Merge #97

97: Prevent file descriptor leak in the spawned child. r=matthiasbeyer a=kosayoda

## Problem
After [forking the child process](https://github.com/rust-cli/rexpect/blob/cee87fc7a2ef743f09e1e880c8cf7b268a59b351/src/process.rs#L98), the file descriptors for the master and slave aren't closed since we created these descriptors ourselves (Rust sets CLOEXEC on any fds created in the stdlib).

I stumbled upon the problem when my child became a orphan after sending SIGINT to its parent. Since the drop handlers weren't run, the child isn't terminated/waited on. The expected scenario (what pexpect does) is that the master side of the pty is closed by the kernel on process exit, and SIGHUP is sent to the child process (which kills the child by default). However, since the controlling terminal is left open by the child, SIGHUP is not sent.

## Reproduction and Fix
```rust
use rexpect::spawn;

fn main() {
    let mut p = spawn("sleep 100", Some(30_000)).unwrap();

    // Hang parent
    let mut s = String::new();
    std::io::stdin().read_line(&mut s).unwrap();
}
```

On the current master:
<img width="375" alt="image" src="https://user-images.githubusercontent.com/41782385/229242068-01c6b68e-30b4-4c3b-a99d-62ed513fd9e9.png">

On the PR branch:
<img width="372" alt="image" src="https://user-images.githubusercontent.com/41782385/229242179-98e6085e-a671-42d2-93fc-9075a2ba72b4.png">


Co-authored-by: kosayoda <[email protected]>

src/process.rs

@@ -7,7 +7,7 @@ use nix::libc::{STDERR_FILENO, STDIN_FILENO, STDOUT_FILENO};
 use nix::pty::{grantpt, posix_openpt, unlockpt, PtyMaster};
 pub use nix::sys::{signal, wait};
 use nix::sys::{stat, termios};
-use nix::unistd::{dup, dup2, fork, setsid, ForkResult, Pid};
+use nix::unistd::{close, dup, dup2, fork, setsid, ForkResult, Pid};
 use std;
 use std::fs::File;
 use std::os::unix::io::{AsRawFd, FromRawFd};
@@ -97,6 +97,9 @@ impl PtyProcess {
 
         match unsafe { fork()? } {
             ForkResult::Child => {
+                // Avoid leaking master fd
+                close(master_fd.as_raw_fd())?;
+
                 setsid()?; // create new session with child as session leader
                 let slave_fd = open(
                     std::path::Path::new(&slave_name),
@@ -109,6 +112,11 @@ impl PtyProcess {
                 dup2(slave_fd, STDOUT_FILENO)?;
                 dup2(slave_fd, STDERR_FILENO)?;
 
+                // Avoid leaking slave fd
+                if slave_fd > STDERR_FILENO {
+                    close(slave_fd)?;
+                }
+
                 // set echo off
                 let mut flags = termios::tcgetattr(STDIN_FILENO)?;
                 flags.local_flags &= !termios::LocalFlags::ECHO;