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    use crate::convert::TryInto;
    use crate::fmt;
    use crate::io::{self, Error, ErrorKind};
    use crate::ptr;
    use crate::sys;
    use crate::sys::cvt;
    use crate::sys::process::process_common::*;

    #[cfg(target_os = "vxworks")]
    use libc::RTP_ID as pid_t;

    #[cfg(not(target_os = "vxworks"))]
    use libc::{c_int, gid_t, pid_t, uid_t};

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

    impl Command {
        pub fn spawn(
            &mut self,
            default: Stdio,
            needs_stdin: bool,
        ) -> io::Result<(Process, StdioPipes)> {
            const CLOEXEC_MSG_FOOTER: [u8; 4] = *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(err) = self.do_exec(theirs, envp.as_ref());
                        let errno = err.raw_os_error().unwrap_or(libc::EINVAL) as u32;
                        let errno = errno.to_be_bytes();
                        let bytes = [
                            errno[0],
                            errno[1],
                            errno[2],
                            errno[3],
                            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
                        rtassert!(output.write(&bytes).is_ok());
                        libc::_exit(1)
                    }
                    n => n,
                }
            };

            let mut p = Process { 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) => {
                        let (errno, footer) = bytes.split_at(4);
                        assert_eq!(
                            CLOEXEC_MSG_FOOTER, footer,
                            "Validation on the CLOEXEC pipe failed: {:?}",
                            bytes
                        );
                        let errno = i32::from_be_bytes(errno.try_into().unwrap());
                        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")
                    }
                }
            }
        }

        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();

                        let Err(e) = self.do_exec(theirs, envp.as_ref());
                        e
                    }
                }
                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>,
        ) -> Result<!, io::Error> {
            use crate::sys::{self, cvt_r};

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

            #[cfg(not(target_os = "l4re"))]
            {
                if let Some(_g) = self.get_groups() {
                    //FIXME: Redox kernel does not support setgroups yet
                    #[cfg(not(target_os = "redox"))]
                    cvt(libc::setgroups(_g.len().try_into().unwrap(), _g.as_ptr()))?;
                }
                if let Some(u) = self.get_gid() {
                    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. We only drop groups
                    // if the current uid is 0 and we weren't given an explicit
                    // set of 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.
                    //FIXME: Redox kernel does not support setgroups yet
                    #[cfg(not(target_os = "redox"))]
                    if libc::getuid() == 0 && self.get_groups().is_none() {
                        cvt(libc::setgroups(0, ptr::null()))?;
                    }
                    cvt(libc::setuid(u as uid_t))?;
                }
            }
            if let Some(ref cwd) = *self.get_cwd() {
                cvt(libc::chdir(cwd.as_ptr()))?;
            }

            // emscripten has no signal support.
            #[cfg(not(target_os = "emscripten"))]
            {
                use crate::mem::MaybeUninit;
                // 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 = MaybeUninit::<libc::sigset_t>::uninit();
                cvt(sigemptyset(set.as_mut_ptr()))?;
                cvt(libc::pthread_sigmask(libc::SIG_SETMASK, set.as_ptr(), ptr::null_mut()))?;
                let ret = sys::signal(libc::SIGPIPE, libc::SIG_DFL);
                if ret == libc::SIG_ERR {
                    return Err(io::Error::last_os_error());
                }
            }

            for callback in self.get_closures().iter_mut() {
                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_program_cstr().as_ptr(), self.get_argv().as_ptr());
            Err(io::Error::last_os_error())
        }

        #[cfg(not(any(
            target_os = "macos",
            target_os = "freebsd",
            all(target_os = "linux", target_env = "gnu"),
            all(target_os = "linux", target_env = "musl"),
        )))]
        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"),
            all(target_os = "linux", target_env = "musl"),
        ))]
        fn posix_spawn(
            &mut self,
            stdio: &ChildPipes,
            envp: Option<&CStringArray>,
        ) -> io::Result<Option<Process>> {
            use crate::mem::MaybeUninit;
            use crate::sys::{self, cvt_nz};

            if self.get_gid().is_some()
                || self.get_uid().is_some()
                || (self.env_saw_path() && !self.program_is_path())
                || !self.get_closures().is_empty()
                || self.get_groups().is_some()
            {
                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);
                }
            }

            // Solaris, glibc 2.29+, and musl 1.24+ can set a new working directory,
            // and maybe others will gain this non-POSIX function too. We'll check
            // for this weak symbol as soon as it's needed, so we can return early
            // otherwise to do a manual chdir before exec.
            weak! {
                fn posix_spawn_file_actions_addchdir_np(
                    *mut libc::posix_spawn_file_actions_t,
                    *const libc::c_char
                ) -> libc::c_int
            }
            let addchdir = match self.get_cwd() {
                Some(cwd) => {
                    if cfg!(target_os = "macos") {
                        // There is a bug in macOS where a relative executable
                        // path like "../myprogram" will cause `posix_spawn` to
                        // successfully launch the program, but erroneously return
                        // ENOENT when used with posix_spawn_file_actions_addchdir_np
                        // which was introduced in macOS 10.15.
                        return Ok(None);
                    }
                    match posix_spawn_file_actions_addchdir_np.get() {
                        Some(f) => Some((f, cwd)),
                        None => return Ok(None),
                    }
                }
                None => None,
            };

