Add support for MQTT server as a notification target (#4474)

This implementation is similar to AMQP notifications:

* Notifications are published on a single topic as a JSON feed
* Topic is configurable, as is the QoS. Uses the paho.mqtt.golang
  library for the mqtt connection, and supports connections over tcp
  and websockets, with optional secure tls support.
* Additionally the minio server configuration has been bumped up
  so mqtt configuration can be added.
* Configuration migration code is added with tests.

MQTT is an ISO standard M2M/IoT messaging protocol and was
originally designed for applications for limited bandwidth
networks. Today it's use is growing in the IoT space.

vendor/github.com/eclipse/paho.mqtt.golang/packets/packets.go

@@ -0,0 +1,322 @@
+package packets
+
+import (
+	"bytes"
+	"encoding/binary"
+	"errors"
+	"fmt"
+	"io"
+)
+
+//ControlPacket defines the interface for structs intended to hold
+//decoded MQTT packets, either from being read or before being
+//written
+type ControlPacket interface {
+	Write(io.Writer) error
+	Unpack(io.Reader) error
+	String() string
+	Details() Details
+}
+
+//PacketNames maps the constants for each of the MQTT packet types
+//to a string representation of their name.
+var PacketNames = map[uint8]string{
+	1:  "CONNECT",
+	2:  "CONNACK",
+	3:  "PUBLISH",
+	4:  "PUBACK",
+	5:  "PUBREC",
+	6:  "PUBREL",
+	7:  "PUBCOMP",
+	8:  "SUBSCRIBE",
+	9:  "SUBACK",
+	10: "UNSUBSCRIBE",
+	11: "UNSUBACK",
+	12: "PINGREQ",
+	13: "PINGRESP",
+	14: "DISCONNECT",
+}
+
+//Below are the constants assigned to each of the MQTT packet types
+const (
+	Connect     = 1
+	Connack     = 2
+	Publish     = 3
+	Puback      = 4
+	Pubrec      = 5
+	Pubrel      = 6
+	Pubcomp     = 7
+	Subscribe   = 8
+	Suback      = 9
+	Unsubscribe = 10
+	Unsuback    = 11
+	Pingreq     = 12
+	Pingresp    = 13
+	Disconnect  = 14
+)
+
+//Below are the const definitions for error codes returned by
+//Connect()
+const (
+	Accepted                        = 0x00
+	ErrRefusedBadProtocolVersion    = 0x01
+	ErrRefusedIDRejected            = 0x02
+	ErrRefusedServerUnavailable     = 0x03
+	ErrRefusedBadUsernameOrPassword = 0x04
+	ErrRefusedNotAuthorised         = 0x05
+	ErrNetworkError                 = 0xFE
+	ErrProtocolViolation            = 0xFF
+)
+
+//ConnackReturnCodes is a map of the error codes constants for Connect()
+//to a string representation of the error
+var ConnackReturnCodes = map[uint8]string{
+	0:   "Connection Accepted",
+	1:   "Connection Refused: Bad Protocol Version",
+	2:   "Connection Refused: Client Identifier Rejected",
+	3:   "Connection Refused: Server Unavailable",
+	4:   "Connection Refused: Username or Password in unknown format",
+	5:   "Connection Refused: Not Authorised",
+	254: "Connection Error",
+	255: "Connection Refused: Protocol Violation",
+}
+
+//ConnErrors is a map of the errors codes constants for Connect()
+//to a Go error
+var ConnErrors = map[byte]error{
+	Accepted:                        nil,
+	ErrRefusedBadProtocolVersion:    errors.New("Unnacceptable protocol version"),
+	ErrRefusedIDRejected:            errors.New("Identifier rejected"),
+	ErrRefusedServerUnavailable:     errors.New("Server Unavailable"),
+	ErrRefusedBadUsernameOrPassword: errors.New("Bad user name or password"),
+	ErrRefusedNotAuthorised:         errors.New("Not Authorized"),
