Don't count a per-view counter going down as a restart of named

After a reconfig that changed named's views, the cache and resolver
counters (summed over views) went down, which was taken for a restart:
every counter then counted its total since named started for one minute.
Only a new boot time means a restart now; a counter that goes down
counts 0.

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

internal/stats/poll.go

@@ -96,23 +96,20 @@ func Flatten(body []byte) (*Snapshot, error) {
 }
 
 // Delta returns the change from prev to cur. When named restarted (its
-// boot time changed or a counter went down), the counters started again
-// from zero, so the current values are the change.
+// boot time changed), the counters started again from zero, so the
+// current values are the change. Without a restart, a counter can still
+// go down: the per-view ones (cache, upstream times, validation) are
+// summed over named's views, and a reconfig that adds, removes or
+// recreates views changes that sum. Such a counter counts 0 for the
+// minute; it must never make every counter count from zero, which would
+// add everything since named started (millions of "queries" in a minute).
 func Delta(prev, cur *Snapshot) map[string]int64 {
 	out := map[string]int64{}
 	restarted := prev.BootTime != cur.BootTime
-	if !restarted {
-		for k, n := range cur.Values {
-			if n < prev.Values[k] {
-				restarted = true
-				break
-			}
-		}
-	}
 	for k, n := range cur.Values {
 		d := n
 		if !restarted {
-			d = n - prev.Values[k]
+			d = max(n-prev.Values[k], 0)
 		}
 		if d != 0 || k == "queries" {
 			out[k] = d

internal/stats/poll_test.go

@@ -47,9 +47,15 @@ func TestFlattenAndDelta(t *testing.T) {
 	if d := Delta(s2, s3); d["queries"] != 3 || d["qtype:A"] != 3 {
 		t.Errorf("restart delta = %v", d)
 	}
-	// A counter going down also means a restart.
-	s4 := &Snapshot{BootTime: s2.BootTime, Values: map[string]int64{"queries": 2}}
-	if d := Delta(s2, s4); d["queries"] != 2 {
-		t.Errorf("reset delta = %v", d)
+	// A counter going down without a restart (views changed in a
+	// reconfig) counts 0, and doesn't make the others count from zero.
+	s4 := &Snapshot{BootTime: s2.BootTime, Values: map[string]int64{}}
+	for k, v := range s2.Values {
+		s4.Values[k] = v
+	}
+	s4.Values["queries"] += 4
+	s4.Values["cache:hits"] = 1 // was 4: a view's cache counters are gone
+	if d := Delta(s2, s4); d["queries"] != 4 || d["cache:hits"] != 0 || len(d) != 1 {
+		t.Errorf("delta with a counter going down = %v", d)
 	}
 }