fix: integration test for #9011 (#9166)

Adds an integration test for the new behavior introduced in
https://github.com/openai/codex/pull/9011. The work to create the test
setup was substantial enough that I thought it merited a separate PR.

This integration test spawns `codex` in TUI mode, which requires
spawning a PTY to run successfully, so I had to introduce quite a bit of
scaffolding in `run_codex_cli()`. I was surprised to discover that we
have not done this in our codebase before, so perhaps this should get
moved to a common location so it can be reused.

The test itself verifies that a malformed `rules` in `$CODEX_HOME`
prints a human-readable error message and exits nonzero.

codex-rs/tui/tests/suite/no_panic_on_startup.rs

@@ -0,0 +1,119 @@
+use std::collections::HashMap;
+use std::path::Path;
+use std::time::Duration;
+use tokio::select;
+use tokio::time::timeout;
+
+/// Regression test for https://github.com/openai/codex/issues/8803.
+#[tokio::test]
+async fn malformed_rules_should_not_panic() -> anyhow::Result<()> {
+    // run_codex_cli() does not work on Windows due to PTY limitations.
+    if cfg!(windows) {
+        return Ok(());
+    }
+
+    let tmp = tempfile::tempdir()?;
+    let codex_home = tmp.path();
+    std::fs::write(
+        codex_home.join("rules"),
+        "rules should be a directory not a file",
+    )?;
+
+    // TODO(mbolin): Figure out why using a temp dir as the cwd causes this test
+    // to hang.
+    let cwd = std::env::current_dir()?;
+    let config_contents = format!(
+        r#"
+# Pick a local provider so the CLI doesn't prompt for OpenAI auth in this test.
+model_provider = "ollama"
+
+[projects]
+"{cwd}" = {{ trust_level = "trusted" }}
+"#,
+        cwd = cwd.display()
+    );
+    std::fs::write(codex_home.join("config.toml"), config_contents)?;
+
+    let CodexCliOutput { exit_code, output } = run_codex_cli(codex_home, cwd).await?;
+    assert_eq!(1, exit_code, "Codex CLI should exit nonzero.");
+    assert!(
+        output.contains("ERROR: Failed to initialize codex:"),
+        "expected startup error in output, got: {output}"
+    );
+    assert!(
+        output.contains("failed to read execpolicy files"),
+        "expected execpolicy read error in output, got: {output}"
+    );
+    Ok(())
+}
+
+struct CodexCliOutput {
+    exit_code: i32,
+    output: String,
+}
+
+async fn run_codex_cli(
+    codex_home: impl AsRef<Path>,
+    cwd: impl AsRef<Path>,
+) -> anyhow::Result<CodexCliOutput> {
+    let codex_cli = codex_utils_cargo_bin::cargo_bin("codex")?;
+    let mut env = HashMap::new();
+    env.insert(
+        "CODEX_HOME".to_string(),
+        codex_home.as_ref().display().to_string(),
+    );
+
+    let args = vec!["-c".to_string(), "analytics_enabled=false".to_string()];
+    let spawned = codex_utils_pty::spawn_pty_process(
+        codex_cli.to_string_lossy().as_ref(),
+        &args,
+        cwd.as_ref(),
+        &env,
+        &None,
+    )
+    .await?;
+    let mut output = Vec::new();
+    let mut output_rx = spawned.output_rx;
+    let mut exit_rx = spawned.exit_rx;
+    let writer_tx = spawned.session.writer_sender();
+    let exit_code_result = timeout(Duration::from_secs(10), async {
+        // Read PTY output until the process exits while replying to cursor
+        // position queries so the TUI can initialize without a real terminal.
+        loop {
+            select! {
+                result = output_rx.recv() => match result {
+                    Ok(chunk) => {
+                        // The TUI asks for the cursor position via ESC[6n.
+                        // Respond with a valid position to unblock startup.
+                        if chunk.windows(4).any(|window| window == b"\x1b[6n") {
+                            let _ = writer_tx.send(b"\x1b[1;1R".to_vec()).await;
+                        }
+                        output.extend_from_slice(&chunk);
+                    }
+                    Err(tokio::sync::broadcast::error::RecvError::Closed) => break exit_rx.await,
+                    Err(tokio::sync::broadcast::error::RecvError::Lagged(_)) => {}
+                },
+                result = &mut exit_rx => break result,
+            }
+        }
+    })
+    .await;
+    let exit_code = match exit_code_result {
+        Ok(Ok(code)) => code,
+        Ok(Err(err)) => return Err(err.into()),
+        Err(_) => {
+            spawned.session.terminate();
+            anyhow::bail!("timed out waiting for codex CLI to exit");
+        }
+    };
+    // Drain any output that raced with the exit notification.
+    while let Ok(chunk) = output_rx.try_recv() {
+        output.extend_from_slice(&chunk);
+    }
+
+    let output = String::from_utf8_lossy(&output);
+    Ok(CodexCliOutput {
+        exit_code,
+        output: output.to_string(),
+    })
+}