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    //! epoll support.
    //!
    //! This is an experiment, and it isn't yet clear whether epoll is the right
    //! level of abstraction at which to introduce safety. But it works fairly well
    //! in simple examples 🙂.
    //!
    //! # Examples
    //!
    //! ```rust,no_run
    //! # #![cfg_attr(io_lifetimes_use_std, feature(io_safety))]
    //! # #[cfg(feature = "net")]
    //! # fn main() -> std::io::Result<()> {
    //! use io_lifetimes::AsFd;
    //! use rustix::io::epoll::{self, Epoll};
    //! use rustix::io::{ioctl_fionbio, read, write};
    //! use rustix::net::{
    //!     accept, bind_v4, listen, socket, AddressFamily, Ipv4Addr, Protocol, SocketAddrV4,
    //!     SocketType,
    //! };
    //! use std::os::unix::io::AsRawFd;
    //!
    //! // Create a socket and listen on it.
    //! let listen_sock = socket(AddressFamily::INET, SocketType::STREAM, Protocol::default())?;
    //! bind_v4(&listen_sock, &SocketAddrV4::new(Ipv4Addr::LOCALHOST, 0))?;
    //! listen(&listen_sock, 1)?;
    //!
    //! // Create an epoll object. Using `Owning` here means the epoll object will
    //! // take ownership of the file descriptors registered with it.
    //! let epoll = Epoll::new(epoll::CreateFlags::CLOEXEC, epoll::Owning::new())?;
    //!
    //! // Remember the socket raw fd, which we use for comparisons only.
    //! let raw_listen_sock = listen_sock.as_fd().as_raw_fd();
    //!
    //! // Register the socket with the epoll object.
    //! epoll.add(listen_sock, epoll::EventFlags::IN)?;
    //!
    //! // Process events.
    //! let mut event_list = epoll::EventVec::with_capacity(4);
    //! loop {
    //!     epoll.wait(&mut event_list, -1)?;
    //!     for (_event_flags, target) in &event_list {
    //!         if target.as_raw_fd() == raw_listen_sock {
    //!             // Accept a new connection, set it to non-blocking, and
    //!             // register to be notified when it's ready to write to.
    //!             let conn_sock = accept(&*target)?;
    //!             ioctl_fionbio(&conn_sock, true)?;
    //!             epoll.add(conn_sock, epoll::EventFlags::OUT | epoll::EventFlags::ET)?;
    //!         } else {
    //!             // Write a message to the stream and then unregister it.
    //!             write(&*target, b"hello\n")?;
    //!             let _ = epoll.del(target)?;
    //!         }
    //!     }
    //! }
    //! # }
    //! # #[cfg(not(feature = "net"))]
    //! # fn main() {}
    //! ```

    #![allow(unsafe_code)]

    use super::super::c;
    use crate::backend::io::syscalls::{epoll_add, epoll_create, epoll_del, epoll_mod, epoll_wait};
    use crate::fd::{AsFd, AsRawFd, OwnedFd};
    #[cfg(feature = "std")]
    use crate::fd::{BorrowedFd, FromRawFd, IntoRawFd, RawFd};
    use crate::io;
    use alloc::vec::Vec;
    use bitflags::bitflags;
    use core::marker::PhantomData;
    use core::ptr::null;

    #[doc(inline)]
    pub use crate::io::context::*;

    bitflags! {
        /// `EPOLL_*` for use with [`Epoll::new`].
        pub struct CreateFlags: c::c_uint {
            /// `EPOLL_CLOEXEC`
            const CLOEXEC = linux_raw_sys::general::EPOLL_CLOEXEC;
        }
    }

    bitflags! {
        /// `EPOLL*` for use with [`Epoll::add`].
        #[derive(Default)]
        pub struct EventFlags: u32 {
            /// `EPOLLIN`
            const IN = linux_raw_sys::general::EPOLLIN as u32;

            /// `EPOLLOUT`
            const OUT = linux_raw_sys::general::EPOLLOUT as u32;

            /// `EPOLLPRI`
            const PRI = linux_raw_sys::general::EPOLLPRI as u32;

            /// `EPOLLERR`
            const ERR = linux_raw_sys::general::EPOLLERR as u32;

            /// `EPOLLHUP`
            const HUP = linux_raw_sys::general::EPOLLHUP as u32;

            /// `EPOLLET`
            const ET = linux_raw_sys::general::EPOLLET as u32;

            /// `EPOLLONESHOT`
            const ONESHOT = linux_raw_sys::general::EPOLLONESHOT as u32;

            /// `EPOLLWAKEUP`
            const WAKEUP = linux_raw_sys::general::EPOLLWAKEUP as u32;

            /// `EPOLLEXCLUSIVE`
            const EXCLUSIVE = linux_raw_sys::general::EPOLLEXCLUSIVE as u32;
        }
    }

