package main

import (
	"context"
	"encoding/hex"
	"encoding/xml"
	"errors"
	"fmt"
	"io"
	"net"
	"net/http"
	"os"
	"os/signal"
	"runtime/debug"
	"sync"
	"syscall"
	"time"

	v2container "git.frostfs.info/TrueCloudLab/frostfs-api-go/v2/container"
	"git.frostfs.info/TrueCloudLab/frostfs-observability/tracing"
	grpctracing "git.frostfs.info/TrueCloudLab/frostfs-observability/tracing/grpc"
	"git.frostfs.info/TrueCloudLab/frostfs-s3-gw/api"
	"git.frostfs.info/TrueCloudLab/frostfs-s3-gw/api/auth"
	"git.frostfs.info/TrueCloudLab/frostfs-s3-gw/api/cache"
	"git.frostfs.info/TrueCloudLab/frostfs-s3-gw/api/handler"
	"git.frostfs.info/TrueCloudLab/frostfs-s3-gw/api/layer"
	s3middleware "git.frostfs.info/TrueCloudLab/frostfs-s3-gw/api/middleware"
	"git.frostfs.info/TrueCloudLab/frostfs-s3-gw/api/notifications"
	"git.frostfs.info/TrueCloudLab/frostfs-s3-gw/api/resolver"
	"git.frostfs.info/TrueCloudLab/frostfs-s3-gw/creds/tokens"
	"git.frostfs.info/TrueCloudLab/frostfs-s3-gw/internal/frostfs"
	"git.frostfs.info/TrueCloudLab/frostfs-s3-gw/internal/frostfs/frostfsid"
	ffidcontract "git.frostfs.info/TrueCloudLab/frostfs-s3-gw/internal/frostfs/frostfsid/contract"
	"git.frostfs.info/TrueCloudLab/frostfs-s3-gw/internal/frostfs/policy"
	"git.frostfs.info/TrueCloudLab/frostfs-s3-gw/internal/frostfs/policy/contract"
	"git.frostfs.info/TrueCloudLab/frostfs-s3-gw/internal/frostfs/services"
	"git.frostfs.info/TrueCloudLab/frostfs-s3-gw/internal/logs"
	"git.frostfs.info/TrueCloudLab/frostfs-s3-gw/internal/version"
	"git.frostfs.info/TrueCloudLab/frostfs-s3-gw/internal/wallet"
	"git.frostfs.info/TrueCloudLab/frostfs-s3-gw/metrics"
	"git.frostfs.info/TrueCloudLab/frostfs-s3-gw/pkg/service/control"
	controlSvc "git.frostfs.info/TrueCloudLab/frostfs-s3-gw/pkg/service/control/server"
	"git.frostfs.info/TrueCloudLab/frostfs-s3-gw/pkg/service/tree"
	"git.frostfs.info/TrueCloudLab/frostfs-sdk-go/netmap"
	"git.frostfs.info/TrueCloudLab/frostfs-sdk-go/pool"
	treepool "git.frostfs.info/TrueCloudLab/frostfs-sdk-go/pool/tree"
	"git.frostfs.info/TrueCloudLab/frostfs-sdk-go/user"
	"github.com/go-chi/chi/v5/middleware"
	"github.com/nspcc-dev/neo-go/pkg/crypto/keys"
	"github.com/spf13/viper"
	"go.uber.org/zap"
	"golang.org/x/exp/slices"
	"golang.org/x/text/encoding/ianaindex"
	"google.golang.org/grpc"
)

const awsDefaultNamespace = "http://s3.amazonaws.com/doc/2006-03-01/"

type (
	// App is the main application structure.
	App struct {
		ctr      s3middleware.Center
		log      *zap.Logger
		cfg      *viper.Viper
		pool     *pool.Pool
		treePool *treepool.Pool
		key      *keys.PrivateKey
		nc       *notifications.Controller
		obj      layer.Client
		api      api.Handler

		frostfsid *frostfsid.FrostFSID

		policyStorage *policy.Storage

		servers       []Server
		unbindServers []ServerInfo
		mu            sync.RWMutex

		controlAPI *grpc.Server

		metrics        *metrics.AppMetrics
		bucketResolver *resolver.BucketResolver
		services       []*Service
		settings       *appSettings

		webDone chan struct{}
		wrkDone chan struct{}
	}

	appSettings struct {
		logLevel            zap.AtomicLevel
		maxClient           maxClientsConfig
		defaultMaxAge       int
		reconnectInterval   time.Duration
		notificatorEnabled  bool
		resolveZoneList     []string
		isResolveListAllow  bool // True if ResolveZoneList contains allowed zones
		frostfsidValidation bool

		mu                            sync.RWMutex
		namespaces                    Namespaces
		defaultXMLNS                  bool
		bypassContentEncodingInChunks bool
		clientCut                     bool
		maxBufferSizeForPut           uint64
		md5Enabled                    bool
		aclEnabled                    bool
		namespaceHeader               string
		defaultNamespaces             []string
		authorizedControlAPIKeys      [][]byte
		policyDenyByDefault           bool
		sourceIPHeader                string
	}

	maxClientsConfig struct {
		deadline time.Duration
		count    int
	}

	Logger struct {
		logger *zap.Logger
		lvl    zap.AtomicLevel
	}
)

func newApp(ctx context.Context, log *Logger, v *viper.Viper) *App {
	objPool, treePool, key := getPools(ctx, log.logger, v)

	cfg := tokens.Config{
		FrostFS:                     frostfs.NewAuthmateFrostFS(frostfs.NewFrostFS(objPool, key)),
		Key:                         key,
		CacheConfig:                 getAccessBoxCacheConfig(v, log.logger),
		RemovingCheckAfterDurations: fetchRemovingCheckInterval(v, log.logger),
	}

