Implementing new initializing OS resolver logic

Since the nameservers that we got during startup are the good ones that
work, saving it for later usage if we could not find available ones.

resolver.go

@@ -85,60 +85,41 @@ func availableNameservers() []string {
 func InitializeOsResolver() []string {
 	return initializeOsResolver(availableNameservers())
 }
+
+// initializeOsResolver performs logic for choosing OS resolver nameserver.
+// The logic:
+//
+// - First available LAN servers are saved and store.
+// - Later calls, if no LAN servers available, the saved servers above will be used.
 func initializeOsResolver(servers []string) []string {
 	var (
-		nss       []string
+		lanNss    []string
 		publicNss []string
 	)
-	var (
-		lastLanServer         netip.Addr
-		curLanServer          netip.Addr
-		curLanServerAvailable bool
-	)
-	if p := or.currentLanServer.Load(); p != nil {
-		curLanServer = *p
-		or.currentLanServer.Store(nil)
-	}
-	if p := or.lastLanServer.Load(); p != nil {
-		lastLanServer = *p
-		or.lastLanServer.Store(nil)
-	}
+
 	for _, ns := range servers {
 		addr, err := netip.ParseAddr(ns)
 		if err != nil {
 			continue
 		}
 		server := net.JoinHostPort(ns, "53")
-		// Always use new public nameserver.
-		if !isLanAddr(addr) {
-			publicNss = append(publicNss, server)
-			nss = append(nss, server)
-			continue
-		}
-		// For LAN server, storing only current and last LAN server if any.
-		if addr.Compare(curLanServer) == 0 {
-			curLanServerAvailable = true
+		if isLanAddr(addr) {
+			lanNss = append(lanNss, server)
 		} else {
-			if addr.Compare(lastLanServer) == 0 {
-				or.lastLanServer.Store(&addr)
-			} else {
-				if or.currentLanServer.CompareAndSwap(nil, &addr) {
-					nss = append(nss, server)
-				}
-			}
+			publicNss = append(publicNss, server)
 		}
 	}
-	// Store current LAN server as last one only if it's still available.
-	if curLanServerAvailable && curLanServer.IsValid() {
-		or.lastLanServer.Store(&curLanServer)
-		nss = append(nss, net.JoinHostPort(curLanServer.String(), "53"))
+	if len(lanNss) > 0 {
+		// Saved first initialized LAN servers.
+		or.initializedLanServers.CompareAndSwap(nil, &lanNss)
 	}
-	if len(publicNss) == 0 {
-		publicNss = append(publicNss, controldPublicDnsWithPort)
-		nss = append(nss, controldPublicDnsWithPort)
+	if len(lanNss) == 0 {
+		or.lanServers.Store(or.initializedLanServers.Load())
+	} else {
+		or.lanServers.Store(&lanNss)
 	}
-	or.publicServer.Store(&publicNss)
-	return nss
+	or.publicServers.Store(&publicNss)
+	return slices.Concat(lanNss, publicNss)
 }
 
 // testPlainDnsNameserver sends a test query to DNS nameserver to check if the server is available.
@@ -185,9 +166,9 @@ func NewResolver(uc *UpstreamConfig) (Resolver, error) {
 }
 
 type osResolver struct {
-	currentLanServer atomic.Pointer[netip.Addr]
-	lastLanServer    atomic.Pointer[netip.Addr]
-	publicServer     atomic.Pointer[[]string]
+	initializedLanServers atomic.Pointer[[]string]
+	lanServers            atomic.Pointer[[]string]
+	publicServers         atomic.Pointer[[]string]
 }
 
 type osResolverResult struct {
@@ -201,13 +182,10 @@ type osResolverResult struct {
 // Query is sent to all nameservers concurrently, and the first
 // success response will be returned.
 func (o *osResolver) Resolve(ctx context.Context, msg *dns.Msg) (*dns.Msg, error) {
-	publicServers := *o.publicServer.Load()
-	nss := make([]string, 0, 2)
-	if p := o.currentLanServer.Load(); p != nil {
-		nss = append(nss, net.JoinHostPort(p.String(), "53"))
-	}
-	if p := o.lastLanServer.Load(); p != nil {
-		nss = append(nss, net.JoinHostPort(p.String(), "53"))
+	publicServers := *o.publicServers.Load()
+	var nss []string
+	if p := o.lanServers.Load(); p != nil {
+		nss = append(nss, (*p)...)
 	}
 	numServers := len(nss) + len(publicServers)
 	if numServers == 0 {
@@ -467,17 +445,19 @@ func NewResolverWithNameserver(nameservers []string) Resolver {
 // The caller must ensure each server in list is formed "ip:53".
 func newResolverWithNameserver(nameservers []string) *osResolver {
 	r := &osResolver{}
-	nss := slices.Sorted(slices.Values(nameservers))
-	for i, ns := range nss {
+	var publicNss []string
+	var lanNss []string
+	for _, ns := range slices.Sorted(slices.Values(nameservers)) {
 		ip, _, _ := net.SplitHostPort(ns)
 		addr, _ := netip.ParseAddr(ip)
 		if isLanAddr(addr) {
-			r.currentLanServer.Store(&addr)
-			nss = slices.Delete(nss, i, i+1)
-			break
+			lanNss = append(lanNss, ns)
+		} else {
+			publicNss = append(publicNss, ns)
 		}
 	}
-	r.publicServer.Store(&nss)
+	r.lanServers.Store(&lanNss)
+	r.publicServers.Store(&publicNss)
 	return r
 }