// Copyright 2018 The etcd Authors
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package proxy

import (
	"context"
	"errors"
	"fmt"
	"io"
	mrand "math/rand"
	"net"
	"net/http"
	"net/url"
	"strconv"
	"strings"
	"sync"
	"time"

	humanize "github.com/dustin/go-humanize"
	"go.uber.org/zap"

	"go.etcd.io/etcd/client/pkg/v3/transport"
)

var (
	defaultDialTimeout   = 3 * time.Second
	defaultBufferSize    = 48 * 1024
	defaultRetryInterval = 10 * time.Millisecond
)

// Server defines proxy server layer that simulates common network faults:
// latency spikes and packet drop or corruption. The proxy overhead is very
// small overhead (<500μs per request). Please run tests to compute actual
// overhead.
type Server interface {
	// From returns proxy source address in "scheme://host:port" format.
	From() string
	// To returns proxy destination address in "scheme://host:port" format.
	To() string

	// Ready returns when proxy is ready to serve.
	Ready() <-chan struct{}
	// Done returns when proxy has been closed.
	Done() <-chan struct{}
	// Error sends errors while serving proxy.
	Error() <-chan error
	// Close closes listener and transport.
	Close() error

	// PauseAccept stops accepting new connections.
	PauseAccept()
	// UnpauseAccept removes pause operation on accepting new connections.
	UnpauseAccept()

	// DelayAccept adds latency ± random variable to accepting
	// new incoming connections.
	DelayAccept(latency, rv time.Duration)
	// UndelayAccept removes sending latencies.
	UndelayAccept()
	// LatencyAccept returns current latency on accepting
	// new incoming connections.
	LatencyAccept() time.Duration

	// DelayTx adds latency ± random variable for "outgoing" traffic
	// in "sending" layer.
	DelayTx(latency, rv time.Duration)
	// UndelayTx removes sending latencies.
	UndelayTx()
	// LatencyTx returns current send latency.
	LatencyTx() time.Duration

	// DelayRx adds latency ± random variable for "incoming" traffic
	// in "receiving" layer.
	DelayRx(latency, rv time.Duration)
	// UndelayRx removes "receiving" latencies.
	UndelayRx()
	// LatencyRx returns current receive latency.
	LatencyRx() time.Duration

	// ModifyTx alters/corrupts/drops "outgoing" packets from the listener
	// with the given edit function.
	ModifyTx(f func(data []byte) []byte)
	// UnmodifyTx removes modify operation on "forwarding".
	UnmodifyTx()

	// ModifyRx alters/corrupts/drops "incoming" packets to client
	// with the given edit function.
	ModifyRx(f func(data []byte) []byte)
	// UnmodifyRx removes modify operation on "receiving".
	UnmodifyRx()

	// BlackholeTx drops all "outgoing" packets before "forwarding".
	// "BlackholeTx" operation is a wrapper around "ModifyTx" with
	// a function that returns empty bytes.
	BlackholeTx()
	// UnblackholeTx removes blackhole operation on "sending".
	UnblackholeTx()

	// BlackholeRx drops all "incoming" packets to client.
	// "BlackholeRx" operation is a wrapper around "ModifyRx" with
	// a function that returns empty bytes.
	BlackholeRx()
	// UnblackholeRx removes blackhole operation on "receiving".
	UnblackholeRx()

	// PauseTx stops "forwarding" packets; "outgoing" traffic blocks.
	PauseTx()
	// UnpauseTx removes "forwarding" pause operation.
	UnpauseTx()

	// PauseRx stops "receiving" packets; "incoming" traffic blocks.
	PauseRx()
	// UnpauseRx removes "receiving" pause operation.
	UnpauseRx()

