Pull request: 4358 fix stats

Merge in DNS/adguard-home from 4358-fix-stats to master

Updates #4358.
Updates #4342.

Squashed commit of the following:

commit 5683cb304688ea639e5ba7f219a7bf12370211a4
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Thu Aug 4 18:20:54 2022 +0300

    stats: rm races test

commit 63dd67650ed64eaf9685b955a4fdf3c0067a7f8c
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Thu Aug 4 17:13:36 2022 +0300

    stats: try to imp test

commit 59a0f249fc00566872db62e362c87bc0c201b333
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Thu Aug 4 16:38:57 2022 +0300

    stats: fix nil ptr deref

commit 7fc3ff18a34a1d0e0fec3ca83a33f499ac752572
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Thu Apr 7 16:02:51 2022 +0300

    stats: fix races finally, imp tests

commit c63f5f4e7929819fe79b3a1e392f6b91cd630846
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Thu Aug 4 00:56:49 2022 +0300

    aghhttp: add register func

commit 61adc7f0e95279c1b7f4a0c0af5ab387ee461411
Merge: edbdb2d4 9b3adac1
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Thu Aug 4 00:36:01 2022 +0300

    Merge branch 'master' into 4358-fix-stats

commit edbdb2d4c6a06dcbf8107a28c4c3a61ba394e907
Merge: a91e4d7a a481ff4c
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Wed Aug 3 21:00:42 2022 +0300

    Merge branch 'master' into 4358-fix-stats

commit a91e4d7af13591eeef45cb7980d1ebc1650a5cb7
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Wed Aug 3 18:46:19 2022 +0300

    stats: imp code, docs

commit c5f3814c5c1a734ca8ff6726cc9ffc1177a055cf
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Wed Aug 3 18:16:13 2022 +0300

    all: log changes

commit 5e6caafc771dddc4c6be07c34658de359106fbe5
Merge: 091ba756 eb8e8166
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Wed Aug 3 18:09:10 2022 +0300

    Merge branch 'master' into 4358-fix-stats

commit 091ba75618d3689b9c04f05431283417c8cc52f9
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Wed Aug 3 18:07:39 2022 +0300

    stats: imp docs, code

commit f2b2de77ce5f0448d6df9232a614a3710f1e2e8a
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Tue Aug 2 17:09:30 2022 +0300

    all: refactor stats & add mutexes

commit b3f11c455ceaa3738ec20eefc46f866ff36ed046
Author: Eugene Burkov <E.Burkov@AdGuard.COM>
Date:   Wed Apr 27 15:30:09 2022 +0300

    WIP

CHANGELOG.md

@@ -25,6 +25,10 @@ and this project adheres to
 - Support for Discovery of Designated Resolvers (DDR) according to the [RFC
   draft][ddr-draft] ([#4463]).
 
+### Fixed
+
+- Data races and concurrent map access in statistics module ([#4358], [#4342]).
+
 ### Deprecated
 
 - Go 1.18 support.  v0.109.0 will require at least Go 1.19 to build.
@@ -35,6 +39,8 @@ and this project adheres to
   ([#4670]).
 
 [#2993]: https://github.com/AdguardTeam/AdGuardHome/issues/2993
+[#4342]: https://github.com/AdguardTeam/AdGuardHome/issues/4342
+[#4358]: https://github.com/AdguardTeam/AdGuardHome/issues/4358
 [#4670]: https://github.com/AdguardTeam/AdGuardHome/issues/4670
 
 [ddr-draft]: https://datatracker.ietf.org/doc/html/draft-ietf-add-ddr-08

internal/aghhttp/aghhttp.go

@@ -9,6 +9,12 @@ import (
 	"github.com/AdguardTeam/golibs/log"
 )
 
+// RegisterFunc is the function that sets the handler to handle the URL for the
+// method.
+//
+// TODO(e.burkov, a.garipov):  Get rid of it.
+type RegisterFunc func(method, url string, handler http.HandlerFunc)
+
 // OK responds with word OK.
 func OK(w http.ResponseWriter) {
 	if _, err := io.WriteString(w, "OK\n"); err != nil {

internal/dhcpd/dhcpd.go

@@ -5,11 +5,11 @@ import (
 	"encoding/json"
 	"fmt"
 	"net"
-	"net/http"
 	"path/filepath"
 	"runtime"
 	"time"
 
+	"github.com/AdguardTeam/AdGuardHome/internal/aghhttp"
 	"github.com/AdguardTeam/golibs/log"
 	"github.com/AdguardTeam/golibs/netutil"
 )
@@ -126,7 +126,7 @@ type ServerConfig struct {
 	ConfigModified func() `yaml:"-"`
 
 	// Register an HTTP handler
-	HTTPRegister func(string, string, func(http.ResponseWriter, *http.Request)) `yaml:"-"`
+	HTTPRegister aghhttp.RegisterFunc `yaml:"-"`
 
 	Enabled       bool   `yaml:"enabled"`
 	InterfaceName string `yaml:"interface_name"`

internal/dnsforward/config.go

@@ -5,12 +5,12 @@ import (
 	"crypto/x509"
 	"fmt"
 	"net"
-	"net/http"
 	"os"
 	"sort"
 	"strings"
 	"time"
 
+	"github.com/AdguardTeam/AdGuardHome/internal/aghhttp"
 	"github.com/AdguardTeam/AdGuardHome/internal/aghtls"
 	"github.com/AdguardTeam/AdGuardHome/internal/filtering"
 	"github.com/AdguardTeam/dnsproxy/proxy"
@@ -193,7 +193,7 @@ type ServerConfig struct {
 	ConfigModified func()
 
 	// Register an HTTP handler
-	HTTPRegister func(string, string, func(http.ResponseWriter, *http.Request))
+	HTTPRegister aghhttp.RegisterFunc
 
 	// ResolveClients signals if the RDNS should resolve clients' addresses.
 	ResolveClients bool

internal/dnsforward/dnsforward.go

@@ -61,7 +61,7 @@ type Server struct {
 	dnsFilter  *filtering.DNSFilter  // DNS filter instance
 	dhcpServer dhcpd.ServerInterface // DHCP server instance (optional)
 	queryLog   querylog.QueryLog     // Query log instance
-	stats      stats.Stats
+	stats      stats.Interface
 	access     *accessCtx
 