    /// An "epoll", an interface to an OS object allowing one to repeatedly wait
    /// for events from a set of file descriptors efficiently.
    pub struct Epoll<Context: self::Context> {
        epoll_fd: OwnedFd,
        context: Context,
    }

    impl<Context: self::Context> Epoll<Context> {
        /// `epoll_create1(flags)`—Creates a new `Epoll`.
        ///
        /// Use the [`CreateFlags::CLOEXEC`] flag to prevent the resulting file
        /// descriptor from being implicitly passed across `exec` boundaries.
        #[inline]
        #[doc(alias = "epoll_create1")]
        pub fn new(flags: CreateFlags, context: Context) -> io::Result<Self> {
            // Safety: We're calling `epoll_create1` via FFI and we know how it
            // behaves.
            Ok(Self {
                epoll_fd: epoll_create(flags)?,
                context,
            })
        }

        /// `epoll_ctl(self, EPOLL_CTL_ADD, data, event)`—Adds an element to an
        /// `Epoll`.
        ///
        /// This registers interest in any of the events set in `events` occurring
        /// on the file descriptor associated with `data`.
        #[doc(alias = "epoll_ctl")]
        pub fn add(
            &self,
            data: Context::Data,
            event_flags: EventFlags,
        ) -> io::Result<Ref<'_, Context::Target>> {
            // Safety: We're calling `epoll_ctl` via FFI and we know how it
            // behaves.
            unsafe {
                let target = self.context.acquire(data);
                let raw_fd = target.as_fd().as_raw_fd();
                let encoded = self.context.encode(target);
                epoll_add(
                    self.epoll_fd.as_fd(),
                    raw_fd,
                    &linux_raw_sys::general::epoll_event {
                        events: event_flags.bits(),
                        data: encoded,
                    },
                )?;
                Ok(self.context.decode(encoded))
            }
        }

        /// `epoll_ctl(self, EPOLL_CTL_MOD, target, event)`—Modifies an element in
        /// this `Epoll`.
        ///
        /// This sets the events of interest with `target` to `events`.
        #[doc(alias = "epoll_ctl")]
        pub fn mod_(
            &self,
            target: Ref<'_, Context::Target>,
            event_flags: EventFlags,
        ) -> io::Result<()> {
            let raw_fd = target.as_fd().as_raw_fd();
            let encoded = self.context.encode(target);
            // Safety: We're calling `epoll_ctl` via FFI and we know how it
            // behaves.
            unsafe {
                epoll_mod(
                    self.epoll_fd.as_fd(),
                    raw_fd,
                    &linux_raw_sys::general::epoll_event {
                        events: event_flags.bits(),
                        data: encoded,
                    },
                )
            }
        }

        /// `epoll_ctl(self, EPOLL_CTL_DEL, target, NULL)`—Removes an element in
        /// this `Epoll`.
        ///
        /// This also returns the owning `Data`.
        #[doc(alias = "epoll_ctl")]
        pub fn del(&self, target: Ref<'_, Context::Target>) -> io::Result<Context::Data> {
            // Safety: We're calling `epoll_ctl` via FFI and we know how it
            // behaves.
            unsafe {
                let raw_fd = target.as_fd().as_raw_fd();
                epoll_del(self.epoll_fd.as_fd(), raw_fd)?;
            }
            Ok(self.context.release(target))
        }

        /// `epoll_wait(self, events, timeout)`—Waits for registered events of
        /// interest.
        ///
        /// For each event of interest, an element is written to `events`. On
        /// success, this returns the number of written elements.
        #[doc(alias = "epoll_wait")]
        pub fn wait<'context>(
            &'context self,
            event_list: &mut EventVec<'context, Context>,
            timeout: c::c_int,
        ) -> io::Result<()> {
            // Safety: We're calling `epoll_wait` via FFI and we know how it
            // behaves.
            unsafe {
                event_list.events.set_len(0);
                let nfds = epoll_wait(
                    self.epoll_fd.as_fd(),
                    event_list.events[..].as_mut_ptr().cast(),
                    event_list.events.capacity(),
                    timeout,
                )?;
                event_list.events.set_len(nfds);
                event_list.context = &self.context;
            }

            Ok(())
        }
    }

    #[cfg(feature = "std")]
    impl<'context, T: AsFd + Into<OwnedFd> + From<OwnedFd>> AsRawFd for Epoll<Owning<'context, T>> {
        fn as_raw_fd(&self) -> RawFd {
            self.epoll_fd.as_raw_fd()
        }
    }

    #[cfg(feature = "std")]
    impl<'context, T: AsFd + Into<OwnedFd> + From<OwnedFd>> IntoRawFd for Epoll<Owning<'context, T>> {
        fn into_raw_fd(self) -> RawFd {
            self.epoll_fd.into_raw_fd()
        }
    }

