Add Linux systemd support and --install-service/--remove-service flags

The service package now has a Linux implementation alongside the Windows
one: systemd unit install/remove (/etc/systemd/system), readiness
notification (READY=1 via go-systemd) sent only after the listeners are
bound, a 30s watchdog, and STOPPING on shutdown. Listeners are pre-bound
so port conflicts fail fast and the readiness signal is truthful. The
notify calls are no-ops without NOTIFY_SOCKET (containers, shells) and
on non-Linux builds. --install-service/--remove-service work on both
platforms; the 'service install|remove' subcommand remains as an alias.

cmd/gpu-turnstile/main.go

@@ -8,6 +8,7 @@ import (
 	"fmt"
 	"io"
 	"log/slog"
+	"net"
 	"net/http"
 	"os"
 	"os/signal"
@@ -34,12 +35,22 @@ var version = "dev"
 const exitCodeUpdate = 3
 
 func main() {
-	configPath, args := splitConfigFlag(os.Args[1:])
+	configPath, install, remove, args := parseFlags(os.Args[1:])
+	switch {
+	case install && remove:
+		fmt.Fprintf(os.Stderr, "gpu-turnstile: --install-service and --remove-service are mutually exclusive\n")
+		os.Exit(2)
+	case install:
+		os.Exit(serviceCommand(configPath, []string{"install"}))
+	case remove:
+		os.Exit(serviceCommand(configPath, []string{"remove"}))
+	}
 	if len(args) > 0 && args[0] == "service" {
 		os.Exit(serviceCommand(configPath, args[1:]))
 	}
 	if len(args) > 0 {
-		fmt.Fprintf(os.Stderr, "usage: gpu-turnstile [-config path] | gpu-turnstile service install|remove [-config path]\n")
+		fmt.Fprintf(os.Stderr, "usage: gpu-turnstile [-config path] [--install-service | --remove-service]\n")
+		fmt.Fprintf(os.Stderr, "       gpu-turnstile service install|remove [-config path]\n")
 		os.Exit(2)
 	}
 
@@ -70,10 +81,10 @@ func main() {
 	}
 }
 
-// splitConfigFlag extracts -config <path> (or -config=<path>) from args.
-func splitConfigFlag(args []string) (string, []string) {
-	var configPath string
-	rest := args[:0]
+// parseFlags extracts -config <path> (or -config=<path>) and the
+// --install-service / --remove-service switches from args.
+func parseFlags(args []string) (configPath string, install, remove bool, rest []string) {
+	rest = args[:0]
 	for i := 0; i < len(args); i++ {
 		switch {
 		case args[i] == "-config" && i+1 < len(args):
@@ -81,11 +92,15 @@ func splitConfigFlag(args []string) (string, []string) {
 			i++
 		case strings.HasPrefix(args[i], "-config="):
 			configPath = strings.TrimPrefix(args[i], "-config=")
+		case args[i] == "--install-service" || args[i] == "-install-service":
+			install = true
+		case args[i] == "--remove-service" || args[i] == "-remove-service":
+			remove = true
 		default:
 			rest = append(rest, args[i])
 		}
 	}
-	return configPath, rest
+	return configPath, install, remove, rest
 }
 
 // defaultConfigPath returns gpu-turnstile.env next to the executable.
@@ -282,21 +297,41 @@ func run(ctx context.Context, cfg config.Config, log *slog.Logger, logOut io.Wri
 	}
 	probeCancel()
 
+	// Bind the listeners up front so a port conflict fails fast and the
+	// readiness notification below really means "accepting connections".
 	var servers []*http.Server
+	var listeners []net.Listener
+	bind := func(addr string, handler http.Handler, consumer string) error {
+		ln, err := net.Listen("tcp", addr)
+		if err != nil {
+			return fmt.Errorf("listen %s on %s: %w", consumer, addr, err)
+		}
+		servers = append(servers, &http.Server{Addr: addr, Handler: handler})
+		listeners = append(listeners, ln)
+		log.Warn("listening", "consumer", consumer, "addr", addr)
+		return nil
+	}
 	if ollamaClient != nil {
-		servers = append(servers, &http.Server{Addr: cfg.ListenOllama, Handler: srv.OllamaHandler()})
-		log.Warn("listening", "consumer", "ollama", "addr", cfg.ListenOllama)
+		if err := bind(cfg.ListenOllama, srv.OllamaHandler(), "ollama"); err != nil {
+			return err
+		}
 	}
 	if comfyClient != nil {
-		servers = append(servers, &http.Server{Addr: cfg.ListenComfy, Handler: srv.ComfyHandler()})
-		log.Warn("listening", "consumer", "comfy", "addr", cfg.ListenComfy)
+		if err := bind(cfg.ListenComfy, srv.ComfyHandler(), "comfy"); err != nil {
+			return err
+		}
 	}
 
 	errCh := make(chan error, len(servers))
-	for _, s := range servers {
-		go func(s *http.Server) { errCh <- s.ListenAndServe() }(s)
+	for i := range servers {
+		go func(s *http.Server, ln net.Listener) { errCh <- s.Serve(ln) }(servers[i], listeners[i])
 	}
 
+	// Tell systemd we are up and start the watchdog pings; both are no-ops
+	// when not running under a notify/watchdog unit.
+	service.NotifyReady()
+	service.StartWatchdog(ctx)
+
 	if cfg.AutoUpdate {
 		go updateLoop(ctx, cfg, log, lk, isService)
 	}