+	ErrNetworkError:                 errors.New("Network Error"),
+	ErrProtocolViolation:            errors.New("Protocol Violation"),
+}
+
+//ReadPacket takes an instance of an io.Reader (such as net.Conn) and attempts
+//to read an MQTT packet from the stream. It returns a ControlPacket
+//representing the decoded MQTT packet and an error. One of these returns will
+//always be nil, a nil ControlPacket indicating an error occurred.
+func ReadPacket(r io.Reader) (cp ControlPacket, err error) {
+	var fh FixedHeader
+	b := make([]byte, 1)
+
+	_, err = io.ReadFull(r, b)
+	if err != nil {
+		return nil, err
+	}
+	fh.unpack(b[0], r)
+	cp = NewControlPacketWithHeader(fh)
+	if cp == nil {
+		return nil, errors.New("Bad data from client")
+	}
+	packetBytes := make([]byte, fh.RemainingLength)
+	_, err = io.ReadFull(r, packetBytes)
+	if err != nil {
+		return nil, err
+	}
+	err = cp.Unpack(bytes.NewBuffer(packetBytes))
+	return cp, err
+}
+
+//NewControlPacket is used to create a new ControlPacket of the type specified
+//by packetType, this is usually done by reference to the packet type constants
+//defined in packets.go. The newly created ControlPacket is empty and a pointer
+//is returned.
+func NewControlPacket(packetType byte) (cp ControlPacket) {
+	switch packetType {
+	case Connect:
+		cp = &ConnectPacket{FixedHeader: FixedHeader{MessageType: Connect}}
+	case Connack:
+		cp = &ConnackPacket{FixedHeader: FixedHeader{MessageType: Connack}}
+	case Disconnect:
+		cp = &DisconnectPacket{FixedHeader: FixedHeader{MessageType: Disconnect}}
+	case Publish:
+		cp = &PublishPacket{FixedHeader: FixedHeader{MessageType: Publish}}
+	case Puback:
+		cp = &PubackPacket{FixedHeader: FixedHeader{MessageType: Puback}}
+	case Pubrec:
+		cp = &PubrecPacket{FixedHeader: FixedHeader{MessageType: Pubrec}}
+	case Pubrel:
+		cp = &PubrelPacket{FixedHeader: FixedHeader{MessageType: Pubrel, Qos: 1}}
+	case Pubcomp:
+		cp = &PubcompPacket{FixedHeader: FixedHeader{MessageType: Pubcomp}}
+	case Subscribe:
+		cp = &SubscribePacket{FixedHeader: FixedHeader{MessageType: Subscribe, Qos: 1}}
+	case Suback:
+		cp = &SubackPacket{FixedHeader: FixedHeader{MessageType: Suback}}
+	case Unsubscribe:
+		cp = &UnsubscribePacket{FixedHeader: FixedHeader{MessageType: Unsubscribe, Qos: 1}}
+	case Unsuback:
+		cp = &UnsubackPacket{FixedHeader: FixedHeader{MessageType: Unsuback}}
+	case Pingreq:
+		cp = &PingreqPacket{FixedHeader: FixedHeader{MessageType: Pingreq}}
+	case Pingresp:
+		cp = &PingrespPacket{FixedHeader: FixedHeader{MessageType: Pingresp}}
+	default:
+		return nil
+	}
+	return cp
+}
+
+//NewControlPacketWithHeader is used to create a new ControlPacket of the type
+//specified within the FixedHeader that is passed to the function.
+//The newly created ControlPacket is empty and a pointer is returned.
+func NewControlPacketWithHeader(fh FixedHeader) (cp ControlPacket) {
+	switch fh.MessageType {
+	case Connect:
+		cp = &ConnectPacket{FixedHeader: fh}
+	case Connack:
+		cp = &ConnackPacket{FixedHeader: fh}
+	case Disconnect:
+		cp = &DisconnectPacket{FixedHeader: fh}
+	case Publish:
+		cp = &PublishPacket{FixedHeader: fh}
+	case Puback:
+		cp = &PubackPacket{FixedHeader: fh}
+	case Pubrec:
+		cp = &PubrecPacket{FixedHeader: fh}
+	case Pubrel:
+		cp = &PubrelPacket{FixedHeader: fh}
+	case Pubcomp:
