Auto merge of #30490 - ipetkov:unix-spawn, r=alexcrichton

* If the requested descriptors to inherit are stdio descriptors there
  are situations where they will not be set correctly
* Example: parent's stdout --> child's stderr
           parent's stderr --> child's stdout
* Solution: if the requested descriptors for the child are stdio
  descriptors, `dup` them before overwriting the child's stdio

Example of a program which exhibits the bug:
```rust
// stdio.rs
use std::io::Write;
use std::io::{stdout, stderr};
use std::process::{Command, Stdio};
use std::os::unix::io::FromRawFd;

fn main() {
    stdout().write_all("parent stdout\n".as_bytes()).unwrap();
    stderr().write_all("parent stderr\n".as_bytes()).unwrap();

    Command::new("sh")
        .arg("-c")
        .arg("echo 'child stdout'; echo 'child stderr' 1>&2")
        .stdin(Stdio::inherit())
        .stdout(unsafe { FromRawFd::from_raw_fd(2) })
        .stderr(unsafe { FromRawFd::from_raw_fd(1) })
        .status()
        .unwrap_or_else(|e| { panic!("failed to execute process: {}", e) });
}
```

Before:
```
$ rustc --version
rustc 1.7.0-nightly (8ad12c3e2 2015-12-19)
$ rustc stdio.rs && ./stdio >out 2>err
$ cat out
parent stdout
$ cat err
parent stderr
child stdout
child stderr
```

After (expected):
```
$ rustc --version
rustc 1.7.0-dev (712eccee2 2015-12-19)
$ rustc stdio.rs && ./stdio >out 2>err
$ cat out
parent stdout
child stderr
$ cat err
parent stderr
child stdout
```

1 files changed, 32 insertions, 0 deletions

diff --git a/src/libstd/sys/unix/process.rs b/src/libstd/sys/unix/process.rs
index 495f45f5c7e..bb9d37f93ab 100644
--- a/src/libstd/sys/unix/process.rs
+++ b/src/libstd/sys/unix/process.rs
@@ -288,6 +288,32 @@ impl Process {
             unsafe { libc::_exit(1) }
         }
 
+        // 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.
+        let maybe_migrate = |src: Stdio, output: &mut AnonPipe| {
+            match src {
+                Stdio::Raw(fd @ libc::STDIN_FILENO) |
+                Stdio::Raw(fd @ libc::STDOUT_FILENO) |
+                Stdio::Raw(fd @ libc::STDERR_FILENO) => {
+                    let fd = match cvt_r(|| libc::dup(fd)) {
+                        Ok(fd) => fd,
+                        Err(_) => fail(output),
+                    };
+                    let fd = FileDesc::new(fd);
+                    fd.set_cloexec();
+                    Stdio::Raw(fd.into_raw())
+                },
+
+                s @ Stdio::None |
+                s @ Stdio::Inherit |
+                s @ Stdio::Raw(_) => s,
+            }
+        };
+
         let setup = |src: Stdio, dst: c_int| {
             match src {
                 Stdio::Inherit => true,
@@ -313,6 +339,12 @@ impl Process {
             }
         };
 
+        // Make sure we migrate all source descriptors before
+        // we start overwriting them
+        let in_fd = maybe_migrate(in_fd, &mut output);
+        let out_fd = maybe_migrate(out_fd, &mut output);
+        let err_fd = maybe_migrate(err_fd, &mut output);
+
         if !setup(in_fd, libc::STDIN_FILENO) { fail(&mut output) }
         if !setup(out_fd, libc::STDOUT_FILENO) { fail(&mut output) }
         if !setup(err_fd, libc::STDERR_FILENO) { fail(&mut output) }