Split mail sender sub package from mailer service package (#32618)

Move all mail sender related codes into a sub package of
services/mailer. Just move, no code change.
Then we just have dependencies on go-mail package in the new sub
package. We can use other package to replace it because it's
unmaintainable. ref #18664

services/mailer/sender/message_test.go

@@ -0,0 +1,114 @@
+// Copyright 2024 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package sender
+
+import (
+	"strings"
+	"testing"
+	"time"
+
+	"code.gitea.io/gitea/modules/setting"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestGenerateMessageID(t *testing.T) {
+	mailService := setting.Mailer{
+		From: "test@gitea.com",
+	}
+
+	setting.MailService = &mailService
+	setting.Domain = "localhost"
+
+	date := time.Date(2000, 1, 2, 3, 4, 5, 6, time.UTC)
+	m := NewMessageFrom("", "display-name", "from-address", "subject", "body")
+	m.Date = date
+	gm := m.ToMessage()
+	assert.Equal(t, "<autogen-946782245000-41e8fc54a8ad3a3f@localhost>", gm.GetHeader("Message-ID")[0])
+
+	m = NewMessageFrom("a@b.com", "display-name", "from-address", "subject", "body")
+	m.Date = date
+	gm = m.ToMessage()
+	assert.Equal(t, "<autogen-946782245000-cc88ce3cfe9bd04f@localhost>", gm.GetHeader("Message-ID")[0])
+
+	m = NewMessageFrom("a@b.com", "display-name", "from-address", "subject", "body")
+	m.SetHeader("Message-ID", "<msg-d@domain.com>")
+	gm = m.ToMessage()
+	assert.Equal(t, "<msg-d@domain.com>", gm.GetHeader("Message-ID")[0])
+}
+
+func TestToMessage(t *testing.T) {
+	oldConf := *setting.MailService
+	defer func() {
+		setting.MailService = &oldConf
+	}()
+	setting.MailService.From = "test@gitea.com"
+
+	m1 := Message{
+		Info:            "info",
+		FromAddress:     "test@gitea.com",
+		FromDisplayName: "Test Gitea",
+		To:              "a@b.com",
+		Subject:         "Issue X Closed",
+		Body:            "Some Issue got closed by Y-Man",
+	}
+
+	buf := &strings.Builder{}
+	_, err := m1.ToMessage().WriteTo(buf)
+	assert.NoError(t, err)
+	header, _ := extractMailHeaderAndContent(t, buf.String())
+	assert.EqualValues(t, map[string]string{
+		"Content-Type":             "multipart/alternative;",
+		"Date":                     "Mon, 01 Jan 0001 00:00:00 +0000",
+		"From":                     "\"Test Gitea\" <test@gitea.com>",
+		"Message-ID":               "<autogen--6795364578871-69c000786adc60dc@localhost>",
+		"Mime-Version":             "1.0",
+		"Subject":                  "Issue X Closed",
+		"To":                       "a@b.com",
+		"X-Auto-Response-Suppress": "All",
+	}, header)
+
+	setting.MailService.OverrideHeader = map[string][]string{
+		"Message-ID":     {""},               // delete message id
+		"Auto-Submitted": {"auto-generated"}, // suppress auto replay
+	}
+
+	buf = &strings.Builder{}
+	_, err = m1.ToMessage().WriteTo(buf)
+	assert.NoError(t, err)
+	header, _ = extractMailHeaderAndContent(t, buf.String())
+	assert.EqualValues(t, map[string]string{
+		"Content-Type":             "multipart/alternative;",
+		"Date":                     "Mon, 01 Jan 0001 00:00:00 +0000",
+		"From":                     "\"Test Gitea\" <test@gitea.com>",
+		"Message-ID":               "",
+		"Mime-Version":             "1.0",
+		"Subject":                  "Issue X Closed",
+		"To":                       "a@b.com",
+		"X-Auto-Response-Suppress": "All",
+		"Auto-Submitted":           "auto-generated",
+	}, header)
+}
+
+func extractMailHeaderAndContent(t *testing.T, mail string) (map[string]string, string) {
+	header := make(map[string]string)
+
+	parts := strings.SplitN(mail, "boundary=", 2)
+	if !assert.Len(t, parts, 2) {
+		return nil, ""
+	}
+	content := strings.TrimSpace("boundary=" + parts[1])
+
+	hParts := strings.Split(parts[0], "\n")
+
+	for _, hPart := range hParts {
+		parts := strings.SplitN(hPart, ":", 2)
+		hk := strings.TrimSpace(parts[0])
+		if hk != "" {
+			header[hk] = strings.TrimSpace(parts[1])
+		}
+	}
+
+	return header, content
+}