Add basic email abstraction layer

(Untested.) This will allow us to send via any supported provider within
WriteFreely.

Ref T731 T905

go.mod

@@ -53,6 +53,8 @@ require (
 	golang.org/x/net v0.30.0
 )
 
+require github.com/xhit/go-simple-mail/v2 v2.16.0
+
 require (
 	code.as/core/socks v1.0.0 // indirect
 	filippo.io/edwards25519 v1.1.0 // indirect
@@ -82,6 +84,7 @@ require (
 	github.com/russross/blackfriday/v2 v2.1.0 // indirect
 	github.com/sasha-s/go-deadlock v0.3.1 // indirect
 	github.com/shurcooL/sanitized_anchor_name v1.0.0 // indirect
+	github.com/toorop/go-dkim v0.0.0-20201103131630-e1cd1a0a5208 // indirect
 	github.com/writeas/go-writeas/v2 v2.0.2 // indirect
 	github.com/writeas/openssl-go v1.0.0 // indirect
 	github.com/writeas/slug v1.2.0 // indirect

go.sum

@@ -177,6 +177,8 @@ github.com/stretchr/testify v1.5.1/go.mod h1:5W2xD1RspED5o8YsWQXVCued0rvSQ+mT+I5
 github.com/stretchr/testify v1.7.0/go.mod h1:6Fq8oRcR53rry900zMqJjRRixrwX3KX962/h/Wwjteg=
 github.com/stretchr/testify v1.9.0 h1:HtqpIVDClZ4nwg75+f6Lvsy/wHu+3BoSGCbBAcpTsTg=
 github.com/stretchr/testify v1.9.0/go.mod h1:r2ic/lqez/lEtzL7wO/rwa5dbSLXVDPFyf8C91i36aY=
+github.com/toorop/go-dkim v0.0.0-20201103131630-e1cd1a0a5208 h1:PM5hJF7HVfNWmCjMdEfbuOBNXSVF2cMFGgQTPdKCbwM=
+github.com/toorop/go-dkim v0.0.0-20201103131630-e1cd1a0a5208/go.mod h1:BzWtXXrXzZUvMacR0oF/fbDDgUPO8L36tDMmRAf14ns=
 github.com/urfave/cli/v2 v2.27.4 h1:o1owoI+02Eb+K107p27wEX9Bb8eqIoZCfLXloLUSWJ8=
 github.com/urfave/cli/v2 v2.27.4/go.mod h1:m4QzxcD2qpra4z7WhzEGn74WZLViBnMpb1ToCAKdGRQ=
 github.com/writeas/activity v0.1.2 h1:Y12B5lIrabfqKE7e7HFCWiXrlfXljr9tlkFm2mp7DgY=
@@ -212,6 +214,8 @@ github.com/writefreely/go-gopher v0.0.0-20220429181814-40127126f83b h1:h3NzB8OZ5
 github.com/writefreely/go-gopher v0.0.0-20220429181814-40127126f83b/go.mod h1:T2UVVzt+R5KSSZe2xRSytnwc2M9AoDegi7foeIsik+M=
 github.com/writefreely/go-nodeinfo v1.2.0 h1:La+YbTCvmpTwFhBSlebWDDL81N88Qf/SCAvRLR7F8ss=
 github.com/writefreely/go-nodeinfo v1.2.0/go.mod h1:UTvE78KpcjYOlRHupZIiSEFcXHioTXuacCbHU+CAcPg=
+github.com/xhit/go-simple-mail/v2 v2.16.0 h1:ouGy/Ww4kuaqu2E2UrDw7SvLaziWTB60ICLkIkNVccA=
+github.com/xhit/go-simple-mail/v2 v2.16.0/go.mod h1:b7P5ygho6SYE+VIqpxA6QkYfv4teeyG4MKqB3utRu98=
 github.com/xrash/smetrics v0.0.0-20240521201337-686a1a2994c1 h1:gEOO8jv9F4OT7lGCjxCBTO/36wtF6j2nSip77qHd4x4=
 github.com/xrash/smetrics v0.0.0-20240521201337-686a1a2994c1/go.mod h1:Ohn+xnUBiLI6FVj/9LpzZWtj1/D6lUovWYBkxHVV3aM=
 github.com/yuin/goldmark v1.2.1/go.mod h1:3hX8gzYuyVAZsxl0MRgGTJEmQBFcNTphYh9decYSb74=

