src/libstd/sys/unix/process.rs - external/github.com/rust-lang/rust - Git at Google

 // Copyright 2014-2015 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 prelude::v1::*;
 use os::unix::prelude::*;

 use collections::hash_map::{HashMap, Entry};
 use env;
 use ffi::{OsString, OsStr, CString, CStr};
 use fmt;
 use io::{self, Error, ErrorKind};
 use libc::{self, pid_t, c_int, gid_t, uid_t, c_char};
 use mem;
 use ptr;
 use sys::fd::FileDesc;
 use sys::fs::{File, OpenOptions};
 use sys::pipe::{self, AnonPipe};
 use sys::{self, cvt, cvt_r};

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

 pub struct Command {
     // Currently we try hard to ensure that the call to `.exec()` doesn't
     // actually allocate any memory. While many platforms try to ensure that
     // memory allocation works after a fork in a multithreaded process, it's
     // been observed to be buggy and somewhat unreliable, so we do our best to
     // just not do it at all!
     //
     // Along those lines, the `argv` and `envp` raw pointers here are exactly
     // what's gonna get passed to `execvp`. The `argv` array starts with the
     // `program` and ends with a NULL, and the `envp` pointer, if present, is
     // also null-terminated.
     //
     // Right now we don't support removing arguments, so there's no much fancy
     // support there, but we support adding and removing environment variables,
     // so a side table is used to track where in the `envp` array each key is
     // located. Whenever we add a key we update it in place if it's already
     // present, and whenever we remove a key we update the locations of all
     // other keys.
     program: CString,
     args: Vec<CString>,
     env: Option<HashMap<OsString, (usize, CString)>>,
     argv: Vec<*const c_char>,
     envp: Option<Vec<*const c_char>>,

     cwd: Option<CString>,
     uid: Option<uid_t>,
     gid: Option<gid_t>,
     session_leader: bool,
     saw_nul: bool,
     closures: Vec<Box<FnMut() -> io::Result<()> + Send + Sync>>,
     stdin: Option<Stdio>,
     stdout: Option<Stdio>,
     stderr: Option<Stdio>,
 }

 // passed back to std::process with the pipes connected to the child, if any
 // were requested
 pub struct StdioPipes {
     pub stdin: Option<AnonPipe>,
     pub stdout: Option<AnonPipe>,
     pub stderr: Option<AnonPipe>,
 }

 // passed to do_exec() with configuration of what the child stdio should look
 // like
 struct ChildPipes {
     stdin: ChildStdio,
     stdout: ChildStdio,
     stderr: ChildStdio,
 }

 enum ChildStdio {
     Inherit,
     Explicit(c_int),
     Owned(FileDesc),
 }

 pub enum Stdio {
     Inherit,
     Null,
     MakePipe,
     Fd(FileDesc),
 }

 impl Command {
     pub fn new(program: &OsStr) -> Command {
         let mut saw_nul = false;
         let program = os2c(program, &mut saw_nul);
         Command {
             argv: vec![program.as_ptr(), 0 as *const _],
             program: program,
             args: Vec::new(),
             env: None,
             envp: None,
             cwd: None,
             uid: None,
             gid: None,
             session_leader: false,
             saw_nul: saw_nul,
             closures: Vec::new(),
             stdin: None,
             stdout: None,
             stderr: None,
         }
     }

     pub fn arg(&mut self, arg: &OsStr) {
         // Overwrite the trailing NULL pointer in `argv` and then add a new null
         // pointer.
         let arg = os2c(arg, &mut self.saw_nul);
         self.argv[self.args.len() + 1] = arg.as_ptr();
         self.argv.push(0 as *const _);

         // Also make sure we keep track of the owned value to schedule a
         // destructor for this memory.
         self.args.push(arg);
     }

     fn init_env_map(&mut self) -> (&mut HashMap<OsString, (usize, CString)>,
                                    &mut Vec<*const c_char>) {
         if self.env.is_none() {
             let mut map = HashMap::new();
             let mut envp = Vec::new();
             for (k, v) in env::vars_os() {
                 let s = pair_to_key(&k, &v, &mut self.saw_nul);
                 envp.push(s.as_ptr());
                 map.insert(k, (envp.len() - 1, s));
             }
             envp.push(0 as *const _);
             self.env = Some(map);
             self.envp = Some(envp);
         }
         (self.env.as_mut().unwrap(), self.envp.as_mut().unwrap())
     }

     pub fn env(&mut self, key: &OsStr, val: &OsStr) {
         let new_key = pair_to_key(key, val, &mut self.saw_nul);
         let (map, envp) = self.init_env_map();

