dnsforward: imp code

internal/dnsforward/upstreams.go

@@ -609,6 +609,10 @@ func (cv *upstreamConfigValidator) status() (results map[string]string) {
 		cv.private,
 	} {
 		for _, res := range results {
+			if _, ok := result[res.original]; ok {
+				log.Debug("dnsforward: warning: duplicate configuration %q", res.original)
+			}
+
 			if res.err != nil {
 				result[res.original] = res.err.Error()
 			} else {

internal/dnsforward/upstreams_internal_test.go

@@ -4,6 +4,7 @@ import (
 	"net"
 	"net/url"
 	"strings"
+	"sync/atomic"
 	"testing"
 	"time"
 
@@ -161,3 +162,48 @@ func TestUpstreamConfigValidator(t *testing.T) {
 		})
 	}
 }
+
+func TestUpstreamConfigValidator_Check_once(t *testing.T) {
+	reqs := atomic.Int32{}
+	reset := func() { reqs.Store(0) }
+
+	hdlr := dns.HandlerFunc(func(w dns.ResponseWriter, m *dns.Msg) {
+		err := w.WriteMsg(new(dns.Msg).SetReply(m))
+		require.NoError(testutil.PanicT{}, err)
+		reqs.Add(1)
+	})
+
+	addr := (&url.URL{
+		Scheme: "tcp",
+		Host:   newLocalUpstreamListener(t, 0, hdlr).String(),
+	}).String()
+	twoAddrs := strings.Join([]string{addr, addr}, " ")
+
+	testCases := []struct {
+		name string
+		ups  []string
+	}{{
+		name: "common",
+		ups:  []string{addr, addr, addr},
+	}, {
+		name: "domain-specific",
+		ups:  []string{"[/one.example/]" + addr, "[/two.example/]" + twoAddrs},
+	}, {
+		name: "both",
+		ups:  []string{addr, "[/one.example/]" + addr, addr, "[/two.example/]" + twoAddrs},
+	}}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Cleanup(reset)
+
+			cv := newUpstreamConfigValidator(tc.ups, nil, nil, &upstream.Options{
+				Timeout: 100 * time.Millisecond,
+			})
+			cv.check()
+			cv.close()
+
+			assert.Equal(t, int32(1), reqs.Load())
+		})
+	}
+}