	// prepare auth center
	ctr := auth.New(tokens.New(cfg), v.GetStringSlice(cfgAllowedAccessKeyIDPrefixes))

	app := &App{
		ctr:      ctr,
		log:      log.logger,
		cfg:      v,
		pool:     objPool,
		treePool: treePool,
		key:      key,

		webDone: make(chan struct{}, 1),
		wrkDone: make(chan struct{}, 1),

		settings: newAppSettings(log, v, key),
	}

	app.init(ctx)

	return app
}

func (a *App) init(ctx context.Context) {
	a.setRuntimeParameters()
	a.initFrostfsID(ctx)
	a.initPolicyStorage(ctx)
	a.initAPI(ctx)
	a.initMetrics()
	a.initControlAPI()
	a.initServers(ctx)
	a.initTracing(ctx)
}

func (a *App) initLayer(ctx context.Context) {
	a.initResolver()

	// prepare random key for anonymous requests
	randomKey, err := keys.NewPrivateKey()
	if err != nil {
		a.log.Fatal(logs.CouldntGenerateRandomKey, zap.Error(err))
	}

	var gateOwner user.ID
	user.IDFromKey(&gateOwner, a.key.PrivateKey.PublicKey)

	layerCfg := &layer.Config{
		Cache: layer.NewCache(getCacheOptions(a.cfg, a.log)),
		AnonKey: layer.AnonymousKey{
			Key: randomKey,
		},
		GateOwner:   gateOwner,
		Resolver:    a.bucketResolver,
		TreeService: tree.NewTree(services.NewPoolWrapper(a.treePool), a.log),
		Features:    a.settings,
	}

	// prepare object layer
	a.obj = layer.NewLayer(a.log, frostfs.NewFrostFS(a.pool, a.key), layerCfg)

	if a.cfg.GetBool(cfgEnableNATS) {
		nopts := getNotificationsOptions(a.cfg, a.log)
		a.nc, err = notifications.NewController(nopts, a.log)
		if err != nil {
			a.log.Fatal(logs.FailedToEnableNotifications, zap.Error(err))
		}

		if err = a.obj.Initialize(ctx, a.nc); err != nil {
			a.log.Fatal(logs.CouldntInitializeLayer, zap.Error(err))
		}
	}
}

func newAppSettings(log *Logger, v *viper.Viper, key *keys.PrivateKey) *appSettings {
	settings := &appSettings{
		logLevel:            log.lvl,
		maxClient:           newMaxClients(v),
		defaultMaxAge:       fetchDefaultMaxAge(v, log.logger),
		reconnectInterval:   fetchReconnectInterval(v),
		notificatorEnabled:  v.GetBool(cfgEnableNATS),
		frostfsidValidation: v.GetBool(cfgFrostfsIDValidationEnabled),
	}

	settings.resolveZoneList = v.GetStringSlice(cfgResolveBucketAllow)
	settings.isResolveListAllow = len(settings.resolveZoneList) > 0
	if !settings.isResolveListAllow {
		settings.resolveZoneList = v.GetStringSlice(cfgResolveBucketDeny)
	}

	settings.update(v, log.logger, key)

	return settings
}

func (s *appSettings) update(v *viper.Viper, log *zap.Logger, key *keys.PrivateKey) {
	s.updateNamespacesSettings(v, log)
	s.useDefaultXMLNamespace(v.GetBool(cfgKludgeUseDefaultXMLNS))
	s.setACLEnabled(v.GetBool(cfgKludgeACLEnabled))
	s.setBypassContentEncodingInChunks(v.GetBool(cfgKludgeBypassContentEncodingCheckInChunks))
	s.setClientCut(v.GetBool(cfgClientCut))
	s.setBufferMaxSizeForPut(v.GetUint64(cfgBufferMaxSizeForPut))
	s.setMD5Enabled(v.GetBool(cfgMD5Enabled))
	s.setAuthorizedControlAPIKeys(append(fetchAuthorizedKeys(log, v), key.PublicKey()))
	s.setPolicyDenyByDefault(v.GetBool(cfgPolicyDenyByDefault))
	s.setSourceIPHeader(v.GetString(cfgSourceIPHeader))
}

func (s *appSettings) updateNamespacesSettings(v *viper.Viper, log *zap.Logger) {
	nsHeader := v.GetString(cfgResolveNamespaceHeader)
	nsConfig, defaultNamespaces := fetchNamespacesConfig(log, v)

	s.mu.Lock()
	defer s.mu.Unlock()