	// ResetListener closes and restarts listener.
	ResetListener() error
}

// ServerConfig defines proxy server configuration.
type ServerConfig struct {
	Logger        *zap.Logger
	From          url.URL
	To            url.URL
	TLSInfo       transport.TLSInfo
	DialTimeout   time.Duration
	BufferSize    int
	RetryInterval time.Duration
}

type server struct {
	lg *zap.Logger

	from     url.URL
	fromPort int
	to       url.URL
	toPort   int

	tlsInfo     transport.TLSInfo
	dialTimeout time.Duration

	bufferSize    int
	retryInterval time.Duration

	readyc chan struct{}
	donec  chan struct{}
	errc   chan error

	closeOnce sync.Once
	closeWg   sync.WaitGroup

	listenerMu sync.RWMutex
	listener   net.Listener

	pauseAcceptMu sync.Mutex
	pauseAcceptc  chan struct{}

	latencyAcceptMu sync.RWMutex
	latencyAccept   time.Duration

	modifyTxMu sync.RWMutex
	modifyTx   func(data []byte) []byte

	modifyRxMu sync.RWMutex
	modifyRx   func(data []byte) []byte

	pauseTxMu sync.Mutex
	pauseTxc  chan struct{}

	pauseRxMu sync.Mutex
	pauseRxc  chan struct{}

	latencyTxMu sync.RWMutex
	latencyTx   time.Duration

	latencyRxMu sync.RWMutex
	latencyRx   time.Duration
}

// NewServer returns a proxy implementation with no iptables/tc dependencies.
// The proxy layer overhead is <1ms.
func NewServer(cfg ServerConfig) Server {
	s := &server{
		lg: cfg.Logger,

		from: cfg.From,
		to:   cfg.To,

		tlsInfo:     cfg.TLSInfo,
		dialTimeout: cfg.DialTimeout,

		bufferSize:    cfg.BufferSize,
		retryInterval: cfg.RetryInterval,

		readyc: make(chan struct{}),
		donec:  make(chan struct{}),
		errc:   make(chan error, 16),

		pauseAcceptc: make(chan struct{}),
		pauseTxc:     make(chan struct{}),
		pauseRxc:     make(chan struct{}),
	}

	_, fromPort, err := net.SplitHostPort(cfg.From.Host)
	if err == nil {
		s.fromPort, _ = strconv.Atoi(fromPort)
	}
	var toPort string
	_, toPort, err = net.SplitHostPort(cfg.To.Host)
	if err == nil {
		s.toPort, _ = strconv.Atoi(toPort)
	}

	if s.dialTimeout == 0 {
		s.dialTimeout = defaultDialTimeout
	}
	if s.bufferSize == 0 {
		s.bufferSize = defaultBufferSize
	}
	if s.retryInterval == 0 {
		s.retryInterval = defaultRetryInterval
	}

	close(s.pauseAcceptc)
	close(s.pauseTxc)
	close(s.pauseRxc)

	if strings.HasPrefix(s.from.Scheme, "http") {
		s.from.Scheme = "tcp"
	}
	if strings.HasPrefix(s.to.Scheme, "http") {
		s.to.Scheme = "tcp"
	}

	addr := fmt.Sprintf(":%d", s.fromPort)
	if s.fromPort == 0 { // unix
		addr = s.from.Host
	}

	var ln net.Listener
	if !s.tlsInfo.Empty() {
		ln, err = transport.NewListener(addr, s.from.Scheme, &s.tlsInfo)
	} else {
		ln, err = net.Listen(s.from.Scheme, addr)
	}
	if err != nil {
		s.errc <- err
		s.Close()
		return s
	}
	s.listener = ln

	s.closeWg.Add(1)
	go s.listenAndServe()

	s.lg.Info("started proxying", zap.String("from", s.From()), zap.String("to", s.To()))
	return s
}

func (s *server) From() string {
	return fmt.Sprintf("%s://%s", s.from.Scheme, s.from.Host)
}

func (s *server) To() string {
	return fmt.Sprintf("%s://%s", s.to.Scheme, s.to.Host)
}

// TODO: implement packet reordering from multiple TCP connections
// buffer packets per connection for awhile, reorder before transmit
// - https://github.com/etcd-io/etcd/issues/5614
// - https://github.com/etcd-io/etcd/pull/6918#issuecomment-264093034

func (s *server) listenAndServe() {
	defer s.closeWg.Done()

	ctx := context.Background()
	s.lg.Info("proxy is listening on", zap.String("from", s.From()))
	close(s.readyc)

	for {
		s.pauseAcceptMu.Lock()
		pausec := s.pauseAcceptc
		s.pauseAcceptMu.Unlock()
		select {
		case <-pausec:
		case <-s.donec:
			return
		}

