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    use io::{self, Error, ErrorKind};
    use libc::{self, c_int, gid_t, pid_t, uid_t};
    use ptr;
    use sys::cvt;
    use sys::process::process_common::*;
    use sys;

    ////////////////////////////////////////////////////////////////////////////////
    // Command
    ////////////////////////////////////////////////////////////////////////////////

    impl Command {
        pub fn spawn(&mut self, default: Stdio, needs_stdin: bool)
                     -> io::Result<(Process, StdioPipes)> {
            const CLOEXEC_MSG_FOOTER: &[u8] = b"NOEX";

            let envp = self.capture_env();

            if self.saw_nul() {
                return Err(io::Error::new(ErrorKind::InvalidInput,
                                          "nul byte found in provided data"));
            }

            let (ours, theirs) = self.setup_io(default, needs_stdin)?;

            if let Some(ret) = self.posix_spawn(&theirs, envp.as_ref())? {
                return Ok((ret, ours))
            }

            let (input, output) = sys::pipe::anon_pipe()?;

            // Whatever happens after the fork is almost for sure going to touch or
            // look at the environment in one way or another (PATH in `execvp` or
            // accessing the `environ` pointer ourselves). Make sure no other thread
            // is accessing the environment when we do the fork itself.
            //
            // Note that as soon as we're done with the fork there's no need to hold
            // a lock any more because the parent won't do anything and the child is
            // in its own process.
            let result = unsafe {
                let _env_lock = sys::os::env_lock();
                cvt(libc::fork())?
            };

            let pid = unsafe {
                match result {
                    0 => {
                        drop(input);
                        let err = self.do_exec(theirs, envp.as_ref());
                        let errno = err.raw_os_error().unwrap_or(libc::EINVAL) as u32;
                        let bytes = [
                            (errno >> 24) as u8,
                            (errno >> 16) as u8,
                            (errno >>  8) as u8,
                            (errno >>  0) as u8,
                            CLOEXEC_MSG_FOOTER[0], CLOEXEC_MSG_FOOTER[1],
                            CLOEXEC_MSG_FOOTER[2], CLOEXEC_MSG_FOOTER[3]
                        ];
                        // pipe I/O up to PIPE_BUF bytes should be atomic, and then
                        // we want to be sure we *don't* run at_exit destructors as
                        // we're being torn down regardless
                        assert!(output.write(&bytes).is_ok());
                        libc::_exit(1)
                    }
                    n => n,
                }
            };

            let mut p = Process { pid: pid, status: None };
            drop(output);
            let mut bytes = [0; 8];

            // loop to handle EINTR
            loop {
                match input.read(&mut bytes) {
                    Ok(0) => return Ok((p, ours)),
                    Ok(8) => {
                        assert!(combine(CLOEXEC_MSG_FOOTER) == combine(&bytes[4.. 8]),
                                "Validation on the CLOEXEC pipe failed: {:?}", bytes);
                        let errno = combine(&bytes[0.. 4]);
                        assert!(p.wait().is_ok(),
                                "wait() should either return Ok or panic");
                        return Err(Error::from_raw_os_error(errno))
                    }
                    Err(ref e) if e.kind() == ErrorKind::Interrupted => {}
                    Err(e) => {
                        assert!(p.wait().is_ok(),
                                "wait() should either return Ok or panic");
                        panic!("the CLOEXEC pipe failed: {:?}", e)
                    },
                    Ok(..) => { // pipe I/O up to PIPE_BUF bytes should be atomic
                        assert!(p.wait().is_ok(),
                                "wait() should either return Ok or panic");
                        panic!("short read on the CLOEXEC pipe")
                    }
                }
            }

            fn combine(arr: &[u8]) -> i32 {
                let a = arr[0] as u32;
                let b = arr[1] as u32;
                let c = arr[2] as u32;
                let d = arr[3] as u32;