         // If `key` is already present then we we just update `envp` in place
         // (and store the owned value), but if it's not there we override the
         // trailing NULL pointer, add a new NULL pointer, and store where we
         // were located.
         match map.entry(key.to_owned()) {
             Entry::Occupied(mut e) => {
                 let (i, ref mut s) = *e.get_mut();
                 envp[i] = new_key.as_ptr();
                 *s = new_key;
             }
             Entry::Vacant(e) => {
                 let len = envp.len();
                 envp[len - 1] = new_key.as_ptr();
                 envp.push(0 as *const _);
                 e.insert((len - 1, new_key));
             }
         }
     }

     pub fn env_remove(&mut self, key: &OsStr) {
         let (map, envp) = self.init_env_map();

         // If we actually ended up removing a key, then we need to update the
         // position of all keys that come after us in `envp` because they're all
         // one element sooner now.
         if let Some((i, _)) = map.remove(key) {
             envp.remove(i);

             for (_, &mut (ref mut j, _)) in map.iter_mut() {
                 if *j >= i {
                     *j -= 1;
                 }
             }
         }
     }

     pub fn env_clear(&mut self) {
         self.env = Some(HashMap::new());
         self.envp = Some(vec![0 as *const _]);
     }

     pub fn cwd(&mut self, dir: &OsStr) {
         self.cwd = Some(os2c(dir, &mut self.saw_nul));
     }
     pub fn uid(&mut self, id: uid_t) {
         self.uid = Some(id);
     }
     pub fn gid(&mut self, id: gid_t) {
         self.gid = Some(id);
     }
     pub fn session_leader(&mut self, session_leader: bool) {
         self.session_leader = session_leader;
     }

     pub fn before_exec(&mut self,
                        f: Box<FnMut() -> io::Result<()> + Send + Sync>) {
         self.closures.push(f);
     }

     pub fn stdin(&mut self, stdin: Stdio) {
         self.stdin = Some(stdin);
     }
     pub fn stdout(&mut self, stdout: Stdio) {
         self.stdout = Some(stdout);
     }
     pub fn stderr(&mut self, stderr: Stdio) {
         self.stderr = Some(stderr);
     }

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

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

         let pid = unsafe {
             match cvt(libc::fork())? {
                 0 => {
                     drop(input);
                     let err = self.do_exec(theirs);
                     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 {
         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 { self.do_exec(theirs) },
             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 OSX 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) -> io::Error {
         macro_rules! try {
             ($e:expr) => (match $e {
                 Ok(e) => e,
                 Err(e) => return e,
             })
         }

         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 let Some(u) = self.gid {
             cvt(libc::setgid(u as gid_t))?;
         }
         if let Some(u) = self.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());

             cvt(libc::setuid(u as uid_t))?;
         }
         if self.session_leader {
             // Don't check the error of setsid because it fails if we're the
             // process leader already. We just forked so it shouldn't return
             // error, but ignore it anyway.
             let _ = libc::setsid();
         }
         if let Some(ref cwd) = self.cwd {
             cvt(libc::chdir(cwd.as_ptr()))?;
         }
         if let Some(ref envp) = self.envp {
             *sys::os::environ() = envp.as_ptr();
         }

         // NaCl has no signal support.
         if cfg!(not(target_os = "nacl")) {
             // 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();
             cvt(libc::sigemptyset(&mut set))?;
             cvt(libc::pthread_sigmask(libc::SIG_SETMASK, &set,
                                            ptr::null_mut()))?;
             let ret = libc::signal(libc::SIGPIPE, libc::SIG_DFL);
             if ret == libc::SIG_ERR {
                 return io::Error::last_os_error()
             }
         }

