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    //! Unix handling of child processes
    //!
    //! Right now the only "fancy" thing about this is how we implement the
    //! `Future` implementation on `Child` to get the exit status. Unix offers
    //! no way to register a child with epoll, and the only real way to get a
    //! notification when a process exits is the SIGCHLD signal.
    //!
    //! Signal handling in general is *super* hairy and complicated, and it's even
    //! more complicated here with the fact that signals are coalesced, so we may
    //! not get a SIGCHLD-per-child.
    //!
    //! Our best approximation here is to check *all spawned processes* for all
    //! SIGCHLD signals received. To do that we create a `Signal`, implemented in
    //! the `tokio-net` crate, which is a stream over signals being received.
    //!
    //! Later when we poll the process's exit status we simply check to see if a
    //! SIGCHLD has happened since we last checked, and while that returns "yes" we
    //! keep trying.
    //!
    //! Note that this means that this isn't really scalable, but then again
    //! processes in general aren't scalable (e.g. millions) so it shouldn't be that
    //! bad in theory...

    pub(crate) mod driver;

    pub(crate) mod orphan;
    use orphan::{OrphanQueue, OrphanQueueImpl, ReapOrphanQueue, Wait};

    mod reap;
    use reap::Reaper;

    use crate::io::PollEvented;
    use crate::process::kill::Kill;
    use crate::process::SpawnedChild;
    use crate::signal::unix::{signal, Signal, SignalKind};

    use mio::event::Source;
    use mio::unix::SourceFd;
    use once_cell::sync::Lazy;
    use std::fmt;
    use std::fs::File;
    use std::future::Future;
    use std::io;
    use std::os::unix::io::{AsRawFd, FromRawFd, IntoRawFd, RawFd};
    use std::pin::Pin;
    use std::process::{Child as StdChild, ExitStatus, Stdio};
    use std::task::Context;
    use std::task::Poll;

    impl Wait for StdChild {
        fn id(&self) -> u32 {
            self.id()
        }

        fn try_wait(&mut self) -> io::Result<Option<ExitStatus>> {
            self.try_wait()
        }
    }

    impl Kill for StdChild {
        fn kill(&mut self) -> io::Result<()> {
            self.kill()
        }
    }

    static ORPHAN_QUEUE: Lazy<OrphanQueueImpl<StdChild>> = Lazy::new(OrphanQueueImpl::new);

    pub(crate) struct GlobalOrphanQueue;

    impl fmt::Debug for GlobalOrphanQueue {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            ORPHAN_QUEUE.fmt(fmt)
        }
    }

    impl ReapOrphanQueue for GlobalOrphanQueue {
        fn reap_orphans(&self) {
            ORPHAN_QUEUE.reap_orphans()
        }
    }

    impl OrphanQueue<StdChild> for GlobalOrphanQueue {
        fn push_orphan(&self, orphan: StdChild) {
            ORPHAN_QUEUE.push_orphan(orphan)
        }
    }

    #[must_use = "futures do nothing unless polled"]
    pub(crate) struct Child {
        inner: Reaper<StdChild, GlobalOrphanQueue, Signal>,
    }

    impl fmt::Debug for Child {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            fmt.debug_struct("Child")
                .field("pid", &self.inner.id())
                .finish()
        }
    }

    pub(crate) fn spawn_child(cmd: &mut std::process::Command) -> io::Result<SpawnedChild> {
        let mut child = cmd.spawn()?;
        let stdin = stdio(child.stdin.take())?;
        let stdout = stdio(child.stdout.take())?;
        let stderr = stdio(child.stderr.take())?;

        let signal = signal(SignalKind::child())?;

