// Copyright 2018 the u-root Authors and Andrea Barberio. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Package nclient6 is a minimum-functionality client for DHCPv6.
package nclient6

import (
	"context"
	"errors"
	"fmt"
	"log"
	"net"
	"os"
	"strings"
	"sync"
	"sync/atomic"
	"time"

	"github.com/insomniacslk/dhcp/dhcpv6"
)

// Broadcast destination IP addresses as defined by RFC 3315
var (
	AllDHCPRelayAgentsAndServers = &net.UDPAddr{
		IP:   net.ParseIP("ff02::1:2"),
		Port: dhcpv6.DefaultServerPort,
	}
	AllDHCPServers = &net.UDPAddr{
		IP:   net.ParseIP("ff05::1:3"),
		Port: dhcpv6.DefaultServerPort,
	}
)

var (
	// ErrNoResponse is returned when no response packet is received.
	ErrNoResponse = errors.New("no matching response packet received")
)

// pendingCh is a channel associated with a pending TransactionID.
type pendingCh struct {
	// SendAndRead closes done to indicate that it wishes for no more
	// messages for this particular XID.
	done <-chan struct{}

	// ch is used by the receive loop to distribute DHCP messages.
	ch chan<- *dhcpv6.Message
}

// Client is a DHCPv6 client.
type Client struct {
	ifaceHWAddr net.HardwareAddr
	conn        net.PacketConn
	timeout     time.Duration
	retry       int
	logger      logger

	// bufferCap is the channel capacity for each TransactionID.
	bufferCap int

	// serverAddr is the UDP address to send all packets to.
	//
	// This may be an actual broadcast address, or a unicast address.
	serverAddr *net.UDPAddr

	// closed is an atomic bool set to 1 when done is closed.
	closed uint32

	// done is closed to unblock the receive loop.
	done chan struct{}

	// wg protects the receiveLoop.
	wg sync.WaitGroup

	// printDropped logs dropped packets to logger if true.
	printDropped bool

	pendingMu sync.Mutex
	// pending stores the distribution channels for each pending
	// TransactionID. receiveLoop uses this map to determine which channel
	// to send a new DHCP message to.
	pending map[dhcpv6.TransactionID]*pendingCh
}

type logger interface {
	Printf(format string, v ...interface{})
	PrintMessage(prefix string, message *dhcpv6.Message)
}

type emptyLogger struct{}

func (e emptyLogger) Printf(format string, v ...interface{})              {}
func (e emptyLogger) PrintMessage(prefix string, message *dhcpv6.Message) {}

type shortSummaryLogger struct {
	*log.Logger
}

func (s shortSummaryLogger) Printf(format string, v ...interface{}) {
	s.Logger.Printf(format, v...)
}
func (s shortSummaryLogger) PrintMessage(prefix string, message *dhcpv6.Message) {
	s.Printf("%s: %s", prefix, message)
}

type debugLogger struct {
	*log.Logger
}

func (d debugLogger) Printf(format string, v ...interface{}) {
	d.Logger.Printf(format, v...)
}
func (d debugLogger) PrintMessage(prefix string, message *dhcpv6.Message) {
	d.Printf("%s: %s", prefix, message.Summary())
}

// NewIPv6UDPConn returns a UDP connection bound to both the interface and port
// given based on a IPv6 DGRAM socket.
func NewIPv6UDPConn(iface string, port int) (net.PacketConn, error) {
	ip, err := dhcpv6.GetLinkLocalAddr(iface)
	if err != nil {
		return nil, err
	}

	return net.ListenUDP("udp6", &net.UDPAddr{
		IP:   ip,
		Port: port,
		Zone: iface,
	})
}

// New returns a new DHCPv6 client for the given network interface.
func New(iface string, opts ...ClientOpt) (*Client, error) {
	c, err := NewIPv6UDPConn(iface, dhcpv6.DefaultClientPort)
	if err != nil {
		return nil, err
	}

	i, err := net.InterfaceByName(iface)
	if err != nil {
		return nil, err
	}
	return NewWithConn(c, i.HardwareAddr, opts...)
}

// NewWithConn creates a new DHCP client that sends and receives packets on the
// given interface.
func NewWithConn(conn net.PacketConn, ifaceHWAddr net.HardwareAddr, opts ...ClientOpt) (*Client, error) {
	c := &Client{
		ifaceHWAddr: ifaceHWAddr,
		timeout:     5 * time.Second,
		retry:       3,
		serverAddr:  AllDHCPRelayAgentsAndServers,
		bufferCap:   5,
		conn:        conn,
		logger:      emptyLogger{},

		done:    make(chan struct{}),
		pending: make(map[dhcpv6.TransactionID]*pendingCh),
	}

	for _, opt := range opts {
		opt(c)
	}

	if c.conn == nil {
		return nil, fmt.Errorf("require a connection")
	}

	c.receiveLoop()
	return c, nil
}

// Close closes the underlying connection.
func (c *Client) Close() error {
	// Make sure not to close done twice.
	if !atomic.CompareAndSwapUint32(&c.closed, 0, 1) {
		return nil
	}

	err := c.conn.Close()