         for callback in self.closures.iter_mut() {
             callback()?;
         }

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


     fn setup_io(&self, default: Stdio, needs_stdin: bool)
                 -> io::Result<(StdioPipes, ChildPipes)> {
         let null = Stdio::Null;
         let default_stdin = if needs_stdin {&default} else {&null};
         let stdin = self.stdin.as_ref().unwrap_or(default_stdin);
         let stdout = self.stdout.as_ref().unwrap_or(&default);
         let stderr = self.stderr.as_ref().unwrap_or(&default);
         let (their_stdin, our_stdin) = stdin.to_child_stdio(true)?;
         let (their_stdout, our_stdout) = stdout.to_child_stdio(false)?;
         let (their_stderr, our_stderr) = stderr.to_child_stdio(false)?;
         let ours = StdioPipes {
             stdin: our_stdin,
             stdout: our_stdout,
             stderr: our_stderr,
         };
         let theirs = ChildPipes {
             stdin: their_stdin,
             stdout: their_stdout,
             stderr: their_stderr,
         };
         Ok((ours, theirs))
     }
 }

 fn os2c(s: &OsStr, saw_nul: &mut bool) -> CString {
     CString::new(s.as_bytes()).unwrap_or_else(|_e| {
         *saw_nul = true;
         CString::new("<string-with-nul>").unwrap()
     })
 }

 impl Stdio {
     fn to_child_stdio(&self, readable: bool)
                       -> io::Result<(ChildStdio, Option<AnonPipe>)> {
         match *self {
             Stdio::Inherit => Ok((ChildStdio::Inherit, None)),

             // Make sure that the source descriptors are not an stdio
             // descriptor, otherwise the order which we set the child's
             // descriptors may blow away a descriptor which we are hoping to
             // save. For example, suppose we want the child's stderr to be the
             // parent's stdout, and the child's stdout to be the parent's
             // stderr. No matter which we dup first, the second will get
             // overwritten prematurely.
             Stdio::Fd(ref fd) => {
                 if fd.raw() >= 0 && fd.raw() <= libc::STDERR_FILENO {
                     Ok((ChildStdio::Owned(fd.duplicate()?), None))
                 } else {
                     Ok((ChildStdio::Explicit(fd.raw()), None))
                 }
             }

             Stdio::MakePipe => {
                 let (reader, writer) = pipe::anon_pipe()?;
                 let (ours, theirs) = if readable {
                     (writer, reader)
                 } else {
                     (reader, writer)
                 };
                 Ok((ChildStdio::Owned(theirs.into_fd()), Some(ours)))
             }

             Stdio::Null => {
                 let mut opts = OpenOptions::new();
                 opts.read(readable);
                 opts.write(!readable);
                 let path = unsafe {
                     CStr::from_ptr("/dev/null\0".as_ptr() as *const _)
                 };
                 let fd = File::open_c(&path, &opts)?;
                 Ok((ChildStdio::Owned(fd.into_fd()), None))
             }
         }
     }
 }

 impl ChildStdio {
     fn fd(&self) -> Option<c_int> {
         match *self {
             ChildStdio::Inherit => None,
             ChildStdio::Explicit(fd) => Some(fd),
             ChildStdio::Owned(ref fd) => Some(fd.raw()),
         }
     }
 }

 fn pair_to_key(key: &OsStr, value: &OsStr, saw_nul: &mut bool) -> CString {
     let (key, value) = (key.as_bytes(), value.as_bytes());
     let mut v = Vec::with_capacity(key.len() + value.len() + 1);
     v.extend(key);
     v.push(b'=');
     v.extend(value);
     CString::new(v).unwrap_or_else(|_e| {
         *saw_nul = true;
         CString::new("foo=bar").unwrap()
     })
 }

 impl fmt::Debug for Command {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         write!(f, "{:?}", self.program)?;
         for arg in &self.args {
             write!(f, " {:?}", arg)?;
         }
         Ok(())
     }
 }

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

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

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

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

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

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

 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)
         }
     }
 }

 /// 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> {
         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(status));
         Ok(ExitStatus(status))
     }
 }

 #[cfg(test)]
 mod tests {
     use super::*;
     use prelude::v1::*;

     use ffi::OsStr;
     use mem;
     use ptr;
     use libc;
     use sys::cvt;

     macro_rules! t {
         ($e:expr) => {
             match $e {
                 Ok(t) => t,
                 Err(e) => panic!("received error for `{}`: {}", stringify!($e), e),
             }
         }
     }

     #[cfg(not(target_os = "android"))]
     extern {
         #[cfg_attr(target_os = "netbsd", link_name = "__sigaddset14")]
         fn sigaddset(set: *mut libc::sigset_t, signum: libc::c_int) -> libc::c_int;
     }

