Fix: tcp dual stack dial

adapters/outbound/util.go

@@ -2,6 +2,7 @@ package adapters
 
 import (
 	"bytes"
+	"context"
 	"crypto/tls"
 	"fmt"
 	"net"
@@ -104,12 +105,75 @@ func dialTimeout(network, address string, timeout time.Duration) (net.Conn, erro
 		return nil, err
 	}
 
-	ip, err := dns.ResolveIP(host)
-	if err != nil {
-		return nil, err
+	dialer := net.Dialer{Timeout: timeout}
+	ctx, cancel := context.WithTimeout(context.Background(), timeout)
+	defer cancel()
+
+	returned := make(chan struct{})
+	defer close(returned)
+
+	type dialResult struct {
+		net.Conn
+		error
+		ipv6 bool
+		done bool
+	}
+	results := make(chan dialResult)
+	var primary, fallback dialResult
+
+	startRacer := func(ctx context.Context, host string, ipv6 bool) {
+		var err error
+
+		var ip net.IP
+		if ipv6 {
+			ip, err = dns.ResolveIPv6(host)
+		} else {
+			ip, err = dns.ResolveIPv4(host)
+		}
+		if err != nil {
+			return
+		}
+
+		var c net.Conn
+		if ipv6 {
+			c, err = dialer.DialContext(ctx, "tcp6", net.JoinHostPort(ip.String(), port))
+		} else {
+			c, err = dialer.DialContext(ctx, "tcp4", net.JoinHostPort(ip.String(), port))
+		}
+		if err != nil {
+			return
+		}
+
+		select {
+		case results <- dialResult{Conn: c, error: err, ipv6: ipv6}:
+		case <-returned:
+			if c != nil {
+				c.Close()
+			}
+		}
 	}
 
-	return net.DialTimeout(network, net.JoinHostPort(ip.String(), port), timeout)
+	go startRacer(ctx, host, false)
+	go startRacer(ctx, host, true)
+
+	for {
+		select {
+		case res := <-results:
+			if res.error == nil {
+				return res.Conn, nil
+			}
+
+			if res.ipv6 {
+				primary = res
+			} else {
+				fallback = res
+			}
+
+			if primary.done && fallback.done {
+				return nil, primary.error
+			}
+		}
+	}
 }
 
 func resolveUDPAddr(network, address string) (*net.UDPAddr, error) {