	// Closing c.done sets off a chain reaction:
	//
	// Any SendAndRead unblocks trying to receive more messages, which
	// means rem() gets called.
	//
	// rem() should be unblocking receiveLoop if it is blocked.
	//
	// receiveLoop should then exit gracefully.
	close(c.done)

	// Wait for receiveLoop to stop.
	c.wg.Wait()

	return err
}

func isErrClosing(err error) bool {
	// Unfortunately, the epoll-connection-closed error is internal to the
	// net library.
	return strings.Contains(err.Error(), "use of closed network connection")
}

func (c *Client) receiveLoop() {
	c.wg.Add(1)
	go func() {
		defer c.wg.Done()
		for {
			// TODO: Clients can send a "max packet size" option in their
			// packets, IIRC. Choose a reasonable size and set it.
			b := make([]byte, 1500)
			n, _, err := c.conn.ReadFrom(b)
			if err != nil {
				if !isErrClosing(err) {
					c.logger.Printf("error reading from UDP connection: %v", err)
				}
				return
			}

			msg, err := dhcpv6.MessageFromBytes(b[:n])
			if err != nil {
				// Not a valid DHCP packet; keep listening.
				if c.printDropped {
					if len(b) > 12 {
						b = b[:12]
					}
					c.logger.Printf("Invalid DHCPv6 message received (len %d bytes), first 12 bytes: %#x", n, b)
				}
				continue
			}

			c.pendingMu.Lock()
			p, ok := c.pending[msg.TransactionID]
			if ok {
				select {
				case <-p.done:
					close(p.ch)
					delete(c.pending, msg.TransactionID)

				// This send may block.
				case p.ch <- msg:
				}
			} else if c.printDropped {
				// The Stringer will print the transaction ID.
				c.logger.Printf("No client waiting for msg with this XID: %s", msg)
			}
			c.pendingMu.Unlock()
		}
	}()
}

// ClientOpt is a function that configures the Client.
type ClientOpt func(*Client)

// WithTimeout configures the retransmission timeout.
//
// Default is 5 seconds.
func WithTimeout(d time.Duration) ClientOpt {
	return func(c *Client) {
		c.timeout = d
	}
}

// WithLogDroppedPackets logs a short message for dropped packets.
func WithLogDroppedPackets() ClientOpt {
	return func(c *Client) {
		c.printDropped = true
	}
}

// WithRetry configures the number of retransmissions to attempt.
//
// Default is 3.
func WithRetry(r int) ClientOpt {
	return func(c *Client) {
		c.retry = r
	}
}

// WithConn configures the packet connection to use.
func WithConn(conn net.PacketConn) ClientOpt {
	return func(c *Client) {
		c.conn = conn
	}
}

// WithBroadcastAddr configures the address to broadcast to.
func WithBroadcastAddr(n *net.UDPAddr) ClientOpt {
	return func(c *Client) {
		c.serverAddr = n
	}
}

// WithSummaryLogger logs one-line DHCPv6 message summarys when sent & received.
func WithSummaryLogger() ClientOpt {
	return func(c *Client) {
		c.logger = shortSummaryLogger{
			Logger: log.New(os.Stderr, "[dhcpv6] ", log.LstdFlags),
		}
	}
}

// WithDebugLogger logs multi-line full DHCPv6 messages when sent & received.
func WithDebugLogger() ClientOpt {
	return func(c *Client) {
		c.logger = debugLogger{
			Logger: log.New(os.Stderr, "[dhcpv6] ", log.LstdFlags),
		}
	}
}

// Matcher matches DHCP packets.
type Matcher func(*dhcpv6.Message) bool

// IsMessageType returns a matcher that checks for the message type.
func IsMessageType(t dhcpv6.MessageType, tt ...dhcpv6.MessageType) Matcher {
	return func(p *dhcpv6.Message) bool {
		if p.MessageType == t {
			return true
		}
		for _, mt := range tt {
			if p.MessageType == mt {
				return true
			}
		}
		return false
	}
}

// RemoteAddr is the default DHCP server address this client sends messages to.
func (c *Client) RemoteAddr() *net.UDPAddr {
	// Make a copy so the caller cannot modify the address once the client
	// is running.
	cop := *c.serverAddr
	return &cop
}

// InterfaceAddr returns the MAC address of the client's interface.
func (c *Client) InterfaceAddr() net.HardwareAddr {
	b := make(net.HardwareAddr, len(c.ifaceHWAddr))
	copy(b, c.ifaceHWAddr)
	return b
}

