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>

internal/proxy/proxy.go

@@ -0,0 +1,190 @@
+// Package proxy accepts Minecraft connections, routes them by hostname and
+// pipes bytes to the chosen backend.
+package proxy
+
+import (
+	"context"
+	"errors"
+	"io"
+	"log/slog"
+	"net"
+	"sync"
+	"sync/atomic"
+	"time"
+
+	"git.veillax.com/Veillax/spinner/internal/config"
+	"git.veillax.com/Veillax/spinner/internal/health"
+	"git.veillax.com/Veillax/spinner/internal/protocol"
+	"git.veillax.com/Veillax/spinner/internal/router"
+)
+
+type Proxy struct {
+	cfg    *config.Manager
+	health *health.Tracker
+	log    *slog.Logger
+	rt     atomic.Pointer[router.Router]
+	Stats  *Stats
+
+	ipMu    sync.Mutex
+	ipConns map[string]int
+}
+
+func New(cfg *config.Manager, h *health.Tracker, log *slog.Logger) *Proxy {
+	p := &Proxy{cfg: cfg, health: h, log: log, Stats: newStats(), ipConns: map[string]int{}}
+	p.rt.Store(router.New(cfg.Current()))
+	cfg.OnChange(func(c *config.Config) { p.rt.Store(router.New(c)) })
+	return p
+}
+
+// Serve accepts connections until ctx ends or the listener fails.
+func (p *Proxy) Serve(ctx context.Context, ln net.Listener) error {
+	go func() { <-ctx.Done(); ln.Close() }()
+	for {
+		c, err := ln.Accept()
+		if err != nil {
+			if ctx.Err() != nil {
+				return nil
+			}
+			var ne net.Error
+			if errors.As(err, &ne) && ne.Timeout() {
+				continue
+			}
+			return err
+		}
+		go p.handle(c)
+	}
+}
+
+func (p *Proxy) acquireIP(ip string, limit int) bool {
+	p.ipMu.Lock()
+	defer p.ipMu.Unlock()
+	if limit > 0 && p.ipConns[ip] >= limit {
+		return false
+	}
+	p.ipConns[ip]++
+	return true
+}
+
+func (p *Proxy) releaseIP(ip string) {
+	p.ipMu.Lock()
+	defer p.ipMu.Unlock()
+	if p.ipConns[ip]--; p.ipConns[ip] <= 0 {
+		delete(p.ipConns, ip)
+	}
+}
+
+// reject answers a status ping with a canned MOTD, or a login with a disconnect message.
+func (p *Proxy) reject(c net.Conn, h protocol.Handshake, msg string) {
+	c.SetDeadline(time.Now().Add(10 * time.Second))
+	if h.IsStatus() {
+		protocol.ServeStatus(c, protocol.StatusJSON(h.Protocol, "Spinner", msg, 0, 0))
+		return
+	}
+	c.Write(protocol.LoginDisconnect(msg))
+}
+
+func (p *Proxy) handle(c net.Conn) {
+	defer c.Close()
+	cfg := p.cfg.Current()
+	p.Stats.total.Add(1)
+
+	ip, _, _ := net.SplitHostPort(c.RemoteAddr().String())
+	if !p.acquireIP(ip, cfg.MaxConnsPerIP) {
+		p.Stats.rejected.Add(1)
+		return
+	}
+	defer p.releaseIP(ip)
+
+	c.SetReadDeadline(time.Now().Add(cfg.HandshakeTimeout()))
+	hs, raw, err := protocol.ReadHandshake(c)
+	if err != nil {
+		p.log.Debug("bad handshake", "remote", ip, "err", err)
+		return
+	}
+	c.SetReadDeadline(time.Time{})
+
+	srv := p.rt.Load().Match(hs.Host)
+	if srv == nil {
+		p.Stats.unknown.Add(1)
+		p.log.Debug("unknown host", "remote", ip, "host", hs.Host)
+		p.reject(c, hs, cfg.UnknownMOTD)
+		return
+	}
+	ss := p.Stats.server(srv.Name)
+	ss.total.Add(1)
+
+	if srv.MaxConnections > 0 && ss.active.Load() >= int64(srv.MaxConnections) {
+		p.Stats.rejected.Add(1)
+		p.reject(c, hs, "§cServer is full")
+		return
+	}
+
+	if hs.IsStatus() {
+		if js, ok := p.health.Cached(srv.Name, 3*cfg.ProbeInterval()); ok {
+			c.SetDeadline(time.Now().Add(10 * time.Second))
+			protocol.ServeStatus(c, js)
+			return
+		}
+	}
+
+	b, err := dialAny(srv.Addresses, cfg.ConnectTimeout())
+	if err != nil {
+		ss.failures.Add(1)
+		p.log.Warn("backend unreachable", "server", srv.Name, "err", err)
+		msg := srv.OfflineMOTD
+		if msg == "" {
+			msg = "§c" + srv.Name + " is offline"
+		}
+		p.reject(c, hs, msg)
+		return
+	}
+	defer b.Close()
+
+	if srv.SendProxyProtocol {
+		if _, err := b.Write(proxyV2Header(c.RemoteAddr(), c.LocalAddr())); err != nil {
+			return
+		}
+	}
+	if _, err := b.Write(raw); err != nil {
+		return
+	}
+
+	p.Stats.active.Add(1)
+	ss.active.Add(1)
+	defer func() { p.Stats.active.Add(-1); ss.active.Add(-1) }()
+	if !hs.IsStatus() {
+		p.log.Info("connected", "remote", ip, "host", hs.Host, "server", srv.Name)
+	}
+	pipe(c, b, ss)
+}
+
+func dialAny(addrs []string, timeout time.Duration) (net.Conn, error) {
+	var errs []error
+	for _, a := range addrs {
+		conn, err := net.DialTimeout("tcp", a, timeout)
+		if err == nil {
+			return conn, nil
+		}
+		errs = append(errs, err)
+	}
+	return nil, errors.Join(errs...)
+}
+
+// pipe copies both ways and tears both connections down when either side ends.
+func pipe(client, backend net.Conn, ss *serverStats) {
+	done := make(chan struct{}, 2)
+	go func() {
+		n, _ := io.Copy(backend, client)
+		ss.bytesIn.Add(n)
+		done <- struct{}{}
+	}()
+	go func() {
+		n, _ := io.Copy(client, backend)
+		ss.bytesOut.Add(n)
+		done <- struct{}{}
+	}()
+	<-done
+	client.Close()
+	backend.Close()
+	<-done
+}