app/vmauth: properly handle the case when zero backend hosts are resolved at SRV DNS

When zero backend hosts are resolved, then vmauth must return 'no backend hosts' error instead of crashing with panic

This is a follow-up for 590aeccd7d and 3a45bbb4e0

Updates https://github.com/VictoriaMetrics/VictoriaMetrics/pull/6401

app/vmauth/auth_config.go

@@ -312,12 +312,18 @@ func (up *URLPrefix) getBackendsCount() int {
 
 // getBackendURL returns the backendURL depending on the load balance policy.
 //
+// It can return nil if there are no backend urls available at the moment.
+//
 // backendURL.put() must be called on the returned backendURL after the request is complete.
 func (up *URLPrefix) getBackendURL() *backendURL {
 	up.discoverBackendAddrsIfNeeded()
 
 	pbus := up.bus.Load()
 	bus := *pbus
+	if len(bus) == 0 {
+		return nil
+	}
+
 	if up.loadBalancingPolicy == "first_available" {
 		return getFirstAvailableBackendURL(bus)
 	}

app/vmauth/main.go

@@ -208,6 +208,9 @@ func processRequest(w http.ResponseWriter, r *http.Request, ui *UserInfo) {
 	maxAttempts := up.getBackendsCount()
 	for i := 0; i < maxAttempts; i++ {
 		bu := up.getBackendURL()
+		if bu == nil {
+			break
+		}
 		targetURL := bu.url
 		// Don't change path and add request_path query param for default route.
 		if isDefault {

app/vmauth/target_url_test.go

@@ -342,6 +342,55 @@ func TestUserInfoGetBackendURL_SRV(t *testing.T) {
 	f(ui, `/test`, "http://non-exist-dns-addr/test")
 }
 
+func TestUserInfoGetBackendURL_SRVZeroBackends(t *testing.T) {
+	f := func(ui *UserInfo, requestURI string) {
+		t.Helper()
+
+		u, err := url.Parse(requestURI)
+		if err != nil {
+			t.Fatalf("cannot parse %q: %s", requestURI, err)
+		}
+		u = normalizeURL(u)
+		up, _ := ui.getURLPrefixAndHeaders(u, nil)
+		if up == nil {
+			t.Fatalf("cannot match available backend: %s", err)
+		}
+		bu := up.getBackendURL()
+		if bu != nil {
+			t.Fatalf("expecting nil backendURL; got %v", bu)
+		}
+	}
+
+	customResolver := &fakeResolver{
+		Resolver: &net.Resolver{},
+		lookupSRVResults: map[string][]*net.SRV{
+			"vmselect": {},
+		},
+	}
+	origResolver := netutil.Resolver
+	netutil.Resolver = customResolver
+	defer func() {
+		netutil.Resolver = origResolver
+	}()
+
+	allowed := true
+	ui := &UserInfo{
+		URLMaps: []URLMap{
+			{
+				SrcPaths:  getRegexs([]string{"/select/.+"}),
+				URLPrefix: mustParseURL("http://srv+vmselect"),
+			},
+		},
+		DiscoverBackendIPs: &allowed,
+		URLPrefix:          mustParseURL("http://non-exist-dns-addr"),
+	}
+	if err := ui.initURLs(); err != nil {
+		t.Fatalf("cannot initialize urls inside UserInfo: %s", err)
+	}
+
+	f(ui, `/select/0/prometheus/api/v1/query?query=up`)
+}
+
 func TestCreateTargetURLFailure(t *testing.T) {
 	f := func(ui *UserInfo, requestURI string) {
 		t.Helper()