    /// use futures::executor::LocalPool;
    ///
    /// let mut pool = LocalPool::new();
    /// # let my_app  = async {};
    ///
    /// // run tasks in the pool until `my_app` completes
    /// pool.run_until(my_app);
    /// ```
    ///
    /// The function will block the calling thread *only* until the future `f`
    /// completes; there may still be incomplete tasks in the pool, which will
    /// be inert after the call completes, but can continue with further use of
    /// one of the pool's run or poll methods. While the function is running,
    /// however, all tasks in the pool will try to make progress.
    pub fn run_until<F: Future>(&mut self, future: F) -> F::Output {
        pin_mut!(future);

        run_executor(|cx| {
            {
                // if our main task is done, so are we
                let result = future.as_mut().poll(cx);
                if let Poll::Ready(output) = result {
                    return Poll::Ready(output);
                }
            }

            let _ = self.poll_pool(cx);
            Poll::Pending
        })
    }

    /// Runs all tasks and returns after completing one future or until no more progress
    /// can be made. Returns `true` if one future was completed, `false` otherwise.
    ///
    /// ```
    /// use futures::executor::LocalPool;
    /// use futures::task::LocalSpawnExt;
    /// use futures::future::{ready, pending};
    ///
    /// let mut pool = LocalPool::new();
    /// let spawner = pool.spawner();
    ///
    /// spawner.spawn_local(ready(())).unwrap();
    /// spawner.spawn_local(ready(())).unwrap();
    /// spawner.spawn_local(pending()).unwrap();
    ///
    /// // Run the two ready tasks and return true for them.
    /// pool.try_run_one(); // returns true after completing one of the ready futures
    /// pool.try_run_one(); // returns true after completing the other ready future
    ///
    /// // the remaining task can not be completed
    /// assert!(!pool.try_run_one()); // returns false
    /// ```
    ///
    /// This function will not block the calling thread and will return the moment
    /// that there are no tasks left for which progress can be made or after exactly one
    /// task was completed; Remaining incomplete tasks in the pool can continue with
    /// further use of one of the pool's run or poll methods.
    /// Though only one task will be completed, progress may be made on multiple tasks.
    pub fn try_run_one(&mut self) -> bool {
        poll_executor(|ctx| {
            loop {
                let ret = self.poll_pool_once(ctx);

                // return if we have executed a future
                if let Poll::Ready(Some(_)) = ret {
                    return true;
                }

                // if there are no new incoming futures
                // then there is no feature that can make progress
                // and we can return without having completed a single future
                if self.incoming.borrow().is_empty() {
                    return false;
                }
            }
        })
    }

    /// Runs all tasks in the pool and returns if no more progress can be made
    /// on any task.
    ///
    /// ```
    /// use futures::executor::LocalPool;
    /// use futures::task::LocalSpawnExt;
    /// use futures::future::{ready, pending};
    ///
    /// let mut pool = LocalPool::new();
    /// let spawner = pool.spawner();
    ///
    /// spawner.spawn_local(ready(())).unwrap();
    /// spawner.spawn_local(ready(())).unwrap();
    /// spawner.spawn_local(pending()).unwrap();
    ///
    /// // Runs the two ready task and returns.
    /// // The empty task remains in the pool.
    /// pool.run_until_stalled();
    /// ```
    ///
    /// This function will not block the calling thread and will return the moment
    /// that there are no tasks left for which progress can be made;
    /// remaining incomplete tasks in the pool can continue with further use of one
    /// of the pool's run or poll methods. While the function is running, all tasks
    /// in the pool will try to make progress.
    pub fn run_until_stalled(&mut self) {
        poll_executor(|ctx| {
            let _ = self.poll_pool(ctx);
        });
    }

    // Make maximal progress on the entire pool of spawned task, returning `Ready`
    // if the pool is empty and `Pending` if no further progress can be made.
    fn poll_pool(&mut self, cx: &mut Context<'_>) -> Poll<()> {
        // state for the FuturesUnordered, which will never be used
        loop {
            let ret = self.poll_pool_once(cx);

            // we queued up some new tasks; add them and poll again
            if !self.incoming.borrow().is_empty() {
                continue;
            }

            // no queued tasks; we may be done
            match ret {
                Poll::Pending => return Poll::Pending,
                Poll::Ready(None) => return Poll::Ready(()),
                _ => {}
            }
        }
    }

    // Try make minimal progress on the pool of spawned tasks
    fn poll_pool_once(&mut self, cx: &mut Context<'_>) -> Poll<Option<()>> {
        // empty the incoming queue of newly-spawned tasks
        {
            let mut incoming = self.incoming.borrow_mut();
            for task in incoming.drain(..) {
                self.pool.push(task)
            }
        }

        // try to execute the next ready future
        self.pool.poll_next_unpin(cx)
    }
}

impl Default for LocalPool {
    fn default() -> Self {
        Self::new()
    }
}

/// Run a future to completion on the current thread.
///
/// This function will block the caller until the given future has completed.
///
/// Use a [`LocalPool`](LocalPool) if you need finer-grained control over
/// spawned tasks.
pub fn block_on<F: Future>(f: F) -> F::Output {
    pin_mut!(f);
    run_executor(|cx| f.as_mut().poll(cx))
}

/// Turn a stream into a blocking iterator.
///
/// When `next` is called on the resulting `BlockingStream`, the caller
/// will be blocked until the next element of the `Stream` becomes available.
pub fn block_on_stream<S: Stream + Unpin>(stream: S) -> BlockingStream<S> {
    BlockingStream { stream }
}

/// An iterator which blocks on values from a stream until they become available.
#[derive(Debug)]
pub struct BlockingStream<S: Stream + Unpin> {
    stream: S,
}

impl<S: Stream + Unpin> Deref for BlockingStream<S> {
    type Target = S;
    fn deref(&self) -> &Self::Target {
        &self.stream
    }
}

impl<S: Stream + Unpin> DerefMut for BlockingStream<S> {
    fn deref_mut(&mut self) -> &mut Self::Target {
        &mut self.stream
    }
}

impl<S: Stream + Unpin> BlockingStream<S> {
    /// Convert this `BlockingStream` into the inner `Stream` type.
    pub fn into_inner(self) -> S {
        self.stream
    }
}

impl<S: Stream + Unpin> Iterator for BlockingStream<S> {
    type Item = S::Item;

    fn next(&mut self) -> Option<Self::Item> {
        LocalPool::new().run_until(self.stream.next())
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        self.stream.size_hint()
    }
}

impl Spawn for LocalSpawner {
    fn spawn_obj(&self, future: FutureObj<'static, ()>) -> Result<(), SpawnError> {
        if let Some(incoming) = self.incoming.upgrade() {
            incoming.borrow_mut().push(future.into());
            Ok(())
        } else {
            Err(SpawnError::shutdown())
        }
    }

    fn status(&self) -> Result<(), SpawnError> {
        if self.incoming.upgrade().is_some() {
            Ok(())
        } else {
            Err(SpawnError::shutdown())
        }
    }
}

impl LocalSpawn for LocalSpawner {
    fn spawn_local_obj(&self, future: LocalFutureObj<'static, ()>) -> Result<(), SpawnError> {
        if let Some(incoming) = self.incoming.upgrade() {
            incoming.borrow_mut().push(future);
            Ok(())
        } else {
            Err(SpawnError::shutdown())
        }
    }

    fn status_local(&self) -> Result<(), SpawnError> {
        if self.incoming.upgrade().is_some() {
            Ok(())
        } else {
            Err(SpawnError::shutdown())
        }
    }
}