// Copyright 2016 The Netstack Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package dhcp

import (
	"bytes"
	"context"
	"crypto/rand"
	"fmt"
	"sync"
	"time"

	"github.com/google/netstack/tcpip"
	"github.com/google/netstack/tcpip/buffer"
	"github.com/google/netstack/tcpip/network/ipv4"
	"github.com/google/netstack/tcpip/stack"
	"github.com/google/netstack/tcpip/transport/udp"
	"github.com/google/netstack/waiter"
)

// Client is a DHCP client.
type Client struct {
	stack        *stack.Stack
	nicid        tcpip.NICID
	linkAddr     tcpip.LinkAddress
	acquiredFunc func(old, new tcpip.Address, cfg Config)

	mu          sync.Mutex
	addr        tcpip.Address
	cfg         Config
	lease       time.Duration
	cancelRenew func()
}

// NewClient creates a DHCP client.
//
// TODO(crawshaw): add s.LinkAddr(nicid) to *stack.Stack.
func NewClient(s *stack.Stack, nicid tcpip.NICID, linkAddr tcpip.LinkAddress, acquiredFunc func(old, new tcpip.Address, cfg Config)) *Client {
	return &Client{
		stack:        s,
		nicid:        nicid,
		linkAddr:     linkAddr,
		acquiredFunc: acquiredFunc,
	}
}

// Run starts the DHCP client.
// It will periodically search for an IP address using the Request method.
func (c *Client) Run(ctx context.Context) {
	go c.run(ctx)
}

func (c *Client) run(ctx context.Context) {
	defer func() {
		c.mu.Lock()
		defer c.mu.Unlock()
		if c.addr != "" {
			c.stack.RemoveAddress(c.nicid, c.addr)
		}
	}()

	var renewAddr tcpip.Address
	for {
		reqCtx, cancel := context.WithTimeout(ctx, 3*time.Second)
		cfg, err := c.Request(reqCtx, renewAddr)
		cancel()
		if err != nil {
			select {
			case <-time.After(1 * time.Second):
				// loop and try again
			case <-ctx.Done():
				return
			}
		}

		c.mu.Lock()
		renewAddr = c.addr
		c.mu.Unlock()

		timer := time.NewTimer(cfg.LeaseLength)
		select {
		case <-ctx.Done():
			timer.Stop()
			return
		case <-timer.C:
			// loop and make a renewal request
		}
	}
}

// Address reports the IP address acquired by the DHCP client.
func (c *Client) Address() tcpip.Address {
	c.mu.Lock()
	defer c.mu.Unlock()
	return c.addr
}

// Config reports the DHCP configuration acquired with the IP address lease.
func (c *Client) Config() Config {
	c.mu.Lock()
	defer c.mu.Unlock()
	return c.cfg
}

// Request executes a DHCP request session.
//
// On success, it adds a new address to this client's TCPIP stack.
// If the server sets a lease limit a timer is set to automatically
// renew it.
func (c *Client) Request(ctx context.Context, requestedAddr tcpip.Address) (cfg Config, reterr error) {
	if err := c.stack.AddAddressWithOptions(c.nicid, ipv4.ProtocolNumber, "\xff\xff\xff\xff", stack.NeverPrimaryEndpoint); err != nil && err != tcpip.ErrDuplicateAddress {
		return Config{}, fmt.Errorf("dhcp: %v", err)
	}
	if err := c.stack.AddAddressWithOptions(c.nicid, ipv4.ProtocolNumber, "\x00\x00\x00\x00", stack.NeverPrimaryEndpoint); err != nil && err != tcpip.ErrDuplicateAddress {
		return Config{}, fmt.Errorf("dhcp: %v", err)
	}
	defer c.stack.RemoveAddress(c.nicid, "\xff\xff\xff\xff")
	defer c.stack.RemoveAddress(c.nicid, "\x00\x00\x00\x00")

	var wq waiter.Queue
	ep, err := c.stack.NewEndpoint(udp.ProtocolNumber, ipv4.ProtocolNumber, &wq)
	if err != nil {
		return Config{}, fmt.Errorf("dhcp: outbound endpoint: %v", err)
	}
	err = ep.Bind(tcpip.FullAddress{
		Addr: "\x00\x00\x00\x00",
		Port: clientPort,
		NIC:  c.nicid,
	}, nil)
	defer ep.Close()
	if err != nil {
		return Config{}, fmt.Errorf("dhcp: connect failed: %v", err)
	}

	epin, err := c.stack.NewEndpoint(udp.ProtocolNumber, ipv4.ProtocolNumber, &wq)
	if err != nil {
		return Config{}, fmt.Errorf("dhcp: inbound endpoint: %v", err)
	}
	err = epin.Bind(tcpip.FullAddress{
		Addr: "\xff\xff\xff\xff",
		Port: clientPort,
		NIC:  c.nicid,
	}, nil)
	defer epin.Close()
	if err != nil {
		return Config{}, fmt.Errorf("dhcp: connect failed: %v", err)
	}

	var xid [4]byte
	rand.Read(xid[:])

