net: make DNSError.Temporary return true on SERVFAIL

Fixes #8434

Change-Id: I323222b4160f3aba35cac1de7f6df93c524b72ec
Reviewed-on: https://go-review.googlesource.com/14169Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/dnsclient.go

@@ -47,8 +47,13 @@ func answer(name, server string, dns *dnsMsg, qtype uint16) (cname string, addrs
 		// None of the error codes make sense
 		// for the query we sent.  If we didn't get
 		// a name error and we didn't get success,
-		// the server is behaving incorrectly.
-		return "", nil, &DNSError{Err: "server misbehaving", Name: name, Server: server}
+		// the server is behaving incorrectly or
+		// having temporary trouble.
+		err := &DNSError{Err: "server misbehaving", Name: name, Server: server}
+		if dns.rcode == dnsRcodeServerFailure {
+			err.IsTemporary = true
+		}
+		return "", nil, err
 	}
 
 	// Look for the name.

src/net/dnsclient_test.go

@@ -67,3 +67,28 @@ func testWeighting(t *testing.T, margin float64) {
 func TestWeighting(t *testing.T) {
 	testWeighting(t, 0.05)
 }
+
+// Issue 8434: verify that Temporary returns true on an error when rcode
+// is SERVFAIL
+func TestIssue8434(t *testing.T) {
+	msg := &dnsMsg{
+		dnsMsgHdr: dnsMsgHdr{
+			rcode: dnsRcodeServerFailure,
+		},
+	}
+
+	_, _, err := answer("golang.org", "foo:53", msg, uint16(dnsTypeSRV))
+	if err == nil {
+		t.Fatal("expected an error")
+	}
+	if ne, ok := err.(Error); !ok {
+		t.Fatalf("err = %#v; wanted something supporting net.Error", err)
+	} else if !ne.Temporary() {
+		t.Fatalf("Temporary = false for err = %#v; want Temporary == true", err)
+	}
+	if de, ok := err.(*DNSError); !ok {
+		t.Fatalf("err = %#v; wanted a *net.DNSError", err)
+	} else if !de.IsTemporary {
+		t.Fatalf("IsTemporary = false for err = %#v; want IsTemporary == true", err)
+	}
+}

src/net/net.go

@@ -524,6 +524,7 @@ type DNSError struct {
 	Name        string // name looked for
 	Server      string // server used
 	IsTimeout   bool   // if true, timed out; not all timeouts set this
+	IsTemporary bool   // if true, error is temporary; not all errors set this
 }
 
 func (e *DNSError) Error() string {
@@ -546,7 +547,7 @@ func (e *DNSError) Timeout() bool { return e.IsTimeout }
 // Temporary reports whether the DNS error is known to be temporary.
 // This is not always known; a DNS lookup may fail due to a temporary
 // error and return a DNSError for which Temporary returns false.
-func (e *DNSError) Temporary() bool { return e.IsTimeout }
+func (e *DNSError) Temporary() bool { return e.IsTimeout || e.IsTemporary }
 
 type writerOnly struct {
 	io.Writer