net: fix LookupTXT of long records on Windows

The response to a TXT lookup is a sequence of RRs,
each of which contains a sequence of string fragments.

The correct handling of the response is to do:

    for each rr {
        list = append(list, strings.Join(rr.fragments, ""))
    }

(like in at dnsRR_TXT.Walk, used on most platforms).

The Windows code incorrectly does:

    for each rr {
        list = append(list, rr.fragments...)
    }

This CL fixes it to concatenate fragments, as it must.

Fixes #21472.

Change-Id: I78cce96f172e5e90da9a212b0343457f6d5f92e8
Reviewed-on: https://go-review.googlesource.com/79555
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/lookup_test.go

@@ -9,7 +9,9 @@ import (
 	"context"
 	"fmt"
 	"internal/testenv"
+	"reflect"
 	"runtime"
+	"sort"
 	"strings"
 	"testing"
 	"time"
@@ -303,6 +305,25 @@ func TestLookupGoogleHost(t *testing.T) {
 	}
 }
 
+func TestLookupLongTXT(t *testing.T) {
+	if testenv.Builder() == "" {
+		testenv.MustHaveExternalNetwork(t)
+	}
+
+	txts, err := LookupTXT("golang.rsc.io")
+	if err != nil {
+		t.Fatal(err)
+	}
+	sort.Strings(txts)
+	want := []string{
+		strings.Repeat("abcdefghijklmnopqrstuvwxyABCDEFGHJIKLMNOPQRSTUVWXY", 10),
+		"gophers rule",
+	}
+	if !reflect.DeepEqual(txts, want) {
+		t.Fatalf("LookupTXT golang.rsc.io incorrect\nhave %q\nwant %q", txts, want)
+	}
+}
+
 var lookupGoogleIPTests = []struct {
 	name string
 }{

src/net/lookup_windows.go

@@ -279,10 +279,11 @@ func (*Resolver) lookupTXT(ctx context.Context, name string) ([]string, error) {
 	txts := make([]string, 0, 10)
 	for _, p := range validRecs(r, syscall.DNS_TYPE_TEXT, name) {
 		d := (*syscall.DNSTXTData)(unsafe.Pointer(&p.Data[0]))
+		s := ""
 		for _, v := range (*[1 << 10]*uint16)(unsafe.Pointer(&(d.StringArray[0])))[:d.StringCount] {
-			s := syscall.UTF16ToString((*[1 << 20]uint16)(unsafe.Pointer(v))[:])
+			s += syscall.UTF16ToString((*[1 << 20]uint16)(unsafe.Pointer(v))[:])
-			txts = append(txts, s)
 		}
+		txts = append(txts, s)
 	}
 	return txts, nil
 }