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

            struct PosixSpawnFileActions<'a>(&'a mut MaybeUninit<libc::posix_spawn_file_actions_t>);

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

            struct PosixSpawnattr<'a>(&'a mut MaybeUninit<libc::posix_spawnattr_t>);

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

            unsafe {
                let mut attrs = MaybeUninit::uninit();
                cvt_nz(libc::posix_spawnattr_init(attrs.as_mut_ptr()))?;
                let attrs = PosixSpawnattr(&mut attrs);

                let mut file_actions = MaybeUninit::uninit();
                cvt_nz(libc::posix_spawn_file_actions_init(file_actions.as_mut_ptr()))?;
                let file_actions = PosixSpawnFileActions(&mut file_actions);

                if let Some(fd) = stdio.stdin.fd() {
                    cvt_nz(libc::posix_spawn_file_actions_adddup2(
                        file_actions.0.as_mut_ptr(),
                        fd,
                        libc::STDIN_FILENO,
                    ))?;
                }
                if let Some(fd) = stdio.stdout.fd() {
                    cvt_nz(libc::posix_spawn_file_actions_adddup2(
                        file_actions.0.as_mut_ptr(),
                        fd,
                        libc::STDOUT_FILENO,
                    ))?;
                }
                if let Some(fd) = stdio.stderr.fd() {
                    cvt_nz(libc::posix_spawn_file_actions_adddup2(
                        file_actions.0.as_mut_ptr(),
                        fd,
                        libc::STDERR_FILENO,
                    ))?;
                }
                if let Some((f, cwd)) = addchdir {
                    cvt_nz(f(file_actions.0.as_mut_ptr(), cwd.as_ptr()))?;
                }

                let mut set = MaybeUninit::<libc::sigset_t>::uninit();
                cvt(sigemptyset(set.as_mut_ptr()))?;
                cvt_nz(libc::posix_spawnattr_setsigmask(attrs.0.as_mut_ptr(), set.as_ptr()))?;
                cvt(sigaddset(set.as_mut_ptr(), libc::SIGPIPE))?;
                cvt_nz(libc::posix_spawnattr_setsigdefault(attrs.0.as_mut_ptr(), set.as_ptr()))?;

                let flags = libc::POSIX_SPAWN_SETSIGDEF | libc::POSIX_SPAWN_SETSIGMASK;
                cvt_nz(libc::posix_spawnattr_setflags(attrs.0.as_mut_ptr(), 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 _);
                cvt_nz(libc::posix_spawnp(
                    &mut p.pid,
                    self.get_program_cstr().as_ptr(),
                    file_actions.0.as_ptr(),
                    attrs.0.as_ptr(),
                    self.get_argv().as_ptr() as *const _,
                    envp as *const _,
                ))?;
                Ok(Some(p))
            }
        }
    }

    ////////////////////////////////////////////////////////////////////////////////
    // 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(drop)
            }
        }

        pub fn wait(&mut self) -> io::Result<ExitStatus> {
            use crate::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)))
            }
        }
    }

    /// Unix exit statuses
    #[derive(PartialEq, Eq, Clone, Copy, Debug)]
    pub struct ExitStatus(c_int);

    impl ExitStatus {
        pub fn new(status: c_int) -> ExitStatus {
            ExitStatus(status)
        }

        fn exited(&self) -> bool {
            libc::WIFEXITED(self.0)
        }

        pub fn success(&self) -> bool {
            self.code() == Some(0)
        }

        pub fn code(&self) -> Option<i32> {
            if self.exited() { Some(libc::WEXITSTATUS(self.0)) } else { None }
        }

        pub fn signal(&self) -> Option<i32> {
            if libc::WIFSIGNALED(self.0) { Some(libc::WTERMSIG(self.0)) } else { None }
        }

        pub fn core_dumped(&self) -> bool {
            libc::WIFSIGNALED(self.0) && libc::WCOREDUMP(self.0)
        }

        pub fn stopped_signal(&self) -> Option<i32> {
            if libc::WIFSTOPPED(self.0) { Some(libc::WSTOPSIG(self.0)) } else { None }
        }

        pub fn continued(&self) -> bool {
            libc::WIFCONTINUED(self.0)
        }

        pub fn into_raw(&self) -> c_int {
            self.0
        }
    }

    /// Converts a raw `c_int` to a type-safe `ExitStatus` by wrapping it without copying.
    impl From<c_int> for ExitStatus {
        fn from(a: c_int) -> ExitStatus {
            ExitStatus(a)
        }
    }

    impl fmt::Display for ExitStatus {
        fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
            if let Some(code) = self.code() {
                write!(f, "exit code: {}", code)
            } else {
                let signal = self.signal().unwrap();
                write!(f, "signal: {}", signal)
            }
        }
    }