                ((a << 24) | (b << 16) | (c << 8) | (d << 0)) as i32
            }
        }

        pub fn exec(&mut self, default: Stdio) -> io::Error {
            let envp = self.capture_env();

            if self.saw_nul() {
                return io::Error::new(ErrorKind::InvalidInput,
                                      "nul byte found in provided data")
            }

            match self.setup_io(default, true) {
                Ok((_, theirs)) => {
                    unsafe {
                        // Similar to when forking, we want to ensure that access to
                        // the environment is synchronized, so make sure to grab the
                        // environment lock before we try to exec.
                        let _lock = sys::os::env_lock();

                        self.do_exec(theirs, envp.as_ref())
                    }
                }
                Err(e) => e,
            }
        }

        // And at this point we've reached a special time in the life of the
        // child. The child must now be considered hamstrung and unable to
        // do anything other than syscalls really. Consider the following
        // scenario:
        //
        //      1. Thread A of process 1 grabs the malloc() mutex
        //      2. Thread B of process 1 forks(), creating thread C
        //      3. Thread C of process 2 then attempts to malloc()
        //      4. The memory of process 2 is the same as the memory of
        //         process 1, so the mutex is locked.
        //
        // This situation looks a lot like deadlock, right? It turns out
        // that this is what pthread_atfork() takes care of, which is
        // presumably implemented across platforms. The first thing that
        // threads to *before* forking is to do things like grab the malloc
        // mutex, and then after the fork they unlock it.
        //
        // Despite this information, libnative's spawn has been witnessed to
        // deadlock on both macOS and FreeBSD. I'm not entirely sure why, but
        // all collected backtraces point at malloc/free traffic in the
        // child spawned process.
        //
        // For this reason, the block of code below should contain 0
        // invocations of either malloc of free (or their related friends).
        //
        // As an example of not having malloc/free traffic, we don't close
        // this file descriptor by dropping the FileDesc (which contains an
        // allocation). Instead we just close it manually. This will never
        // have the drop glue anyway because this code never returns (the
        // child will either exec() or invoke libc::exit)
        unsafe fn do_exec(
            &mut self,
            stdio: ChildPipes,
            maybe_envp: Option<&CStringArray>
        ) -> io::Error {
            use sys::{self, cvt_r};

            macro_rules! t {
                ($e:expr) => (match $e {
                    Ok(e) => e,
                    Err(e) => return e,
                })
            }

            if let Some(fd) = stdio.stdin.fd() {
                t!(cvt_r(|| libc::dup2(fd, libc::STDIN_FILENO)));
            }
            if let Some(fd) = stdio.stdout.fd() {
                t!(cvt_r(|| libc::dup2(fd, libc::STDOUT_FILENO)));
            }
            if let Some(fd) = stdio.stderr.fd() {
                t!(cvt_r(|| libc::dup2(fd, libc::STDERR_FILENO)));
            }

            if cfg!(not(any(target_os = "l4re"))) {
                if let Some(u) = self.get_gid() {
                    t!(cvt(libc::setgid(u as gid_t)));
                }
                if let Some(u) = self.get_uid() {
                    // When dropping privileges from root, the `setgroups` call
                    // will remove any extraneous groups. If we don't call this,
                    // then even though our uid has dropped, we may still have
                    // groups that enable us to do super-user things. This will
                    // fail if we aren't root, so don't bother checking the
                    // return value, this is just done as an optimistic
                    // privilege dropping function.
                    let _ = libc::setgroups(0, ptr::null());

                    t!(cvt(libc::setuid(u as uid_t)));
                }
            }
            if let Some(ref cwd) = *self.get_cwd() {
                t!(cvt(libc::chdir(cwd.as_ptr())));
            }

