Add --headless mode (no stdin read, runs until SIGTERM/exit) + systemd/NixOS examples

cli/tests/cli.rs

@@ -39,3 +39,34 @@ fn bare_command_without_a_world_starts_init() {
     // It asks before it creates anything.
     assert!(!dir.join("world").exists());
 }
+
+/// `--headless` never reads stdin: with stdin at EOF the world stays up, and
+/// SIGTERM stops it cleanly (exit code 0).
+#[test]
+fn headless_ignores_stdin_eof_and_stops_cleanly_on_sigterm() {
+    use std::io::{BufRead, BufReader};
+    let dir = empty_dir("headless");
+    let world = dir.join("world");
+    sausage::World::create(&world).unwrap();
+    let mut child = Command::new(env!("CARGO_BIN_EXE_sausage"))
+        .args(["run", "--headless"])
+        .arg(&world)
+        .stdin(Stdio::null())
+        .stdout(Stdio::piped())
+        .stderr(Stdio::null())
+        .spawn()
+        .unwrap();
+    let mut lines = BufReader::new(child.stdout.take().unwrap()).lines();
+    // The host prints this once it runs and the signal handlers are installed.
+    let line = lines.next().expect("host exited before it started").unwrap();
+    assert!(line.contains("headless"), "unexpected first line: {line}");
+    // The host reaches EOF on stdin immediately; it must still be running.
+    std::thread::sleep(std::time::Duration::from_millis(500));
+    assert!(child.try_wait().unwrap().is_none(), "headless host exited on stdin EOF");
+    let pid = child.id().to_string();
+    assert!(Command::new("kill").args(["-TERM", &pid]).status().unwrap().success());
+    let rest: Vec<String> = lines.map(|l| l.unwrap()).collect();
+    let status = child.wait().unwrap();
+    assert!(status.success(), "exit status {status:?}, output {rest:?}");
+    assert!(rest.iter().any(|l| l.contains("Stopping actors")), "output {rest:?}");
+}