 	// localDomainSuffix is the suffix used to detect internal hosts.  It
@@ -107,7 +107,7 @@ const defaultLocalDomainSuffix = "lan"
 // DNSCreateParams are parameters to create a new server.
 type DNSCreateParams struct {
 	DNSFilter   *filtering.DNSFilter
-	Stats       stats.Stats
+	Stats       stats.Interface
 	QueryLog    querylog.QueryLog
 	DHCPServer  dhcpd.ServerInterface
 	PrivateNets netutil.SubnetSet

internal/dnsforward/stats_test.go

@@ -34,7 +34,7 @@ func (l *testQueryLog) Add(p *querylog.AddParams) {
 type testStats struct {
 	// Stats is embedded here simply to make testStats a stats.Stats without
 	// actually implementing all methods.
-	stats.Stats
+	stats.Interface
 
 	lastEntry stats.Entry
 }

internal/filtering/filtering.go

@@ -6,7 +6,6 @@ import (
 	"fmt"
 	"io/fs"
 	"net"
-	"net/http"
 	"os"
 	"runtime"
 	"runtime/debug"
@@ -14,6 +13,7 @@ import (
 	"sync"
 	"sync/atomic"
 
+	"github.com/AdguardTeam/AdGuardHome/internal/aghhttp"
 	"github.com/AdguardTeam/AdGuardHome/internal/aghnet"
 	"github.com/AdguardTeam/dnsproxy/upstream"
 	"github.com/AdguardTeam/golibs/cache"
@@ -94,7 +94,7 @@ type Config struct {
 	ConfigModified func() `yaml:"-"`
 
 	// Register an HTTP handler
-	HTTPRegister func(string, string, func(http.ResponseWriter, *http.Request)) `yaml:"-"`
+	HTTPRegister aghhttp.RegisterFunc `yaml:"-"`
 
 	// CustomResolver is the resolver used by DNSFilter.
 	CustomResolver Resolver `yaml:"-"`

internal/home/clients.go

@@ -2,6 +2,7 @@ package home
 
 import (
 	"bytes"
+	"encoding"
 	"fmt"
 	"net"
 	"sort"
@@ -60,6 +61,33 @@ const (
 	ClientSourceHostsFile
 )
 
+var _ fmt.Stringer = clientSource(0)
+
+// String returns a human-readable name of cs.
+func (cs clientSource) String() (s string) {
+	switch cs {
+	case ClientSourceWHOIS:
+		return "WHOIS"
+	case ClientSourceARP:
+		return "ARP"
+	case ClientSourceRDNS:
+		return "rDNS"
+	case ClientSourceDHCP:
+		return "DHCP"
+	case ClientSourceHostsFile:
+		return "etc/hosts"
+	default:
+		return ""
+	}
+}
+
+var _ encoding.TextMarshaler = clientSource(0)
+
+// MarshalText implements encoding.TextMarshaler for the clientSource.
+func (cs clientSource) MarshalText() (text []byte, err error) {
+	return []byte(cs.String()), nil
+}
+
 // clientSourceConf is used to configure where the runtime clients will be
 // obtained from.
 type clientSourcesConf struct {
@@ -397,6 +425,7 @@ func (clients *clientsContainer) Find(id string) (c *Client, ok bool) {
 	c.Tags = stringutil.CloneSlice(c.Tags)
 	c.BlockedServices = stringutil.CloneSlice(c.BlockedServices)
 	c.Upstreams = stringutil.CloneSlice(c.Upstreams)
+
 	return c, true
 }
 

internal/home/clientshttp.go

@@ -47,9 +47,9 @@ type clientJSON struct {
 type runtimeClientJSON struct {
 	WHOISInfo *RuntimeClientWHOISInfo `json:"whois_info"`
 
-	Name   string `json:"name"`
+	Name   string       `json:"name"`
-	Source string `json:"source"`
+	Source clientSource `json:"source"`
-	IP     net.IP `json:"ip"`
+	IP     net.IP       `json:"ip"`
 }
 
 type clientListJSON struct {
@@ -81,20 +81,9 @@ func (clients *clientsContainer) handleGetClients(w http.ResponseWriter, r *http
 		cj := runtimeClientJSON{
 			WHOISInfo: rc.WHOISInfo,
 
-			Name: rc.Host,
+			Name:   rc.Host,
-			IP:   ip,
+			Source: rc.Source,
-		}
+			IP:     ip,
-
-		cj.Source = "etc/hosts"
-		switch rc.Source {
-		case ClientSourceDHCP:
-			cj.Source = "DHCP"
-		case ClientSourceRDNS:
-			cj.Source = "rDNS"
-		case ClientSourceARP:
-			cj.Source = "ARP"
-		case ClientSourceWHOIS:
-			cj.Source = "WHOIS"
 		}
 
 		data.RuntimeClients = append(data.RuntimeClients, cj)
@@ -107,13 +96,7 @@ func (clients *clientsContainer) handleGetClients(w http.ResponseWriter, r *http
 	w.Header().Set("Content-Type", "application/json")
 	e := json.NewEncoder(w).Encode(data)
 	if e != nil {
-		aghhttp.Error(
+		aghhttp.Error(r, w, http.StatusInternalServerError, "failed to encode to json: %v", e)
-			r,
-			w,
-			http.StatusInternalServerError,
-			"Failed to encode to json: %v",
-			e,
-		)
 
 		return
 	}
@@ -279,9 +262,9 @@ func (clients *clientsContainer) handleFindClient(w http.ResponseWriter, r *http
 func (clients *clientsContainer) findRuntime(ip net.IP, idStr string) (cj *clientJSON) {
 	rc, ok := clients.FindRuntimeClient(ip)
 	if !ok {
-		// It is still possible that the IP used to be in the runtime
+		// It is still possible that the IP used to be in the runtime clients
-		// clients list, but then the server was reloaded.  So, check
+		// list, but then the server was reloaded.  So, check the DNS server's
-		// the DNS server's blocked IP list.
+		// blocked IP list.
 		//
 		// See https://github.com/AdguardTeam/AdGuardHome/issues/2428.
 		disallowed, rule := clients.dnsServer.IsBlockedClient(ip, idStr)

internal/home/control.go

@@ -189,7 +189,7 @@ func registerControlHandlers() {
 	RegisterAuthHandlers()
 }
 
-func httpRegister(method, url string, handler func(http.ResponseWriter, *http.Request)) {
+func httpRegister(method, url string, handler http.HandlerFunc) {
 	if method == "" {
 		// "/dns-query" handler doesn't need auth, gzip and isn't restricted by 1 HTTP method
 		Context.mux.HandleFunc(url, postInstall(handler))

internal/home/home.go

@@ -47,7 +47,7 @@ type homeContext struct {
 	// --
 
 	clients    clientsContainer     // per-client-settings module
-	stats      stats.Stats          // statistics module
+	stats      stats.Interface      // statistics module
 	queryLog   querylog.QueryLog    // query log module
 	dnsServer  *dnsforward.Server   // DNS module
 	rdns       *RDNS                // rDNS module

internal/querylog/querylog.go

@@ -2,10 +2,10 @@ package querylog
 
 import (
 	"net"
-	"net/http"
 	"path/filepath"
 	"time"
 
