net: support no-reload option for unix go resolver

It adds support for no-reload option, as specified in resolv.conf(5):
 no-reload (since glibc 2.26)
                     Sets RES_NORELOAD in _res.options.  This option
                     disables automatic reloading of a changed
                     configuration file.

Change-Id: I11182c5829434503f719ed162014f2301e3ba8d4
GitHub-Last-Rev: 7ae44be2d5
GitHub-Pull-Request: golang/go#56489
Reviewed-on: https://go-review.googlesource.com/c/go/+/446555
Reviewed-by: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>

src/net/dnsclient_unix.go

@@ -369,6 +369,10 @@ func (conf *resolverConfig) init() {
 func (conf *resolverConfig) tryUpdate(name string) {
 	conf.initOnce.Do(conf.init)
 
+	if conf.dnsConfig.noReload {
+		return
+	}
+
 	// Ensure only one update at a time checks resolv.conf.
 	if !conf.tryAcquireSema() {
 		return

src/net/dnsclient_unix_test.go

@@ -247,7 +247,7 @@ func newResolvConfTest() (*resolvConfTest, error) {
 	return conf, nil
 }
 
-func (conf *resolvConfTest) writeAndUpdate(lines []string) error {
+func (conf *resolvConfTest) write(lines []string) error {
 	f, err := os.OpenFile(conf.path, os.O_CREATE|os.O_TRUNC|os.O_WRONLY, 0600)
 	if err != nil {
 		return err
@@ -257,10 +257,18 @@ func (conf *resolvConfTest) writeAndUpdate(lines []string) error {
 		return err
 	}
 	f.Close()
-	if err := conf.forceUpdate(conf.path, time.Now().Add(time.Hour)); err != nil {
+	return nil
+}
+
+func (conf *resolvConfTest) writeAndUpdate(lines []string) error {
+	return conf.writeAndUpdateWithLastCheckedTime(lines, time.Now().Add(time.Hour))
+}
+
+func (conf *resolvConfTest) writeAndUpdateWithLastCheckedTime(lines []string, lastChecked time.Time) error {
+	if err := conf.write(lines); err != nil {
 		return err
 	}
-	return nil
+	return conf.forceUpdate(conf.path, lastChecked)
 }
 
 func (conf *resolvConfTest) forceUpdate(name string, lastChecked time.Time) error {
@@ -2409,3 +2417,36 @@ func TestDNSTrustAD(t *testing.T) {
 		t.Errorf("lookup failed: %v", err)
 	}
 }
+
+func TestDNSConfigNoReload(t *testing.T) {
+	r := &Resolver{PreferGo: true, Dial: func(ctx context.Context, network, address string) (Conn, error) {
+		if address != "192.0.2.1:53" {
+			return nil, errors.New("configuration unexpectedly changed")
+		}
+		return fakeDNSServerSuccessful.DialContext(ctx, network, address)
+	}}
+
+	conf, err := newResolvConfTest()
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer conf.teardown()
+
+	err = conf.writeAndUpdateWithLastCheckedTime([]string{"nameserver 192.0.2.1", "options no-reload"}, time.Now().Add(-time.Hour))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if _, err = r.LookupHost(context.Background(), "go.dev"); err != nil {
+		t.Fatal(err)
+	}
+
+	err = conf.write([]string{"nameserver 192.0.2.200"})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if _, err = r.LookupHost(context.Background(), "go.dev"); err != nil {
+		t.Fatal(err)
+	}
+}

src/net/dnsconfig.go

@@ -30,6 +30,7 @@ type dnsConfig struct {
 	singleRequest bool          // use sequential A and AAAA queries instead of parallel queries
 	useTCP        bool          // force usage of TCP for DNS resolutions
 	trustAD       bool          // add AD flag to queries
+	noReload      bool          // do not check for config file updates
 }
 
 // serverOffset returns an offset that can be used to determine

src/net/dnsconfig_unix.go

@@ -118,6 +118,8 @@ func dnsReadConfig(filename string) *dnsConfig {
 				case s == "edns0":
 					// We use EDNS by default.
 					// Ignore this option.
+				case s == "no-reload":
+					conf.noReload = true
 				default:
 					conf.unknownOpt = true
 				}