		s.latencyAcceptMu.RLock()
		lat := s.latencyAccept
		s.latencyAcceptMu.RUnlock()
		if lat > 0 {
			select {
			case <-time.After(lat):
			case <-s.donec:
				return
			}
		}

		s.listenerMu.RLock()
		ln := s.listener
		s.listenerMu.RUnlock()

		in, err := ln.Accept()
		if err != nil {
			select {
			case s.errc <- err:
				select {
				case <-s.donec:
					return
				default:
				}
			case <-s.donec:
				return
			}
			s.lg.Debug("listener accept error", zap.Error(err))

			if strings.HasSuffix(err.Error(), "use of closed network connection") {
				select {
				case <-time.After(s.retryInterval):
				case <-s.donec:
					return
				}
				s.lg.Debug("listener is closed; retry listening on", zap.String("from", s.From()))

				if err = s.ResetListener(); err != nil {
					select {
					case s.errc <- err:
						select {
						case <-s.donec:
							return
						default:
						}
					case <-s.donec:
						return
					}
					s.lg.Warn("failed to reset listener", zap.Error(err))
				}
			}

			continue
		}

		var out net.Conn
		if !s.tlsInfo.Empty() {
			var tp *http.Transport
			tp, err = transport.NewTransport(s.tlsInfo, s.dialTimeout)
			if err != nil {
				select {
				case s.errc <- err:
					select {
					case <-s.donec:
						return
					default:
					}
				case <-s.donec:
					return
				}
				continue
			}
			out, err = tp.DialContext(ctx, s.to.Scheme, s.to.Host)
		} else {
			out, err = net.Dial(s.to.Scheme, s.to.Host)
		}
		if err != nil {
			select {
			case s.errc <- err:
				select {
				case <-s.donec:
					return
				default:
				}
			case <-s.donec:
				return
			}
			s.lg.Debug("failed to dial", zap.Error(err))
			continue
		}

		s.closeWg.Add(2)
		go func() {
			defer s.closeWg.Done()
			// read incoming bytes from listener, dispatch to outgoing connection
			s.transmit(out, in)
			out.Close()
			in.Close()
		}()
		go func() {
			defer s.closeWg.Done()
			// read response from outgoing connection, write back to listener
			s.receive(in, out)
			in.Close()
			out.Close()
		}()
	}
}

func (s *server) transmit(dst io.Writer, src io.Reader) {
	s.ioCopy(dst, src, proxyTx)
}

func (s *server) receive(dst io.Writer, src io.Reader) {
	s.ioCopy(dst, src, proxyRx)
}

type proxyType uint8

const (
	proxyTx proxyType = iota
	proxyRx
)

func (s *server) ioCopy(dst io.Writer, src io.Reader, ptype proxyType) {
	buf := make([]byte, s.bufferSize)
	for {
		nr1, err := src.Read(buf)
		if err != nil {
			if errors.Is(err, io.EOF) {
				return
			}
			// connection already closed
			if strings.HasSuffix(err.Error(), "read: connection reset by peer") {
				return
			}
			if strings.HasSuffix(err.Error(), "use of closed network connection") {
				return
			}
			select {
			case s.errc <- err:
				select {
				case <-s.donec:
					return
				default:
				}
			case <-s.donec:
				return
			}
			s.lg.Debug("failed to read", zap.Error(err))
			return
		}
		if nr1 == 0 {
			return
		}
		data := buf[:nr1]

		// alters/corrupts/drops data
		switch ptype {
		case proxyTx:
			s.modifyTxMu.RLock()
			if s.modifyTx != nil {
				data = s.modifyTx(data)
			}
			s.modifyTxMu.RUnlock()
		case proxyRx:
			s.modifyRxMu.RLock()
			if s.modifyRx != nil {
				data = s.modifyRx(data)
			}
			s.modifyRxMu.RUnlock()
		default:
			panic("unknown proxy type")
		}
		nr2 := len(data)
		switch ptype {
		case proxyTx:
			s.lg.Debug(
				"modified tx",
				zap.String("data-received", humanize.Bytes(uint64(nr1))),
				zap.String("data-modified", humanize.Bytes(uint64(nr2))),
				zap.String("from", s.From()),
				zap.String("to", s.To()),
			)
		case proxyRx:
			s.lg.Debug(
				"modified rx",
				zap.String("data-received", humanize.Bytes(uint64(nr1))),
				zap.String("data-modified", humanize.Bytes(uint64(nr2))),
				zap.String("from", s.To()),
				zap.String("to", s.From()),
			)
		default:
			panic("unknown proxy type")
		}