        Ok(SpawnedChild {
            child: Child {
                inner: Reaper::new(child, GlobalOrphanQueue, signal),
            },
            stdin,
            stdout,
            stderr,
        })
    }

    impl Child {
        pub(crate) fn id(&self) -> u32 {
            self.inner.id()
        }

        pub(crate) fn try_wait(&mut self) -> io::Result<Option<ExitStatus>> {
            self.inner.inner_mut().try_wait()
        }
    }

    impl Kill for Child {
        fn kill(&mut self) -> io::Result<()> {
            self.inner.kill()
        }
    }

    impl Future for Child {
        type Output = io::Result<ExitStatus>;

        fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
            Pin::new(&mut self.inner).poll(cx)
        }
    }

    #[derive(Debug)]
    pub(crate) struct Pipe {
        // Actually a pipe and not a File. However, we are reusing `File` to get
        // close on drop. This is a similar trick as `mio`.
        fd: File,
    }

    impl<T: IntoRawFd> From<T> for Pipe {
        fn from(fd: T) -> Self {
            let fd = unsafe { File::from_raw_fd(fd.into_raw_fd()) };
            Self { fd }
        }
    }

    impl<'a> io::Read for &'a Pipe {
        fn read(&mut self, bytes: &mut [u8]) -> io::Result<usize> {
            (&self.fd).read(bytes)
        }
    }

    impl<'a> io::Write for &'a Pipe {
        fn write(&mut self, bytes: &[u8]) -> io::Result<usize> {
            (&self.fd).write(bytes)
        }

        fn flush(&mut self) -> io::Result<()> {
            (&self.fd).flush()
        }
    }

    impl AsRawFd for Pipe {
        fn as_raw_fd(&self) -> RawFd {
            self.fd.as_raw_fd()
        }
    }

    pub(crate) fn convert_to_stdio(io: PollEvented<Pipe>) -> io::Result<Stdio> {
        let mut fd = io.into_inner()?.fd;

        // Ensure that the fd to be inherited is set to *blocking* mode, as this
        // is the default that virtually all programs expect to have. Those
        // programs that know how to work with nonblocking stdio will know how to
        // change it to nonblocking mode.
        set_nonblocking(&mut fd, false)?;

        Ok(Stdio::from(fd))
    }

    impl Source for Pipe {
        fn register(
            &mut self,
            registry: &mio::Registry,
            token: mio::Token,
            interest: mio::Interest,
        ) -> io::Result<()> {
            SourceFd(&self.as_raw_fd()).register(registry, token, interest)
        }

        fn reregister(
            &mut self,
            registry: &mio::Registry,
            token: mio::Token,
            interest: mio::Interest,
        ) -> io::Result<()> {
            SourceFd(&self.as_raw_fd()).reregister(registry, token, interest)
        }

        fn deregister(&mut self, registry: &mio::Registry) -> io::Result<()> {
            SourceFd(&self.as_raw_fd()).deregister(registry)
        }
    }

    pub(crate) type ChildStdin = PollEvented<Pipe>;
    pub(crate) type ChildStdout = PollEvented<Pipe>;
    pub(crate) type ChildStderr = PollEvented<Pipe>;

    fn set_nonblocking<T: AsRawFd>(fd: &mut T, nonblocking: bool) -> io::Result<()> {
        unsafe {
            let fd = fd.as_raw_fd();
            let previous = libc::fcntl(fd, libc::F_GETFL);
            if previous == -1 {
                return Err(io::Error::last_os_error());
            }

            let new = if nonblocking {
                previous | libc::O_NONBLOCK
            } else {
                previous & !libc::O_NONBLOCK
            };

            let r = libc::fcntl(fd, libc::F_SETFL, new);
            if r == -1 {
                return Err(io::Error::last_os_error());
            }
        }

        Ok(())
    }

    fn stdio<T>(option: Option<T>) -> io::Result<Option<PollEvented<Pipe>>>
    where
        T: IntoRawFd,
    {
        let io = match option {
            Some(io) => io,
            None => return Ok(None),
        };

        // Set the fd to nonblocking before we pass it to the event loop
        let mut pipe = Pipe::from(io);
        set_nonblocking(&mut pipe, true)?;

        Ok(Some(PollEvented::new(pipe)?))
    }