Add background forwarding mode

tests/integration_smoke.rs

@@ -626,6 +626,59 @@ fn native_local_forward_echo_smoke() {
     );
 }
 
+#[test]
+fn native_local_forward_background_smoke() {
+    let dir = tempfile::tempdir().unwrap();
+    let port = free_udp_port();
+    let config = write_server_config(&dir, port);
+    write_native_client_auth(&dir, &config);
+    let echo_port = start_echo_server();
+    let local_port = free_tcp_port();
+    let pid_file = dir.path().join("background.pid");
+    let mut server = start_server(&dir, &config);
+    let client_bin = env!("CARGO_BIN_EXE_dosh-client");
+    let output = Command::new(client_bin)
+        .arg("--auth")
+        .arg("native")
+        .arg("--no-cache")
+        .arg("-f")
+        .arg("-N")
+        .arg("-L")
+        .arg(format!("{local_port}:127.0.0.1:{echo_port}"))
+        .arg("--session")
+        .arg("default")
+        .arg("--dosh-host")
+        .arg("127.0.0.1")
+        .arg("--dosh-port")
+        .arg(port.to_string())
+        .arg("local")
+        .env("HOME", dir.path())
+        .env("DOSH_BACKGROUND_PID_FILE", &pid_file)
+        .output()
+        .unwrap();
+    assert!(
+        output.status.success(),
+        "stderr={}",
+        String::from_utf8_lossy(&output.stderr)
+    );
+
+    let mut stream = TcpStream::connect(("127.0.0.1", local_port)).unwrap();
+    stream
+        .set_read_timeout(Some(Duration::from_secs(3)))
+        .unwrap();
+    stream.write_all(b"dosh-background-forward-ping").unwrap();
+    let mut buf = [0u8; 64];
+    let n = stream.read(&mut buf).unwrap();
+
+    if let Ok(pid) = fs::read_to_string(&pid_file) {
+        let _ = Command::new("kill").arg(pid.trim()).status();
+    }
+    let _ = server.kill();
+    let _ = server.wait();
+
+    assert_eq!(&buf[..n], b"dosh-background-forward-ping");
+}
+
 #[test]
 fn native_remote_forward_echo_smoke() {
     let dir = tempfile::tempdir().unwrap();