net: concatenate multiple TXT strings in single TXT record

When go resolver was changed to use dnsmessage.Parser, LookupTXT
returned two strings in one record as two different records. This change
reverts back to concatenating multiple strings in a single
TXT record.

Fixes #27763

Change-Id: Ice226fcb2be4be58853de34ed35b4627acb429ea
Reviewed-on: https://go-review.googlesource.com/136955
Reviewed-by: Ian Gudger <igudger@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Ian Gudger <igudger@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/dnsclient_unix_test.go

@@ -1568,3 +1568,56 @@ func TestDNSDialTCP(t *testing.T) {
 		t.Fatal("exhange failed:", err)
 	}
 }
+
+// Issue 27763: verify that two strings in one TXT record are concatenated.
+func TestTXTRecordTwoStrings(t *testing.T) {
+	fake := fakeDNSServer{
+		rh: func(n, _ string, q dnsmessage.Message, _ time.Time) (dnsmessage.Message, error) {
+			r := dnsmessage.Message{
+				Header: dnsmessage.Header{
+					ID:       q.Header.ID,
+					Response: true,
+					RCode:    dnsmessage.RCodeSuccess,
+				},
+				Questions: q.Questions,
+				Answers: []dnsmessage.Resource{
+					{
+						Header: dnsmessage.ResourceHeader{
+							Name:  q.Questions[0].Name,
+							Type:  dnsmessage.TypeA,
+							Class: dnsmessage.ClassINET,
+						},
+						Body: &dnsmessage.TXTResource{
+							TXT: []string{"string1 ", "string2"},
+						},
+					},
+					{
+						Header: dnsmessage.ResourceHeader{
+							Name:  q.Questions[0].Name,
+							Type:  dnsmessage.TypeA,
+							Class: dnsmessage.ClassINET,
+						},
+						Body: &dnsmessage.TXTResource{
+							TXT: []string{"onestring"},
+						},
+					},
+				},
+			}
+			return r, nil
+		},
+	}
+	r := Resolver{PreferGo: true, Dial: fake.DialContext}
+	txt, err := r.lookupTXT(context.Background(), "golang.org")
+	if err != nil {
+		t.Fatal("LookupTXT failed:", err)
+	}
+	if want := 2; len(txt) != want {
+		t.Fatalf("len(txt), got %d, want %d", len(txt), want)
+	}
+	if want := "string1 string2"; txt[0] != want {
+		t.Errorf("txt[0], got %q, want %q", txt[0], want)
+	}
+	if want := "onestring"; txt[1] != want {
+		t.Errorf("txt[1], got %q, want %q", txt[1], want)
+	}
+}