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    // Copyright 2015 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    use io;
    use libc::{self, c_int};
    use mem;
    use sync::atomic::{AtomicBool, ATOMIC_BOOL_INIT, Ordering};
    use sys::fd::FileDesc;
    use sys::{cvt, cvt_r};

    ////////////////////////////////////////////////////////////////////////////////
    // Anonymous pipes
    ////////////////////////////////////////////////////////////////////////////////

    pub struct AnonPipe(FileDesc);

    pub fn anon_pipe() -> io::Result<(AnonPipe, AnonPipe)> {
        weak! { fn pipe2(*mut c_int, c_int) -> c_int }
        static INVALID: AtomicBool = ATOMIC_BOOL_INIT;

        let mut fds = [0; 2];

        // Unfortunately the only known way right now to create atomically set the
        // CLOEXEC flag is to use the `pipe2` syscall on Linux. This was added in
        // 2.6.27, however, and because we support 2.6.18 we must detect this
        // support dynamically.
        if cfg!(any(target_os = "dragonfly",
                    target_os = "freebsd",
                    target_os = "linux",
                    target_os = "netbsd",
                    target_os = "openbsd")) &&
           !INVALID.load(Ordering::SeqCst)
        {

            if let Some(pipe) = pipe2.get() {
                // Note that despite calling a glibc function here we may still
                // get ENOSYS. Glibc has `pipe2` since 2.9 and doesn't try to
                // emulate on older kernels, so if you happen to be running on
                // an older kernel you may see `pipe2` as a symbol but still not
                // see the syscall.
                match cvt(unsafe { pipe(fds.as_mut_ptr(), libc::O_CLOEXEC) }) {
                    Ok(_) => {
                        return Ok((AnonPipe(FileDesc::new(fds[0])),
                                   AnonPipe(FileDesc::new(fds[1]))));
                    }
                    Err(ref e) if e.raw_os_error() == Some(libc::ENOSYS) => {
                        INVALID.store(true, Ordering::SeqCst);
                    }
                    Err(e) => return Err(e),
                }
            }
        }
        cvt(unsafe { libc::pipe(fds.as_mut_ptr()) })?;

        let fd0 = FileDesc::new(fds[0]);
        let fd1 = FileDesc::new(fds[1]);
        fd0.set_cloexec()?;
        fd1.set_cloexec()?;
        Ok((AnonPipe(fd0), AnonPipe(fd1)))
    }

    impl AnonPipe {
        pub fn read(&self, buf: &mut [u8]) -> io::Result<usize> {
            self.0.read(buf)
        }

        pub fn write(&self, buf: &[u8]) -> io::Result<usize> {
            self.0.write(buf)
        }

        pub fn fd(&self) -> &FileDesc { &self.0 }
        pub fn into_fd(self) -> FileDesc { self.0 }
    }

    pub fn read2(p1: AnonPipe,
                 v1: &mut Vec<u8>,
                 p2: AnonPipe,
                 v2: &mut Vec<u8>) -> io::Result<()> {

        // Set both pipes into nonblocking mode as we're gonna be reading from both
        // in the `select` loop below, and we wouldn't want one to block the other!
        let p1 = p1.into_fd();
        let p2 = p2.into_fd();
        p1.set_nonblocking(true)?;
        p2.set_nonblocking(true)?;

        let mut fds: [libc::pollfd; 2] = unsafe { mem::zeroed() };
        fds[0].fd = p1.raw();
        fds[0].events = libc::POLLIN;
        fds[1].fd = p2.raw();
        fds[1].events = libc::POLLIN;
        loop {
            // wait for either pipe to become readable using `poll`
            cvt_r(|| unsafe { libc::poll(fds.as_mut_ptr(), 2, -1) })?;

            // Read as much as we can from each pipe, ignoring EWOULDBLOCK or
            // EAGAIN. If we hit EOF, then this will happen because the underlying
            // reader will return Ok(0), in which case we'll see `Ok` ourselves. In
            // this case we flip the other fd back into blocking mode and read
            // whatever's leftover on that file descriptor.
            let read = |fd: &FileDesc, dst: &mut Vec<u8>| {
                match fd.read_to_end(dst) {
                    Ok(_) => Ok(true),
                    Err(e) => {
                        if e.raw_os_error() == Some(libc::EWOULDBLOCK) ||
                           e.raw_os_error() == Some(libc::EAGAIN) {
                            Ok(false)
                        } else {
                            Err(e)
                        }
                    }
                }
            };
            if fds[0].revents != 0 && read(&p1, v1)? {
                p2.set_nonblocking(false)?;
                return p2.read_to_end(v2).map(|_| ());
            }
            if fds[1].revents != 0 && read(&p2, v2)? {
                p1.set_nonblocking(false)?;
                return p1.read_to_end(v1).map(|_| ());
            }
        }
    }

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