About whether to support wait_timeout_until function in WaitQueue

[hky1999]

Background

In current implementation of wait_queue, wait_timeout_until function supports blocking the current task and put it into the wait queue, until the given condition becomes true, or the given duration has elapsed.

    pub fn wait_timeout_until<F>(&self, dur: core::time::Duration, condition: F) -> bool
    where
        F: Fn() -> bool,
    {}

This function is awkward because a AxTaskRef may co-exist in both wait queue and timer list, which brings complexity in cancel_events.

Question

When I try to port Condvar from rust std library, in function wait_timeout_while, I found it difficult to reuse the wait_timeout_until from wait queue.

    #[stable(feature = "wait_timeout_until", since = "1.42.0")]
    pub fn wait_timeout_while<'a, T, F>(
        &self,
        mut guard: MutexGuard<'a, T>,
        dur: Duration,
        mut condition: F,
    ) -> LockResult<(MutexGuard<'a, T>, WaitTimeoutResult)>
    where
        F: FnMut(&mut T) -> bool,
    {
        let start = Instant::now();
        loop {
            if !condition(&mut *guard) {
                return Ok((guard, WaitTimeoutResult(false)));
            }
            let timeout = match dur.checked_sub(start.elapsed()) {
                Some(timeout) => timeout,
                None => return Ok((guard, WaitTimeoutResult(true))),
            };
            guard = self.wait_timeout(guard, timeout)?.0;
        }
    }

It only needs to rely on wait_timeout, if we do want to reuse wait_timeout_until, we still need to mind the dropping and re-acquiring of MutexGuard.