+	"github.com/AdguardTeam/AdGuardHome/internal/aghhttp"
 	"github.com/AdguardTeam/AdGuardHome/internal/aghnet"
 	"github.com/AdguardTeam/AdGuardHome/internal/filtering"
 	"github.com/AdguardTeam/golibs/errors"
@@ -38,7 +38,7 @@ type Config struct {
 	ConfigModified func()
 
 	// HTTPRegister registers an HTTP handler.
-	HTTPRegister func(string, string, func(http.ResponseWriter, *http.Request))
+	HTTPRegister aghhttp.RegisterFunc
 
 	// FindClient returns client information by their IDs.
 	FindClient func(ids []string) (c *Client, err error)

internal/stats/http.go

@@ -39,34 +39,21 @@ type statsResponse struct {
 }
 
 // handleStats is a handler for getting statistics.
-func (s *statsCtx) handleStats(w http.ResponseWriter, r *http.Request) {
+func (s *StatsCtx) handleStats(w http.ResponseWriter, r *http.Request) {
 	start := time.Now()
 
 	var resp statsResponse
-	if s.conf.limit == 0 {
+	var ok bool
-		resp = statsResponse{
+	resp, ok = s.getData()
-			TimeUnits: "days",
 
-			TopBlocked: []topAddrs{},
+	log.Debug("stats: prepared data in %v", time.Since(start))
-			TopClients: []topAddrs{},
-			TopQueried: []topAddrs{},
 
-			BlockedFiltering:     []uint64{},
+	if !ok {
-			DNSQueries:           []uint64{},
+		// Don't bring the message to the lower case since it's a part of UI
-			ReplacedParental:     []uint64{},
+		// text for the moment.
-			ReplacedSafebrowsing: []uint64{},
+		aghhttp.Error(r, w, http.StatusInternalServerError, "Couldn't get statistics data")
-		}
-	} else {
-		var ok bool
-		resp, ok = s.getData()
 
-		log.Debug("stats: prepared data in %v", time.Since(start))
+		return
-
-		if !ok {
-			aghhttp.Error(r, w, http.StatusInternalServerError, "Couldn't get statistics data")
-
-			return
-		}
 	}
 
 	w.Header().Set("Content-Type", "application/json")
@@ -84,9 +71,9 @@ type config struct {
 }
 
 // Get configuration
-func (s *statsCtx) handleStatsInfo(w http.ResponseWriter, r *http.Request) {
+func (s *StatsCtx) handleStatsInfo(w http.ResponseWriter, r *http.Request) {
 	resp := config{}
-	resp.IntervalDays = s.conf.limit / 24
+	resp.IntervalDays = s.limitHours / 24
 
 	data, err := json.Marshal(resp)
 	if err != nil {
@@ -102,7 +89,7 @@ func (s *statsCtx) handleStatsInfo(w http.ResponseWriter, r *http.Request) {
 }
 
 // Set configuration
-func (s *statsCtx) handleStatsConfig(w http.ResponseWriter, r *http.Request) {
+func (s *StatsCtx) handleStatsConfig(w http.ResponseWriter, r *http.Request) {
 	reqData := config{}
 	err := json.NewDecoder(r.Body).Decode(&reqData)
 	if err != nil {
@@ -118,22 +105,22 @@ func (s *statsCtx) handleStatsConfig(w http.ResponseWriter, r *http.Request) {
 	}
 
 	s.setLimit(int(reqData.IntervalDays))
-	s.conf.ConfigModified()
+	s.configModified()
 }
 
 // Reset data
-func (s *statsCtx) handleStatsReset(w http.ResponseWriter, r *http.Request) {
+func (s *StatsCtx) handleStatsReset(w http.ResponseWriter, r *http.Request) {
 	s.clear()
 }
 
 // Register web handlers
-func (s *statsCtx) initWeb() {
+func (s *StatsCtx) initWeb() {
-	if s.conf.HTTPRegister == nil {
+	if s.httpRegister == nil {
 		return
 	}
 
-	s.conf.HTTPRegister(http.MethodGet, "/control/stats", s.handleStats)
+	s.httpRegister(http.MethodGet, "/control/stats", s.handleStats)
-	s.conf.HTTPRegister(http.MethodPost, "/control/stats_reset", s.handleStatsReset)
+	s.httpRegister(http.MethodPost, "/control/stats_reset", s.handleStatsReset)
-	s.conf.HTTPRegister(http.MethodPost, "/control/stats_config", s.handleStatsConfig)
+	s.httpRegister(http.MethodPost, "/control/stats_config", s.handleStatsConfig)
-	s.conf.HTTPRegister(http.MethodGet, "/control/stats_info", s.handleStatsInfo)
+	s.httpRegister(http.MethodGet, "/control/stats_info", s.handleStatsInfo)
 }

internal/stats/stats.go

@@ -4,75 +4,85 @@ package stats
 
 import (
 	"net"
-	"net/http"
+
+	"github.com/AdguardTeam/AdGuardHome/internal/aghhttp"
 )
 
-type unitIDCallback func() uint32
+// UnitIDGenFunc is the signature of a function that generates a unique ID for
+// the statistics unit.
+type UnitIDGenFunc func() (id uint32)
 
-// DiskConfig - configuration settings that are stored on disk
+// DiskConfig is the configuration structure that is stored in file.
 type DiskConfig struct {
-	Interval uint32 `yaml:"statistics_interval"` // time interval for statistics (in days)
+	// Interval is the number of days for which the statistics are collected
+	// before flushing to the database.
+	Interval uint32 `yaml:"statistics_interval"`
 }
 
-// Config - module configuration
+// Config is the configuration structure for the statistics collecting.
 type Config struct {
-	Filename  string         // database file name
+	// UnitID is the function to generate the identifier for current unit.  If
-	LimitDays uint32         // time limit (in days)
+	// nil, the default function is used, see newUnitID.
-	UnitID    unitIDCallback // user function to get the current unit ID.  If nil, the current time hour is used.
+	UnitID UnitIDGenFunc
 