     #[cfg(target_os = "android")]
     unsafe fn sigaddset(set: *mut libc::sigset_t, signum: libc::c_int) -> libc::c_int {
         use slice;

         let raw = slice::from_raw_parts_mut(set as *mut u8, mem::size_of::<libc::sigset_t>());
         let bit = (signum - 1) as usize;
         raw[bit / 8] |= 1 << (bit % 8);
         return 0;
     }

     // See #14232 for more information, but it appears that signal delivery to a
     // newly spawned process may just be raced in the OSX, so to prevent this
     // test from being flaky we ignore it on OSX.
     #[test]
     #[cfg_attr(target_os = "macos", ignore)]
     #[cfg_attr(target_os = "nacl", ignore)] // no signals on NaCl.
     fn test_process_mask() {
         unsafe {
             // Test to make sure that a signal mask does not get inherited.
             let mut cmd = Command::new(OsStr::new("cat"));

             let mut set: libc::sigset_t = mem::uninitialized();
             let mut old_set: libc::sigset_t = mem::uninitialized();
             t!(cvt(libc::sigemptyset(&mut set)));
             t!(cvt(sigaddset(&mut set, libc::SIGINT)));
             t!(cvt(libc::pthread_sigmask(libc::SIG_SETMASK, &set, &mut old_set)));

             cmd.stdin(Stdio::MakePipe);
             cmd.stdout(Stdio::MakePipe);

             let (mut cat, mut pipes) = t!(cmd.spawn(Stdio::Null, true));
             let stdin_write = pipes.stdin.take().unwrap();
             let stdout_read = pipes.stdout.take().unwrap();

             t!(cvt(libc::pthread_sigmask(libc::SIG_SETMASK, &old_set,
                                          ptr::null_mut())));

             t!(cvt(libc::kill(cat.id() as libc::pid_t, libc::SIGINT)));
             // We need to wait until SIGINT is definitely delivered. The
             // easiest way is to write something to cat, and try to read it
             // back: if SIGINT is unmasked, it'll get delivered when cat is
             // next scheduled.
             let _ = stdin_write.write(b"Hello");
             drop(stdin_write);

             // Either EOF or failure (EPIPE) is okay.
             let mut buf = [0; 5];
             if let Ok(ret) = stdout_read.read(&mut buf) {
                 assert!(ret == 0);
             }

             t!(cat.wait());
         }
     }
 }
	// Copyright 2014-2015 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 prelude::v1::*;
	use os::unix::prelude::*;

	use collections::hash_map::{HashMap, Entry};
	use env;
	use ffi::{OsString, OsStr, CString, CStr};
	use fmt;
	use io::{self, Error, ErrorKind};
	use libc::{self, pid_t, c_int, gid_t, uid_t, c_char};
	use mem;
	use ptr;
	use sys::fd::FileDesc;
	use sys::fs::{File, OpenOptions};
	use sys::pipe::{self, AnonPipe};
	use sys::{self, cvt, cvt_r};

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

	pub struct Command {
	// Currently we try hard to ensure that the call to `.exec()` doesn't
	// actually allocate any memory. While many platforms try to ensure that
	// memory allocation works after a fork in a multithreaded process, it's
	// been observed to be buggy and somewhat unreliable, so we do our best to
	// just not do it at all!
	//
	// Along those lines, the `argv` and `envp` raw pointers here are exactly
	// what's gonna get passed to `execvp`. The `argv` array starts with the
	// `program` and ends with a NULL, and the `envp` pointer, if present, is
	// also null-terminated.
	//
	// Right now we don't support removing arguments, so there's no much fancy
	// support there, but we support adding and removing environment variables,
	// so a side table is used to track where in the `envp` array each key is
	// located. Whenever we add a key we update it in place if it's already
	// present, and whenever we remove a key we update the locations of all
	// other keys.
	program: CString,
	args: Vec<CString>,
	env: Option<HashMap<OsString, (usize, CString)>>,
	argv: Vec<*const c_char>,
	envp: Option<Vec<*const c_char>>,

	cwd: Option<CString>,
	uid: Option<uid_t>,
	gid: Option<gid_t>,
	session_leader: bool,
	saw_nul: bool,
	closures: Vec<Box<FnMut() -> io::Result<()> + Send + Sync>>,
	stdin: Option<Stdio>,
	stdout: Option<Stdio>,
	stderr: Option<Stdio>,
	}

