net: respect resolv.conf rotate option

Instead of ranging over servers in the config, grab an offset
from the config that is used to determine indices.

When the rotate option is enabled, the offset increases which
rotates queries through servers. Otherwise, it is always 0
which uses servers in config order.

Fixes #17126

Change-Id: If233f6de7bfa42f88570055b9ab631be08a76b3e
Reviewed-on: https://go-review.googlesource.com/29233Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/net/dnsclient_unix.go

@@ -125,7 +125,7 @@ func (d *Dialer) dialDNS(ctx context.Context, network, server string) (dnsConn,
 	// Calling Dial here is scary -- we have to be sure not to
 	// dial a name that will require a DNS lookup, or Dial will
 	// call back here to translate it. The DNS config parser has
-	// already checked that all the cfg.servers[i] are IP
+	// already checked that all the cfg.servers are IP
 	// addresses, which Dial will use without a DNS lookup.
 	c, err := d.DialContext(ctx, network, server)
 	if err != nil {
@@ -182,13 +182,14 @@ func exchange(ctx context.Context, server, name string, qtype uint16, timeout ti
 // Do a lookup for a single name, which must be rooted
 // (otherwise answer will not find the answers).
 func tryOneName(ctx context.Context, cfg *dnsConfig, name string, qtype uint16) (string, []dnsRR, error) {
-	if len(cfg.servers) == 0 {
-		return "", nil, &DNSError{Err: "no DNS servers", Name: name}
-	}
-
 	var lastErr error
+	serverOffset := cfg.serverOffset()
+	sLen := uint32(len(cfg.servers))
+
 	for i := 0; i < cfg.attempts; i++ {
-		for _, server := range cfg.servers {
+		for j := uint32(0); j < sLen; j++ {
+			server := cfg.servers[(serverOffset+j)%sLen]
+
 			msg, err := exchange(ctx, server, name, qtype, cfg.timeout)
 			if err != nil {
 				lastErr = &DNSError{

src/net/dnsclient_unix_test.go

@@ -797,3 +797,73 @@ func TestRetryTimeout(t *testing.T) {
 		t.Error("deadline0 still zero", deadline0)
 	}
 }
+
+func TestRotate(t *testing.T) {
+	// without rotation, always uses the first server
+	testRotate(t, false, []string{"192.0.2.1", "192.0.2.2"}, []string{"192.0.2.1:53", "192.0.2.1:53", "192.0.2.1:53"})
+
+	// with rotation, rotates through back to first
+	testRotate(t, true, []string{"192.0.2.1", "192.0.2.2"}, []string{"192.0.2.1:53", "192.0.2.2:53", "192.0.2.1:53"})
+}
+
+func testRotate(t *testing.T, rotate bool, nameservers, wantServers []string) {
+	origTestHookDNSDialer := testHookDNSDialer
+	defer func() { testHookDNSDialer = origTestHookDNSDialer }()
+
+	conf, err := newResolvConfTest()
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer conf.teardown()
+
+	var confLines []string
+	for _, ns := range nameservers {
+		confLines = append(confLines, "nameserver "+ns)
+	}
+	if rotate {
+		confLines = append(confLines, "options rotate")
+	}
+
+	if err := conf.writeAndUpdate(confLines); err != nil {
+		t.Fatal(err)
+	}
+
+	d := &fakeDNSDialer{}
+	testHookDNSDialer = func() dnsDialer { return d }
+
+	var usedServers []string
+	d.rh = func(s string, q *dnsMsg, _ time.Time) (*dnsMsg, error) {
+		usedServers = append(usedServers, s)
+
+		r := &dnsMsg{
+			dnsMsgHdr: dnsMsgHdr{
+				id:                  q.id,
+				response:            true,
+				recursion_available: true,
+			},
+			question: q.question,
+			answer: []dnsRR{
+				&dnsRR_CNAME{
+					Hdr: dnsRR_Header{
+						Name:   q.question[0].Name,
+						Rrtype: dnsTypeCNAME,
+						Class:  dnsClassINET,
+					},
+					Cname: "golang.org",
+				},
+			},
+		}
+		return r, nil
+	}
+
+	// len(nameservers) + 1 to allow rotation to get back to start
+	for i := 0; i < len(nameservers)+1; i++ {
+		if _, err := goLookupCNAME(context.Background(), "www.golang.org"); err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	if !reflect.DeepEqual(usedServers, wantServers) {
+		t.Fatalf("rotate=%t got used servers:\n%v\nwant:\n%v", rotate, usedServers, wantServers)
+	}
+}

src/net/dnsconfig_unix.go

@@ -10,6 +10,7 @@ package net
 
 import (
 	"os"
+	"sync/atomic"
 	"time"
 )
 
@@ -29,6 +30,7 @@ type dnsConfig struct {
 	lookup     []string      // OpenBSD top-level database "lookup" order
 	err        error         // any error that occurs during open of resolv.conf
 	mtime      time.Time     // time of resolv.conf modification
+	soffset    uint32        // used by serverOffset
 }
 
 // See resolv.conf(5) on a Linux machine.
@@ -136,6 +138,17 @@ func dnsReadConfig(filename string) *dnsConfig {
 	return conf
 }
 
+// serverOffset returns an offset that can be used to determine
+// indices of servers in c.servers when making queries.
+// When the rotate option is enabled, this offset increases.
+// Otherwise it is always 0.
+func (c *dnsConfig) serverOffset() uint32 {
+	if c.rotate {
+		return atomic.AddUint32(&c.soffset, 1) - 1 // return 0 to start
+	}
+	return 0
+}
+
 func dnsDefaultSearch() []string {
 	hn, err := getHostname()
 	if err != nil {