	s.namespaceHeader = nsHeader
	s.defaultNamespaces = defaultNamespaces
	s.namespaces = nsConfig.Namespaces
}

func (s *appSettings) BypassContentEncodingInChunks() bool {
	s.mu.RLock()
	defer s.mu.RUnlock()
	return s.bypassContentEncodingInChunks
}

func (s *appSettings) setBypassContentEncodingInChunks(bypass bool) {
	s.mu.Lock()
	s.bypassContentEncodingInChunks = bypass
	s.mu.Unlock()
}

func (s *appSettings) ClientCut() bool {
	s.mu.RLock()
	defer s.mu.RUnlock()
	return s.clientCut
}

func (s *appSettings) setClientCut(clientCut bool) {
	s.mu.Lock()
	s.clientCut = clientCut
	s.mu.Unlock()
}

func (s *appSettings) BufferMaxSizeForPut() uint64 {
	s.mu.RLock()
	defer s.mu.RUnlock()
	return s.maxBufferSizeForPut
}

func (s *appSettings) setBufferMaxSizeForPut(size uint64) {
	s.mu.Lock()
	s.maxBufferSizeForPut = size
	s.mu.Unlock()
}

func (s *appSettings) DefaultPlacementPolicy(namespace string) netmap.PlacementPolicy {
	s.mu.RLock()
	defer s.mu.RUnlock()
	return s.namespaces[namespace].LocationConstraints[defaultConstraintName]
}

func (s *appSettings) PlacementPolicy(namespace, constraint string) (netmap.PlacementPolicy, bool) {
	s.mu.RLock()
	placementPolicy, ok := s.namespaces[namespace].LocationConstraints[constraint]
	s.mu.RUnlock()

	return placementPolicy, ok
}

func (s *appSettings) CopiesNumbers(namespace, constraint string) ([]uint32, bool) {
	s.mu.RLock()
	copiesNumbers, ok := s.namespaces[namespace].CopiesNumbers[constraint]
	s.mu.RUnlock()

	return copiesNumbers, ok
}

func (s *appSettings) DefaultCopiesNumbers(namespace string) []uint32 {
	s.mu.RLock()
	defer s.mu.RUnlock()
	return s.namespaces[namespace].CopiesNumbers[defaultConstraintName]
}

func (s *appSettings) NewXMLDecoder(r io.Reader) *xml.Decoder {
	dec := xml.NewDecoder(r)
	dec.CharsetReader = func(charset string, reader io.Reader) (io.Reader, error) {
		enc, err := ianaindex.IANA.Encoding(charset)
		if err != nil {
			return nil, fmt.Errorf("charset %s: %w", charset, err)
		}
		return enc.NewDecoder().Reader(reader), nil
	}

	s.mu.RLock()
	if s.defaultXMLNS {
		dec.DefaultSpace = awsDefaultNamespace
	}
	s.mu.RUnlock()

	return dec
}

func (s *appSettings) useDefaultXMLNamespace(useDefaultNamespace bool) {
	s.mu.Lock()
	s.defaultXMLNS = useDefaultNamespace
	s.mu.Unlock()
}

func (s *appSettings) DefaultMaxAge() int {
	return s.defaultMaxAge
}

func (s *appSettings) NotificatorEnabled() bool {
	return s.notificatorEnabled
}

func (s *appSettings) ResolveZoneList() []string {
	return s.resolveZoneList
}

func (s *appSettings) IsResolveListAllow() bool {
	return s.isResolveListAllow
}

func (s *appSettings) MD5Enabled() bool {
	s.mu.RLock()
	defer s.mu.RUnlock()
	return s.md5Enabled
}

func (s *appSettings) setMD5Enabled(md5Enabled bool) {
	s.mu.Lock()
	s.md5Enabled = md5Enabled
	s.mu.Unlock()
}

func (s *appSettings) setACLEnabled(enableACL bool) {
	s.mu.Lock()
	s.aclEnabled = enableACL
	s.mu.Unlock()
}

func (s *appSettings) ACLEnabled() bool {
	s.mu.RLock()
	defer s.mu.RUnlock()
	return s.aclEnabled
}

func (s *appSettings) NamespaceHeader() string {
	s.mu.RLock()
	defer s.mu.RUnlock()
	return s.namespaceHeader
}

func (s *appSettings) FormContainerZone(ns string) (zone string, isDefault bool) {
	if len(ns) == 0 {
		return v2container.SysAttributeZoneDefault, true
	}

	return ns + ".ns", false
}

func (s *appSettings) isDefaultNamespace(ns string) bool {
	s.mu.RLock()
	namespaces := s.defaultNamespaces
	s.mu.RUnlock()
	return slices.Contains(namespaces, ns)
}

func (s *appSettings) FetchRawKeys() [][]byte {
	s.mu.RLock()
	defer s.mu.RUnlock()
	return s.authorizedControlAPIKeys
}

func (s *appSettings) setAuthorizedControlAPIKeys(keys keys.PublicKeys) {
	rawPubs := make([][]byte, len(keys))
	for i := range keys {
		rawPubs[i] = keys[i].Bytes()
	}

