fix: port Windows VPN DNS settling fix to master

cmd/cli/vpn_dns_test.go

@@ -0,0 +1,88 @@
+package cli
+
+import (
+	"context"
+	"testing"
+
+	"github.com/Control-D-Inc/ctrld"
+)
+
+func withVPNDNSSettlingEnabled(t *testing.T) {
+	t.Helper()
+	old := vpnDNSSettlingEnabled
+	vpnDNSSettlingEnabled = true
+	t.Cleanup(func() { vpnDNSSettlingEnabled = old })
+}
+
+func TestVPNDNSRefreshRetainsStateForOneGuardedEmptyDiscovery(t *testing.T) {
+	withVPNDNSSettlingEnabled(t)
+	var gotExemptions []vpnDNSExemption
+	m := newVPNDNSManager(&mainLog, func(exemptions []vpnDNSExemption) error {
+		gotExemptions = exemptions
+		return nil
+	})
+	m.configs = []ctrld.VPNDNSConfig{{
+		InterfaceName: "Ethernet 6",
+		Servers:       []string{"10.25.37.21", "10.25.37.22"},
+	}}
+	m.domainlessServers = []string{"10.25.37.21", "10.25.37.22"}
+
+	m.Refresh(context.Background(), true)
+
+	if got := m.DomainlessServers(); len(got) != 2 {
+		t.Fatalf("expected retained domainless servers, got %v", got)
+	}
+	if len(gotExemptions) != 2 {
+		t.Fatalf("expected retained exemptions to be re-applied, got %v", gotExemptions)
+	}
+	if !m.retainedAfterEmptyDiscovery {
+		t.Fatal("expected empty discovery retention to be marked")
+	}
+}
+
+func TestVPNDNSRefreshClearsOnSecondGuardedEmptyDiscovery(t *testing.T) {
+	withVPNDNSSettlingEnabled(t)
+	var gotExemptions []vpnDNSExemption
+	m := newVPNDNSManager(&mainLog, func(exemptions []vpnDNSExemption) error {
+		gotExemptions = exemptions
+		return nil
+	})
+	m.configs = []ctrld.VPNDNSConfig{{
+		InterfaceName: "Ethernet 6",
+		Servers:       []string{"10.25.37.21"},
+	}}
+	m.domainlessServers = []string{"10.25.37.21"}
+	m.retainedAfterEmptyDiscovery = true
+
+	m.Refresh(context.Background(), true)
+
+	if got := m.DomainlessServers(); len(got) != 0 {
+		t.Fatalf("expected domainless servers to be cleared on second empty discovery, got %v", got)
+	}
+	if len(gotExemptions) != 0 {
+		t.Fatalf("expected empty exemptions after clearing stale state, got %v", gotExemptions)
+	}
+	if m.retainedAfterEmptyDiscovery {
+		t.Fatal("expected retained empty-discovery marker to be cleared with stale state")
+	}
+}
+
+func TestVPNDNSTransportFailureSuppressesFallbackOnlyWhileRetainingState(t *testing.T) {
+	withVPNDNSSettlingEnabled(t)
+	m := newVPNDNSManager(&mainLog, nil)
+	m.domainlessServers = []string{"10.25.37.21"}
+
+	if m.ShouldFailClosedAfterVPNDNSTransportFailure("splunk.aws.arena.net.", []string{"10.25.37.21"}) {
+		t.Fatal("did not expect transport failure to suppress OS fallback outside retained empty-discovery state")
+	}
+
+	m.retainedAfterEmptyDiscovery = true
+	if !m.ShouldFailClosedAfterVPNDNSTransportFailure("splunk.aws.arena.net.", []string{"10.25.37.21"}) {
+		t.Fatal("expected transport failure to suppress OS fallback while retained state is active")
+	}
+
+	m.VPNDNSReachable()
+	if m.retainedAfterEmptyDiscovery {
+		t.Fatal("expected reachable DNS response to clear retained empty-discovery state")
+	}
+}