Use posix_spawn_file_actions_addchdir_np when possible

This is a non-POSIX extension implemented in Solaris and in glibc 2.29.
With this we can still use `posix_spawn()` when `Command::current_dir()`
has been set, otherwise we fallback to `fork(); chdir(); exec()`.

src/libstd/sys/unix/process/process_unix.rs

@@ -281,8 +281,7 @@ impl Command {
  use mem;
  use sys;
 
- if self.get_cwd().is_some() ||
- self.get_gid().is_some() ||
+ if self.get_gid().is_some() ||
  self.get_uid().is_some() ||
  self.env_saw_path() ||
  self.get_closures().len() != 0 {
@@ -301,6 +300,24 @@ impl Command {
  }
  }
 
+ // 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(libc::posix_spawn_file_actions_t);
@@ -345,6 +362,9 @@ impl Command {
  fd,
  libc::STDERR_FILENO))?;
  }
+ if let Some((f, cwd)) = addchdir {
+ cvt(f(&mut file_actions.0, cwd.as_ptr()))?;
+ }
 
  let mut set: libc::sigset_t = mem::uninitialized();
  cvt(libc::sigemptyset(&mut set))?;