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    #![cfg_attr(not(feature = "rt"), allow(dead_code))]

    //! Process driver

    use crate::park::Park;
    use crate::process::unix::orphan::ReapOrphanQueue;
    use crate::process::unix::GlobalOrphanQueue;
    use crate::signal::unix::driver::Driver as SignalDriver;
    use crate::signal::unix::{signal_with_handle, InternalStream, Signal, SignalKind};
    use crate::sync::mpsc::error::TryRecvError;

    use std::io;
    use std::time::Duration;

    /// Responsible for cleaning up orphaned child processes on Unix platforms.
    #[derive(Debug)]
    pub(crate) struct Driver {
        park: SignalDriver,
        inner: CoreDriver<Signal, GlobalOrphanQueue>,
    }

    #[derive(Debug)]
    struct CoreDriver<S, Q> {
        sigchild: S,
        orphan_queue: Q,
    }

    // ===== impl CoreDriver =====

    impl<S, Q> CoreDriver<S, Q>
    where
        S: InternalStream,
        Q: ReapOrphanQueue,
    {
        fn got_signal(&mut self) -> bool {
            match self.sigchild.try_recv() {
                Ok(()) => true,
                Err(TryRecvError::Empty) => false,
                Err(TryRecvError::Closed) => panic!("signal was deregistered"),
            }
        }

        fn process(&mut self) {
            if self.got_signal() {
                // Drain all notifications which may have been buffered
                // so we can try to reap all orphans in one batch
                while self.got_signal() {}

                self.orphan_queue.reap_orphans();
            }
        }
    }

    // ===== impl Driver =====

    impl Driver {
        /// Creates a new signal `Driver` instance that delegates wakeups to `park`.
        pub(crate) fn new(park: SignalDriver) -> io::Result<Self> {
            let sigchild = signal_with_handle(SignalKind::child(), park.handle())?;
            let inner = CoreDriver {
                sigchild,
                orphan_queue: GlobalOrphanQueue,
            };

            Ok(Self { park, inner })
        }
    }

    // ===== impl Park for Driver =====

    impl Park for Driver {
        type Unpark = <SignalDriver as Park>::Unpark;
        type Error = io::Error;

        fn unpark(&self) -> Self::Unpark {
            self.park.unpark()
        }

        fn park(&mut self) -> Result<(), Self::Error> {
            self.park.park()?;
            self.inner.process();
            Ok(())
        }

        fn park_timeout(&mut self, duration: Duration) -> Result<(), Self::Error> {
            self.park.park_timeout(duration)?;
            self.inner.process();
            Ok(())
        }

        fn shutdown(&mut self) {
            self.park.shutdown()
        }
    }

    #[cfg(test)]
    mod test {
        use super::*;
        use crate::process::unix::orphan::test::MockQueue;
        use crate::sync::mpsc::error::TryRecvError;
        use std::task::{Context, Poll};

        struct MockStream {
            total_try_recv: usize,
            values: Vec<Option<()>>,
        }

        impl MockStream {
            fn new(values: Vec<Option<()>>) -> Self {
                Self {
                    total_try_recv: 0,
                    values,
                }
            }
        }

        impl InternalStream for MockStream {
            fn poll_recv(&mut self, _cx: &mut Context<'_>) -> Poll<Option<()>> {
                unimplemented!();
            }

            fn try_recv(&mut self) -> Result<(), TryRecvError> {
                self.total_try_recv += 1;
                match self.values.remove(0) {
                    Some(()) => Ok(()),
                    None => Err(TryRecvError::Empty),
                }
            }
        }

        #[test]
        fn no_reap_if_no_signal() {
            let mut driver = CoreDriver {
                sigchild: MockStream::new(vec![None]),
                orphan_queue: MockQueue::<()>::new(),
            };

            driver.process();

            assert_eq!(1, driver.sigchild.total_try_recv);
            assert_eq!(0, driver.orphan_queue.total_reaps.get());
        }

        #[test]
        fn coalesce_signals_before_reaping() {
            let mut driver = CoreDriver {
                sigchild: MockStream::new(vec![Some(()), Some(()), None]),
                orphan_queue: MockQueue::<()>::new(),
            };

            driver.process();

            assert_eq!(3, driver.sigchild.total_try_recv);
            assert_eq!(1, driver.orphan_queue.total_reaps.get());
        }
    }