s3: S3 IPv6 support with option "use_dual_stack" (bool)

dualstack_endpoint=true enables IPv6 DNS lookup for S3 endpoints
in s3.go, add Options.DualstackEndpoint to support IPv6 on S3

backend/s3/s3_test.go

@@ -2,6 +2,9 @@
 package s3
 
 import (
+	"context"
+	"net/http"
+	"strings"
 	"testing"
 
 	"github.com/rclone/rclone/fs"
@@ -9,6 +12,13 @@ import (
 	"github.com/rclone/rclone/fstest/fstests"
 )
 
+func SetupS3Test(t *testing.T) (context.Context, *Options, *http.Client) {
+	ctx, opt := context.Background(), new(Options)
+	opt.Provider = "AWS"
+	client := getClient(ctx, opt)
+	return ctx, opt, client
+}
+
 // TestIntegration runs integration tests against the remote
 func TestIntegration(t *testing.T) {
 	fstests.Run(t, &fstests.Opt{
@@ -39,6 +49,28 @@ func TestIntegration2(t *testing.T) {
 	})
 }
 
+func TestAWSDualStackOption(t *testing.T) {
+	{
+		// test enabled
+		ctx, opt, client := SetupS3Test(t)
+		opt.UseDualStack = true
+		s3Conn, _, _ := s3Connection(ctx, opt, client)
+		if !strings.Contains(s3Conn.Endpoint, "dualstack") {
+			t.Errorf("dualstack failed got: %s, wanted: dualstack", s3Conn.Endpoint)
+			t.Fail()
+		}
+	}
+	{
+		// test default case
+		ctx, opt, client := SetupS3Test(t)
+		s3Conn, _, _ := s3Connection(ctx, opt, client)
+		if strings.Contains(s3Conn.Endpoint, "dualstack") {
+			t.Errorf("dualstack failed got: %s, NOT wanted: dualstack", s3Conn.Endpoint)
+			t.Fail()
+		}
+	}
+}
+
 func (f *Fs) SetUploadChunkSize(cs fs.SizeSuffix) (fs.SizeSuffix, error) {
 	return f.setUploadChunkSize(cs)
 }