[feature] Email notifications for new / closed moderation reports (#1628)

* start fiddling about with email sending to allow multiple recipients

* do some fiddling

* notifs working

* notify on closed report

* finishing up

* envparsing

* use strings.ContainsAny

internal/email/common.go

@@ -0,0 +1,112 @@
+// GoToSocial
+// Copyright (C) GoToSocial Authors admin@gotosocial.org
+// SPDX-License-Identifier: AGPL-3.0-or-later
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package email
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"net/smtp"
+	"os"
+	"path/filepath"
+	"strings"
+	"text/template"
+
+	"github.com/superseriousbusiness/gotosocial/internal/config"
+	"github.com/superseriousbusiness/gotosocial/internal/gtserror"
+)
+
+func (s *sender) sendTemplate(template string, subject string, data any, toAddresses ...string) error {
+	buf := &bytes.Buffer{}
+	if err := s.template.ExecuteTemplate(buf, template, data); err != nil {
+		return err
+	}
+
+	msg, err := assembleMessage(subject, buf.String(), s.from, toAddresses...)
+	if err != nil {
+		return err
+	}
+
+	if err := smtp.SendMail(s.hostAddress, s.auth, s.from, toAddresses, msg); err != nil {
+		return gtserror.SetType(err, gtserror.TypeSMTP)
+	}
+
+	return nil
+}
+
+func loadTemplates(templateBaseDir string) (*template.Template, error) {
+	if !filepath.IsAbs(templateBaseDir) {
+		cwd, err := os.Getwd()
+		if err != nil {
+			return nil, fmt.Errorf("error getting current working directory: %s", err)
+		}
+		templateBaseDir = filepath.Join(cwd, templateBaseDir)
+	}
+
+	// look for all templates that start with 'email_'
+	return template.ParseGlob(filepath.Join(templateBaseDir, "email_*"))
+}
+
+// assembleMessage assembles a valid email message following:
+//   - https://datatracker.ietf.org/doc/html/rfc2822
+//   - https://pkg.go.dev/net/smtp#SendMail
+func assembleMessage(mailSubject string, mailBody string, mailFrom string, mailTo ...string) ([]byte, error) {
+	if strings.ContainsAny(mailSubject, "\r\n") {
+		return nil, errors.New("email subject must not contain newline characters")
+	}
+
+	if strings.ContainsAny(mailFrom, "\r\n") {
+		return nil, errors.New("email from address must not contain newline characters")
+	}
+
+	for _, to := range mailTo {
+		if strings.ContainsAny(to, "\r\n") {
+			return nil, errors.New("email to address must not contain newline characters")
+		}
+	}
+
+	// Normalize the message body to use CRLF line endings
+	const CRLF = "\r\n"
+	mailBody = strings.ReplaceAll(mailBody, CRLF, "\n")
+	mailBody = strings.ReplaceAll(mailBody, "\n", CRLF)
+
+	msg := bytes.Buffer{}
+	switch {
+	case len(mailTo) == 1:
+		// Address email directly to the one recipient.
+		msg.WriteString("To: " + mailTo[0] + CRLF)
+	case config.GetSMTPDiscloseRecipients():
+		// Simply address To all recipients.
+		msg.WriteString("To: " + strings.Join(mailTo, ", ") + CRLF)
+	default:
+		// Address To anonymous group.
+		//
+		// Email will be sent to all recipients but we shouldn't include Bcc header.
+		//
+		// From the smtp.SendMail function: 'Sending "Bcc" messages is accomplished by
+		// including an email address in the to parameter but not including it in the
+		// msg headers.'
+		msg.WriteString("To: Undisclosed Recipients:;" + CRLF)
+	}
+	msg.WriteString("Subject: " + mailSubject + CRLF)
+	msg.WriteString(CRLF)
+	msg.WriteString(mailBody)
+	msg.WriteString(CRLF)
+
+	return msg.Bytes(), nil
+}