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 (8ad12c3 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 (712ecce 2015-12-19)
$ rustc stdio.rs && ./stdio >out 2>err
$ cat out
parent stdout
child stderr
$ cat err
parent stderr
child stdout
```

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) }

src/test/run-pass/issue-30490.rs

@@ -0,0 +1,107 @@
+// Copyright 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.
+
+// Previously libstd would set stdio descriptors of a child process
+// by `dup`ing the requested descriptors to inherit directly into the
+// stdio descriptors. This, however, would incorrectly handle cases
+// where the descriptors to inherit were already stdio descriptors.
+// This test checks to avoid that regression.
+
+#![cfg_attr(unix, feature(libc))]
+#![cfg_attr(windows, allow(unused_imports))]
+
+#[cfg(unix)]
+extern crate libc;
+
+use std::fs::File;
+use std::io::{Read, Write};
+use std::io::{stdout, stderr};
+use std::process::{Command, Stdio};
+
+#[cfg(unix)]
+use std::os::unix::io::FromRawFd;
+
+#[cfg(not(unix))]
+fn main() {
+    // Bug not present in Windows
+}
+
+#[cfg(unix)]
+fn main() {
+    let mut args = std::env::args();
+    let name = args.next().unwrap();
+    let args: Vec<String> = args.collect();
+    if let Some("--child") = args.get(0).map(|s| &**s) {
+        return child();
+    } else if !args.is_empty() {
+        panic!("unknown options");
+    }
+
+    let stdout_backup = unsafe { libc::dup(libc::STDOUT_FILENO) };
+    let stderr_backup = unsafe { libc::dup(libc::STDERR_FILENO) };
+    assert!(stdout_backup > -1);
+    assert!(stderr_backup > -1);
+
+    let (stdout_reader, stdout_writer) = pipe();
+    let (stderr_reader, stderr_writer) = pipe();
+    assert!(unsafe { libc::dup2(stdout_writer, libc::STDOUT_FILENO) } > -1);
+    assert!(unsafe { libc::dup2(stderr_writer, libc::STDERR_FILENO) } > -1);
+
+    // Make sure we close any duplicates of the writer end of the pipe,
+    // otherwise we can get stuck reading from the pipe which has open
+    // writers but no one supplying any input
+    assert_eq!(unsafe { libc::close(stdout_writer) }, 0);
+    assert_eq!(unsafe { libc::close(stderr_writer) }, 0);
+
+    stdout().write_all("parent stdout\n".as_bytes()).expect("failed to write to stdout");
+    stderr().write_all("parent stderr\n".as_bytes()).expect("failed to write to stderr");
+
+    let child = {
+        Command::new(name)
+            .arg("--child")
+            .stdin(Stdio::inherit())
+            .stdout(unsafe { FromRawFd::from_raw_fd(libc::STDERR_FILENO) })
+            .stderr(unsafe { FromRawFd::from_raw_fd(libc::STDOUT_FILENO) })
+            .spawn()
+    };
+
+    // The Stdio passed into the Command took over (and closed) std{out, err}
+    // so we should restore them as they were.
+    assert!(unsafe { libc::dup2(stdout_backup, libc::STDOUT_FILENO) } > -1);
+    assert!(unsafe { libc::dup2(stderr_backup, libc::STDERR_FILENO) } > -1);
+
+    // Using File as a shim around the descriptor
+    let mut read = String::new();
+    let mut f: File = unsafe { FromRawFd::from_raw_fd(stdout_reader) };
+    f.read_to_string(&mut read).expect("failed to read from stdout file");
+    assert_eq!(read, "parent stdout\nchild stderr\n");
+
+    // Using File as a shim around the descriptor
+    read.clear();
+    let mut f: File = unsafe { FromRawFd::from_raw_fd(stderr_reader) };
+    f.read_to_string(&mut read).expect("failed to read from stderr file");
+    assert_eq!(read, "parent stderr\nchild stdout\n");
+
+    assert!(child.expect("failed to execute child process").wait().unwrap().success());
+}
+
+#[cfg(unix)]
+fn child() {
+    stdout().write_all("child stdout\n".as_bytes()).expect("child failed to write to stdout");
+    stderr().write_all("child stderr\n".as_bytes()).expect("child failed to write to stderr");
+}
+
+#[cfg(unix)]
+/// Returns a pipe (reader, writer combo)
+fn pipe() -> (i32, i32) {
+     let mut fds = [0; 2];
+     assert_eq!(unsafe { libc::pipe(fds.as_mut_ptr()) }, 0);
+     (fds[0], fds[1])
+}