mailer/mailer.go

@@ -0,0 +1,98 @@
+/*
+ * Copyright © 2024 Musing Studio LLC.
+ *
+ * This file is part of WriteFreely.
+ *
+ * WriteFreely is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License, included
+ * in the LICENSE file in this source code package.
+ */
+
+package mailer
+
+import (
+	"fmt"
+	"github.com/mailgun/mailgun-go"
+	"github.com/writefreely/writefreely/config"
+	mail "github.com/xhit/go-simple-mail/v2"
+)
+
+type (
+	// Mailer holds configurations for the preferred mailing provider.
+	Mailer struct {
+		smtp    *mail.SMTPServer
+		mailGun *mailgun.MailgunImpl
+	}
+
+	// Message holds the email contents and metadata for the preferred mailing provider.
+	Message struct {
+		mgMsg   *mailgun.Message
+		smtpMsg *mail.Email
+	}
+)
+
+// New creates a new Mailer from the instance's config.EmailCfg, returning an error if not properly configured.
+func New(eCfg *config.EmailCfg) (*Mailer, error) {
+	m := &Mailer{}
+	if eCfg.Domain != "" && eCfg.MailgunPrivate != "" {
+		m.mailGun = mailgun.NewMailgun(eCfg.Domain, eCfg.MailgunPrivate)
+	} else if eCfg.Username != "" && eCfg.Password != "" && eCfg.Host != "" && eCfg.Port > 0 {
+		m.smtp = mail.NewSMTPClient()
+		m.smtp.Host = eCfg.Host
+		m.smtp.Port = eCfg.Port
+		m.smtp.Username = eCfg.Username
+		m.smtp.Password = eCfg.Password
+		if eCfg.EnableStartTLS {
+			m.smtp.Encryption = mail.EncryptionSTARTTLS
+		}
+	} else {
+		return nil, fmt.Errorf("no email provider is configured")
+	}
+
+	return m, nil
+}
+
+// NewMessage creates a new Message from the given parameters.
+func (m *Mailer) NewMessage(from, subject, text string, to ...string) (*Message, error) {
+	msg := &Message{}
+	if m.mailGun != nil {
+		msg.mgMsg = m.mailGun.NewMessage(from, subject, text, to...)
+	} else if m.smtp != nil {
+		msg.smtpMsg = mail.NewMSG()
+		msg.smtpMsg.SetFrom(from)
+		msg.smtpMsg.AddTo(to...)
+		msg.smtpMsg.SetSubject(subject)
+		msg.smtpMsg.AddAlternative(mail.TextPlain, text)
+
+		if msg.smtpMsg.Error != nil {
+			return nil, msg.smtpMsg.Error
+		}
+	}
+	return msg, nil
+}
+
+// SetHTML sets the body of the message.
+func (m *Message) SetHTML(html string) {
+	if m.smtpMsg != nil {
+		m.smtpMsg.SetBody(mail.TextHTML, html)
+	} else if m.mgMsg != nil {
+		m.mgMsg.SetHtml(html)
+	}
+}
+
+// Send sends the given message via the preferred provider.
+func (m *Mailer) Send(msg *Message) error {
+	if m.smtp != nil {
+		client, err := m.smtp.Connect()
+		if err != nil {
+			return err
+		}
+		return msg.smtpMsg.Send(client)
+	} else if m.mailGun != nil {
+		_, _, err := m.mailGun.Send(msg.mgMsg)
+		if err != nil {
+			return err
+		}
+	}
+	return nil
+}