	// DHCPDISCOVERY
	discOpts := options{
		{optDHCPMsgType, []byte{byte(dhcpDISCOVER)}},
		{optParamReq, []byte{
			1,  // request subnet mask
			3,  // request router
			15, // domain name
			6,  // domain name server
		}},
	}
	if requestedAddr != "" {
		discOpts = append(discOpts, option{optReqIPAddr, []byte(requestedAddr)})
	}
	var clientID []byte
	if len(c.linkAddr) == 6 {
		clientID = make([]byte, 7)
		clientID[0] = 1 // htype: ARP Ethernet from RFC 1700
		copy(clientID[1:], c.linkAddr)
		discOpts = append(discOpts, option{optClientID, clientID})
	}
	h := make(header, headerBaseSize+discOpts.len()+1)
	h.init()
	h.setOp(opRequest)
	copy(h.xidbytes(), xid[:])
	h.setBroadcast()
	copy(h.chaddr(), c.linkAddr)
	h.setOptions(discOpts)

	serverAddr := &tcpip.FullAddress{
		Addr: "\xff\xff\xff\xff",
		Port: serverPort,
		NIC:  c.nicid,
	}
	if _, err := ep.Write(buffer.View(h), serverAddr); err != nil {
		return Config{}, fmt.Errorf("dhcp discovery write: %v", err)
	}

	we, ch := waiter.NewChannelEntry(nil)
	wq.EventRegister(&we, waiter.EventIn)
	defer wq.EventUnregister(&we)

	// DHCPOFFER
	var opts options
	for {
		var addr tcpip.FullAddress
		v, e := epin.Read(&addr)
		if e == tcpip.ErrWouldBlock {
			select {
			case <-ch:
				continue
			case <-ctx.Done():
				return Config{}, fmt.Errorf("reading dhcp offer: %v", tcpip.ErrAborted)
			}
		}
		h = header(v)
		var valid bool
		var err error
		opts, valid, err = loadDHCPReply(h, dhcpOFFER, xid[:])
		if !valid {
			if err != nil {
				// TODO: report malformed server responses
			}
			continue
		}
		break
	}

	var ack bool
	if err := cfg.decode(opts); err != nil {
		return Config{}, fmt.Errorf("dhcp offer: %v", err)
	}

	// DHCPREQUEST
	addr := tcpip.Address(h.yiaddr())
	if err := c.stack.AddAddressWithOptions(c.nicid, ipv4.ProtocolNumber, addr, stack.FirstPrimaryEndpoint); err != nil {
		if err != tcpip.ErrDuplicateAddress {
			return Config{}, fmt.Errorf("adding address: %v", err)
		}
	}
	defer func() {
		if !ack || reterr != nil {
			c.stack.RemoveAddress(c.nicid, addr)
			addr = ""
			cfg = Config{Error: reterr}
		}

		c.mu.Lock()
		oldAddr := c.addr
		c.addr = addr
		c.cfg = cfg
		c.mu.Unlock()

		// Clean up broadcast addresses before calling acquiredFunc
		// so nothing else uses them by mistake.
		//
		// (The deferred RemoveAddress calls above silently error.)
		c.stack.RemoveAddress(c.nicid, "\xff\xff\xff\xff")
		c.stack.RemoveAddress(c.nicid, "\x00\x00\x00\x00")

		if c.acquiredFunc != nil {
			c.acquiredFunc(oldAddr, addr, cfg)
		}
		if requestedAddr != "" && requestedAddr != addr {
			c.stack.RemoveAddress(c.nicid, requestedAddr)
		}
	}()
	h.init()
	h.setOp(opRequest)
	for i, b := 0, h.yiaddr(); i < len(b); i++ {
		b[i] = 0
	}
	for i, b := 0, h.siaddr(); i < len(b); i++ {
		b[i] = 0
	}
	for i, b := 0, h.giaddr(); i < len(b); i++ {
		b[i] = 0
	}
	reqOpts := []option{
		{optDHCPMsgType, []byte{byte(dhcpREQUEST)}},
		{optReqIPAddr, []byte(addr)},
		{optDHCPServer, []byte(cfg.ServerAddress)},
	}
	if len(clientID) != 0 {
		reqOpts = append(reqOpts, option{optClientID, clientID})
	}
	h.setOptions(reqOpts)
	if _, err := ep.Write([]byte(h), serverAddr); err != nil {
		return Config{}, fmt.Errorf("dhcp discovery write: %v", err)
	}

	// DHCPACK
	for {
		var addr tcpip.FullAddress
		v, e := epin.Read(&addr)
		if e == tcpip.ErrWouldBlock {
			select {
			case <-ch:
				continue
			case <-ctx.Done():
				return Config{}, fmt.Errorf("reading dhcp ack: %v", tcpip.ErrAborted)
			}
		}
		h = header(v)
		var valid bool
		var err error
		opts, valid, err = loadDHCPReply(h, dhcpACK, xid[:])
		if !valid {
			if err != nil {
				// TODO: report malformed server responses
			}
			if opts, valid, _ = loadDHCPReply(h, dhcpNAK, xid[:]); valid {
				if msg := opts.message(); msg != "" {
					return Config{}, fmt.Errorf("dhcp: NAK %q", msg)
				}
				return Config{}, fmt.Errorf("dhcp: NAK with no message")
			}
			continue
		}
		break
	}
	ack = true
	return cfg, nil
}

func loadDHCPReply(h header, typ dhcpMsgType, xid []byte) (opts options, valid bool, err error) {
	if !h.isValid() || h.op() != opReply || !bytes.Equal(h.xidbytes(), xid[:]) {
		return nil, false, nil
	}
	opts, e := h.options()
	if e != nil {
		return nil, false, fmt.Errorf("dhcp ack: %v", e)
	}
	msgtype, e := opts.dhcpMsgType()
	if e != nil {
		return nil, false, fmt.Errorf("dhcp ack: %v", e)
	}
	if msgtype != typ {
		return nil, false, nil
	}
	return opts, true, nil
}