Add Spinner, a Minecraft hostname-routing reverse proxy

Routes Java Edition connections by the hostname in the handshake, with
wildcard domains, backend failover, PROXY protocol v2, cached server-list
status from health probes, hot-reloaded config, and an admin page and
metrics endpoint. Includes CI, tagged releases and a container image.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

cmd/spinner/main.go

@@ -0,0 +1,114 @@
+// Command spinner routes Minecraft Java connections to backend servers by hostname.
+package main
+
+import (
+	"context"
+	"errors"
+	"flag"
+	"log/slog"
+	"net"
+	"net/http"
+	"os"
+	"os/signal"
+	"syscall"
+	"time"
+
+	"git.veillax.com/Veillax/reef/dotenv"
+	"git.veillax.com/Veillax/spinner/internal/admin"
+	"git.veillax.com/Veillax/spinner/internal/config"
+	"git.veillax.com/Veillax/spinner/internal/health"
+	"git.veillax.com/Veillax/spinner/internal/proxy"
+)
+
+// version is set at build time (deploy/build.sh).
+var version = "dev"
+
+const starterConfig = `{
+  "bind": ":25565",
+  "admin_bind": ":8080",
+  "servers": []
+}
+`
+
+func main() {
+	envFile := dotenv.ArgPath(os.Args[1:], ".env")
+	if _, err := dotenv.Load(envFile); err != nil {
+		slog.Error("reading env file", "err", err)
+		os.Exit(1)
+	}
+	cfgPath := flag.String("config", envOr("SPINNER_CONFIG", "spinner.json"), "config file")
+	flag.String("env", ".env", "env file with SPINNER_ADMIN_TOKEN")
+	debug := flag.Bool("debug", false, "verbose logging")
+	showVersion := flag.Bool("version", false, "print the version and exit")
+	flag.Parse()
+	if *showVersion {
+		os.Stdout.WriteString("spinner " + version + "\n")
+		return
+	}
+
+	level := slog.LevelInfo
+	if *debug {
+		level = slog.LevelDebug
+	}
+	log := slog.New(slog.NewTextHandler(os.Stderr, &slog.HandlerOptions{Level: level}))
+
+	// First run, for example in a fresh container volume: write a starter config so the admin page can fill it in.
+	if _, err := os.Stat(*cfgPath); errors.Is(err, os.ErrNotExist) {
+		if err := os.WriteFile(*cfgPath, []byte(starterConfig), 0o644); err != nil {
+			log.Error("writing starter config", "path", *cfgPath, "err", err)
+			os.Exit(1)
+		}
+		log.Info("wrote a starter config", "path", *cfgPath)
+	}
+
+	mgr, err := config.NewManager(*cfgPath, log)
+	if err != nil {
+		log.Error("loading config", "err", err)
+		os.Exit(1)
+	}
+	cfg := mgr.Current()
+
+	ctx, stop := signal.NotifyContext(context.Background(), os.Interrupt, syscall.SIGTERM)
+	defer stop()
+
+	tracker := health.New(log)
+	p := proxy.New(mgr, tracker, log)
+	go mgr.Watch(ctx)
+	go tracker.Run(ctx, mgr)
+
+	if cfg.AdminBind != "" {
+		token := os.Getenv("SPINNER_ADMIN_TOKEN")
+		if token == "" {
+			log.Warn("admin_bind is set but SPINNER_ADMIN_TOKEN is empty; admin UI disabled")
+		} else {
+			a := &admin.Admin{Cfg: mgr, Health: tracker, Proxy: p, Token: token}
+			srv := &http.Server{Addr: cfg.AdminBind, Handler: a.Handler(), ReadHeaderTimeout: 5 * time.Second}
+			go func() {
+				log.Info("admin listening", "addr", cfg.AdminBind)
+				if err := srv.ListenAndServe(); err != http.ErrServerClosed {
+					log.Error("admin server", "err", err)
+				}
+			}()
+			go func() { <-ctx.Done(); srv.Close() }()
+		}
+	}
+
+	ln, err := net.Listen("tcp", cfg.Bind)
+	if err != nil {
+		log.Error("listen", "addr", cfg.Bind, "err", err)
+		os.Exit(1)
+	}
+	log.Info("spinner listening", "addr", cfg.Bind, "servers", len(cfg.Servers))
+	if err := p.Serve(ctx, ln); err != nil {
+		log.Error("serve", "err", err)
+		os.Exit(1)
+	}
+	log.Info("shutting down")
+}
+
+func envOr(k, def string) string {
+	if v := os.Getenv(k); v != "" {
+		return v
+	}
+	return def
+}