    #[cfg(feature = "std")]
    impl<'context, T: AsFd + Into<OwnedFd> + From<OwnedFd>> FromRawFd for Epoll<Owning<'context, T>> {
        unsafe fn from_raw_fd(fd: RawFd) -> Self {
            Self {
                epoll_fd: OwnedFd::from_raw_fd(fd),
                context: Owning::new(),
            }
        }
    }

    #[cfg(feature = "std")]
    impl<'context, T: AsFd + Into<OwnedFd> + From<OwnedFd>> AsFd for Epoll<Owning<'context, T>> {
        fn as_fd(&self) -> BorrowedFd<'_> {
            self.epoll_fd.as_fd()
        }
    }

    #[cfg(feature = "std")]
    impl<'context, T: AsFd + Into<OwnedFd> + From<OwnedFd>> From<Epoll<Owning<'context, T>>>
        for OwnedFd
    {
        fn from(epoll: Epoll<Owning<'context, T>>) -> Self {
            epoll.epoll_fd
        }
    }

    #[cfg(feature = "std")]
    impl<'context, T: AsFd + Into<OwnedFd> + From<OwnedFd>> From<OwnedFd>
        for Epoll<Owning<'context, T>>
    {
        fn from(fd: OwnedFd) -> Self {
            Self {
                epoll_fd: fd,
                context: Owning::new(),
            }
        }
    }

    /// An iterator over the `Event`s in an `EventVec`.
    pub struct Iter<'context, Context: self::Context> {
        iter: core::slice::Iter<'context, Event>,
        context: *const Context,
        _phantom: PhantomData<&'context Context>,
    }

    impl<'context, Context: self::Context> Iterator for Iter<'context, Context> {
        type Item = (EventFlags, Ref<'context, Context::Target>);

        fn next(&mut self) -> Option<Self::Item> {
            self.iter.next().map(|event| {
                // Safety: `self.context` is guaranteed to be valid because we hold
                // `'context` for it. And we know this event is associated with this
                // context because `wait` sets both.
                let decoded = unsafe { (*self.context).decode(event.encoded) };

                (event.event_flags, decoded)
            })
        }
    }

    /// A record of an event that occurred.
    #[repr(C)]
    #[cfg_attr(target_arch = "x86_64", repr(packed))]
    struct Event {
        // Match the layout of `linux_raw_sys::general::epoll_event`. We just use a
        // `u64` instead of the full union; `Context` implementations will simply
        // need to deal with casting the value into and out of the `u64`
        // themselves.
        event_flags: EventFlags,
        encoded: u64,
    }

    /// A vector of `Event`s, plus context for interpreting them.
    pub struct EventVec<'context, Context: self::Context> {
        events: Vec<Event>,
        context: *const Context,
        _phantom: PhantomData<&'context Context>,
    }

    impl<'context, Context: self::Context> EventVec<'context, Context> {
        /// Constructs an `EventVec` with memory for `capacity` `Event`s.
        #[inline]
        pub fn with_capacity(capacity: usize) -> Self {
            Self {
                events: Vec::with_capacity(capacity),
                context: null(),
                _phantom: PhantomData,
            }
        }

        /// Returns the current `Event` capacity of this `EventVec`.
        #[inline]
        pub fn capacity(&self) -> usize {
            self.events.capacity()
        }

        /// Reserves enough memory for at least `additional` more `Event`s.
        #[inline]
        pub fn reserve(&mut self, additional: usize) {
            self.events.reserve(additional);
        }

        /// Reserves enough memory for exactly `additional` more `Event`s.
        #[inline]
        pub fn reserve_exact(&mut self, additional: usize) {
            self.events.reserve_exact(additional);
        }

        /// Clears all the `Events` out of this `EventVec`.
        #[inline]
        pub fn clear(&mut self) {
            self.events.clear();
        }

        /// Shrinks the capacity of this `EventVec` as much as possible.
        #[inline]
        pub fn shrink_to_fit(&mut self) {
            self.events.shrink_to_fit();
        }

        /// Returns an iterator over the `Event`s in this `EventVec`.
        #[inline]
        pub fn iter(&self) -> Iter<'_, Context> {
            Iter {
                iter: self.events.iter(),
                context: self.context,
                _phantom: PhantomData,
            }
        }

        /// Returns the number of `Event`s logically contained in this `EventVec`.
        #[inline]
        pub fn len(&mut self) -> usize {
            self.events.len()
        }

        /// Tests whether this `EventVec` is logically empty.
        #[inline]
        pub fn is_empty(&mut self) -> bool {
            self.events.is_empty()
        }
    }

    impl<'context, Context: self::Context> IntoIterator for &'context EventVec<'context, Context> {
        type IntoIter = Iter<'context, Context>;
        type Item = (EventFlags, Ref<'context, Context::Target>);

        #[inline]
        fn into_iter(self) -> Self::IntoIter {
            self.iter()
        }
    }