-	// Called when the configuration is changed by HTTP request
+	// ConfigModified will be called each time the configuration changed via web
+	// interface.
 	ConfigModified func()
 
-	// Register an HTTP handler
+	// HTTPRegister is the function that registers handlers for the stats
-	HTTPRegister func(string, string, func(http.ResponseWriter, *http.Request))
+	// endpoints.
+	HTTPRegister aghhttp.RegisterFunc
 
-	limit uint32 // maximum time we need to keep data for (in hours)
+	// Filename is the name of the database file.
+	Filename string
+
+	// LimitDays is the maximum number of days to collect statistics into the
+	// current unit.
+	LimitDays uint32
 }
 
-// New - create object
+// Interface is the statistics interface to be used by other packages.
-func New(conf Config) (Stats, error) {
+type Interface interface {
-	return createObject(conf)
+	// Start begins the statistics collecting.
-}
-
-// Stats - main interface
-type Stats interface {
 	Start()
 
-	// Close object.
+	// Close stops the statistics collecting.
-	// This function is not thread safe
-	//  (can't be called in parallel with any other function of this interface).
 	Close()
 
-	// Update counters
+	// Update collects the incoming statistics data.
 	Update(e Entry)
 
-	// Get IP addresses of the clients with the most number of requests
+	// GetTopClientIP returns at most limit IP addresses corresponding to the
+	// clients with the most number of requests.
 	GetTopClientsIP(limit uint) []net.IP
 
-	// WriteDiskConfig - write configuration
+	// WriteDiskConfig puts the Interface's configuration to the dc.
 	WriteDiskConfig(dc *DiskConfig)
 }
 
-// TimeUnit - time unit
+// TimeUnit is the unit of measuring time while aggregating the statistics.
 type TimeUnit int
 
-// Supported time units
+// Supported TimeUnit values.
 const (
 	Hours TimeUnit = iota
 	Days
 )
 
-// Result of DNS request processing
+// Result is the resulting code of processing the DNS request.
 type Result int
 
-// Supported result values
+// Supported Result values.
+//
+// TODO(e.burkov):  Think about better naming.
 const (
 	RNotFiltered Result = iota + 1
 	RFiltered
 	RSafeBrowsing
 	RSafeSearch
 	RParental
-	rLast
+
+	resultLast = RParental + 1
 )
 
 // Entry is a statistics data entry.
@@ -82,7 +92,12 @@ type Entry struct {
 	// TODO(a.garipov): Make this a {net.IP, string} enum?
 	Client string
 
+	// Domain is the domain name requested.
 	Domain string
+
+	// Result is the result of processing the request.
 	Result Result
-	Time   uint32 // processing time (msec)
+
+	// Time is the duration of the request processing in milliseconds.
+	Time uint32
 }

internal/stats/stats_test.go

@@ -37,7 +37,7 @@ func TestStats(t *testing.T) {
 		LimitDays: 1,
 	}
 
-	s, err := createObject(conf)
+	s, err := New(conf)
 	require.NoError(t, err)
 	testutil.CleanupAndRequireSuccess(t, func() (err error) {
 		s.clear()
@@ -110,7 +110,7 @@ func TestLargeNumbers(t *testing.T) {
 		LimitDays: 1,
 		UnitID:    newID,
 	}
-	s, err := createObject(conf)
+	s, err := New(conf)
 	require.NoError(t, err)
 	testutil.CleanupAndRequireSuccess(t, func() (err error) {
 		s.Close()

internal/stats/unit.go

@@ -9,11 +9,13 @@ import (
 	"os"
 	"sort"
 	"sync"
+	"sync/atomic"
 	"time"
 
+	"github.com/AdguardTeam/AdGuardHome/internal/aghhttp"
 	"github.com/AdguardTeam/golibs/errors"
 	"github.com/AdguardTeam/golibs/log"
-	bolt "go.etcd.io/bbolt"
+	"go.etcd.io/bbolt"
 )
 
 // TODO(a.garipov): Rewrite all of this.  Add proper error handling and
@@ -24,47 +26,130 @@ const (
 	maxClients = 100 // max number of top clients to store in file or return via Get()
 )
 
-// statsCtx - global context
+// StatsCtx collects the statistics and flushes it to the database.  Its default
-type statsCtx struct {
+// flushing interval is one hour.
-	// mu protects unit.
+//
-	mu *sync.Mutex
+// TODO(e.burkov):  Use atomic.Pointer for accessing curr and db in go1.19.
-	// current is the actual statistics collection result.
+type StatsCtx struct {
-	current *unit
+	// currMu protects the current unit.
+	currMu *sync.Mutex
+	// curr is the actual statistics collection result.
+	curr *unit
 
-	db   *bolt.DB
+	// dbMu protects db.
-	conf *Config
+	dbMu *sync.Mutex
+	// db is the opened statistics database, if any.
+	db *bbolt.DB
+
+	// unitIDGen is the function that generates an identifier for the current
+	// unit.  It's here for only testing purposes.
+	unitIDGen UnitIDGenFunc
+
+	// httpRegister is used to set HTTP handlers.
+	httpRegister aghhttp.RegisterFunc
+
+	// configModified is called whenever the configuration is modified via web
+	// interface.
+	configModified func()
+
+	// filename is the name of database file.
+	filename string
+
+	// limitHours is the maximum number of hours to collect statistics into the
+	// current unit.
+	limitHours uint32
 }
 
-// data for 1 time unit
+// unit collects the statistics data for a specific period of time.
 type unit struct {
-	id uint32 // unit ID.  Default: absolute hour since Jan 1, 1970
+	// mu protects all the fields of a unit.
+	mu *sync.RWMutex
 
-	nTotal  uint64   // total requests
+	// id is the unique unit's identifier.  It's set to an absolute hour number
-	nResult []uint64 // number of requests per one result
+	// since the beginning of UNIX time by the default ID generating function.
-	timeSum uint64   // sum of processing time of all requests (usec)
+	id uint32
 
-	// top:
+	// nTotal stores the total number of requests.
-	domains        map[string]uint64 // number of requests per domain
+	nTotal uint64
-	blockedDomains map[string]uint64 // number of blocked requests per domain
+	// nResult stores the number of requests grouped by it's result.
-	clients        map[string]uint64 // number of requests per client
+	nResult []uint64
+	// timeSum stores the sum of processing time in milliseconds of each request
+	// written by the unit.
+	timeSum uint64
+
+	// domains stores the number of requests for each domain.
+	domains map[string]uint64
+	// blockedDomains stores the number of requests for each domain that has
+	// been blocked.
+	blockedDomains map[string]uint64
+	// clients stores the number of requests from each client.
+	clients map[string]uint64
 }
 
