rollup merge of #19454: nodakai/libstd-reap-failed-child

After the library successfully called `fork(2)`, the child does several
setup works such as setting UID, GID and current directory before it
calls `exec(2)`.  When those setup works failed, the child exits but the
parent didn't call `waitpid(2)` and left it as a zombie.

This patch also add several sanity checks.  They shouldn't make any
noticeable impact to runtime performance.

The new test case in `libstd/io/process.rs` calls the ps command to check
if the new code can really reap a zombie.
The output of `ps -A -o pid,sid,command` should look like this:

```
  PID   SID COMMAND
    1     1 /sbin/init
    2     0 [kthreadd]
    3     0 [ksoftirqd/0]
...
12562  9237 ./spawn-failure
12563  9237 [spawn-failure] <defunct>
12564  9237 [spawn-failure] <defunct>
...
12592  9237 [spawn-failure] <defunct>
12593  9237 ps -A -o pid,sid,command
12884 12884 /bin/zsh
12922 12922 /bin/zsh
...
```

where `./spawn-failure` is my test program which intentionally leaves many
zombies.  Filtering the output with the "SID" (session ID) column is
a quick way to tell if a process (zombie) was spawned by my own test
program.  Then the number of "defunct" lines is the number of zombie
children.

src/test/run-pass/wait-forked-but-failed-child.rs

@@ -0,0 +1,79 @@
+// Copyright 2014 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.
+
+
+extern crate libc;
+
+use std::io::process::Command;
+use std::iter::IteratorExt;
+
+use libc::funcs::posix88::unistd;
+
+
+// "ps -A -o pid,sid,command" with GNU ps should output something like this:
+//   PID   SID COMMAND
+//     1     1 /sbin/init
+//     2     0 [kthreadd]
+//     3     0 [ksoftirqd/0]
+// ...
+// 12562  9237 ./spawn-failure
+// 12563  9237 [spawn-failure] <defunct>
+// 12564  9237 [spawn-failure] <defunct>
+// ...
+// 12592  9237 [spawn-failure] <defunct>
+// 12593  9237 ps -A -o pid,sid,command
+// 12884 12884 /bin/zsh
+// 12922 12922 /bin/zsh
+// ...
+
+#[cfg(unix)]
+fn find_zombies() {
+    // http://man.freebsd.org/ps(1)
+    // http://man7.org/linux/man-pages/man1/ps.1.html
+    #[cfg(not(target_os = "macos"))]
+    const FIELDS: &'static str = "pid,sid,command";
+
+    // https://developer.apple.com/library/mac/documentation/Darwin/
+    // Reference/ManPages/man1/ps.1.html
+    #[cfg(target_os = "macos")]
+    const FIELDS: &'static str = "pid,sess,command";
+
+    let my_sid = unsafe { unistd::getsid(0) };
+
+    let ps_cmd_output = Command::new("ps").args(&["-A", "-o", FIELDS]).output().unwrap();
+    let ps_output = String::from_utf8_lossy(ps_cmd_output.output.as_slice());
+
+    let found = ps_output.split('\n').enumerate().any(|(line_no, line)|
+        0 < line_no && 0 < line.len() &&
+        my_sid == from_str(line.split(' ').filter(|w| 0 < w.len()).nth(1)
+            .expect("1st column should be Session ID")
+            ).expect("Session ID string into integer") &&
+        line.contains("defunct") && {
+            println!("Zombie child {}", line);
+            true
+        }
+    );
+
+    assert!( ! found, "Found at least one zombie child");
+}
+
+#[cfg(windows)]
+fn find_zombies() { }
+
+fn main() {
+    let too_long = format!("/NoSuchCommand{:0300}", 0u8);
+
+    for _ in range(0u32, 100) {
+        let invalid = Command::new(too_long.as_slice()).spawn();
+        assert!(invalid.is_err());
+    }
+
+    find_zombies();
+}