allow codex to be run from pid 1 (#4200)

Previously it was not possible for codex to run commands as the init
process (pid 1) in linux. Commands run in containers tend to see their
own pid as 1. See https://github.com/openai/codex/issues/4198

This pr implements the solution mentioned in that issue.

Co-authored-by: Eric Traut <etraut@openai.com>

codex-rs/core/src/spawn.rs

@@ -67,7 +67,8 @@ pub(crate) async fn spawn_child_async(
     // This relies on prctl(2), so it only works on Linux.
     #[cfg(target_os = "linux")]
     unsafe {
-        cmd.pre_exec(|| {
+        let parent_pid = libc::getpid();
+        cmd.pre_exec(move || {
             // This prctl call effectively requests, "deliver SIGTERM when my
             // current parent dies."
             if libc::prctl(libc::PR_SET_PDEATHSIG, libc::SIGTERM) == -1 {
@@ -76,9 +77,10 @@ pub(crate) async fn spawn_child_async(
 
             // Though if there was a race condition and this pre_exec() block is
             // run _after_ the parent (i.e., the Codex process) has already
-            // exited, then the parent is the _init_ process (which will never
-            // die), so we should just terminate the child process now.
-            if libc::getppid() == 1 {
+            // exited, then parent will be the closest configured "subreaper"
+            // ancestor process, or PID 1 (init). If the Codex process has exited
+            // already, so should the child process.
+            if libc::getppid() != parent_pid {
                 libc::raise(libc::SIGTERM);
             }
             Ok(())