A systemd stop or restart logged "Server shutdown failed: use of closed
network connection | xray is not running" on every clean exit. httpServer.Shutdown
already closes the listener, so the second Close's net.ErrClosed is now ignored,
and StopXray's not-running case is a sentinel (ErrXrayNotRunning) that Stop
treats as done.
The shipped units now use KillMode=mixed, so SIGTERM reaches only the panel,
which stops Xray and the sidecars itself; systemd still SIGKILLs whatever is left
after the stop timeout.
Rouzbeh†
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