Avoid hang when tool's process spawns grandchild that shares stderr/stdout (#6575)

We've received many reports of codex hanging when calling certain tools.
[Here](https://github.com/openai/codex/issues/3204) is one example. This
is likely a major cause. The problem occurs when
`consume_truncated_output` waits for `stdout` and `stderr` to be closed
once the child process terminates. This normally works fine, but it
doesn't handle the case where the child has spawned grandchild processes
that inherits `stdout` and `stderr`.

The fix was originally written by @md-oai in [this
PR](https://github.com/openai/codex/pull/1852), which has gone stale.
I've copied the original fix (which looks sound to me) and added an
integration test to prevent future regressions.

codex-rs/core/src/exec.rs

@@ -532,8 +532,52 @@ async fn consume_truncated_output(
         }
     };
 
-    let stdout = stdout_handle.await??;
-    let stderr = stderr_handle.await??;
+    // Wait for the stdout/stderr collection tasks but guard against them
+    // hanging forever. In the normal case, both pipes are closed once the child
+    // terminates so the tasks exit quickly. However, if the child process
+    // spawned grandchildren that inherited its stdout/stderr file descriptors
+    // those pipes may stay open after we `kill` the direct child on timeout.
+    // That would cause the `read_capped` tasks to block on `read()`
+    // indefinitely, effectively hanging the whole agent.
+
+    const IO_DRAIN_TIMEOUT_MS: u64 = 2_000; // 2 s should be plenty for local pipes
+
+    // We need mutable bindings so we can `abort()` them on timeout.
+    use tokio::task::JoinHandle;
+
+    async fn await_with_timeout(
+        handle: &mut JoinHandle<std::io::Result<StreamOutput<Vec<u8>>>>,
+        timeout: Duration,
+    ) -> std::io::Result<StreamOutput<Vec<u8>>> {
+        match tokio::time::timeout(timeout, &mut *handle).await {
+            Ok(join_res) => match join_res {
+                Ok(io_res) => io_res,
+                Err(join_err) => Err(std::io::Error::other(join_err)),
+            },
+            Err(_elapsed) => {
+                // Timeout: abort the task to avoid hanging on open pipes.
+                handle.abort();
+                Ok(StreamOutput {
+                    text: Vec::new(),
+                    truncated_after_lines: None,
+                })
+            }
+        }
+    }
+
+    let mut stdout_handle = stdout_handle;
+    let mut stderr_handle = stderr_handle;
+
+    let stdout = await_with_timeout(
+        &mut stdout_handle,
+        Duration::from_millis(IO_DRAIN_TIMEOUT_MS),
+    )
+    .await?;
+    let stderr = await_with_timeout(
+        &mut stderr_handle,
+        Duration::from_millis(IO_DRAIN_TIMEOUT_MS),
+    )
+    .await?;
 
     drop(agg_tx);
 

codex-rs/core/tests/suite/tools.rs

@@ -1,6 +1,10 @@
 #![cfg(not(target_os = "windows"))]
 #![allow(clippy::unwrap_used, clippy::expect_used)]
 
+use std::fs;
+use std::time::Duration;
+use std::time::Instant;
+
 use anyhow::Context;
 use anyhow::Result;
 use codex_core::features::Feature;
@@ -458,6 +462,102 @@ async fn shell_timeout_includes_timeout_prefix_and_metadata() -> Result<()> {
     Ok(())
 }
 
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn shell_timeout_handles_background_grandchild_stdout() -> Result<()> {
+    skip_if_no_network!(Ok(()));
+
+    let server = start_mock_server().await;
+    let mut builder = test_codex().with_config(|config| {
+        config.model = "gpt-5".to_string();
+        config.model_family = find_family_for_model("gpt-5").expect("gpt-5 is a valid model");
+        config.sandbox_policy = SandboxPolicy::DangerFullAccess;
+    });
+    let test = builder.build(&server).await?;
+
+    let call_id = "shell-grandchild-timeout";
+    let pid_path = test.cwd.path().join("grandchild_pid.txt");
+    let script_path = test.cwd.path().join("spawn_detached.py");
+    let script = format!(
+        r#"import subprocess
+import time
+from pathlib import Path
+
+# Spawn a detached grandchild that inherits stdout/stderr so the pipe stays open.
+proc = subprocess.Popen(["/bin/sh", "-c", "sleep 60"], start_new_session=True)
+Path({pid_path:?}).write_text(str(proc.pid))
+time.sleep(60)
+"#
+    );
+    fs::write(&script_path, script)?;
+
+    let args = json!({
+        "command": ["python3", script_path.to_string_lossy()],
+        "timeout_ms": 200,
+    });
+
+    mount_sse_once(
+        &server,
+        sse(vec![
+            ev_response_created("resp-1"),
+            ev_function_call(call_id, "shell", &serde_json::to_string(&args)?),
+            ev_completed("resp-1"),
+        ]),
+    )
+    .await;
+    let second_mock = mount_sse_once(
+        &server,
+        sse(vec![
+            ev_assistant_message("msg-1", "done"),
+            ev_completed("resp-2"),
+        ]),
+    )
+    .await;
+
+    let start = Instant::now();
+    let output_str = tokio::time::timeout(Duration::from_secs(10), async {
+        submit_turn(
+            &test,
+            "run a command with a detached grandchild",
+            AskForApproval::Never,
+            SandboxPolicy::DangerFullAccess,
+        )
+        .await?;
+        let timeout_item = second_mock.single_request().function_call_output(call_id);
+        timeout_item
+            .get("output")
+            .and_then(Value::as_str)
+            .map(str::to_string)
+            .context("timeout output string")
+    })
+    .await
+    .context("exec call should not hang waiting for grandchild pipes to close")??;
+    let elapsed = start.elapsed();
+
+    if let Ok(output_json) = serde_json::from_str::<Value>(&output_str) {
+        assert_eq!(
+            output_json["metadata"]["exit_code"].as_i64(),
+            Some(124),
+            "expected timeout exit code 124",
+        );
+    } else {
+        let timeout_pattern = r"(?is)command timed out|timeout";
+        assert_regex_match(timeout_pattern, &output_str);
+    }
+
+    assert!(
+        elapsed < Duration::from_secs(9),
+        "command should return shortly after timeout even with live grandchildren: {elapsed:?}"
+    );
+
+    if let Ok(pid_str) = fs::read_to_string(&pid_path)
+        && let Ok(pid) = pid_str.trim().parse::<libc::pid_t>()
+    {
+        unsafe { libc::kill(pid, libc::SIGKILL) };
+    }
+
+    Ok(())
+}
+
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn shell_spawn_failure_truncates_exec_error() -> Result<()> {
     skip_if_no_network!(Ok(()));