Pull request: all: make stats disableable, imp code

Updates #2141.

Squashed commit of the following:

commit d8f3bcd9927b00a1d4b8b60b43144bc4b4469311
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu Jun 17 19:10:53 2021 +0300

    stats: imp docs, names

commit 97eae3c2da5585467ca024bdacdbf922bcc8b444
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Thu Jun 17 18:51:49 2021 +0300

    all: make stats disableable, imp code

internal/stats/http.go

@@ -19,6 +19,10 @@ func httpError(r *http.Request, w http.ResponseWriter, code int, format string,
 	http.Error(w, text, code)
 }
 
+// topAddrs is an alias for the types of the TopFoo fields of statsResponse.
+// The key is either a client's address or a requested address.
+type topAddrs = map[string]uint64
+
 // statsResponse is a response for getting statistics.
 type statsResponse struct {
 	TimeUnits string `json:"time_units"`
@@ -31,9 +35,9 @@ type statsResponse struct {
 
 	AvgProcessingTime float64 `json:"avg_processing_time"`
 
-	TopQueried []map[string]uint64 `json:"top_queried_domains"`
-	TopClients []map[string]uint64 `json:"top_clients"`
-	TopBlocked []map[string]uint64 `json:"top_blocked_domains"`
+	TopQueried []topAddrs `json:"top_queried_domains"`
+	TopClients []topAddrs `json:"top_clients"`
+	TopBlocked []topAddrs `json:"top_blocked_domains"`
 
 	DNSQueries []uint64 `json:"dns_queries"`
 
@@ -45,17 +49,37 @@ type statsResponse struct {
 // handleStats is a handler for getting statistics.
 func (s *statsCtx) handleStats(w http.ResponseWriter, r *http.Request) {
 	start := time.Now()
-	response, ok := s.getData()
-	log.Debug("Stats: prepared data in %v", time.Since(start))
 
-	if !ok {
-		httpError(r, w, http.StatusInternalServerError, "Couldn't get statistics data")
+	var resp statsResponse
+	if s.conf.limit == 0 {
+		resp = statsResponse{
+			TimeUnits: "days",
 
-		return
+			TopBlocked: []topAddrs{},
+			TopClients: []topAddrs{},
+			TopQueried: []topAddrs{},
+
+			BlockedFiltering:     []uint64{},
+			DNSQueries:           []uint64{},
+			ReplacedParental:     []uint64{},
+			ReplacedSafebrowsing: []uint64{},
+		}
+	} else {
+		var ok bool
+		resp, ok = s.getData()
+
+		log.Debug("stats: prepared data in %v", time.Since(start))
+
+		if !ok {
+			httpError(r, w, http.StatusInternalServerError, "Couldn't get statistics data")
+
+			return
+		}
 	}
 
 	w.Header().Set("Content-Type", "application/json")
-	err := json.NewEncoder(w).Encode(response)
+
+	err := json.NewEncoder(w).Encode(resp)
 	if err != nil {
 		httpError(r, w, http.StatusInternalServerError, "json encode: %s", err)
 

internal/stats/unit.go

@@ -147,7 +147,7 @@ func (s *statsCtx) Start() {
 }
 
 func checkInterval(days uint32) bool {
-	return days == 1 || days == 7 || days == 30 || days == 90
+	return days == 0 || days == 1 || days == 7 || days == 30 || days == 90
 }
 
 func (s *statsCtx) dbOpen() bool {
@@ -251,7 +251,7 @@ func (s *statsCtx) periodicFlush() {
 		}
 
 		id := s.conf.UnitID()
-		if ptr.id == id {
+		if ptr.id == id || s.conf.limit == 0 {
 			time.Sleep(time.Second)
 
 			continue
@@ -412,9 +412,11 @@ func convertTopSlice(a []countPair) []map[string]uint64 {
 }
 
 func (s *statsCtx) setLimit(limitDays int) {
-	conf := *s.conf
-	conf.limit = uint32(limitDays) * 24
-	s.conf = &conf
+	s.conf.limit = uint32(limitDays) * 24
+	if limitDays == 0 {
+		s.clear()
+	}
+
 	log.Debug("stats: set limit: %d", limitDays)
 }
 
@@ -488,6 +490,10 @@ func (s *statsCtx) getClientIP(ip net.IP) (clientIP net.IP) {
 }
 
 func (s *statsCtx) Update(e Entry) {
+	if s.conf.limit == 0 {
+		return
+	}
+
 	if e.Result == 0 ||
 		e.Result >= rLast ||
 		e.Domain == "" ||
@@ -695,6 +701,10 @@ func (s *statsCtx) getData() (statsResponse, bool) {
 }
 
 func (s *statsCtx) GetTopClientsIP(maxCount uint) []net.IP {
+	if s.conf.limit == 0 {
+		return nil
+	}
+
 	units, _ := s.loadUnits(s.conf.limit)
 	if units == nil {
 		return nil