// RapidSolicit sends a solicitation message with the RapidCommit option and
// returns the first valid reply received.
func (c *Client) RapidSolicit(ctx context.Context, modifiers ...dhcpv6.Modifier) (*dhcpv6.Message, error) {
	solicit, err := dhcpv6.NewSolicit(c.ifaceHWAddr, append(modifiers, dhcpv6.WithRapidCommit)...)
	if err != nil {
		return nil, err
	}
	msg, err := c.SendAndRead(ctx, c.serverAddr, solicit, IsMessageType(dhcpv6.MessageTypeReply, dhcpv6.MessageTypeAdvertise))
	if err != nil {
		return nil, err
	}

	switch msg.MessageType {
	case dhcpv6.MessageTypeReply:
		// We got RapidCommitted.
		return msg, nil

	case dhcpv6.MessageTypeAdvertise:
		// We didn't get RapidCommitted. Request regular lease.
		return c.Request(ctx, msg, modifiers...)

	default:
		return nil, fmt.Errorf("invalid message type: cannot happen")
	}
}

// Solicit sends a solicitation message and returns the first valid
// advertisement received.
func (c *Client) Solicit(ctx context.Context, modifiers ...dhcpv6.Modifier) (*dhcpv6.Message, error) {
	solicit, err := dhcpv6.NewSolicit(c.ifaceHWAddr, modifiers...)
	if err != nil {
		return nil, err
	}
	msg, err := c.SendAndRead(ctx, c.serverAddr, solicit, IsMessageType(dhcpv6.MessageTypeAdvertise))
	if err != nil {
		return nil, err
	}
	return msg, nil
}

// Request requests an IP Assignment from peer given an advertise message.
func (c *Client) Request(ctx context.Context, advertise *dhcpv6.Message, modifiers ...dhcpv6.Modifier) (*dhcpv6.Message, error) {
	request, err := dhcpv6.NewRequestFromAdvertise(advertise, modifiers...)
	if err != nil {
		return nil, err
	}
	return c.SendAndRead(ctx, c.serverAddr, request, nil)
}

// send sends p to destination and returns a response channel.
//
// The returned function must be called after all desired responses have been
// received.
//
// Responses will be matched by transaction ID.
func (c *Client) send(dest net.Addr, msg *dhcpv6.Message) (<-chan *dhcpv6.Message, func(), error) {
	c.pendingMu.Lock()
	if _, ok := c.pending[msg.TransactionID]; ok {
		c.pendingMu.Unlock()
		return nil, nil, fmt.Errorf("transaction ID %s already in use", msg.TransactionID)
	}

	ch := make(chan *dhcpv6.Message, c.bufferCap)
	done := make(chan struct{})
	c.pending[msg.TransactionID] = &pendingCh{done: done, ch: ch}
	c.pendingMu.Unlock()

	cancel := func() {
		// Why can't we just close ch here?
		//
		// Because receiveLoop may potentially be blocked trying to
		// send on ch. We gotta unblock it first, so it'll unlock the
		// lock, and then we can take the lock and remove the XID from
		// the pending transaction map.
		close(done)

		c.pendingMu.Lock()
		if p, ok := c.pending[msg.TransactionID]; ok {
			close(p.ch)
			delete(c.pending, msg.TransactionID)
		}
		c.pendingMu.Unlock()
	}

	if _, err := c.conn.WriteTo(msg.ToBytes(), dest); err != nil {
		cancel()
		return nil, nil, fmt.Errorf("error writing packet to connection: %v", err)
	}
	return ch, cancel, nil
}

// This should never be visible to a user.
var errDeadlineExceeded = errors.New("INTERNAL ERROR: deadline exceeded")

// SendAndRead sends a packet p to a destination dest and waits for the first
// response matching `match` as well as its Transaction ID.
//
// If match is nil, the first packet matching the Transaction ID is returned.
func (c *Client) SendAndRead(ctx context.Context, dest *net.UDPAddr, msg *dhcpv6.Message, match Matcher) (*dhcpv6.Message, error) {
	var response *dhcpv6.Message
	err := c.retryFn(func(timeout time.Duration) error {
		ch, rem, err := c.send(dest, msg)
		if err != nil {
			return err
		}
		c.logger.PrintMessage("sent message", msg)
		defer rem()

		for {
			select {
			case <-c.done:
				return ErrNoResponse

			case <-time.After(timeout):
				return errDeadlineExceeded

			case <-ctx.Done():
				return ctx.Err()

			case packet := <-ch:
				if match == nil || match(packet) {
					c.logger.PrintMessage("received message", packet)
					response = packet
					return nil
				}
			}
		}
	})
	if err == errDeadlineExceeded {
		return nil, ErrNoResponse
	}
	if err != nil {
		return nil, err
	}
	return response, nil
}

func (c *Client) retryFn(fn func(timeout time.Duration) error) error {
	timeout := c.timeout

	// Each retry takes the amount of timeout at worst.
	for i := 0; i < c.retry || c.retry < 0; i++ {
		switch err := fn(timeout); err {
		case nil:
			// Got it!
			return nil

		case errDeadlineExceeded:
			// Double timeout, then retry.
			timeout *= 2

		default:
			return err
		}
	}

	return errDeadlineExceeded
}