Socket‑activated service stalls on upgrade: Type=simple vs Type=notify behavior

Goal

Migrate a small network service on Arch Linux without dropping existing connections by using systemd socket activation.

Constraints

• Pacman replaces the service binary and libraries during pacman -Syu.
• Running processes keep open file descriptors to the old binary until they are restarted.
• The service is started via a systemd socket unit; the manager must hand new connections to the updated process.

Uncertainty

The choice between Type=simple (default) and Type=notify influences when systemd considers the service ready after a reload. With Type=simple the manager assumes readiness immediately, while Type=notify waits for the daemon to send a readiness notification.

Questions

1. Does Type=notify guarantee that new connections are routed to the upgraded process without interrupting existing ones?
2. What are the practical implications of staying with Type=simple for a socket‑activated service during a package upgrade?
3. How can an administrator verify that the new binary is actually serving traffic after a systemctl reload?