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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 cmp;
    use io;
    use libc::{self, c_int};
    use mem;
    use ptr;
    use sys::cvt_r;
    use sys::fd::FileDesc;

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

    pub struct AnonPipe(FileDesc);

    pub fn anon_pipe() -> io::Result<(AnonPipe, AnonPipe)> {
        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!(target_os = "linux") {
            weak! { fn pipe2(*mut c_int, c_int) -> c_int }
            if let Some(pipe) = pipe2.get() {
                match cvt_r(|| 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) => {}
                    Err(e) => return Err(e),
                }
            }
        }
        if unsafe { libc::pipe(fds.as_mut_ptr()) == 0 } {
            let fd0 = FileDesc::new(fds[0]);
            let fd1 = FileDesc::new(fds[1]);
            Ok((AnonPipe::from_fd(fd0)?, AnonPipe::from_fd(fd1)?))
        } else {
            Err(io::Error::last_os_error())
        }
    }

    impl AnonPipe {
        pub fn from_fd(fd: FileDesc) -> io::Result<AnonPipe> {
            fd.set_cloexec()?;
            Ok(AnonPipe(fd))
        }

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

        pub fn read_to_end(&self, buf: &mut Vec<u8>) -> io::Result<usize> {
            self.0.read_to_end(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 max = cmp::max(p1.raw(), p2.raw());
        loop {
            // wait for either pipe to become readable using `select`
            cvt_r(|| unsafe {
                let mut read: libc::fd_set = mem::zeroed();
                libc::FD_SET(p1.raw(), &mut read);
                libc::FD_SET(p2.raw(), &mut read);
                libc::select(max + 1, &mut read, ptr::null_mut(), ptr::null_mut(),
                             ptr::null_mut())
            })?;

            // 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 read(&p1, v1)? {
                p2.set_nonblocking(false)?;
                return p2.read_to_end(v2).map(|_| ());
            }
            if read(&p2, v2)? {
                p1.set_nonblocking(false)?;
                return p1.read_to_end(v1).map(|_| ());
            }
        }
    }

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