[tailscale] net: don't wait 5 seconds to re-read /etc/resolv.conf

If a Go process starts up, finds /etc/resolv.conf empty, then the DHCP
client writes /etc/resolv.conf, the Go program would find itself
broken for up to 5 seconds.

Updates tailscale/corp#22206

Signed-off-by: Brad Fitzpatrick <[email protected]>
(cherry picked from commit d05ca2a)

Author: bradfitz

src/net/conf_test.go

@@ -396,7 +396,7 @@ func TestConfHostLookupOrder(t *testing.T) {
 	defer conf.teardown()
 
 	for _, tt := range tests {
-		if !conf.forceUpdateConf(tt.resolv, time.Now().Add(time.Hour)) {
+		if !conf.forceUpdateConf(tt.resolv, distantFuture) {
 			t.Errorf("%s: failed to change resolv config", tt.name)
 		}
 		for _, ht := range tt.hostTests {

src/net/dnsclient_unix.go

@@ -381,13 +381,18 @@ func (conf *resolverConfig) init() {
 	conf.ch = make(chan struct{}, 1)
 }
 
+// distantFuture is a sentinel time used for tests to signal that
+// resolv.conf should not be rechecked.
+var distantFuture = time.Now().Add(1000 * time.Hour)
+
 // tryUpdate tries to update conf with the named resolv.conf file.
 // The name variable only exists for testing. It is otherwise always
 // "/etc/resolv.conf".
 func (conf *resolverConfig) tryUpdate(name string) {
 	conf.initOnce.Do(conf.init)
 
-	if conf.dnsConfig.Load().noReload {
+	dc := conf.dnsConfig.Load()
+	if dc.noReload {
 		return
 	}
 
@@ -398,7 +403,7 @@ func (conf *resolverConfig) tryUpdate(name string) {
 	defer conf.releaseSema()
 
 	now := time.Now()
-	if conf.lastChecked.After(now.Add(-5 * time.Second)) {
+	if len(dc.servers) > 0 && conf.lastChecked.After(now.Add(-5*time.Second)) || conf.lastChecked == distantFuture {
 		return
 	}
 	conf.lastChecked = now