mailer.go

// Copyright (c) 2019, Mohlmann Solutions SRL. All rights reserved.
// Use of this source code is governed by a License that can be found in the LICENSE file.
// SPDX-License-Identifier: BSD-3-Clause

//Package mailer is a functional wrapper around the standard "net/smtp" and "html/template" packages.
package mailer

import (
	"bytes"
	"fmt"
	"html/template"
	"log"
	"net/smtp"
	"strings"
	"time"
)

const (
	// GlobalHeaders is set on every message.
	GlobalHeaders = "MIME-Version: 1.0\r\nContent-type: text/html; charset=\"UTF-8\"\r\n\r\n"
)

// Debug mode prints the outgoing mail before sending
var Debug bool

// Header for mail message
type Header struct {
	Key    string
	Values []string
}

func (h Header) String() string {
	if h.Values == nil {
		return ""
	}
	return fmt.Sprintf(
		"%s: %s\r\n",
		strings.Title(h.Key),
		strings.Join(h.Values, ","),
	)
}

func mailHeaders(headers []Header) *bytes.Buffer {
	var msg bytes.Buffer
	for _, h := range headers {
		msg.WriteString(h.String())
	}
	msg.WriteString(GlobalHeaders)
	return &msg
}

// Mailer holds a html template, server and authentication information for efficient reuse.
type Mailer struct {
	tmpl *template.Template
	addr string
	from string
	auth smtp.Auth
}

// New returns a reusable mailer.
// Tmpl should hold a collection of parsed templates.
// Addr is the hostname and port used by smtp.SendMail. For example:
//   "mail.host.com:587"
// From is used in every subsequent SendMail invocation and set as message header.
// If auth is nil, connections will omit authentication.
func New(tmpl *template.Template, addr, from string, auth smtp.Auth) *Mailer {
	return &Mailer{tmpl, addr, from, auth}
}

// Send renders the headers and named template with passed data.
// The rendered message is sent using smtp.SendMail, to all the recipients.
//
// Headers keys are rendered Title cased, and the values are joined with a comma separator.
// Each entry becomes a CRLF separated line. For example:
//   {"to", []string{"test@test.mailu.io", "admin@test.mailu.io"}}
// Results in:
//   To: test@test.mailu.io,admin@test.mailu.io
//
// The current time is automatically appended as timestamp header.
func (m *Mailer) Send(headers []Header, tmplName string, data interface{}, recipients ...string) error {
	headers = append(headers,
		Header{"from", []string{m.from}},
		Header{"date", []string{time.Now().Format(time.RFC822Z)}},
	)

	msg := mailHeaders(headers)
	if err := m.tmpl.ExecuteTemplate(msg, tmplName, data); err != nil {
		return err
	}
	if Debug {
		log.Printf("mailer: %+v;\n------------\n%s", m, msg)
	}
	return smtp.SendMail(m.addr, m.auth, m.from, recipients, msg.Bytes())
}