	s.mu.Lock()
	s.authorizedControlAPIKeys = rawPubs
	s.mu.Unlock()
}

func (s *appSettings) ResolveNamespaceAlias(namespace string) string {
	if s.isDefaultNamespace(namespace) {
		return defaultNamespace
	}

	return namespace
}

func (s *appSettings) PolicyDenyByDefault() bool {
	s.mu.RLock()
	defer s.mu.RUnlock()
	return s.policyDenyByDefault
}

func (s *appSettings) setPolicyDenyByDefault(policyDenyByDefault bool) {
	s.mu.Lock()
	s.policyDenyByDefault = policyDenyByDefault
	s.mu.Unlock()
}

func (s *appSettings) setSourceIPHeader(header string) {
	s.mu.Lock()
	s.sourceIPHeader = header
	s.mu.Unlock()
}

func (s *appSettings) SourceIPHeader() string {
	s.mu.RLock()
	defer s.mu.RUnlock()
	return s.sourceIPHeader
}

func (a *App) initAPI(ctx context.Context) {
	a.initLayer(ctx)
	a.initHandler()
}

func (a *App) initControlAPI() {
	svc := controlSvc.New(
		controlSvc.WithSettings(a.settings),
		controlSvc.WithLogger(a.log),
		controlSvc.WithChainStorage(a.policyStorage.LocalStorage()),
	)

	a.controlAPI = grpc.NewServer()

	control.RegisterControlServiceServer(a.controlAPI, svc)
}

func (a *App) initMetrics() {
	cfg := metrics.AppMetricsConfig{
		Logger:         a.log,
		PoolStatistics: frostfs.NewPoolStatistic(a.pool),
		Enabled:        a.cfg.GetBool(cfgPrometheusEnabled),
	}

	a.metrics = metrics.NewAppMetrics(cfg)
	a.metrics.State().SetHealth(metrics.HealthStatusStarting)
}

func (a *App) initFrostfsID(ctx context.Context) {
	cli, err := ffidcontract.New(ctx, ffidcontract.Config{
		RPCAddress:    a.cfg.GetString(cfgRPCEndpoint),
		Contract:      a.cfg.GetString(cfgFrostfsIDContract),
		ProxyContract: a.cfg.GetString(cfgProxyContract),
		Key:           a.key,
	})
	if err != nil {
		a.log.Fatal(logs.InitFrostfsIDContractFailed, zap.Error(err))
	}

	a.frostfsid, err = frostfsid.NewFrostFSID(frostfsid.Config{
		Cache:     cache.NewFrostfsIDCache(getFrostfsIDCacheConfig(a.cfg, a.log)),
		FrostFSID: cli,
		Logger:    a.log,
	})
	if err != nil {
		a.log.Fatal(logs.InitFrostfsIDContractFailed, zap.Error(err))
	}
}

func (a *App) initPolicyStorage(ctx context.Context) {
	policyContract, err := contract.New(ctx, contract.Config{
		RPCAddress:    a.cfg.GetString(cfgRPCEndpoint),
		Contract:      a.cfg.GetString(cfgPolicyContract),
		ProxyContract: a.cfg.GetString(cfgProxyContract),
		Key:           a.key,
	})
	if err != nil {
		a.log.Fatal(logs.InitPolicyContractFailed, zap.Error(err))
	}

	a.policyStorage = policy.NewStorage(policy.StorageConfig{
		Contract: policyContract,
		Cache:    cache.NewMorphPolicyCache(getMorphPolicyCacheConfig(a.cfg, a.log)),
		Log:      a.log,
	})
}

func (a *App) initResolver() {
	var err error
	a.bucketResolver, err = resolver.NewBucketResolver(a.getResolverOrder(), a.getResolverConfig())
	if err != nil {
		a.log.Fatal(logs.FailedToCreateResolver, zap.Error(err))
	}
}

func (a *App) getResolverConfig() *resolver.Config {
	return &resolver.Config{
		FrostFS:    frostfs.NewResolverFrostFS(a.pool),
		RPCAddress: a.cfg.GetString(cfgRPCEndpoint),
		Settings:   a.settings,
	}
}

func (a *App) getResolverOrder() []string {
	order := a.cfg.GetStringSlice(cfgResolveOrder)
	if a.cfg.GetString(cfgRPCEndpoint) == "" {
		order = remove(order, resolver.NNSResolver)
		a.log.Warn(logs.ResolverNNSWontBeUsedSinceRPCEndpointIsntProvided)
	}

	if len(order) == 0 {
		a.log.Info(logs.ContainerResolverWillBeDisabled)
	}

	return order
}

func (a *App) initTracing(ctx context.Context) {
	instanceID := ""
	if len(a.servers) > 0 {
		instanceID = a.servers[0].Address()
	}
	cfg := tracing.Config{
		Enabled:    a.cfg.GetBool(cfgTracingEnabled),
		Exporter:   tracing.Exporter(a.cfg.GetString(cfgTracingExporter)),
		Endpoint:   a.cfg.GetString(cfgTracingEndpoint),
		Service:    "frostfs-s3-gw",
		InstanceID: instanceID,
		Version:    version.Version,
	}
	updated, err := tracing.Setup(ctx, cfg)
	if err != nil {
		a.log.Warn(logs.FailedToInitializeTracing, zap.Error(err))
	}
	if updated {
		a.log.Info(logs.TracingConfigUpdated)
	}
}

