dns: make more recent DHCP leases overwrite older ones

fixes #20

internal/dns/dns.go

@@ -247,6 +247,17 @@ func (s *Server) SetLeases(leases []dhcp4d.Lease) {
 	defer s.mu.Unlock()
 	s.initHostsLocked()
 	now := time.Now()
+	{
+		// defensive copy
+		slice := make([]dhcp4d.Lease, len(leases))
+		copy(slice, leases)
+		leases = slice
+	}
+	// First entry wins, so we order by expiration descendingly to put the
+	// newest entry for any given name into s.hostsByName.
+	sort.Slice(leases, func(i, j int) bool {
+		return !leases[i].Expiry.Before(leases[j].Expiry)
+	})
 	for _, l := range leases {
 		if l.Expired(now) {
 			continue

internal/dns/dns_test.go

@@ -228,9 +228,7 @@ func TestHostname(t *testing.T) {
 			t.Run(hostname, func(t *testing.T) {
 				m := new(dns.Msg)
 				m.SetQuestion(hostname+".lan.", dns.TypeA)
-				log.Printf("before ServeDNS")
 				s.Mux.ServeDNS(r, m)
-				log.Printf("after ServeDNS")
 				if got, want := len(r.response.Answer), 1; got != want {
 					t.Fatalf("unexpected number of answers for %v: got %d, want %d", m.Question, got, want)
 				}
@@ -260,6 +258,51 @@ func TestHostname(t *testing.T) {
 			t.Fatalf("unexpected response record: got %q, want %q", got, want)
 		}
 	})
+
+	t.Run("MoreRecent", func(t *testing.T) {
+		now := time.Now()
+		older := dhcp4d.Lease{
+			Hostname: "xps",
+			Addr:     net.IP{192, 168, 42, 23},
+			Expiry:   now.Add(1 * time.Second),
+		}
+		newer := dhcp4d.Lease{
+			Hostname: "xps",
+			Addr:     net.IP{192, 168, 42, 42},
+			Expiry:   now.Add(2 * time.Second),
+		}
+
+		for _, tt := range []struct {
+			name  string
+			order []dhcp4d.Lease
+		}{
+			{
+				name:  "older, newer",
+				order: []dhcp4d.Lease{older, newer},
+			},
+			{
+				name:  "newer, older",
+				order: []dhcp4d.Lease{newer, older},
+			},
+		} {
+			t.Run(tt.name, func(t *testing.T) {
+				s.SetLeases(tt.order)
+				m := new(dns.Msg)
+				m.SetQuestion("xps.lan.", dns.TypeA)
+				s.Mux.ServeDNS(r, m)
+				if got, want := len(r.response.Answer), 1; got != want {
+					t.Fatalf("unexpected number of answers for %v: got %d, want %d", m.Question, got, want)
+				}
+				a := r.response.Answer[0]
+				if _, ok := a.(*dns.A); !ok {
+					t.Fatalf("unexpected response type: got %T, want dns.A", a)
+				}
+				if got, want := a.(*dns.A).A, net.ParseIP("192.168.42.42"); !got.Equal(want) {
+					t.Fatalf("unexpected response IP: got %v, want %v", got, want)
+				}
+			})
+		}
+	})
 }
 
 func TestHostnameDHCP(t *testing.T) {