	// passed back to std::process with the pipes connected to the child, if any
	// were requested
	pub struct StdioPipes {
	pub stdin: Option<AnonPipe>,
	pub stdout: Option<AnonPipe>,
	pub stderr: Option<AnonPipe>,
	}

	// passed to do_exec() with configuration of what the child stdio should look
	// like
	struct ChildPipes {
	stdin: ChildStdio,
	stdout: ChildStdio,
	stderr: ChildStdio,
	}

	enum ChildStdio {
	Inherit,
	Explicit(c_int),
	Owned(FileDesc),
	}

	pub enum Stdio {
	Inherit,
	Null,
	MakePipe,
	Fd(FileDesc),
	}

	impl Command {
	pub fn new(program: &OsStr) -> Command {
	let mut saw_nul = false;
	let program = os2c(program, &mut saw_nul);
	Command {
	argv: vec![program.as_ptr(), 0 as *const _],
	program: program,
	args: Vec::new(),
	env: None,
	envp: None,
	cwd: None,
	uid: None,
	gid: None,
	session_leader: false,
	saw_nul: saw_nul,
	closures: Vec::new(),
	stdin: None,
	stdout: None,
	stderr: None,
	}
	}

	pub fn arg(&mut self, arg: &OsStr) {
	// Overwrite the trailing NULL pointer in `argv` and then add a new null
	// pointer.
	let arg = os2c(arg, &mut self.saw_nul);
	self.argv[self.args.len() + 1] = arg.as_ptr();
	self.argv.push(0 as *const _);

	// Also make sure we keep track of the owned value to schedule a
	// destructor for this memory.
	self.args.push(arg);
	}

	fn init_env_map(&mut self) -> (&mut HashMap<OsString, (usize, CString)>,
	&mut Vec<*const c_char>) {
	if self.env.is_none() {
	let mut map = HashMap::new();
	let mut envp = Vec::new();
	for (k, v) in env::vars_os() {
	let s = pair_to_key(&k, &v, &mut self.saw_nul);
	envp.push(s.as_ptr());
	map.insert(k, (envp.len() - 1, s));
	}
	envp.push(0 as *const _);
	self.env = Some(map);
	self.envp = Some(envp);
	}
	(self.env.as_mut().unwrap(), self.envp.as_mut().unwrap())
	}

	pub fn env(&mut self, key: &OsStr, val: &OsStr) {
	let new_key = pair_to_key(key, val, &mut self.saw_nul);
	let (map, envp) = self.init_env_map();

	// If `key` is already present then we we just update `envp` in place
	// (and store the owned value), but if it's not there we override the
	// trailing NULL pointer, add a new NULL pointer, and store where we
	// were located.
	match map.entry(key.to_owned()) {
	Entry::Occupied(mut e) => {
	let (i, ref mut s) = *e.get_mut();
	envp[i] = new_key.as_ptr();
	*s = new_key;
	}
	Entry::Vacant(e) => {
	let len = envp.len();
	envp[len - 1] = new_key.as_ptr();
	envp.push(0 as *const _);
	e.insert((len - 1, new_key));
	}
	}
	}

	pub fn env_remove(&mut self, key: &OsStr) {
	let (map, envp) = self.init_env_map();

	// If we actually ended up removing a key, then we need to update the
	// position of all keys that come after us in `envp` because they're all
	// one element sooner now.
	if let Some((i, _)) = map.remove(key) {
	envp.remove(i);

	for (_, &mut (ref mut j, _)) in map.iter_mut() {
	if *j >= i {
	*j -= 1;
	}
	}
	}
	}

	pub fn env_clear(&mut self) {
	self.env = Some(HashMap::new());
	self.envp = Some(vec![0 as *const _]);
	}

	pub fn cwd(&mut self, dir: &OsStr) {
	self.cwd = Some(os2c(dir, &mut self.saw_nul));
	}
	pub fn uid(&mut self, id: uid_t) {
	self.uid = Some(id);
	}
	pub fn gid(&mut self, id: gid_t) {
	self.gid = Some(id);
	}
	pub fn session_leader(&mut self, session_leader: bool) {
	self.session_leader = session_leader;
	}

	pub fn before_exec(&mut self,
	f: Box<FnMut() -> io::Result<()> + Send + Sync>) {
	self.closures.push(f);
	}

	pub fn stdin(&mut self, stdin: Stdio) {
	self.stdin = Some(stdin);
	}
	pub fn stdout(&mut self, stdout: Stdio) {
	self.stdout = Some(stdout);
	}
	pub fn stderr(&mut self, stderr: Stdio) {
	self.stderr = Some(stderr);
	}

