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use std::cell::UnsafeCell;
use std::mem::MaybeUninit;
use std::sync::{Condvar, Mutex, Once};
use std::time::{Duration as StdDuration, Instant as StdInstant};
use std::{mem, ptr, thread};
use atomic_polyfill::{AtomicU8, Ordering};
use crate::driver::{AlarmHandle, Driver};
const ALARM_COUNT: usize = 4;
struct AlarmState {
timestamp: u64,
// This is really a Option<(fn(*mut ()), *mut ())>
// but fn pointers aren't allowed in const yet
callback: *const (),
ctx: *mut (),
}
unsafe impl Send for AlarmState {}
impl AlarmState {
const fn new() -> Self {
Self {
timestamp: u64::MAX,
callback: ptr::null(),
ctx: ptr::null_mut(),
}
}
}
struct TimeDriver {
alarm_count: AtomicU8,
once: Once,
alarms: UninitCell<Mutex<[AlarmState; ALARM_COUNT]>>,
zero_instant: UninitCell<StdInstant>,
signaler: UninitCell<Signaler>,
}
const ALARM_NEW: AlarmState = AlarmState::new();
crate::time_driver_impl!(static DRIVER: TimeDriver = TimeDriver {
alarm_count: AtomicU8::new(0),
once: Once::new(),
alarms: UninitCell::uninit(),
zero_instant: UninitCell::uninit(),
signaler: UninitCell::uninit(),
});
impl TimeDriver {
fn init(&self) {
self.once.call_once(|| unsafe {
self.alarms.write(Mutex::new([ALARM_NEW; ALARM_COUNT]));
self.zero_instant.write(StdInstant::now());
self.signaler.write(Signaler::new());
thread::spawn(Self::alarm_thread);
});
}
fn alarm_thread() {
let zero = unsafe { DRIVER.zero_instant.read() };
loop {
let now = DRIVER.now();
let mut next_alarm = u64::MAX;
{
let alarms = &mut *unsafe { DRIVER.alarms.as_ref() }.lock().unwrap();
for alarm in alarms {
if alarm.timestamp <= now {
alarm.timestamp = u64::MAX;
// Call after clearing alarm, so the callback can set another alarm.
// safety:
// - we can ignore the possiblity of `f` being unset (null) because of the safety contract of `allocate_alarm`.
// - other than that we only store valid function pointers into alarm.callback
let f: fn(*mut ()) = unsafe { mem::transmute(alarm.callback) };
f(alarm.ctx);
} else {
next_alarm = next_alarm.min(alarm.timestamp);
}
}
}
// Ensure we don't overflow
let until = zero
.checked_add(StdDuration::from_micros(next_alarm))
.unwrap_or_else(|| StdInstant::now() + StdDuration::from_secs(1));
unsafe { DRIVER.signaler.as_ref() }.wait_until(until);
}
}
}
impl Driver for TimeDriver {
fn now(&self) -> u64 {
self.init();
let zero = unsafe { self.zero_instant.read() };
StdInstant::now().duration_since(zero).as_micros() as u64
}
unsafe fn allocate_alarm(&self) -> Option<AlarmHandle> {
let id = self.alarm_count.fetch_update(Ordering::AcqRel, Ordering::Acquire, |x| {
if x < ALARM_COUNT as u8 {
Some(x + 1)
} else {
None
}
});
match id {
Ok(id) => Some(AlarmHandle::new(id)),
Err(_) => None,
}
}
fn set_alarm_callback(&self, alarm: AlarmHandle, callback: fn(*mut ()), ctx: *mut ()) {
self.init();
let mut alarms = unsafe { self.alarms.as_ref() }.lock().unwrap();
let alarm = &mut alarms[alarm.id() as usize];
alarm.callback = callback as *const ();
alarm.ctx = ctx;
}
fn set_alarm(&self, alarm: AlarmHandle, timestamp: u64) {
self.init();
let mut alarms = unsafe { self.alarms.as_ref() }.lock().unwrap();
let alarm = &mut alarms[alarm.id() as usize];
alarm.timestamp = timestamp;
unsafe { self.signaler.as_ref() }.signal();
}
}
struct Signaler {
mutex: Mutex<bool>,
condvar: Condvar,
}
impl Signaler {
fn new() -> Self {
Self {
mutex: Mutex::new(false),
condvar: Condvar::new(),
}
}
fn wait_until(&self, until: StdInstant) {
let mut signaled = self.mutex.lock().unwrap();
while !*signaled {
let now = StdInstant::now();
if now >= until {
break;
}
let dur = until - now;
let (signaled2, timeout) = self.condvar.wait_timeout(signaled, dur).unwrap();
signaled = signaled2;
if timeout.timed_out() {
break;
}
}
*signaled = false;
}
fn signal(&self) {
let mut signaled = self.mutex.lock().unwrap();
*signaled = true;
self.condvar.notify_one();
}
}
pub(crate) struct UninitCell<T>(MaybeUninit<UnsafeCell<T>>);
unsafe impl<T> Send for UninitCell<T> {}
unsafe impl<T> Sync for UninitCell<T> {}
impl<T> UninitCell<T> {
pub const fn uninit() -> Self {
Self(MaybeUninit::uninit())
}
pub unsafe fn as_ptr(&self) -> *const T {
(*self.0.as_ptr()).get()
}
pub unsafe fn as_mut_ptr(&self) -> *mut T {
(*self.0.as_ptr()).get()
}
pub unsafe fn as_ref(&self) -> &T {
&*self.as_ptr()
}
pub unsafe fn write(&self, val: T) {
ptr::write(self.as_mut_ptr(), val)
}
}
impl<T: Copy> UninitCell<T> {
pub unsafe fn read(&self) -> T {
ptr::read(self.as_mut_ptr())
}
}
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