std: Implement CommandExt::exec

This commit implements the `exec` function proposed in [RFC 1359][rfc] which is
a function on the `CommandExt` trait to execute all parts of a `Command::spawn`
without the `fork` on Unix. More details on the function itself can be found in
the comments in the commit.

[rfc]: rust-lang/rfcs#1359

cc #31398

Author: alexcrichton

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

@@ -75,6 +75,28 @@ pub trait CommandExt {
     #[unstable(feature = "process_exec", issue = "31398")]
     fn before_exec<F>(&mut self, f: F) -> &mut process::Command
         where F: FnMut() -> io::Result<()> + Send + Sync + 'static;
+
+    /// Performs all the required setup by this `Command`, followed by calling
+    /// the `execvp` syscall.
+    ///
+    /// On success this function will not return, and otherwise it will return
+    /// an error indicating why the exec (or another part of the setup of the
+    /// `Command`) failed.
+    ///
+    /// This function, unlike `spawn`, will **not** `fork` the process to create
+    /// a new child. Like spawn, however, the default behavior for the stdio
+    /// descriptors will be to inherited from the current process.
+    ///
+    /// # Notes
+    ///
+    /// The process may be in a "broken state" if this function returns in
+    /// error. For example the working directory, environment variables, signal
+    /// handling settings, various user/group information, or aspects of stdio
+    /// file descriptors may have changed. If a "transactional spawn" is
+    /// required to gracefully handle errors it is recommended to use the
+    /// cross-platform `spawn` instead.
+    #[unstable(feature = "process_exec", issue = "31398")]
+    fn exec(&mut self) -> io::Error;
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -100,6 +122,10 @@ impl CommandExt for process::Command {
         self.as_inner_mut().before_exec(Box::new(f));
         self
     }
+
+    fn exec(&mut self) -> io::Error {
+        self.as_inner_mut().exec(sys::process::Stdio::Inherit)
+    }
 }
 
 /// Unix-specific extensions to `std::process::ExitStatus`

src/libstd/sys/unix/process.rs

@@ -230,7 +230,7 @@ impl Command {
             match try!(cvt(libc::fork())) {
                 0 => {
                     drop(input);
-                    let err = self.exec(theirs);
+                    let err = self.do_exec(theirs);
                     let errno = err.raw_os_error().unwrap_or(libc::EINVAL) as u32;
                     let bytes = [
                         (errno >> 24) as u8,
@@ -290,6 +290,18 @@ impl Command {
         }
     }
 
+    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) {
+            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
@@ -320,7 +332,7 @@ impl Command {
     // 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 exec(&mut self, stdio: ChildPipes) -> io::Error {
+    unsafe fn do_exec(&mut self, stdio: ChildPipes) -> io::Error {
         macro_rules! try {
             ($e:expr) => (match $e {
                 Ok(e) => e,

src/test/run-pass/command-exec.rs

@@ -0,0 +1,72 @@
+// Copyright 2016 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.
+
+// ignore-windows - this is a unix-specific test
+// ignore-pretty
+
+#![feature(process_exec)]
+
+use std::env;
+use std::os::unix::process::CommandExt;
+use std::process::Command;
+
+fn main() {
+    let mut args = env::args();
+    let me = args.next().unwrap();
+
+    if let Some(arg) = args.next() {
+        match &arg[..] {
+            "test1" => println!("passed"),
+
+            "exec-test1" => {
+                let err = Command::new(&me).arg("test1").exec();
+                panic!("failed to spawn: {}", err);
+            }
+
+            "exec-test2" => {
+                Command::new("/path/to/nowhere").exec();
+                println!("passed");
+            }
+
+            "exec-test3" => {
+                Command::new(&me).arg("bad\0").exec();
+                println!("passed");
+            }
+
+            "exec-test4" => {
+                Command::new(&me).current_dir("/path/to/nowhere").exec();
+                println!("passed");
+            }
+
+            _ => panic!("unknown argument: {}", arg),
+        }
+        return
+    }
+
+    let output = Command::new(&me).arg("exec-test1").output().unwrap();
+    assert!(output.status.success());
+    assert!(output.stderr.is_empty());
+    assert_eq!(output.stdout, b"passed\n");
+
+    let output = Command::new(&me).arg("exec-test2").output().unwrap();
+    assert!(output.status.success());
+    assert!(output.stderr.is_empty());
+    assert_eq!(output.stdout, b"passed\n");
+
+    let output = Command::new(&me).arg("exec-test3").output().unwrap();
+    assert!(output.status.success());
+    assert!(output.stderr.is_empty());
+    assert_eq!(output.stdout, b"passed\n");
+
+    let output = Command::new(&me).arg("exec-test4").output().unwrap();
+    assert!(output.status.success());
+    assert!(output.stderr.is_empty());
+    assert_eq!(output.stdout, b"passed\n");
+}