		// pause before packet dropping, blocking, and forwarding
		var pausec chan struct{}
		switch ptype {
		case proxyTx:
			s.pauseTxMu.Lock()
			pausec = s.pauseTxc
			s.pauseTxMu.Unlock()
		case proxyRx:
			s.pauseRxMu.Lock()
			pausec = s.pauseRxc
			s.pauseRxMu.Unlock()
		default:
			panic("unknown proxy type")
		}
		select {
		case <-pausec:
		case <-s.donec:
			return
		}

		// pause first, and then drop packets
		if nr2 == 0 {
			continue
		}

		// block before forwarding
		var lat time.Duration
		switch ptype {
		case proxyTx:
			s.latencyTxMu.RLock()
			lat = s.latencyTx
			s.latencyTxMu.RUnlock()
		case proxyRx:
			s.latencyRxMu.RLock()
			lat = s.latencyRx
			s.latencyRxMu.RUnlock()
		default:
			panic("unknown proxy type")
		}
		if lat > 0 {
			select {
			case <-time.After(lat):
			case <-s.donec:
				return
			}
		}

		// now forward packets to target
		var nw int
		nw, err = dst.Write(data)
		if err != nil {
			if errors.Is(err, io.EOF) {
				return
			}
			select {
			case s.errc <- err:
				select {
				case <-s.donec:
					return
				default:
				}
			case <-s.donec:
				return
			}
			switch ptype {
			case proxyTx:
				s.lg.Debug("write fail on tx", zap.Error(err))
			case proxyRx:
				s.lg.Debug("write fail on rx", zap.Error(err))
			default:
				panic("unknown proxy type")
			}
			return
		}

		if nr2 != nw {
			select {
			case s.errc <- io.ErrShortWrite:
				select {
				case <-s.donec:
					return
				default:
				}
			case <-s.donec:
				return
			}
			switch ptype {
			case proxyTx:
				s.lg.Debug(
					"write fail on tx; read/write bytes are different",
					zap.Int("read-bytes", nr1),
					zap.Int("write-bytes", nw),
					zap.Error(io.ErrShortWrite),
				)
			case proxyRx:
				s.lg.Debug(
					"write fail on rx; read/write bytes are different",
					zap.Int("read-bytes", nr1),
					zap.Int("write-bytes", nw),
					zap.Error(io.ErrShortWrite),
				)
			default:
				panic("unknown proxy type")
			}
			return
		}

		switch ptype {
		case proxyTx:
			s.lg.Debug(
				"transmitted",
				zap.String("data-size", humanize.Bytes(uint64(nr1))),
				zap.String("from", s.From()),
				zap.String("to", s.To()),
			)
		case proxyRx:
			s.lg.Debug(
				"received",
				zap.String("data-size", humanize.Bytes(uint64(nr1))),
				zap.String("from", s.To()),
				zap.String("to", s.From()),
			)
		default:
			panic("unknown proxy type")
		}
	}
}

func (s *server) Ready() <-chan struct{} { return s.readyc }
func (s *server) Done() <-chan struct{}  { return s.donec }
func (s *server) Error() <-chan error    { return s.errc }
func (s *server) Close() (err error) {
	s.closeOnce.Do(func() {
		close(s.donec)
		s.listenerMu.Lock()
		if s.listener != nil {
			err = s.listener.Close()
			s.lg.Info(
				"closed proxy listener",
				zap.String("from", s.From()),
				zap.String("to", s.To()),
			)
		}
		s.lg.Sync()
		s.listenerMu.Unlock()
	})
	s.closeWg.Wait()
	return err
}

func (s *server) PauseAccept() {
	s.pauseAcceptMu.Lock()
	s.pauseAcceptc = make(chan struct{})
	s.pauseAcceptMu.Unlock()

	s.lg.Info(
		"paused accept",
		zap.String("from", s.From()),
		zap.String("to", s.To()),
	)
}

