[codex] Handle Ctrl-C for non-TTY unified exec (#26734)

## Why

A long-running unified exec process started with `tty: false` could not
be interrupted via `write_stdin`: ordinary non-TTY stdin writes are
rejected once stdin is closed, but an exact U+0003 payload should still
map to a process interrupt. The interrupt should flow through the same
process lifecycle path as a real signal so Codex preserves
process-reported output and exit metadata instead of fabricating a
Ctrl-C exit code or tearing down the session early.

## What Changed

- Add `process/signal` to exec-server with `ProcessSignal::Interrupt`
and an empty response.
- Add a non-consuming `ProcessHandle::signal` path for spawned
processes; on Unix it sends SIGINT to the process group and leaves
terminate/hard-kill unchanged.
- Route non-TTY U+0003 `write_stdin` through `process.signal(...)`
instead of `terminate`, then let the normal post-write collection path
drain output and observe exit.
- Add exec-server coverage where a shell `trap INT` handler prints the
signal and exits with its own code.
- Add unified exec coverage where a `tty: false` process traps SIGINT,
emits output, and exits with its own code.

## Validation

- `just test -p codex-exec-server
exec_process_signal_interrupts_process`
- `just test -p codex-exec-server`
- `just test -p codex-core
write_stdin_ctrl_c_interrupts_non_tty_session`

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

@@ -1995,6 +1995,249 @@ async fn write_stdin_returns_exit_metadata_and_clears_session() -> Result<()> {
     Ok(())
 }
 
