Stats heatmap: empty hours same height; cname_chain only for long chains

The "Busiest times" grid's hours without data used the class "empty",
which picked up the padding of empty messages. One CNAME to another no
longer warns; three or more get an info note showing the chain.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/validate/records.go

@@ -385,8 +385,16 @@ func (c *checker) checkCNAMETarget(z *model.Zone, ix *zoneIndex, r *model.Record
 	if !ix.inZoneTarget(target) {
 		return
 	}
-	// Follow the chain inside the zone to detect loops.
+	// Follow the chain inside the zone to detect loops. One CNAME to
+	// another is common and costs nothing (named answers the chain in
+	// one response); a longer chain gets a note.
 	seen := map[string]bool{r.Name: true}
+	chain := []string{disp(r.Name), disp(target)}
+	note := func() {
+		if len(chain) > 3 {
+			c.recProblem(z, r, Info, "cname_chain", "%s reaches its address through %d CNAMEs (%s); fine, but each one is another lookup for resolvers that don't get the chain at once", disp(r.Name), len(chain)-1, strings.Join(chain, " → "))
+		}
+	}
 	cur := target
 	for hops := 0; hops < 20; hops++ {
 		if seen[cur] {
@@ -398,18 +406,18 @@ func (c *checker) checkCNAMETarget(z *model.Zone, ix *zoneIndex, r *model.Record
 		if len(next) == 0 {
 			break
 		}
-		if hops == 0 {
-			c.recProblem(z, r, Warning, "cname_chain", "%s is a CNAME to %s, which is itself a CNAME; chains slow down resolution", disp(r.Name), disp(target))
-		}
 		p := ix.parsed[next[0].ID]
 		if p == nil {
 			break
 		}
 		cur = p.RR.(*dns.CNAME).Target
+		chain = append(chain, disp(cur))
 		if !ix.inZoneTarget(cur) {
+			note()
 			return
 		}
 	}
+	note()
 	if len(ix.byName[target]) == 0 && !ix.hasAddress(target) && !hasDescendant(ix, target) {
 		c.recProblem(z, r, Warning, "cname_dangling", "%s is a CNAME to %s, which does not exist in the zone", disp(r.Name), disp(target))
 	}

internal/validate/validate_test.go

@@ -161,8 +161,9 @@ func TestRecordConflicts(t *testing.T) {
 		}, "cname_loop", Error},
 		{"cname chain", func(b *builder, z *model.Zone) {
 			b.rec(z, "a", "CNAME", "b")
-			b.rec(z, "b", "CNAME", "ns1")
-		}, "cname_chain", Warning},
+			b.rec(z, "b", "CNAME", "c")
+			b.rec(z, "c", "CNAME", "ns1")
+		}, "cname_chain", Info},
 		{"dangling cname", func(b *builder, z *model.Zone) { b.rec(z, "a", "CNAME", "nothing") }, "cname_dangling", Warning},
 		{"null mx not alone", func(b *builder, z *model.Zone) {
 			b.rec(z, "@", "MX", "0 .")
@@ -380,3 +381,16 @@ func TestProblemStringDisplaysZone(t *testing.T) {
 		t.Errorf("String() = %q, want %q", got, want)
 	}
 }
+
+// One CNAME to another is common and says nothing.
+func TestShortCNAMEChain(t *testing.T) {
+	b := newBuilder()
+	z := b.primary("example.com", nil)
+	b.rec(z, "songshare", "CNAME", "navidrome")
+	b.rec(z, "navidrome", "CNAME", "ns1")
+	for _, p := range State(b.s, Options{}) {
+		if p.Code == "cname_chain" {
+			t.Errorf("one extra CNAME: %s %s", p.Severity, p.Message)
+		}
+	}
+}