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

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

	let pid = unsafe {
	match cvt(libc::fork())? {
	0 => {
	drop(input);
	let err = self.do_exec(theirs);
	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 {
	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 { self.do_exec(theirs) },
	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 OSX 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) -> io::Error {
	macro_rules! try {
	($e:expr) => (match $e {
	Ok(e) => e,
	Err(e) => return e,
	})
	}

	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 let Some(u) = self.gid {
	cvt(libc::setgid(u as gid_t))?;
	}
	if let Some(u) = self.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());

	cvt(libc::setuid(u as uid_t))?;
	}
	if self.session_leader {
	// Don't check the error of setsid because it fails if we're the
	// process leader already. We just forked so it shouldn't return
	// error, but ignore it anyway.
	let _ = libc::setsid();
	}
	if let Some(ref cwd) = self.cwd {
	cvt(libc::chdir(cwd.as_ptr()))?;
	}
	if let Some(ref envp) = self.envp {
	*sys::os::environ() = envp.as_ptr();
	}

	// NaCl has no signal support.
	if cfg!(not(target_os = "nacl")) {
	// 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();
	cvt(libc::sigemptyset(&mut set))?;
	cvt(libc::pthread_sigmask(libc::SIG_SETMASK, &set,
	ptr::null_mut()))?;
	let ret = libc::signal(libc::SIGPIPE, libc::SIG_DFL);
	if ret == libc::SIG_ERR {
	return io::Error::last_os_error()
	}
	}

	for callback in self.closures.iter_mut() {
	callback()?;
	}

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


	fn setup_io(&self, default: Stdio, needs_stdin: bool)
	-> io::Result<(StdioPipes, ChildPipes)> {
	let null = Stdio::Null;
	let default_stdin = if needs_stdin {&default} else {&null};
	let stdin = self.stdin.as_ref().unwrap_or(default_stdin);
	let stdout = self.stdout.as_ref().unwrap_or(&default);
	let stderr = self.stderr.as_ref().unwrap_or(&default);
	let (their_stdin, our_stdin) = stdin.to_child_stdio(true)?;
	let (their_stdout, our_stdout) = stdout.to_child_stdio(false)?;
	let (their_stderr, our_stderr) = stderr.to_child_stdio(false)?;
	let ours = StdioPipes {
	stdin: our_stdin,
	stdout: our_stdout,
	stderr: our_stderr,
	};
	let theirs = ChildPipes {
	stdin: their_stdin,
	stdout: their_stdout,
	stderr: their_stderr,
	};
	Ok((ours, theirs))
	}
	}

	fn os2c(s: &OsStr, saw_nul: &mut bool) -> CString {
	CString::new(s.as_bytes()).unwrap_or_else(\|_e\| {
	*saw_nul = true;
	CString::new("<string-with-nul>").unwrap()
	})
	}

	impl Stdio {
	fn to_child_stdio(&self, readable: bool)
	-> io::Result<(ChildStdio, Option<AnonPipe>)> {
	match *self {
	Stdio::Inherit => Ok((ChildStdio::Inherit, None)),

	// Make sure that the source descriptors are not an stdio
	// descriptor, otherwise the order which we set the child's
	// descriptors may blow away a descriptor which we are hoping to
	// save. For example, suppose we want the child's stderr to be the
	// parent's stdout, and the child's stdout to be the parent's
	// stderr. No matter which we dup first, the second will get
	// overwritten prematurely.
	Stdio::Fd(ref fd) => {
	if fd.raw() >= 0 && fd.raw() <= libc::STDERR_FILENO {
	Ok((ChildStdio::Owned(fd.duplicate()?), None))
	} else {
	Ok((ChildStdio::Explicit(fd.raw()), None))
	}
	}

	Stdio::MakePipe => {
	let (reader, writer) = pipe::anon_pipe()?;
	let (ours, theirs) = if readable {
	(writer, reader)
	} else {
	(reader, writer)
	};
	Ok((ChildStdio::Owned(theirs.into_fd()), Some(ours)))
	}