func (s *server) UnpauseAccept() {
	s.pauseAcceptMu.Lock()
	select {
	case <-s.pauseAcceptc: // already unpaused
	case <-s.donec:
		s.pauseAcceptMu.Unlock()
		return
	default:
		close(s.pauseAcceptc)
	}
	s.pauseAcceptMu.Unlock()

	s.lg.Info(
		"unpaused accept",
		zap.String("from", s.From()),
		zap.String("to", s.To()),
	)
}

func (s *server) DelayAccept(latency, rv time.Duration) {
	if latency <= 0 {
		return
	}
	d := computeLatency(latency, rv)
	s.latencyAcceptMu.Lock()
	s.latencyAccept = d
	s.latencyAcceptMu.Unlock()

	s.lg.Info(
		"set accept latency",
		zap.Duration("latency", d),
		zap.Duration("given-latency", latency),
		zap.Duration("given-latency-random-variable", rv),
		zap.String("from", s.From()),
		zap.String("to", s.To()),
	)
}

func (s *server) UndelayAccept() {
	s.latencyAcceptMu.Lock()
	d := s.latencyAccept
	s.latencyAccept = 0
	s.latencyAcceptMu.Unlock()

	s.lg.Info(
		"removed accept latency",
		zap.Duration("latency", d),
		zap.String("from", s.From()),
		zap.String("to", s.To()),
	)
}

func (s *server) LatencyAccept() time.Duration {
	s.latencyAcceptMu.RLock()
	d := s.latencyAccept
	s.latencyAcceptMu.RUnlock()
	return d
}

func (s *server) DelayTx(latency, rv time.Duration) {
	if latency <= 0 {
		return
	}
	d := computeLatency(latency, rv)
	s.latencyTxMu.Lock()
	s.latencyTx = d
	s.latencyTxMu.Unlock()

	s.lg.Info(
		"set transmit latency",
		zap.Duration("latency", d),
		zap.Duration("given-latency", latency),
		zap.Duration("given-latency-random-variable", rv),
		zap.String("from", s.From()),
		zap.String("to", s.To()),
	)
}

func (s *server) UndelayTx() {
	s.latencyTxMu.Lock()
	d := s.latencyTx
	s.latencyTx = 0
	s.latencyTxMu.Unlock()

	s.lg.Info(
		"removed transmit latency",
		zap.Duration("latency", d),
		zap.String("from", s.From()),
		zap.String("to", s.To()),
	)
}

func (s *server) LatencyTx() time.Duration {
	s.latencyTxMu.RLock()
	d := s.latencyTx
	s.latencyTxMu.RUnlock()
	return d
}

func (s *server) DelayRx(latency, rv time.Duration) {
	if latency <= 0 {
		return
	}
	d := computeLatency(latency, rv)
	s.latencyRxMu.Lock()
	s.latencyRx = d
	s.latencyRxMu.Unlock()

	s.lg.Info(
		"set receive latency",
		zap.Duration("latency", d),
		zap.Duration("given-latency", latency),
		zap.Duration("given-latency-random-variable", rv),
		zap.String("from", s.To()),
		zap.String("to", s.From()),
	)
}

func (s *server) UndelayRx() {
	s.latencyRxMu.Lock()
	d := s.latencyRx
	s.latencyRx = 0
	s.latencyRxMu.Unlock()

	s.lg.Info(
		"removed receive latency",
		zap.Duration("latency", d),
		zap.String("from", s.To()),
		zap.String("to", s.From()),
	)
}

func (s *server) LatencyRx() time.Duration {
	s.latencyRxMu.RLock()
	d := s.latencyRx
	s.latencyRxMu.RUnlock()
	return d
}

func computeLatency(lat, rv time.Duration) time.Duration {
	if rv == 0 {
		return lat
	}
	if rv < 0 {
		rv *= -1
	}
	if rv > lat {
		rv = lat / 10
	}
	now := time.Now()
	sign := 1
	if now.Second()%2 == 0 {
		sign = -1
	}
	return lat + time.Duration(int64(sign)*mrand.Int63n(rv.Nanoseconds()))
}

