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use std::cell::UnsafeCell;
use std::sync::atomic::AtomicUsize;
use std::sync::atomic::Ordering::SeqCst;
/// A "lock" around data `D`, which employs a *helping* strategy.
///
/// Used to ensure that concurrent `unpark` invocations lead to (1) `poll` being
/// invoked on only a single thread at a time (2) `poll` being invoked at least
/// once after each `unpark` (unless the future has completed).
pub(crate) struct UnparkMutex<D> {
// The state of task execution (state machine described below)
status: AtomicUsize,
// The actual task data, accessible only in the POLLING state
inner: UnsafeCell<Option<D>>,
}
// `UnparkMutex<D>` functions in many ways like a `Mutex<D>`, except that on
// acquisition failure, the current lock holder performs the desired work --
// re-polling.
//
// As such, these impls mirror those for `Mutex<D>`. In particular, a reference
// to `UnparkMutex` can be used to gain `&mut` access to the inner data, which
// must therefore be `Send`.
unsafe impl<D: Send> Send for UnparkMutex<D> {}
unsafe impl<D: Send> Sync for UnparkMutex<D> {}
// There are four possible task states, listed below with their possible
// transitions:
// The task is blocked, waiting on an event
const WAITING: usize = 0; // --> POLLING
// The task is actively being polled by a thread; arrival of additional events
// of interest should move it to the REPOLL state
const POLLING: usize = 1; // --> WAITING, REPOLL, or COMPLETE
// The task is actively being polled, but will need to be re-polled upon
// completion to ensure that all events were observed.
const REPOLL: usize = 2; // --> POLLING
// The task has finished executing (either successfully or with an error/panic)
const COMPLETE: usize = 3; // No transitions out
impl<D> UnparkMutex<D> {
pub(crate) fn new() -> Self {
Self { status: AtomicUsize::new(WAITING), inner: UnsafeCell::new(None) }
}
/// Attempt to "notify" the mutex that a poll should occur.
///
/// An `Ok` result indicates that the `POLLING` state has been entered, and
/// the caller can proceed to poll the future. An `Err` result indicates
/// that polling is not necessary (because the task is finished or the
/// polling has been delegated).
pub(crate) fn notify(&self) -> Result<D, ()> {
let mut status = self.status.load(SeqCst);
loop {
match status {
// The task is idle, so try to run it immediately.
WAITING => {
match self.status.compare_exchange(WAITING, POLLING, SeqCst, SeqCst) {
Ok(_) => {
let data = unsafe {
// SAFETY: we've ensured mutual exclusion via
// the status protocol; we are the only thread
// that has transitioned to the POLLING state,
// and we won't transition back to QUEUED until
// the lock is "released" by this thread. See
// the protocol diagram above.
(*self.inner.get()).take().unwrap()
};
return Ok(data);
}
Err(cur) => status = cur,
}
}
// The task is being polled, so we need to record that it should
// be *repolled* when complete.
POLLING => match self.status.compare_exchange(POLLING, REPOLL, SeqCst, SeqCst) {
Ok(_) => return Err(()),
Err(cur) => status = cur,
},
// The task is already scheduled for polling, or is complete, so
// we've got nothing to do.
_ => return Err(()),
}
}
}
/// Alert the mutex that polling is about to begin, clearing any accumulated
/// re-poll requests.
///
/// # Safety
///
/// Callable only from the `POLLING`/`REPOLL` states, i.e. between
/// successful calls to `notify` and `wait`/`complete`.
pub(crate) unsafe fn start_poll(&self) {
self.status.store(POLLING, SeqCst);
}
/// Alert the mutex that polling completed with `Pending`.
///
/// # Safety
///
/// Callable only from the `POLLING`/`REPOLL` states, i.e. between
/// successful calls to `notify` and `wait`/`complete`.
pub(crate) unsafe fn wait(&self, data: D) -> Result<(), D> {
*self.inner.get() = Some(data);
match self.status.compare_exchange(POLLING, WAITING, SeqCst, SeqCst) {
// no unparks came in while we were running
Ok(_) => Ok(()),
// guaranteed to be in REPOLL state; just clobber the
// state and run again.
Err(status) => {
assert_eq!(status, REPOLL);
self.status.store(POLLING, SeqCst);
Err((*self.inner.get()).take().unwrap())
}
}
}
/// Alert the mutex that the task has completed execution and should not be
/// notified again.
///
/// # Safety
///
/// Callable only from the `POLLING`/`REPOLL` states, i.e. between
/// successful calls to `notify` and `wait`/`complete`.
pub(crate) unsafe fn complete(&self) {
self.status.store(COMPLETE, SeqCst);
}
}
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