src/libstd/sys/unix/process/process_unix.rs - toolchain/rustc - Git at Google

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

 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] = 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 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();

                     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))?;
         }

         if cfg!(not(any(target_os = "l4re"))) {
             if let Some(u) = self.get_gid() {
                 cvt(libc::setgid(u as gid_t))?;
             }
             if let Some(u) = self.get_uid() {
                 //FIXME: Redox kernel does not support setgroups yet
                 if cfg!(not(target_os = "redox")) {
                     // 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());
                 }

                 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(any(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();
             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
                 set.as_mut_ptr().write_bytes(0u8, 1);
             } else {
                 cvt(libc::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_argv()[0], 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"))))]
     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 crate::mem::MaybeUninit;
         use crate::sys;

         if self.get_gid().is_some() ||
             self.get_uid().is_some() ||
             self.env_saw_path() ||
             !self.get_closures().is_empty() {
             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 and glibc 2.29+ 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) => 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(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(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 file_actions = PosixSpawnFileActions(MaybeUninit::uninit());
             let mut attrs = PosixSpawnattr(MaybeUninit::uninit());

             libc::posix_spawnattr_init(attrs.0.as_mut_ptr());
             libc::posix_spawn_file_actions_init(file_actions.0.as_mut_ptr());

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

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

             let flags = libc::POSIX_SPAWN_SETSIGDEF |
                 libc::POSIX_SPAWN_SETSIGMASK;
             cvt(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 _);
             let ret = libc::posix_spawnp(
                 &mut p.pid,
                 self.get_argv()[0],
                 file_actions.0.as_ptr(),
                 attrs.0.as_ptr(),
                 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 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)))
         }
     }
 }
	use crate::io::{self, Error, ErrorKind};
	use crate::ptr;
	use crate::sys::cvt;
	use crate::sys::process::process_common::*;
	use crate::sys;

	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] = 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 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();

	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))?;
	}

	if cfg!(not(any(target_os = "l4re"))) {
	if let Some(u) = self.get_gid() {
	cvt(libc::setgid(u as gid_t))?;
	}
	if let Some(u) = self.get_uid() {
	//FIXME: Redox kernel does not support setgroups yet
	if cfg!(not(target_os = "redox")) {
	// 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());
	}

	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(any(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();
	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
	set.as_mut_ptr().write_bytes(0u8, 1);
	} else {
	cvt(libc::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_argv()[0], 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"))))]
	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 crate::mem::MaybeUninit;
	use crate::sys;

	if self.get_gid().is_some() \|\|
	self.get_uid().is_some() \|\|
	self.env_saw_path() \|\|
	!self.get_closures().is_empty() {
	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 and glibc 2.29+ 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) => 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(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(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 file_actions = PosixSpawnFileActions(MaybeUninit::uninit());
	let mut attrs = PosixSpawnattr(MaybeUninit::uninit());

	libc::posix_spawnattr_init(attrs.0.as_mut_ptr());
	libc::posix_spawn_file_actions_init(file_actions.0.as_mut_ptr());

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

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

	let flags = libc::POSIX_SPAWN_SETSIGDEF \|
	libc::POSIX_SPAWN_SETSIGMASK;
	cvt(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 _);
	let ret = libc::posix_spawnp(
	&mut p.pid,
	self.get_argv()[0],
	file_actions.0.as_ptr(),
	attrs.0.as_ptr(),
	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 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)))
	}
	}
	}