	Stdio::Null => {
	let mut opts = OpenOptions::new();
	opts.read(readable);
	opts.write(!readable);
	let path = unsafe {
	CStr::from_ptr("/dev/null\0".as_ptr() as *const _)
	};
	let fd = File::open_c(&path, &opts)?;
	Ok((ChildStdio::Owned(fd.into_fd()), None))
	}
	}
	}
	}

	impl ChildStdio {
	fn fd(&self) -> Option<c_int> {
	match *self {
	ChildStdio::Inherit => None,
	ChildStdio::Explicit(fd) => Some(fd),
	ChildStdio::Owned(ref fd) => Some(fd.raw()),
	}
	}
	}

	fn pair_to_key(key: &OsStr, value: &OsStr, saw_nul: &mut bool) -> CString {
	let (key, value) = (key.as_bytes(), value.as_bytes());
	let mut v = Vec::with_capacity(key.len() + value.len() + 1);
	v.extend(key);
	v.push(b'=');
	v.extend(value);
	CString::new(v).unwrap_or_else(\|_e\| {
	*saw_nul = true;
	CString::new("foo=bar").unwrap()
	})
	}

	impl fmt::Debug for Command {
	fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
	write!(f, "{:?}", self.program)?;
	for arg in &self.args {
	write!(f, " {:?}", arg)?;
	}
	Ok(())
	}
	}

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

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

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

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

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

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

	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)
	}
	}
	}

	/// 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> {
	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(status));
	Ok(ExitStatus(status))
	}
	}

	#[cfg(test)]
	mod tests {
	use super::*;
	use prelude::v1::*;

	use ffi::OsStr;
	use mem;
	use ptr;
	use libc;
	use sys::cvt;

	macro_rules! t {
	($e:expr) => {
	match $e {
	Ok(t) => t,
	Err(e) => panic!("received error for `{}`: {}", stringify!($e), e),
	}
	}
	}

	#[cfg(not(target_os = "android"))]
	extern {
	#[cfg_attr(target_os = "netbsd", link_name = "__sigaddset14")]
	fn sigaddset(set: *mut libc::sigset_t, signum: libc::c_int) -> libc::c_int;
	}

	#[cfg(target_os = "android")]
	unsafe fn sigaddset(set: *mut libc::sigset_t, signum: libc::c_int) -> libc::c_int {
	use slice;

	let raw = slice::from_raw_parts_mut(set as *mut u8, mem::size_of::<libc::sigset_t>());
	let bit = (signum - 1) as usize;
	raw[bit / 8] \|= 1 << (bit % 8);
	return 0;
	}

	// See #14232 for more information, but it appears that signal delivery to a
	// newly spawned process may just be raced in the OSX, so to prevent this
	// test from being flaky we ignore it on OSX.
	#[test]
	#[cfg_attr(target_os = "macos", ignore)]
	#[cfg_attr(target_os = "nacl", ignore)] // no signals on NaCl.
	fn test_process_mask() {
	unsafe {
	// Test to make sure that a signal mask does not get inherited.
	let mut cmd = Command::new(OsStr::new("cat"));

	let mut set: libc::sigset_t = mem::uninitialized();
	let mut old_set: libc::sigset_t = mem::uninitialized();
	t!(cvt(libc::sigemptyset(&mut set)));
	t!(cvt(sigaddset(&mut set, libc::SIGINT)));
	t!(cvt(libc::pthread_sigmask(libc::SIG_SETMASK, &set, &mut old_set)));

	cmd.stdin(Stdio::MakePipe);
	cmd.stdout(Stdio::MakePipe);

	let (mut cat, mut pipes) = t!(cmd.spawn(Stdio::Null, true));
	let stdin_write = pipes.stdin.take().unwrap();
	let stdout_read = pipes.stdout.take().unwrap();

	t!(cvt(libc::pthread_sigmask(libc::SIG_SETMASK, &old_set,
	ptr::null_mut())));

	t!(cvt(libc::kill(cat.id() as libc::pid_t, libc::SIGINT)));
	// We need to wait until SIGINT is definitely delivered. The
	// easiest way is to write something to cat, and try to read it
	// back: if SIGINT is unmasked, it'll get delivered when cat is
	// next scheduled.
	let _ = stdin_write.write(b"Hello");
	drop(stdin_write);

	// Either EOF or failure (EPIPE) is okay.
	let mut buf = [0; 5];
	if let Ok(ret) = stdout_read.read(&mut buf) {
	assert!(ret == 0);
	}

	t!(cat.wait());
	}
	}
	}