iOS Push notification service in Go

apns.go

package main

import (
	"bytes"
	"crypto/tls"
	"encoding/binary"
	"encoding/hex"
	"encoding/json"
	"fmt"
	"net"
	"time"
)

type ApnNotification struct {
	DeviceToken string
	Payload     map[string]string
}

func (n *ApnNotification) Connect() (*tls.Conn, error) {
	cer, err := tls.LoadX509KeyPair("cert.pem", "key.pem")
	if err != nil {
		return nil, &PushError{Text: fmt.Sprintf("Error loading certificate: %s\n", err.Error())}
	}

	cfg := &tls.Config{
		Certificates: []tls.Certificate{cer},
	}

	c, err := net.Dial("tcp", "gateway.sandbox.push.apple.com:2195")
	if err != nil {
		return nil, &PushError{Text: fmt.Sprintf("Error connecting: %s\n", err.Error())}
	}

	result := tls.Client(c, cfg)

	err = result.Handshake()
	if err != nil {
		return nil, &PushError{Text: fmt.Sprintf("Error authenticating: %s\n", err.Error())}
	}

	return result, nil
}

func (n *ApnNotification) BuildMessage() []byte {
	// prepare binary payload from JSON structure
	p := make(map[string]interface{})
	p["aps"] = n.Payload

	bp, _ := json.Marshal(p)

	// decode hexadecimal push device token to binary byte array
	t, _ := hex.DecodeString(n.DeviceToken)

	// build the actual message
	buf := bytes.NewBuffer([]byte{})

	binary.Write(buf, binary.BigEndian, uint8(1))           // Command byte (1)
	binary.Write(buf, binary.BigEndian, uint32(1))          // Optional transaction ID
	binary.Write(buf, binary.BigEndian, uint32(60*60*24*7)) // Expiration (1 week)
	binary.Write(buf, binary.BigEndian, uint16(len(t)))     // Device token length
	binary.Write(buf, binary.BigEndian, t)                  // Device token
	binary.Write(buf, binary.BigEndian, uint16(len(bp)))    // Payload length
	binary.Write(buf, binary.BigEndian, bp)                 // Payload

	return buf.Bytes()
}

func (n *ApnNotification) ParseError(r []byte) error {
	code, _ := binary.Uvarint(r)

	switch code {
	case 0x01:
		return &PushError{Text: "Processing error"}
	case 0x02:
		return &PushError{Text: "Missing device token"}
	case 0x03:
		return &PushError{Text: "Missing topic"}
	case 0x04:
		return &PushError{Text: "Missing payload"}
	case 0x05:
		return &PushError{Text: "Invalid token size"}
	case 0x06:
		return &PushError{Text: "Invalid topic size"}
	case 0x07:
		return &PushError{Text: "Invalid payload size"}
	case 0x08:
		return &PushError{Text: "Invalid token"}
	}

	return nil
}

func (n *ApnNotification) Send() error {
	c, err := n.Connect()
	defer c.Close()

	if err != nil {
		return err
	}

	m := n.BuildMessage()

	// Write the message on the connection
	_, err = c.Write(m)
	if err != nil {
		return &PushError{Text: fmt.Sprintf("Error writing: %s\n", err.Error())}
	}

	// Get errors
	d, _ := time.ParseDuration("5s")
	c.SetReadDeadline(time.Now().Add(d))

	res := [6]byte{}

	r, err := c.Read(res[:])
	if r > 0 {
		return n.ParseError(res[1:2])
	}

	return nil
}