func (s *server) ModifyTx(f func([]byte) []byte) {
	s.modifyTxMu.Lock()
	s.modifyTx = f
	s.modifyTxMu.Unlock()

	s.lg.Info(
		"modifying tx",
		zap.String("from", s.From()),
		zap.String("to", s.To()),
	)
}

func (s *server) UnmodifyTx() {
	s.modifyTxMu.Lock()
	s.modifyTx = nil
	s.modifyTxMu.Unlock()

	s.lg.Info(
		"unmodifyed tx",
		zap.String("from", s.From()),
		zap.String("to", s.To()),
	)
}

func (s *server) ModifyRx(f func([]byte) []byte) {
	s.modifyRxMu.Lock()
	s.modifyRx = f
	s.modifyRxMu.Unlock()
	s.lg.Info(
		"modifying rx",
		zap.String("from", s.To()),
		zap.String("to", s.From()),
	)
}

func (s *server) UnmodifyRx() {
	s.modifyRxMu.Lock()
	s.modifyRx = nil
	s.modifyRxMu.Unlock()

	s.lg.Info(
		"unmodifyed rx",
		zap.String("from", s.To()),
		zap.String("to", s.From()),
	)
}

func (s *server) BlackholeTx() {
	s.ModifyTx(func([]byte) []byte { return nil })
	s.lg.Info(
		"blackholed tx",
		zap.String("from", s.From()),
		zap.String("to", s.To()),
	)
}

func (s *server) UnblackholeTx() {
	s.UnmodifyTx()
	s.lg.Info(
		"unblackholed tx",
		zap.String("from", s.From()),
		zap.String("to", s.To()),
	)
}

func (s *server) BlackholeRx() {
	s.ModifyRx(func([]byte) []byte { return nil })
	s.lg.Info(
		"blackholed rx",
		zap.String("from", s.To()),
		zap.String("to", s.From()),
	)
}

func (s *server) UnblackholeRx() {
	s.UnmodifyRx()
	s.lg.Info(
		"unblackholed rx",
		zap.String("from", s.To()),
		zap.String("to", s.From()),
	)
}

func (s *server) PauseTx() {
	s.pauseTxMu.Lock()
	s.pauseTxc = make(chan struct{})
	s.pauseTxMu.Unlock()

	s.lg.Info(
		"paused tx",
		zap.String("from", s.From()),
		zap.String("to", s.To()),
	)
}

func (s *server) UnpauseTx() {
	s.pauseTxMu.Lock()
	select {
	case <-s.pauseTxc: // already unpaused
	case <-s.donec:
		s.pauseTxMu.Unlock()
		return
	default:
		close(s.pauseTxc)
	}
	s.pauseTxMu.Unlock()

	s.lg.Info(
		"unpaused tx",
		zap.String("from", s.From()),
		zap.String("to", s.To()),
	)
}

func (s *server) PauseRx() {
	s.pauseRxMu.Lock()
	s.pauseRxc = make(chan struct{})
	s.pauseRxMu.Unlock()

	s.lg.Info(
		"paused rx",
		zap.String("from", s.To()),
		zap.String("to", s.From()),
	)
}

func (s *server) UnpauseRx() {
	s.pauseRxMu.Lock()
	select {
	case <-s.pauseRxc: // already unpaused
	case <-s.donec:
		s.pauseRxMu.Unlock()
		return
	default:
		close(s.pauseRxc)
	}
	s.pauseRxMu.Unlock()

	s.lg.Info(
		"unpaused rx",
		zap.String("from", s.To()),
		zap.String("to", s.From()),
	)
}

func (s *server) ResetListener() error {
	s.listenerMu.Lock()
	defer s.listenerMu.Unlock()

	if err := s.listener.Close(); err != nil {
		// already closed
		if !strings.HasSuffix(err.Error(), "use of closed network connection") {
			return err
		}
	}

	var ln net.Listener
	var err error
	if !s.tlsInfo.Empty() {
		ln, err = transport.NewListener(s.from.Host, s.from.Scheme, &s.tlsInfo)
	} else {
		ln, err = net.Listen(s.from.Scheme, s.from.Host)
	}
	if err != nil {
		return err
	}
	s.listener = ln

	s.lg.Info(
		"reset listener on",
		zap.String("from", s.From()),
	)
	return nil
}