-// name-count pair
+// ongoing returns the current unit.  It's safe for concurrent use.
+//
+// Note that the unit itself should be locked before accessing.
+func (s *StatsCtx) ongoing() (u *unit) {
+	s.currMu.Lock()
+	defer s.currMu.Unlock()
+
+	return s.curr
+}
+
+// swapCurrent swaps the current unit with another and returns it.  It's safe
+// for concurrent use.
+func (s *StatsCtx) swapCurrent(with *unit) (old *unit) {
+	s.currMu.Lock()
+	defer s.currMu.Unlock()
+
+	old, s.curr = s.curr, with
+
+	return old
+}
+
+// database returns the database if it's opened.  It's safe for concurrent use.
+func (s *StatsCtx) database() (db *bbolt.DB) {
+	s.dbMu.Lock()
+	defer s.dbMu.Unlock()
+
+	return s.db
+}
+
+// swapDatabase swaps the database with another one and returns it.  It's safe
+// for concurrent use.
+func (s *StatsCtx) swapDatabase(with *bbolt.DB) (old *bbolt.DB) {
+	s.dbMu.Lock()
+	defer s.dbMu.Unlock()
+
+	old, s.db = s.db, with
+
+	return old
+}
+
+// countPair is a single name-number pair for deserializing statistics data into
+// the database.
 type countPair struct {
 	Name  string
 	Count uint64
 }
 
-// structure for storing data in file
+// unitDB is the structure for deserializing statistics data into the database.
 type unitDB struct {
-	NTotal  uint64
+	// NTotal is the total number of requests.
+	NTotal uint64
+	// NResult is the number of requests by the result's kind.
 	NResult []uint64
 
-	Domains        []countPair
+	// Domains is the number of requests for each domain name.
+	Domains []countPair
+	// BlockedDomains is the number of requests blocked for each domain name.
 	BlockedDomains []countPair
-	Clients        []countPair
+	// Clients is the number of requests from each client.
+	Clients []countPair
 
-	TimeAvg uint32 // usec
+	// TimeAvg is the average of processing times in milliseconds of all the
+	// requests in the unit.
+	TimeAvg uint32
 }
 
 // withRecovered turns the value recovered from panic if any into an error and
@@ -86,34 +171,40 @@ func withRecovered(orig *error) {
 	*orig = errors.WithDeferred(*orig, err)
 }
 
-// createObject creates s from conf and properly initializes it.
+// isEnabled is a helper that check if the statistics collecting is enabled.
-func createObject(conf Config) (s *statsCtx, err error) {
+func (s *StatsCtx) isEnabled() (ok bool) {
+	return atomic.LoadUint32(&s.limitHours) != 0
+}
+
+// New creates s from conf and properly initializes it.  Don't use s before
+// calling it's Start method.
+func New(conf Config) (s *StatsCtx, err error) {
 	defer withRecovered(&err)
 
-	s = &statsCtx{
+	s = &StatsCtx{
-		mu: &sync.Mutex{},
+		currMu:         &sync.Mutex{},
+		dbMu:           &sync.Mutex{},
+		filename:       conf.Filename,
+		configModified: conf.ConfigModified,
+		httpRegister:   conf.HTTPRegister,
 	}
-	if !checkInterval(conf.LimitDays) {
+	if s.limitHours = conf.LimitDays * 24; !checkInterval(conf.LimitDays) {
-		conf.LimitDays = 1
+		s.limitHours = 24
+	}
+	if s.unitIDGen = newUnitID; conf.UnitID != nil {
+		s.unitIDGen = conf.UnitID
 	}
 
-	s.conf = &Config{}
+	if err = s.dbOpen(); err != nil {
-	*s.conf = conf
+		return nil, fmt.Errorf("opening database: %w", err)
-	s.conf.limit = conf.LimitDays * 24
-	if conf.UnitID == nil {
-		s.conf.UnitID = newUnitID
 	}
 
-	if !s.dbOpen() {
+	id := s.unitIDGen()
-		return nil, fmt.Errorf("open database")
+	tx := beginTxn(s.db, true)
-	}
-
-	id := s.conf.UnitID()
-	tx := s.beginTxn(true)
 	var udb *unitDB
 	if tx != nil {
 		log.Tracef("Deleting old units...")
-		firstID := id - s.conf.limit - 1
+		firstID := id - s.limitHours - 1
 		unitDel := 0
 
 		err = tx.ForEach(newBucketWalker(tx, &unitDel, firstID))
@@ -133,12 +224,11 @@ func createObject(conf Config) (s *statsCtx, err error) {
 		}
 	}
 
-	u := unit{}
+	u := newUnit(id)
-	s.initUnit(&u, id)
+	// This use of deserialize is safe since the accessed unit has just been
-	if udb != nil {
+	// created.
-		deserialize(&u, udb)
+	u.deserialize(udb)
-	}
+	s.curr = u
-	s.current = &u
 
 	log.Debug("stats: initialized")
 
@@ -153,11 +243,11 @@ const errStop errors.Error = "stop iteration"
 // integer that unitDelPtr points to is incremented for every successful
 // deletion.  If the bucket isn't deleted, f returns errStop.
 func newBucketWalker(
-	tx *bolt.Tx,
+	tx *bbolt.Tx,
 	unitDelPtr *int,
 	firstID uint32,
-) (f func(name []byte, b *bolt.Bucket) (err error)) {
+) (f func(name []byte, b *bbolt.Bucket) (err error)) {
-	return func(name []byte, _ *bolt.Bucket) (err error) {
+	return func(name []byte, _ *bbolt.Bucket) (err error) {
 		nameID, ok := unitNameToID(name)
 		if !ok || nameID < firstID {
 			err = tx.DeleteBucket(name)
@@ -178,80 +268,92 @@ func newBucketWalker(
 	}
 }
 
-func (s *statsCtx) Start() {
+// Start makes s process the incoming data.
+func (s *StatsCtx) Start() {
 	s.initWeb()
 	go s.periodicFlush()
 }
 
-func checkInterval(days uint32) bool {
+// checkInterval returns true if days is valid to be used as statistics
+// retention interval.  The valid values are 0, 1, 7, 30 and 90.
+func checkInterval(days uint32) (ok bool) {
 	return days == 0 || days == 1 || days == 7 || days == 30 || days == 90
 }
 
