perf: excessive torrent tracker dns resolve (#634)

internal/protocol/bt/dns_cache_resolver.go

@@ -0,0 +1,49 @@
+package bt
+
+import (
+	"context"
+	"net"
+	"time"
+
+	"github.com/rs/dnscache"
+)
+
+// DnsCacheResolver resolves DNS requests for an HTTP client using an in-memory cache.
+type DnsCacheResolver struct {
+	RefreshTimeout time.Duration
+
+	resolver dnscache.Resolver
+}
+
+func (r *DnsCacheResolver) DialContext(ctx context.Context, network, address string) (net.Conn, error) {
+	host, port, err := net.SplitHostPort(address)
+	if err != nil {
+		return nil, err
+	}
+	ips, err := r.resolver.LookupHost(ctx, host)
+	if err != nil {
+		return nil, err
+	}
+	var conn net.Conn
+	for _, ip := range ips {
+		var dialer net.Dialer
+		conn, err = dialer.DialContext(ctx, network, net.JoinHostPort(ip, port))
+		if err == nil {
+			break
+		}
+	}
+	return conn, err
+}
+
+func (r *DnsCacheResolver) Run(ctx context.Context) {
+	ticker := time.NewTicker(r.RefreshTimeout)
+	defer ticker.Stop()
+	for {
+		select {
+		case <-ctx.Done():
+			return
+		case <-ticker.C:
+			r.resolver.Refresh(true)
+		}
+	}
+}