            // emscripten has no signal support.
            #[cfg(not(any(target_os = "emscripten")))]
            {
                use mem;
                // Reset signal handling so the child process starts in a
                // standardized state. libstd ignores SIGPIPE, and signal-handling
                // libraries often set a mask. Child processes inherit ignored
                // signals and the signal mask from their parent, but most
                // UNIX programs do not reset these things on their own, so we
                // need to clean things up now to avoid confusing the program
                // we're about to run.
                let mut set: libc::sigset_t = mem::uninitialized();
                if cfg!(target_os = "android") {
                    // Implementing sigemptyset allow us to support older Android
                    // versions. See the comment about Android and sig* functions in
                    // process_common.rs
                    libc::memset(&mut set as *mut _ as *mut _,
                                 0,
                                 mem::size_of::<libc::sigset_t>());
                } else {
                    t!(cvt(libc::sigemptyset(&mut set)));
                }
                t!(cvt(libc::pthread_sigmask(libc::SIG_SETMASK, &set,
                                             ptr::null_mut())));
                let ret = sys::signal(libc::SIGPIPE, libc::SIG_DFL);
                if ret == libc::SIG_ERR {
                    return io::Error::last_os_error()
                }
            }

            for callback in self.get_closures().iter_mut() {
                t!(callback());
            }

            // Although we're performing an exec here we may also return with an
            // error from this function (without actually exec'ing) in which case we
            // want to be sure to restore the global environment back to what it
            // once was, ensuring that our temporary override, when free'd, doesn't
            // corrupt our process's environment.
            let mut _reset = None;
            if let Some(envp) = maybe_envp {
                struct Reset(*const *const libc::c_char);

                impl Drop for Reset {
                    fn drop(&mut self) {
                        unsafe {
                            *sys::os::environ() = self.0;
                        }
                    }
                }

                _reset = Some(Reset(*sys::os::environ()));
                *sys::os::environ() = envp.as_ptr();
            }

            libc::execvp(self.get_argv()[0], self.get_argv().as_ptr());
            io::Error::last_os_error()
        }

        #[cfg(not(any(target_os = "macos", target_os = "freebsd",
                      all(target_os = "linux", target_env = "gnu"))))]
        fn posix_spawn(&mut self, _: &ChildPipes, _: Option<&CStringArray>)
            -> io::Result<Option<Process>>
        {
            Ok(None)
        }

        // Only support platforms for which posix_spawn() can return ENOENT
        // directly.
        #[cfg(any(target_os = "macos", target_os = "freebsd",
                  all(target_os = "linux", target_env = "gnu")))]
        fn posix_spawn(&mut self, stdio: &ChildPipes, envp: Option<&CStringArray>)
            -> io::Result<Option<Process>>
        {
            use mem;
            use sys;

            if self.get_cwd().is_some() ||
                self.get_gid().is_some() ||
                self.get_uid().is_some() ||
                self.env_saw_path() ||
                self.get_closures().len() != 0 {
                return Ok(None)
            }

            // Only glibc 2.24+ posix_spawn() supports returning ENOENT directly.
            #[cfg(all(target_os = "linux", target_env = "gnu"))]
            {
                if let Some(version) = sys::os::glibc_version() {
                    if version < (2, 24) {
                        return Ok(None)
                    }
                } else {
                    return Ok(None)
                }
            }

            let mut p = Process { pid: 0, status: None };

            struct PosixSpawnFileActions(libc::posix_spawn_file_actions_t);

            impl Drop for PosixSpawnFileActions {
                fn drop(&mut self) {
                    unsafe {
                        libc::posix_spawn_file_actions_destroy(&mut self.0);
                    }
                }
            }

            struct PosixSpawnattr(libc::posix_spawnattr_t);

            impl Drop for PosixSpawnattr {
                fn drop(&mut self) {
                    unsafe {
                        libc::posix_spawnattr_destroy(&mut self.0);
                    }
                }
            }

            unsafe {
                let mut file_actions = PosixSpawnFileActions(mem::uninitialized());
                let mut attrs = PosixSpawnattr(mem::uninitialized());