func (a *App) shutdownTracing() {
	const tracingShutdownTimeout = 5 * time.Second
	shdnCtx, cancel := context.WithTimeout(context.Background(), tracingShutdownTimeout)
	defer cancel()

	if err := tracing.Shutdown(shdnCtx); err != nil {
		a.log.Warn(logs.FailedToShutdownTracing, zap.Error(err))
	}
}

func newMaxClients(cfg *viper.Viper) maxClientsConfig {
	config := maxClientsConfig{}

	config.count = fetchMaxClientsCount(cfg)

	config.deadline = fetchMaxClientsDeadline(cfg)

	return config
}

func getPools(ctx context.Context, logger *zap.Logger, cfg *viper.Viper) (*pool.Pool, *treepool.Pool, *keys.PrivateKey) {
	var prm pool.InitParameters
	var prmTree treepool.InitParameters

	password := wallet.GetPassword(cfg, cfgWalletPassphrase)
	key, err := wallet.GetKeyFromPath(cfg.GetString(cfgWalletPath), cfg.GetString(cfgWalletAddress), password)
	if err != nil {
		logger.Fatal(logs.CouldNotLoadFrostFSPrivateKey, zap.Error(err))
	}

	prm.SetKey(&key.PrivateKey)
	prmTree.SetKey(key)
	logger.Info(logs.UsingCredentials, zap.String("FrostFS", hex.EncodeToString(key.PublicKey().Bytes())))

	for _, peer := range fetchPeers(logger, cfg) {
		prm.AddNode(peer)
		prmTree.AddNode(peer)
	}

	connTimeout := fetchConnectTimeout(cfg)
	prm.SetNodeDialTimeout(connTimeout)
	prmTree.SetNodeDialTimeout(connTimeout)

	streamTimeout := fetchStreamTimeout(cfg)
	prm.SetNodeStreamTimeout(streamTimeout)
	prmTree.SetNodeStreamTimeout(streamTimeout)

	healthCheckTimeout := fetchHealthCheckTimeout(cfg)
	prm.SetHealthcheckTimeout(healthCheckTimeout)
	prmTree.SetHealthcheckTimeout(healthCheckTimeout)

	rebalanceInterval := fetchRebalanceInterval(cfg)
	prm.SetClientRebalanceInterval(rebalanceInterval)
	prmTree.SetClientRebalanceInterval(rebalanceInterval)

	errorThreshold := fetchErrorThreshold(cfg)
	prm.SetErrorThreshold(errorThreshold)
	prm.SetLogger(logger)
	prmTree.SetLogger(logger)

	prmTree.SetMaxRequestAttempts(cfg.GetInt(cfgTreePoolMaxAttempts))

	var apiGRPCDialOpts []grpc.DialOption
	var treeGRPCDialOpts []grpc.DialOption
	if cfg.GetBool(cfgTracingEnabled) {
		interceptors := []grpc.DialOption{
			grpc.WithUnaryInterceptor(grpctracing.NewUnaryClientInteceptor()),
			grpc.WithStreamInterceptor(grpctracing.NewStreamClientInterceptor()),
		}
		treeGRPCDialOpts = append(treeGRPCDialOpts, interceptors...)
		apiGRPCDialOpts = append(apiGRPCDialOpts, interceptors...)
	}
	prm.SetGRPCDialOptions(apiGRPCDialOpts...)
	prmTree.SetGRPCDialOptions(treeGRPCDialOpts...)

	p, err := pool.NewPool(prm)
	if err != nil {
		logger.Fatal(logs.FailedToCreateConnectionPool, zap.Error(err))
	}

	if err = p.Dial(ctx); err != nil {
		logger.Fatal(logs.FailedToDialConnectionPool, zap.Error(err))
	}

	treePool, err := treepool.NewPool(prmTree)
	if err != nil {
		logger.Fatal(logs.FailedToCreateTreePool, zap.Error(err))
	}
	if err = treePool.Dial(ctx); err != nil {
		logger.Fatal(logs.FailedToDialTreePool, zap.Error(err))
	}

	return p, treePool, key
}

func remove(list []string, element string) []string {
	for i, item := range list {
		if item == element {
			return append(list[:i], list[i+1:]...)
		}
	}
	return list
}

// Wait waits for an application to finish.
//
// Pre-logs a message about the launch of the application mentioning its
// version (version.Version) and its name (frostfs-s3-gw). At the end, it writes
// about the stop to the log.
func (a *App) Wait() {
	a.log.Info(logs.ApplicationStarted,
		zap.String("name", "frostfs-s3-gw"),
		zap.String("version", version.Version),
	)

	a.metrics.State().SetVersion(version.Version)
	a.setHealthStatus()

	<-a.webDone // wait for web-server to be stopped

	a.log.Info(logs.ApplicationFinished)
}

func (a *App) setHealthStatus() {
	a.metrics.State().SetHealth(metrics.HealthStatusReady)
}