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn write_stdin_ctrl_c_interrupts_non_tty_session() -> Result<()> {
+    assert_write_stdin_ctrl_c_interrupts_non_tty_session(
+        "trap",
+        "trap 'echo INT-TRAP; exit 42' INT; echo READY; while true; do sleep 30; done",
+        /*expected_exit_code*/ 42,
+        Some("INT-TRAP"),
+    )
+    .await
+}
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn write_stdin_ctrl_c_default_interrupt_reports_130_for_non_tty_session() -> Result<()> {
+    assert_write_stdin_ctrl_c_interrupts_non_tty_session(
+        "default",
+        "echo READY; exec sleep 30",
+        /*expected_exit_code*/ 130,
+        /*expected_interrupt_output*/ None,
+    )
+    .await
+}
+
+async fn assert_write_stdin_ctrl_c_interrupts_non_tty_session(
+    test_name: &str,
+    command: &str,
+    expected_exit_code: i32,
+    expected_interrupt_output: Option<&str>,
+) -> Result<()> {
+    skip_if_no_network!(Ok(()));
+    skip_if_sandbox!(Ok(()));
+    skip_if_windows!(Ok(()));
+
+    let server = start_mock_server().await;
+
+    let mut builder = test_codex().with_config(|config| {
+        if let Err(err) = config.features.enable(Feature::UnifiedExec) {
+            panic!("test config should allow feature update: {err}");
+        }
+    });
+    let test = builder.build_with_remote_env(&server).await?;
+
+    let start_call_id = format!("uexec-non-tty-interrupt-{test_name}-start");
+    let interrupt_call_id = format!("uexec-non-tty-interrupt-{test_name}");
+
+    let start_args = serde_json::json!({
+        "cmd": command,
+        "yield_time_ms": 250,
+        "tty": false,
+    });
+    let interrupt_args = serde_json::json!({
+        "chars": "\u{3}",
+        "session_id": 1000,
+        "yield_time_ms": 1000,
+    });
+
+    let responses = vec![
+        sse(vec![
+            ev_response_created("resp-1"),
+            ev_function_call(
+                &start_call_id,
+                "exec_command",
+                &serde_json::to_string(&start_args)?,
+            ),
+            ev_completed("resp-1"),
+        ]),
+        sse(vec![
+            ev_response_created("resp-2"),
+            ev_function_call(
+                &interrupt_call_id,
+                "write_stdin",
+                &serde_json::to_string(&interrupt_args)?,
+            ),
+            ev_completed("resp-2"),
+        ]),
+        sse(vec![
+            ev_assistant_message("msg-1", "done"),
+            ev_completed("resp-3"),
+        ]),
+    ];
+    let request_log = mount_sse_sequence(&server, responses).await;
+
+    submit_unified_exec_turn(
+        &test,
+        "interrupt non-tty unified exec",
+        PermissionProfile::Disabled,
+    )
+    .await?;
+
+    wait_for_event(&test.codex, |event| {
+        matches!(event, EventMsg::TurnComplete(_))
+    })
+    .await;
+
+    let requests = request_log.requests();
+    assert!(!requests.is_empty(), "expected at least one POST request");
+    let bodies = requests
+        .into_iter()
+        .map(|request| request.body_json())
+        .collect::<Vec<_>>();
+
+    let outputs = collect_tool_outputs(&bodies)?;
+
+    let start_output = outputs
+        .get(&start_call_id)
+        .with_context(|| format!("missing start output for exec_command {start_call_id}"))?;
+    assert_eq!(
+        start_output.process_id.as_deref(),
+        Some("1000"),
+        "exec_command should leave a running non-TTY session"
+    );
+    assert!(
+        start_output.exit_code.is_none(),
+        "initial exec_command should not include exit_code while session is running"
+    );
+    assert!(
+        start_output.output.contains("READY"),
+        "start output should include command readiness marker, got {:?}",
+        start_output.output
+    );
+
+    let interrupt_output = outputs
+        .get(&interrupt_call_id)
+        .with_context(|| format!("missing interrupt output for write_stdin {interrupt_call_id}"))?;
+    assert!(
+        interrupt_output.process_id.is_none(),
+        "interrupted process should be cleared from the session map"
+    );
+    assert_eq!(
+        interrupt_output.exit_code,
+        Some(expected_exit_code),
+        "interrupt should preserve the process-reported exit code"
+    );
+    if let Some(expected_interrupt_output) = expected_interrupt_output {
+        assert!(
+            interrupt_output.output.contains(expected_interrupt_output),
+            "interrupt should drain output from the signal handler, got {:?}",
+            interrupt_output.output
+        );
+    }
+
+    Ok(())
+}
+
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+#[cfg_attr(not(windows), ignore = "Windows-only unified exec interrupt test")]
+async fn write_stdin_ctrl_c_reports_unsupported_interrupt_to_model_on_windows() -> Result<()> {
+    skip_if_no_network!(Ok(()));
+    skip_if_sandbox!(Ok(()));
+
+    let server = start_mock_server().await;
+
+    let mut builder = test_codex().with_config(|config| {
+        config
+            .features
+            .enable(Feature::UnifiedExec)
+            .expect("test config should allow feature update");
+    });
+    let test = builder.build_with_remote_env(&server).await?;
+
+    let start_call_id = "uexec-windows-interrupt-start";
+    let interrupt_call_id = "uexec-windows-interrupt";
+
+    let start_args = serde_json::json!({
+        "shell": "cmd",
+        "cmd": "echo READY && ping -n 30 127.0.0.1 >NUL",
+        "yield_time_ms": 250,
+        "tty": false,
+    });
+    let interrupt_args = serde_json::json!({
+        "chars": "\u{3}",
+        "session_id": 1000,
+        "yield_time_ms": 1000,
+    });
+
+    let responses = vec![
+        sse(vec![
+            ev_response_created("resp-1"),
+            ev_function_call(
+                start_call_id,
+                "exec_command",
+                &serde_json::to_string(&start_args)?,
+            ),
+            ev_completed("resp-1"),
+        ]),
+        sse(vec![
+            ev_response_created("resp-2"),
+            ev_function_call(
+                interrupt_call_id,
+                "write_stdin",
+                &serde_json::to_string(&interrupt_args)?,
+            ),
+            ev_completed("resp-2"),
+        ]),
+        sse(vec![
+            ev_response_created("resp-3"),
+            ev_assistant_message("msg-1", "done"),
+            ev_completed("resp-3"),
+        ]),
+    ];
+    let request_log = mount_sse_sequence(&server, responses).await;
+
+    submit_unified_exec_turn(
+        &test,
+        "interrupt non-tty unified exec on Windows",
+        PermissionProfile::Disabled,
+    )
+    .await?;
+
+    wait_for_event(&test.codex, |event| {
+        matches!(event, EventMsg::TurnComplete(_))
+    })
+    .await;
+
+    let start_output = request_log
+        .function_call_output_text(start_call_id)
+        .expect("missing start output for exec_command");
+    let start_output = parse_unified_exec_output(&start_output)?;
+    assert_eq!(
+        start_output.process_id.as_deref(),
+        Some("1000"),
+        "exec_command should leave a running non-TTY session"
+    );
+    assert!(
+        start_output.output.contains("READY"),
+        "start output should include command readiness marker, got {:?}",
+        start_output.output
+    );
+
+    let interrupt_output = request_log
+        .function_call_output_text(interrupt_call_id)
+        .expect("missing interrupt output for write_stdin");
+    assert!(
+        interrupt_output.contains("write_stdin failed"),
+        "model-visible write_stdin output should report failure, got {interrupt_output:?}"
+    );
+    assert!(
+        interrupt_output.contains("process interrupt is not supported by this process backend"),
+        "model-visible write_stdin output should explain unsupported interrupt, got {interrupt_output:?}"
+    );
+
+    Ok(())
+}
+
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn unified_exec_emits_end_event_when_session_dies_via_stdin() -> Result<()> {
     skip_if_no_network!(Ok(()));