                libc::posix_spawnattr_init(&mut attrs.0);
                libc::posix_spawn_file_actions_init(&mut file_actions.0);

                if let Some(fd) = stdio.stdin.fd() {
                    cvt(libc::posix_spawn_file_actions_adddup2(&mut file_actions.0,
                                                               fd,
                                                               libc::STDIN_FILENO))?;
                }
                if let Some(fd) = stdio.stdout.fd() {
                    cvt(libc::posix_spawn_file_actions_adddup2(&mut file_actions.0,
                                                               fd,
                                                               libc::STDOUT_FILENO))?;
                }
                if let Some(fd) = stdio.stderr.fd() {
                    cvt(libc::posix_spawn_file_actions_adddup2(&mut file_actions.0,
                                                               fd,
                                                               libc::STDERR_FILENO))?;
                }

                let mut set: libc::sigset_t = mem::uninitialized();
                cvt(libc::sigemptyset(&mut set))?;
                cvt(libc::posix_spawnattr_setsigmask(&mut attrs.0,
                                                     &set))?;
                cvt(libc::sigaddset(&mut set, libc::SIGPIPE))?;
                cvt(libc::posix_spawnattr_setsigdefault(&mut attrs.0,
                                                        &set))?;

                let flags = libc::POSIX_SPAWN_SETSIGDEF |
                    libc::POSIX_SPAWN_SETSIGMASK;
                cvt(libc::posix_spawnattr_setflags(&mut attrs.0, flags as _))?;

                // Make sure we synchronize access to the global `environ` resource
                let _env_lock = sys::os::env_lock();
                let envp = envp.map(|c| c.as_ptr())
                    .unwrap_or_else(|| *sys::os::environ() as *const _);
                let ret = libc::posix_spawnp(
                    &mut p.pid,
                    self.get_argv()[0],
                    &file_actions.0,
                    &attrs.0,
                    self.get_argv().as_ptr() as *const _,
                    envp as *const _,
                );
                if ret == 0 {
                    Ok(Some(p))
                } else {
                    Err(io::Error::from_raw_os_error(ret))
                }
            }
        }
    }

    ////////////////////////////////////////////////////////////////////////////////
    // Processes
    ////////////////////////////////////////////////////////////////////////////////

    /// The unique id of the process (this should never be negative).
    pub struct Process {
        pid: pid_t,
        status: Option<ExitStatus>,
    }

    impl Process {
        pub fn id(&self) -> u32 {
            self.pid as u32
        }

        pub fn kill(&mut self) -> io::Result<()> {
            // If we've already waited on this process then the pid can be recycled
            // and used for another process, and we probably shouldn't be killing
            // random processes, so just return an error.
            if self.status.is_some() {
                Err(Error::new(ErrorKind::InvalidInput,
                               "invalid argument: can't kill an exited process"))
            } else {
                cvt(unsafe { libc::kill(self.pid, libc::SIGKILL) }).map(|_| ())
            }
        }

        pub fn wait(&mut self) -> io::Result<ExitStatus> {
            use sys::cvt_r;
            if let Some(status) = self.status {
                return Ok(status)
            }
            let mut status = 0 as c_int;
            cvt_r(|| unsafe { libc::waitpid(self.pid, &mut status, 0) })?;
            self.status = Some(ExitStatus::new(status));
            Ok(ExitStatus::new(status))
        }

        pub fn try_wait(&mut self) -> io::Result<Option<ExitStatus>> {
            if let Some(status) = self.status {
                return Ok(Some(status))
            }
            let mut status = 0 as c_int;
            let pid = cvt(unsafe {
                libc::waitpid(self.pid, &mut status, libc::WNOHANG)
            })?;
            if pid == 0 {
                Ok(None)
            } else {
                self.status = Some(ExitStatus::new(status));
                Ok(Some(ExitStatus::new(status)))
            }
        }
    }

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