// Serve runs HTTP server to handle S3 API requests.
func (a *App) Serve(ctx context.Context) {
	// Attach S3 API:
	domains := a.cfg.GetStringSlice(cfgListenDomains)
	a.log.Info(logs.FetchDomainsPrepareToUseAPI, zap.Strings("domains", domains))

	cfg := api.Config{
		Throttle: middleware.ThrottleOpts{
			Limit:          a.settings.maxClient.count,
			BacklogTimeout: a.settings.maxClient.deadline,
		},
		Handler: a.api,
		Center:  a.ctr,
		Log:     a.log,
		Metrics: a.metrics,
		Domains: domains,

		MiddlewareSettings: a.settings,
		PolicyChecker:      a.policyStorage,

		FrostfsID:           a.frostfsid,
		FrostFSIDValidation: a.settings.frostfsidValidation,

		XMLDecoder: a.settings,
		Tagging:    a.obj,
	}

	chiRouter := api.NewRouter(cfg)

	// Use mux.Router as http.Handler
	srv := new(http.Server)
	srv.Handler = chiRouter
	srv.ErrorLog = zap.NewStdLog(a.log)
	srv.ReadTimeout = a.cfg.GetDuration(cfgWebReadTimeout)
	srv.ReadHeaderTimeout = a.cfg.GetDuration(cfgWebReadHeaderTimeout)
	srv.WriteTimeout = a.cfg.GetDuration(cfgWebWriteTimeout)
	srv.IdleTimeout = a.cfg.GetDuration(cfgWebIdleTimeout)

	a.startServices()

	servs := a.getServers()

	for i := range servs {
		go func(i int) {
			a.log.Info(logs.StartingServer, zap.String("address", servs[i].Address()))

			if err := srv.Serve(servs[i].Listener()); err != nil && err != http.ErrServerClosed {
				a.metrics.MarkUnhealthy(servs[i].Address())
				a.log.Fatal(logs.ListenAndServe, zap.Error(err))
			}
		}(i)
	}

	if len(a.unbindServers) != 0 {
		a.scheduleReconnect(ctx, srv)
	}

	go func() {
		address := a.cfg.GetString(cfgControlGRPCEndpoint)
		a.log.Info(logs.StartingControlAPI, zap.String("address", address))
		if listener, err := net.Listen("tcp", address); err != nil {
			a.log.Fatal(logs.ListenAndServe, zap.Error(err))
		} else if err = a.controlAPI.Serve(listener); err != nil {
			a.log.Fatal(logs.ListenAndServe, zap.Error(err))
		}
	}()

	sigs := make(chan os.Signal, 1)
	signal.Notify(sigs, syscall.SIGHUP)

LOOP:
	for {
		select {
		case <-ctx.Done():
			break LOOP
		case <-sigs:
			a.configReload(ctx)
		}
	}

	ctx, cancel := shutdownContext()
	defer cancel()

	a.log.Info(logs.StoppingServer, zap.Error(srv.Shutdown(ctx)))

	a.stopControlAPI()
	a.metrics.Shutdown()
	a.stopServices()
	a.shutdownTracing()

	close(a.webDone)
}

func shutdownContext() (context.Context, context.CancelFunc) {
	return context.WithTimeout(context.Background(), defaultShutdownTimeout)
}

func (a *App) stopControlAPI() {
	ctx, cancel := shutdownContext()
	defer cancel()

	go func() {
		a.controlAPI.GracefulStop()
		cancel()
	}()

	<-ctx.Done()

	if errors.Is(ctx.Err(), context.DeadlineExceeded) {
		a.log.Info(logs.ControlAPICannotShutdownGracefully)
		a.controlAPI.Stop()
	}

	a.log.Info(logs.ControlAPIServiceStopped)
}

func (a *App) configReload(ctx context.Context) {
	a.log.Info(logs.SIGHUPConfigReloadStarted)

	if !a.cfg.IsSet(cmdConfig) && !a.cfg.IsSet(cmdConfigDir) {
		a.log.Warn(logs.FailedToReloadConfigBecauseItsMissed)
		return
	}
	if err := readInConfig(a.cfg); err != nil {
		a.log.Warn(logs.FailedToReloadConfig, zap.Error(err))
		return
	}

	if err := a.bucketResolver.UpdateResolvers(a.getResolverOrder()); err != nil {
		a.log.Warn(logs.FailedToReloadResolvers, zap.Error(err))
	}

	if err := a.updateServers(); err != nil {
		a.log.Warn(logs.FailedToReloadServerParameters, zap.Error(err))
	}

	a.setRuntimeParameters()

	a.stopServices()
	a.startServices()

	a.updateSettings()

	a.metrics.SetEnabled(a.cfg.GetBool(cfgPrometheusEnabled))
	a.initTracing(ctx)
	a.setHealthStatus()

	a.log.Info(logs.SIGHUPConfigReloadCompleted)
}

func (a *App) updateSettings() {
	if lvl, err := getLogLevel(a.cfg); err != nil {
		a.log.Warn(logs.LogLevelWontBeUpdated, zap.Error(err))
	} else {
		a.settings.logLevel.SetLevel(lvl)
	}