-func (s *statsCtx) dbOpen() bool {
+// dbOpen returns an error if the database can't be opened from the specified
-	var err error
+// file.  It's safe for concurrent use.
+func (s *StatsCtx) dbOpen() (err error) {
 	log.Tracef("db.Open...")
-	s.db, err = bolt.Open(s.conf.Filename, 0o644, nil)
+
+	s.dbMu.Lock()
+	defer s.dbMu.Unlock()
+
+	s.db, err = bbolt.Open(s.filename, 0o644, nil)
 	if err != nil {
-		log.Error("stats: open DB: %s: %s", s.conf.Filename, err)
+		log.Error("stats: open DB: %s: %s", s.filename, err)
 		if err.Error() == "invalid argument" {
 			log.Error("AdGuard Home cannot be initialized due to an incompatible file system.\nPlease read the explanation here: https://github.com/AdguardTeam/AdGuardHome/wiki/Getting-Started#limitations")
 		}
-		return false
+
+		return err
 	}
+
 	log.Tracef("db.Open")
-	return true
+
+	return nil
 }
 
-// Atomically swap the currently active unit with a new value
+// newUnitID is the default UnitIDGenFunc that generates the unique id hourly.
-// Return old value
+func newUnitID() (id uint32) {
-func (s *statsCtx) swapUnit(new *unit) (u *unit) {
+	const secsInHour = int64(time.Hour / time.Second)
-	s.mu.Lock()
-	defer s.mu.Unlock()
 
-	u = s.current
+	return uint32(time.Now().Unix() / secsInHour)
-	s.current = new
-
-	return u
 }
 
-// Get unit ID for the current hour
+// newUnit allocates the new *unit.
-func newUnitID() uint32 {
+func newUnit(id uint32) (u *unit) {
-	return uint32(time.Now().Unix() / (60 * 60))
+	return &unit{
+		mu:             &sync.RWMutex{},
+		id:             id,
+		nResult:        make([]uint64, resultLast),
+		domains:        make(map[string]uint64),
+		blockedDomains: make(map[string]uint64),
+		clients:        make(map[string]uint64),
+	}
 }
 
-// Initialize a unit
+// beginTxn opens a new database transaction.  If writable is true, the
-func (s *statsCtx) initUnit(u *unit, id uint32) {
+// transaction will be opened for writing, and for reading otherwise.  It
-	u.id = id
+// returns nil if the transaction can't be created.
-	u.nResult = make([]uint64, rLast)
+func beginTxn(db *bbolt.DB, writable bool) (tx *bbolt.Tx) {
-	u.domains = make(map[string]uint64)
-	u.blockedDomains = make(map[string]uint64)
-	u.clients = make(map[string]uint64)
-}
-
-// Open a DB transaction
-func (s *statsCtx) beginTxn(wr bool) *bolt.Tx {
-	db := s.db
 	if db == nil {
 		return nil
 	}
 
-	log.Tracef("db.Begin...")
+	log.Tracef("opening a database transaction")
-	tx, err := db.Begin(wr)
+
+	tx, err := db.Begin(writable)
 	if err != nil {
-		log.Error("db.Begin: %s", err)
+		log.Error("stats: opening a transaction: %s", err)
+
 		return nil
 	}
-	log.Tracef("db.Begin")
+
+	log.Tracef("transaction has been opened")
+
 	return tx
 }
 
-func (s *statsCtx) commitTxn(tx *bolt.Tx) {
+// commitTxn applies the changes made in tx to the database.
+func (s *StatsCtx) commitTxn(tx *bbolt.Tx) {
 	err := tx.Commit()
 	if err != nil {
-		log.Debug("tx.Commit: %s", err)
+		log.Error("stats: committing a transaction: %s", err)
+
 		return
 	}
-	log.Tracef("tx.Commit")
+
+	log.Tracef("transaction has been committed")
 }
 
 // bucketNameLen is the length of a bucket, a 64-bit unsigned integer.
@@ -262,10 +364,10 @@ const bucketNameLen = 8
 
 // idToUnitName converts a numerical ID into a database unit name.
 func idToUnitName(id uint32) (name []byte) {
-	name = make([]byte, bucketNameLen)
+	n := [bucketNameLen]byte{}
-	binary.BigEndian.PutUint64(name, uint64(id))
+	binary.BigEndian.PutUint64(n[:], uint64(id))
 
-	return name
+	return n[:]
 }
 
 // unitNameToID converts a database unit name into a numerical ID.  ok is false
@@ -278,13 +380,6 @@ func unitNameToID(name []byte) (id uint32, ok bool) {
 	return uint32(binary.BigEndian.Uint64(name)), true
 }
 
-func (s *statsCtx) ongoing() (u *unit) {
-	s.mu.Lock()
-	defer s.mu.Unlock()
-
-	return s.current
-}
-
 // Flush the current unit to DB and delete an old unit when a new hour is started
 // If a unit must be flushed:
 // . lock DB
@@ -293,34 +388,29 @@ func (s *statsCtx) ongoing() (u *unit) {
 // . write the unit to DB
 // . remove the stale unit from DB
 // . unlock DB
-func (s *statsCtx) periodicFlush() {
+func (s *StatsCtx) periodicFlush() {
-	for {
+	for ptr := s.ongoing(); ptr != nil; ptr = s.ongoing() {
-		ptr := s.ongoing()
+		id := s.unitIDGen()
-		if ptr == nil {
+		// Access the unit's ID with atomic to avoid locking the whole unit.
-			break
+		if !s.isEnabled() || atomic.LoadUint32(&ptr.id) == id {
-		}
-
-		id := s.conf.UnitID()
-		if ptr.id == id || s.conf.limit == 0 {
 			time.Sleep(time.Second)
 
 			continue
 		}
 
-		tx := s.beginTxn(true)
+		tx := beginTxn(s.database(), true)
 
-		nu := unit{}
+		nu := newUnit(id)
-		s.initUnit(&nu, id)
+		u := s.swapCurrent(nu)
-		u := s.swapUnit(&nu)
+		udb := u.serialize()
-		udb := serialize(u)
 
 		if tx == nil {
 			continue
 		}
 
-		ok1 := s.flushUnitToDB(tx, u.id, udb)
+		flushOK := flushUnitToDB(tx, u.id, udb)
-		ok2 := s.deleteUnit(tx, id-s.conf.limit)
+		delOK := s.deleteUnit(tx, id-atomic.LoadUint32(&s.limitHours))
-		if ok1 || ok2 {
+		if flushOK || delOK {
 			s.commitTxn(tx)
 		} else {
 			_ = tx.Rollback()
@@ -330,8 +420,8 @@ func (s *statsCtx) periodicFlush() {
 	log.Tracef("periodicFlush() exited")
 }
 