+		cp = &PubcompPacket{FixedHeader: fh}
+	case Subscribe:
+		cp = &SubscribePacket{FixedHeader: fh}
+	case Suback:
+		cp = &SubackPacket{FixedHeader: fh}
+	case Unsubscribe:
+		cp = &UnsubscribePacket{FixedHeader: fh}
+	case Unsuback:
+		cp = &UnsubackPacket{FixedHeader: fh}
+	case Pingreq:
+		cp = &PingreqPacket{FixedHeader: fh}
+	case Pingresp:
+		cp = &PingrespPacket{FixedHeader: fh}
+	default:
+		return nil
+	}
+	return cp
+}
+
+//Details struct returned by the Details() function called on
+//ControlPackets to present details of the Qos and MessageID
+//of the ControlPacket
+type Details struct {
+	Qos       byte
+	MessageID uint16
+}
+
+//FixedHeader is a struct to hold the decoded information from
+//the fixed header of an MQTT ControlPacket
+type FixedHeader struct {
+	MessageType     byte
+	Dup             bool
+	Qos             byte
+	Retain          bool
+	RemainingLength int
+}
+
+func (fh FixedHeader) String() string {
+	return fmt.Sprintf("%s: dup: %t qos: %d retain: %t rLength: %d", PacketNames[fh.MessageType], fh.Dup, fh.Qos, fh.Retain, fh.RemainingLength)
+}
+
+func boolToByte(b bool) byte {
+	switch b {
+	case true:
+		return 1
+	default:
+		return 0
+	}
+}
+
+func (fh *FixedHeader) pack() bytes.Buffer {
+	var header bytes.Buffer
+	header.WriteByte(fh.MessageType<<4 | boolToByte(fh.Dup)<<3 | fh.Qos<<1 | boolToByte(fh.Retain))
+	header.Write(encodeLength(fh.RemainingLength))
+	return header
+}
+
+func (fh *FixedHeader) unpack(typeAndFlags byte, r io.Reader) {
+	fh.MessageType = typeAndFlags >> 4
+	fh.Dup = (typeAndFlags>>3)&0x01 > 0
+	fh.Qos = (typeAndFlags >> 1) & 0x03
+	fh.Retain = typeAndFlags&0x01 > 0
+	fh.RemainingLength = decodeLength(r)
+}
+
+func decodeByte(b io.Reader) byte {
+	num := make([]byte, 1)
+	b.Read(num)
+	return num[0]
+}
+
+func decodeUint16(b io.Reader) uint16 {
+	num := make([]byte, 2)
+	b.Read(num)
+	return binary.BigEndian.Uint16(num)
+}
+
+func encodeUint16(num uint16) []byte {
+	bytes := make([]byte, 2)
+	binary.BigEndian.PutUint16(bytes, num)
+	return bytes
+}
+
+func encodeString(field string) []byte {
+	fieldLength := make([]byte, 2)
+	binary.BigEndian.PutUint16(fieldLength, uint16(len(field)))
+	return append(fieldLength, []byte(field)...)
+}
+
+func decodeString(b io.Reader) string {
+	fieldLength := decodeUint16(b)
+	field := make([]byte, fieldLength)
+	b.Read(field)
+	return string(field)
+}
+
+func decodeBytes(b io.Reader) []byte {
+	fieldLength := decodeUint16(b)
+	field := make([]byte, fieldLength)
+	b.Read(field)
+	return field
+}
+
+func encodeBytes(field []byte) []byte {
+	fieldLength := make([]byte, 2)
+	binary.BigEndian.PutUint16(fieldLength, uint16(len(field)))
+	return append(fieldLength, field...)
+}
+
+func encodeLength(length int) []byte {
+	var encLength []byte
+	for {
+		digit := byte(length % 128)
+		length /= 128
+		if length > 0 {
+			digit |= 0x80
+		}
+		encLength = append(encLength, digit)
+		if length == 0 {
+			break
+		}
+	}
+	return encLength
+}
+
+func decodeLength(r io.Reader) int {
+	var rLength uint32
+	var multiplier uint32
+	b := make([]byte, 1)
+	for multiplier < 27 { //fix: Infinite '(digit & 128) == 1' will cause the dead loop
+		io.ReadFull(r, b)
+		digit := b[0]
+		rLength |= uint32(digit&127) << multiplier
+		if (digit & 128) == 0 {
+			break
+		}
+		multiplier += 7
+	}
+	return int(rLength)
+}