	a.settings.update(a.cfg, a.log, a.key)
}

func (a *App) startServices() {
	a.services = a.services[:0]

	pprofService := NewPprofService(a.cfg, a.log)
	a.services = append(a.services, pprofService)
	go pprofService.Start()

	prometheusService := NewPrometheusService(a.cfg, a.log, a.metrics.Handler())
	a.services = append(a.services, prometheusService)
	go prometheusService.Start()
}

func (a *App) initServers(ctx context.Context) {
	serversInfo := fetchServers(a.cfg, a.log)

	a.servers = make([]Server, 0, len(serversInfo))
	for _, serverInfo := range serversInfo {
		fields := []zap.Field{
			zap.String("address", serverInfo.Address), zap.Bool("tls enabled", serverInfo.TLS.Enabled),
			zap.String("tls cert", serverInfo.TLS.CertFile), zap.String("tls key", serverInfo.TLS.KeyFile),
		}
		srv, err := newServer(ctx, serverInfo)
		if err != nil {
			a.unbindServers = append(a.unbindServers, serverInfo)
			a.metrics.MarkUnhealthy(serverInfo.Address)
			a.log.Warn(logs.FailedToAddServer, append(fields, zap.Error(err))...)
			continue
		}
		a.metrics.MarkHealthy(serverInfo.Address)

		a.servers = append(a.servers, srv)
		a.log.Info(logs.AddServer, fields...)
	}

	if len(a.servers) == 0 {
		a.log.Fatal(logs.NoHealthyServers)
	}
}

func (a *App) updateServers() error {
	serversInfo := fetchServers(a.cfg, a.log)

	a.mu.Lock()
	defer a.mu.Unlock()

	var found bool
	for _, serverInfo := range serversInfo {
		ser := a.getServer(serverInfo.Address)
		if ser != nil {
			if serverInfo.TLS.Enabled {
				if err := ser.UpdateCert(serverInfo.TLS.CertFile, serverInfo.TLS.KeyFile); err != nil {
					return fmt.Errorf("failed to update tls certs: %w", err)
				}
				found = true
			}
		} else if unbind := a.updateUnbindServerInfo(serverInfo); unbind {
			found = true
		}
	}

	if !found {
		return fmt.Errorf("invalid servers configuration: no known server found")
	}

	return nil
}

func (a *App) stopServices() {
	ctx, cancel := shutdownContext()
	defer cancel()

	for _, svc := range a.services {
		svc.ShutDown(ctx)
	}
}

func getNotificationsOptions(v *viper.Viper, l *zap.Logger) *notifications.Options {
	cfg := notifications.Options{}
	cfg.URL = v.GetString(cfgNATSEndpoint)
	cfg.Timeout = fetchNATSTimeout(v, l)
	cfg.TLSCertFilepath = v.GetString(cfgNATSTLSCertFile)
	cfg.TLSAuthPrivateKeyFilePath = v.GetString(cfgNATSAuthPrivateKeyFile)
	cfg.RootCAFiles = v.GetStringSlice(cfgNATSRootCAFiles)

	return &cfg
}

func getCacheOptions(v *viper.Viper, l *zap.Logger) *layer.CachesConfig {
	cacheCfg := layer.DefaultCachesConfigs(l)

	cacheCfg.Objects.Lifetime = fetchCacheLifetime(v, l, cfgObjectsCacheLifetime, cacheCfg.Objects.Lifetime)
	cacheCfg.Objects.Size = fetchCacheSize(v, l, cfgObjectsCacheSize, cacheCfg.Objects.Size)

	cacheCfg.ObjectsList.Lifetime = fetchCacheLifetime(v, l, cfgListObjectsCacheLifetime, cacheCfg.ObjectsList.Lifetime)
	cacheCfg.ObjectsList.Size = fetchCacheSize(v, l, cfgListObjectsCacheSize, cacheCfg.ObjectsList.Size)

	cacheCfg.SessionList.Lifetime = fetchCacheLifetime(v, l, cfgSessionListCacheLifetime, cacheCfg.SessionList.Lifetime)
	cacheCfg.SessionList.Size = fetchCacheSize(v, l, cfgSessionListCacheSize, cacheCfg.SessionList.Size)

	cacheCfg.Buckets.Lifetime = fetchCacheLifetime(v, l, cfgBucketsCacheLifetime, cacheCfg.Buckets.Lifetime)
	cacheCfg.Buckets.Size = fetchCacheSize(v, l, cfgBucketsCacheSize, cacheCfg.Buckets.Size)

	cacheCfg.Names.Lifetime = fetchCacheLifetime(v, l, cfgNamesCacheLifetime, cacheCfg.Names.Lifetime)
	cacheCfg.Names.Size = fetchCacheSize(v, l, cfgNamesCacheSize, cacheCfg.Names.Size)

	cacheCfg.System.Lifetime = fetchCacheLifetime(v, l, cfgSystemCacheLifetime, cacheCfg.System.Lifetime)
	cacheCfg.System.Size = fetchCacheSize(v, l, cfgSystemCacheSize, cacheCfg.System.Size)

