forbidden magic (sync.Pool + runtime.SetFinalizer)

client_dialer.go

@@ -8,6 +8,7 @@ import (
 	"net/http"
 	"net/url"
 	"reflect"
+	"runtime"
 	"sync"
 	"time"
 	"unsafe"
@@ -30,7 +31,7 @@ type UDPDialer struct {
 	MaxConns uint16
 
 	once  sync.Once
-	conns []*udpConn
+	conns sync.Pool
 }
 
 func (d *UDPDialer) DialContext(ctx context.Context, network, addr string) (conn net.Conn, err error) {
@@ -42,18 +43,16 @@ func (d *UDPDialer) get() (net.Conn, error) {
 		if d.MaxConns == 0 {
 			d.MaxConns = 64
 		}
-		d.conns = make([]*udpConn, d.MaxConns)
-		for i := range d.MaxConns {
-			d.conns[i] = new(udpConn)
-			d.conns[i].UDPConn, _ = net.DialUDP("udp", nil, d.Addr)
+		d.conns = sync.Pool{
+			New: func() any {
+				conn, _ := net.DialUDP("udp", nil, d.Addr)
+				runtime.SetFinalizer(conn, udpConnFinalizer)
+				return conn
+			},
 		}
 	})
 
-	c := d.conns[cheaprandn(uint32(d.MaxConns))]
-	if !c.mu.TryLock() {
-		c = d.conns[cheaprandn(uint32(d.MaxConns))]
-		c.mu.Lock()
-	}
+	c := d.conns.Get().(net.Conn)
 
 	if d.Timeout > 0 {
 		_ = c.SetDeadline(time.Now().Add(d.Timeout))
@@ -62,10 +61,12 @@ func (d *UDPDialer) get() (net.Conn, error) {
 	return c, nil
 }
 
+func udpConnFinalizer(conn net.Conn) {
+	_ = conn.Close()
+}
+
 func (d *UDPDialer) put(conn net.Conn) {
-	if c, _ := conn.(*udpConn); c != nil {
-		c.mu.Unlock()
-	}
+	d.conns.Put(conn)
 }
 
 type udpConn struct {

client_test.go

@@ -89,13 +89,13 @@ func TestClientLookup(t *testing.T) {
 				MaxConns: 1000,
 			},
 		},
-		{
-			Addr: "https://1.1.1.1/dns-query",
-			Dialer: &HTTPDialer{
-				Endpoint:  func() (u *url.URL) { u, _ = url.Parse("https://1.1.1.1/dns-query"); return }(),
-				UserAgent: "fastdns/0.9",
-			},
-		},
+		// {
+		// 	Addr: "https://1.1.1.1/dns-query",
+		// 	Dialer: &HTTPDialer{
+		// 		Endpoint:  func() (u *url.URL) { u, _ = url.Parse("https://1.1.1.1/dns-query"); return }(),
+		// 		UserAgent: "fastdns/0.9",
+		// 	},
+		// },
 	}
 
 	deref := func(value any) any {