core: prevent shell_snapshot from inheriting stdin (#9735)

Fixes #9559.

When `shell_snapshot` runs, it may execute user startup files (e.g.
`.bashrc`). If those files read from stdin (or if stdin is an
interactive TTY under job control), the snapshot subprocess can block or
receive `SIGTTIN` (as reported over SSH).

This change explicitly sets `stdin` to `Stdio::null()` for the snapshot
subprocess, so it can't read from the terminal.

Regression test added that would hang/timeout without this change.
Tests: `ulimit -n 4096 && cargo test -p codex-core`.

cc @dongdongbh @etraut-openai

---------

Co-authored-by: Skylar Graika <sgraika127@gmail.com>

codex-rs/core/src/shell_snapshot.rs

@@ -1,6 +1,7 @@
 use std::io::ErrorKind;
 use std::path::Path;
 use std::path::PathBuf;
+use std::process::Stdio;
 use std::sync::Arc;
 use std::time::Duration;
 use std::time::SystemTime;
@@ -187,6 +188,7 @@ async fn run_script_with_timeout(
     // returns a ref of handler.
     let mut handler = Command::new(&args[0]);
     handler.args(&args[1..]);
+    handler.stdin(Stdio::null());
     #[cfg(unix)]
     unsafe {
         handler.pre_exec(|| {
@@ -477,6 +479,62 @@ mod tests {
 
     use tempfile::tempdir;
 
+    #[cfg(unix)]
+    struct BlockingStdinPipe {
+        original: i32,
+        write_end: i32,
+    }
+
+    #[cfg(unix)]
+    impl BlockingStdinPipe {
+        fn install() -> Result<Self> {
+            let mut fds = [0i32; 2];
+            if unsafe { libc::pipe(fds.as_mut_ptr()) } == -1 {
+                return Err(std::io::Error::last_os_error()).context("create stdin pipe");
+            }
+
+            let original = unsafe { libc::dup(libc::STDIN_FILENO) };
+            if original == -1 {
+                let err = std::io::Error::last_os_error();
+                unsafe {
+                    libc::close(fds[0]);
+                    libc::close(fds[1]);
+                }
+                return Err(err).context("dup stdin");
+            }
+
+            if unsafe { libc::dup2(fds[0], libc::STDIN_FILENO) } == -1 {
+                let err = std::io::Error::last_os_error();
+                unsafe {
+                    libc::close(fds[0]);
+                    libc::close(fds[1]);
+                    libc::close(original);
+                }
+                return Err(err).context("replace stdin");
+            }
+
+            unsafe {
+                libc::close(fds[0]);
+            }
+
+            Ok(Self {
+                original,
+                write_end: fds[1],
+            })
+        }
+    }
+
+    #[cfg(unix)]
+    impl Drop for BlockingStdinPipe {
+        fn drop(&mut self) {
+            unsafe {
+                libc::dup2(self.original, libc::STDIN_FILENO);
+                libc::close(self.original);
+                libc::close(self.write_end);
+            }
+        }
+    }
+
     #[cfg(not(target_os = "windows"))]
     fn assert_posix_snapshot_sections(snapshot: &str) {
         assert!(snapshot.contains("# Snapshot file"));
@@ -557,6 +615,38 @@ mod tests {
         Ok(())
     }
 
+    #[cfg(unix)]
+    #[tokio::test]
+    async fn snapshot_shell_does_not_inherit_stdin() -> Result<()> {
+        let _stdin_guard = BlockingStdinPipe::install()?;
+
+        let dir = tempdir()?;
+        let home = dir.path();
+        fs::write(home.join(".bashrc"), "read -r ignored\n").await?;
+
+        let shell = Shell {
+            shell_type: ShellType::Bash,
+            shell_path: PathBuf::from("/bin/bash"),
+            shell_snapshot: crate::shell::empty_shell_snapshot_receiver(),
+        };
+
+        let home_display = home.display();
+        let script = format!(
+            "HOME=\"{home_display}\"; export HOME; {}",
+            bash_snapshot_script()
+        );
+        let output = run_script_with_timeout(&shell, &script, Duration::from_millis(500), true)
+            .await
+            .context("run snapshot command")?;
+
+        assert!(
+            output.contains("# Snapshot file"),
+            "expected snapshot marker in output; output={output:?}"
+        );
+
+        Ok(())
+    }
+
     #[cfg(target_os = "linux")]
     #[tokio::test]
     async fn timed_out_snapshot_shell_is_terminated() -> Result<()> {