	cacheCfg.AccessControl.Lifetime = fetchCacheLifetime(v, l, cfgAccessControlCacheLifetime, cacheCfg.AccessControl.Lifetime)
	cacheCfg.AccessControl.Size = fetchCacheSize(v, l, cfgAccessControlCacheSize, cacheCfg.AccessControl.Size)

	return cacheCfg
}

func getAccessBoxCacheConfig(v *viper.Viper, l *zap.Logger) *cache.Config {
	cacheCfg := cache.DefaultAccessBoxConfig(l)

	cacheCfg.Lifetime = fetchCacheLifetime(v, l, cfgAccessBoxCacheLifetime, cacheCfg.Lifetime)
	cacheCfg.Size = fetchCacheSize(v, l, cfgAccessBoxCacheSize, cacheCfg.Size)

	return cacheCfg
}

func getMorphPolicyCacheConfig(v *viper.Viper, l *zap.Logger) *cache.Config {
	cacheCfg := cache.DefaultMorphPolicyConfig(l)

	cacheCfg.Lifetime = fetchCacheLifetime(v, l, cfgMorphPolicyCacheLifetime, cacheCfg.Lifetime)
	cacheCfg.Size = fetchCacheSize(v, l, cfgMorphPolicyCacheSize, cacheCfg.Size)

	return cacheCfg
}

func getFrostfsIDCacheConfig(v *viper.Viper, l *zap.Logger) *cache.Config {
	cacheCfg := cache.DefaultFrostfsIDConfig(l)

	cacheCfg.Lifetime = fetchCacheLifetime(v, l, cfgFrostfsIDCacheLifetime, cacheCfg.Lifetime)
	cacheCfg.Size = fetchCacheSize(v, l, cfgFrostfsIDCacheSize, cacheCfg.Size)

	return cacheCfg
}

func (a *App) initHandler() {
	var err error

	a.api, err = handler.New(a.log, a.obj, a.nc, a.settings, a.policyStorage, a.frostfsid)
	if err != nil {
		a.log.Fatal(logs.CouldNotInitializeAPIHandler, zap.Error(err))
	}
}

func (a *App) getServer(address string) Server {
	for i := range a.servers {
		if a.servers[i].Address() == address {
			return a.servers[i]
		}
	}
	return nil
}

func (a *App) updateUnbindServerInfo(info ServerInfo) bool {
	for i := range a.unbindServers {
		if a.unbindServers[i].Address == info.Address {
			a.unbindServers[i] = info
			return true
		}
	}
	return false
}

func (a *App) getServers() []Server {
	a.mu.RLock()
	defer a.mu.RUnlock()
	return a.servers
}

func (a *App) setRuntimeParameters() {
	if len(os.Getenv("GOMEMLIMIT")) != 0 {
		// default limit < yaml limit < app env limit < GOMEMLIMIT
		a.log.Warn(logs.RuntimeSoftMemoryDefinedWithGOMEMLIMIT)
		return
	}

	softMemoryLimit := fetchSoftMemoryLimit(a.cfg)
	previous := debug.SetMemoryLimit(softMemoryLimit)
	if softMemoryLimit != previous {
		a.log.Info(logs.RuntimeSoftMemoryLimitUpdated,
			zap.Int64("new_value", softMemoryLimit),
			zap.Int64("old_value", previous))
	}
}

func (a *App) scheduleReconnect(ctx context.Context, srv *http.Server) {
	go func() {
		t := time.NewTicker(a.settings.reconnectInterval)
		defer t.Stop()
		for {
			select {
			case <-t.C:
				if a.tryReconnect(ctx, srv) {
					return
				}
				t.Reset(a.settings.reconnectInterval)
			case <-ctx.Done():
				return
			}
		}
	}()
}

func (a *App) tryReconnect(ctx context.Context, sr *http.Server) bool {
	a.mu.Lock()
	defer a.mu.Unlock()

	a.log.Info(logs.ServerReconnecting)
	var failedServers []ServerInfo

	for _, serverInfo := range a.unbindServers {
		fields := []zap.Field{
			zap.String("address", serverInfo.Address), zap.Bool("tls enabled", serverInfo.TLS.Enabled),
			zap.String("tls cert", serverInfo.TLS.CertFile), zap.String("tls key", serverInfo.TLS.KeyFile),
		}

		srv, err := newServer(ctx, serverInfo)
		if err != nil {
			a.log.Warn(logs.ServerReconnectFailed, zap.Error(err))
			failedServers = append(failedServers, serverInfo)
			a.metrics.MarkUnhealthy(serverInfo.Address)
			continue
		}

		go func() {
			a.log.Info(logs.StartingServer, zap.String("address", srv.Address()))
			a.metrics.MarkHealthy(serverInfo.Address)
			if err = sr.Serve(srv.Listener()); err != nil && !errors.Is(err, http.ErrServerClosed) {
				a.log.Warn(logs.ListenAndServe, zap.Error(err))
				a.metrics.MarkUnhealthy(serverInfo.Address)
			}
		}()

		a.servers = append(a.servers, srv)
		a.log.Info(logs.ServerReconnectedSuccessfully, fields...)
	}

	a.unbindServers = failedServers

	return len(a.unbindServers) == 0
}