-// Delete unit's data from file
+// deleteUnit removes the unit by it's id from the database the tx belongs to.
-func (s *statsCtx) deleteUnit(tx *bolt.Tx, id uint32) bool {
+func (s *StatsCtx) deleteUnit(tx *bbolt.Tx, id uint32) bool {
 	err := tx.DeleteBucket(idToUnitName(id))
 	if err != nil {
 		log.Tracef("stats: bolt DeleteBucket: %s", err)
@@ -347,10 +437,7 @@ func (s *statsCtx) deleteUnit(tx *bolt.Tx, id uint32) bool {
 func convertMapToSlice(m map[string]uint64, max int) []countPair {
 	a := []countPair{}
 	for k, v := range m {
-		pair := countPair{}
+		a = append(a, countPair{Name: k, Count: v})
-		pair.Name = k
-		pair.Count = v
-		a = append(a, pair)
 	}
 	less := func(i, j int) bool {
 		return a[j].Count < a[i].Count
@@ -370,41 +457,46 @@ func convertSliceToMap(a []countPair) map[string]uint64 {
 	return m
 }
 
-func serialize(u *unit) *unitDB {
+// serialize converts u to the *unitDB.  It's safe for concurrent use.
-	udb := unitDB{}
+func (u *unit) serialize() (udb *unitDB) {
-	udb.NTotal = u.nTotal
+	u.mu.RLock()
-
+	defer u.mu.RUnlock()
-	udb.NResult = append(udb.NResult, u.nResult...)
 
+	var timeAvg uint32 = 0
 	if u.nTotal != 0 {
-		udb.TimeAvg = uint32(u.timeSum / u.nTotal)
+		timeAvg = uint32(u.timeSum / u.nTotal)
 	}
 
-	udb.Domains = convertMapToSlice(u.domains, maxDomains)
+	return &unitDB{
-	udb.BlockedDomains = convertMapToSlice(u.blockedDomains, maxDomains)
+		NTotal:         u.nTotal,
-	udb.Clients = convertMapToSlice(u.clients, maxClients)
+		NResult:        append([]uint64{}, u.nResult...),
-
+		Domains:        convertMapToSlice(u.domains, maxDomains),
-	return &udb
+		BlockedDomains: convertMapToSlice(u.blockedDomains, maxDomains),
+		Clients:        convertMapToSlice(u.clients, maxClients),
+		TimeAvg:        timeAvg,
+	}
 }
 
-func deserialize(u *unit, udb *unitDB) {
+// deserealize assigns the appropriate values from udb to u.  u must not be nil.
+// It's safe for concurrent use.
+func (u *unit) deserialize(udb *unitDB) {
+	if udb == nil {
+		return
+	}
+
+	u.mu.Lock()
+	defer u.mu.Unlock()
+
 	u.nTotal = udb.NTotal
-
+	u.nResult = make([]uint64, resultLast)
-	n := len(udb.NResult)
+	copy(u.nResult, udb.NResult)
-	if n < len(u.nResult) {
-		n = len(u.nResult) // n = min(len(udb.NResult), len(u.nResult))
-	}
-	for i := 1; i < n; i++ {
-		u.nResult[i] = udb.NResult[i]
-	}
-
 	u.domains = convertSliceToMap(udb.Domains)
 	u.blockedDomains = convertSliceToMap(udb.BlockedDomains)
 	u.clients = convertSliceToMap(udb.Clients)
-	u.timeSum = uint64(udb.TimeAvg) * u.nTotal
+	u.timeSum = uint64(udb.TimeAvg) * udb.NTotal
 }
 
-func (s *statsCtx) flushUnitToDB(tx *bolt.Tx, id uint32, udb *unitDB) bool {
+func flushUnitToDB(tx *bbolt.Tx, id uint32, udb *unitDB) bool {
 	log.Tracef("Flushing unit %d", id)
 
 	bkt, err := tx.CreateBucketIfNotExists(idToUnitName(id))
@@ -430,7 +522,7 @@ func (s *statsCtx) flushUnitToDB(tx *bolt.Tx, id uint32, udb *unitDB) bool {
 	return true
 }
 
-func (s *statsCtx) loadUnitFromDB(tx *bolt.Tx, id uint32) *unitDB {
+func (s *StatsCtx) loadUnitFromDB(tx *bbolt.Tx, id uint32) *unitDB {
 	bkt := tx.Bucket(idToUnitName(id))
 	if bkt == nil {
 		return nil
@@ -451,44 +543,44 @@ func (s *statsCtx) loadUnitFromDB(tx *bolt.Tx, id uint32) *unitDB {
 	return &udb
 }
 
-func convertTopSlice(a []countPair) []map[string]uint64 {
+func convertTopSlice(a []countPair) (m []map[string]uint64) {
-	m := []map[string]uint64{}
+	m = make([]map[string]uint64, 0, len(a))
 	for _, it := range a {
-		ent := map[string]uint64{}
+		m = append(m, map[string]uint64{it.Name: it.Count})
-		ent[it.Name] = it.Count
-		m = append(m, ent)
 	}
+
 	return m
 }
 
-func (s *statsCtx) setLimit(limitDays int) {
+func (s *StatsCtx) setLimit(limitDays int) {
-	s.conf.limit = uint32(limitDays) * 24
+	atomic.StoreUint32(&s.limitHours, uint32(24*limitDays))
 	if limitDays == 0 {
 		s.clear()
 	}
 
-	log.Debug("stats: set limit: %d", limitDays)
+	log.Debug("stats: set limit: %d days", limitDays)
 }
 
-func (s *statsCtx) WriteDiskConfig(dc *DiskConfig) {
+func (s *StatsCtx) WriteDiskConfig(dc *DiskConfig) {
-	dc.Interval = s.conf.limit / 24
+	dc.Interval = atomic.LoadUint32(&s.limitHours) / 24
 }
 
-func (s *statsCtx) Close() {
+func (s *StatsCtx) Close() {
-	u := s.swapUnit(nil)
+	u := s.swapCurrent(nil)
-	udb := serialize(u)
+
-	tx := s.beginTxn(true)
+	db := s.database()
-	if tx != nil {
+	if tx := beginTxn(db, true); tx != nil {
-		if s.flushUnitToDB(tx, u.id, udb) {
+		udb := u.serialize()
+		if flushUnitToDB(tx, u.id, udb) {
 			s.commitTxn(tx)
 		} else {
 			_ = tx.Rollback()
 		}
 	}
 
