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//! Parker implementation based on a Mutex and Condvar.
use crate::pin::Pin;
use crate::sync::atomic::AtomicUsize;
use crate::sync::atomic::Ordering::SeqCst;
use crate::sync::{Condvar, Mutex};
use crate::time::Duration;
const EMPTY: usize = 0;
const PARKED: usize = 1;
const NOTIFIED: usize = 2;
pub struct Parker {
state: AtomicUsize,
lock: Mutex<()>,
cvar: Condvar,
}
impl Parker {
/// Construct the generic parker. The UNIX parker implementation
/// requires this to happen in-place.
pub unsafe fn new_in_place(parker: *mut Parker) {
parker.write(Parker {
state: AtomicUsize::new(EMPTY),
lock: Mutex::new(()),
cvar: Condvar::new(),
});
}
// This implementation doesn't require `unsafe` and `Pin`, but other implementations do.
pub unsafe fn park(self: Pin<&Self>) {
// If we were previously notified then we consume this notification and
// return quickly.
if self.state.compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst).is_ok() {
return;
}
// Otherwise we need to coordinate going to sleep
let mut m = self.lock.lock().unwrap();
match self.state.compare_exchange(EMPTY, PARKED, SeqCst, SeqCst) {
Ok(_) => {}
Err(NOTIFIED) => {
// We must read here, even though we know it will be `NOTIFIED`.
// This is because `unpark` may have been called again since we read
// `NOTIFIED` in the `compare_exchange` above. We must perform an
// acquire operation that synchronizes with that `unpark` to observe
// any writes it made before the call to unpark. To do that we must
// read from the write it made to `state`.
let old = self.state.swap(EMPTY, SeqCst);
assert_eq!(old, NOTIFIED, "park state changed unexpectedly");
return;
} // should consume this notification, so prohibit spurious wakeups in next park.
Err(_) => panic!("inconsistent park state"),
}
loop {
m = self.cvar.wait(m).unwrap();
match self.state.compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst) {
Ok(_) => return, // got a notification
Err(_) => {} // spurious wakeup, go back to sleep
}
}
}
// This implementation doesn't require `unsafe` and `Pin`, but other implementations do.
pub unsafe fn park_timeout(self: Pin<&Self>, dur: Duration) {
// Like `park` above we have a fast path for an already-notified thread, and
// afterwards we start coordinating for a sleep.
// return quickly.
if self.state.compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst).is_ok() {
return;
}
let m = self.lock.lock().unwrap();
match self.state.compare_exchange(EMPTY, PARKED, SeqCst, SeqCst) {
Ok(_) => {}
Err(NOTIFIED) => {
// We must read again here, see `park`.
let old = self.state.swap(EMPTY, SeqCst);
assert_eq!(old, NOTIFIED, "park state changed unexpectedly");
return;
} // should consume this notification, so prohibit spurious wakeups in next park.
Err(_) => panic!("inconsistent park_timeout state"),
}
// Wait with a timeout, and if we spuriously wake up or otherwise wake up
// from a notification we just want to unconditionally set the state back to
// empty, either consuming a notification or un-flagging ourselves as
// parked.
let (_m, _result) = self.cvar.wait_timeout(m, dur).unwrap();
match self.state.swap(EMPTY, SeqCst) {
NOTIFIED => {} // got a notification, hurray!
PARKED => {} // no notification, alas
n => panic!("inconsistent park_timeout state: {n}"),
}
}
// This implementation doesn't require `Pin`, but other implementations do.
pub fn unpark(self: Pin<&Self>) {
// To ensure the unparked thread will observe any writes we made
// before this call, we must perform a release operation that `park`
// can synchronize with. To do that we must write `NOTIFIED` even if
// `state` is already `NOTIFIED`. That is why this must be a swap
// rather than a compare-and-swap that returns if it reads `NOTIFIED`
// on failure.
match self.state.swap(NOTIFIED, SeqCst) {
EMPTY => return, // no one was waiting
NOTIFIED => return, // already unparked
PARKED => {} // gotta go wake someone up
_ => panic!("inconsistent state in unpark"),
}
// There is a period between when the parked thread sets `state` to
// `PARKED` (or last checked `state` in the case of a spurious wake
// up) and when it actually waits on `cvar`. If we were to notify
// during this period it would be ignored and then when the parked
// thread went to sleep it would never wake up. Fortunately, it has
// `lock` locked at this stage so we can acquire `lock` to wait until
// it is ready to receive the notification.
//
// Releasing `lock` before the call to `notify_one` means that when the
// parked thread wakes it doesn't get woken only to have to wait for us
// to release `lock`.
drop(self.lock.lock().unwrap());
self.cvar.notify_one()
}
}