-	if s.db != nil {
+	if db != nil {
 		log.Tracef("db.Close...")
-		_ = s.db.Close()
+		_ = db.Close()
 		log.Tracef("db.Close")
 	}
 
@@ -496,11 +588,11 @@ func (s *statsCtx) Close() {
 }
 
 // Reset counters and clear database
-func (s *statsCtx) clear() {
+func (s *StatsCtx) clear() {
-	tx := s.beginTxn(true)
+	db := s.database()
+	tx := beginTxn(db, true)
 	if tx != nil {
-		db := s.db
+		_ = s.swapDatabase(nil)
-		s.db = nil
 		_ = tx.Rollback()
 		// the active transactions can continue using database,
 		//  but no new transactions will be opened
@@ -509,11 +601,10 @@ func (s *statsCtx) clear() {
 		// all active transactions are now closed
 	}
 
-	u := unit{}
+	u := newUnit(s.unitIDGen())
-	s.initUnit(&u, s.conf.UnitID())
+	_ = s.swapCurrent(u)
-	_ = s.swapUnit(&u)
 
-	err := os.Remove(s.conf.Filename)
+	err := os.Remove(s.filename)
 	if err != nil {
 		log.Error("os.Remove: %s", err)
 	}
@@ -523,13 +614,13 @@ func (s *statsCtx) clear() {
 	log.Debug("stats: cleared")
 }
 
-func (s *statsCtx) Update(e Entry) {
+func (s *StatsCtx) Update(e Entry) {
-	if s.conf.limit == 0 {
+	if !s.isEnabled() {
 		return
 	}
 
 	if e.Result == 0 ||
-		e.Result >= rLast ||
+		e.Result >= resultLast ||
 		e.Domain == "" ||
 		e.Client == "" {
 		return
@@ -540,13 +631,15 @@ func (s *statsCtx) Update(e Entry) {
 		clientID = ip.String()
 	}
 
-	s.mu.Lock()
+	u := s.ongoing()
-	defer s.mu.Unlock()
+	if u == nil {
+		return
+	}
 
-	u := s.current
+	u.mu.Lock()
+	defer u.mu.Unlock()
 
 	u.nResult[e.Result]++
-
 	if e.Result == RNotFiltered {
 		u.domains[e.Domain]++
 	} else {
@@ -558,14 +651,19 @@ func (s *statsCtx) Update(e Entry) {
 	u.nTotal++
 }
 
-func (s *statsCtx) loadUnits(limit uint32) ([]*unitDB, uint32) {
+func (s *StatsCtx) loadUnits(limit uint32) ([]*unitDB, uint32) {
-	tx := s.beginTxn(false)
+	tx := beginTxn(s.database(), false)
 	if tx == nil {
 		return nil, 0
 	}
 
 	cur := s.ongoing()
-	curID := cur.id
+	var curID uint32
+	if cur != nil {
+		curID = atomic.LoadUint32(&cur.id)
+	} else {
+		curID = s.unitIDGen()
+	}
 
 	// Per-hour units.
 	units := []*unitDB{}
@@ -574,14 +672,16 @@ func (s *statsCtx) loadUnits(limit uint32) ([]*unitDB, uint32) {
 		u := s.loadUnitFromDB(tx, i)
 		if u == nil {
 			u = &unitDB{}
-			u.NResult = make([]uint64, rLast)
+			u.NResult = make([]uint64, resultLast)
 		}
 		units = append(units, u)
 	}
 
 	_ = tx.Rollback()
 
-	units = append(units, serialize(cur))
+	if cur != nil {
+		units = append(units, cur.serialize())
+	}
 
 	if len(units) != int(limit) {
 		log.Fatalf("len(units) != limit: %d %d", len(units), limit)
@@ -628,13 +728,13 @@ func statsCollector(units []*unitDB, firstID uint32, timeUnit TimeUnit, ng numsG
 // pairsGetter is a signature for topsCollector argument.
 type pairsGetter func(u *unitDB) (pairs []countPair)
 
-// topsCollector collects statistics about highest values fro the given *unitDB
+// topsCollector collects statistics about highest values from the given *unitDB
 // slice using pg to retrieve data.
 func topsCollector(units []*unitDB, max int, pg pairsGetter) []map[string]uint64 {
 	m := map[string]uint64{}
 	for _, u := range units {
-		for _, it := range pg(u) {
+		for _, cp := range pg(u) {
-			m[it.Name] += it.Count
+			m[cp.Name] += cp.Count
 		}
 	}
 	a2 := convertMapToSlice(m, max)
@@ -668,8 +768,22 @@ func topsCollector(units []*unitDB, max int, pg pairsGetter) []map[string]uint64
   * parental-blocked
   These values are just the sum of data for all units.
 */
-func (s *statsCtx) getData() (statsResponse, bool) {
+func (s *StatsCtx) getData() (statsResponse, bool) {
-	limit := s.conf.limit
+	limit := atomic.LoadUint32(&s.limitHours)
+	if limit == 0 {
+		return statsResponse{
+			TimeUnits: "days",
+
+			TopBlocked: []topAddrs{},
+			TopClients: []topAddrs{},
+			TopQueried: []topAddrs{},
+
+			BlockedFiltering:     []uint64{},
+			DNSQueries:           []uint64{},
+			ReplacedParental:     []uint64{},
+			ReplacedSafebrowsing: []uint64{},
+		}, true
+	}
 
 	timeUnit := Hours
 	if limit/24 > 7 {
@@ -698,7 +812,7 @@ func (s *statsCtx) getData() (statsResponse, bool) {
 
 	// Total counters:
 	sum := unitDB{
-		NResult: make([]uint64, rLast),
+		NResult: make([]uint64, resultLast),
 	}
 	timeN := 0
 	for _, u := range units {
@@ -731,12 +845,12 @@ func (s *statsCtx) getData() (statsResponse, bool) {
 	return data, true
 }
 
-func (s *statsCtx) GetTopClientsIP(maxCount uint) []net.IP {
+func (s *StatsCtx) GetTopClientsIP(maxCount uint) []net.IP {
-	if s.conf.limit == 0 {
+	if !s.isEnabled() {
 		return nil
 	}
 
-	units, _ := s.loadUnits(s.conf.limit)
+	units, _ := s.loadUnits(atomic.LoadUint32(&s.limitHours))
 	if units == nil {
 		return nil
 	}