lib/email: set the header Date field on NewMultipart

The Date field value is set to current time on the system that
generated the message.

The date format is set to "Mon, 2 Jan 2006 15:04:05 -0700" according
to RFC 5322 section 3.3.

3 files changed, 43 insertions, 2 deletions

diff --git a/lib/email/email.go b/lib/email/email.go
index 4471a39e..0e56fb4d 100644
--- a/lib/email/email.go
+++ b/lib/email/email.go
@@ -4,10 +4,13 @@
 
 package email
 
+import "time"
+
 const (
 	contentTypeMultipartAlternative = "multipart/alternative"
 	contentTypeTextPlain            = `text/plain; charset="utf-8"`
 	contentTypeTextHTML             = `text/html; charset="utf-8"`
+	DateFormat                      = "Mon, 2 Jan 2006 15:04:05 -0700"
 	encodingQuotedPrintable         = "quoted-printable"
 	mimeVersion1                    = "1.0"
 )
@@ -17,4 +20,14 @@ const (
 	lf byte = '\n'
 )
 
+//
+// Epoch return the UNIX timestamp in seconds.
+//
+// This variable is exported to allow function that use date and/or time can
+// be tested with fixed, predictable value.
+//
+var Epoch = func() int64 {
+	return time.Now().Unix()
+}
+
 var boundSeps = []byte{'-', '-'}
diff --git a/lib/email/message.go b/lib/email/message.go
index 4bc4de12..eafdc144 100644
--- a/lib/email/message.go
+++ b/lib/email/message.go
@@ -10,6 +10,7 @@ import (
 	"fmt"
 	"io/ioutil"
 	"strings"
+	"time"
 
 	libbytes "github.com/shuLhan/share/lib/bytes"
 	"github.com/shuLhan/share/lib/email/dkim"
@@ -31,11 +32,22 @@ type Message struct {
 func NewMultipart(from, to, subject, bodyText, bodyHTML []byte) (
 	msg *Message, err error,
 ) {
+	var (
+		logp      = "NewMultipart"
+		timeNow   = time.Unix(Epoch(), 0)
+		dateValue = timeNow.Format(DateFormat)
+	)
+
 	msg = &Message{
 		Header: &Header{},
 		Body:   &Body{},
 	}
 
+	err = msg.Header.Set(FieldTypeDate, []byte(dateValue))
+	if err != nil {
+		return nil, fmt.Errorf("%s: %w", logp, err)
+	}
+
 	err = msg.Header.Set(FieldTypeFrom, from)
 	if err != nil {
 		return nil, fmt.Errorf("email.NewMultipart: %w", err)
diff --git a/lib/smtp/mail_tx_example_test.go b/lib/smtp/mail_tx_example_test.go
index 28e3653f..0b4dda52 100644
--- a/lib/smtp/mail_tx_example_test.go
+++ b/lib/smtp/mail_tx_example_test.go
@@ -9,6 +9,8 @@ import (
 	"fmt"
 	"log"
 	"math/rand"
+	"regexp"
+	"time"
 
 	"github.com/shuLhan/share/lib/email"
 )
@@ -18,13 +20,20 @@ func ExampleNewMailTx() {
 	//
 	// [1] github.com/shuLhan/share/lib/email
 
+	// Overwrite the email.Epoch to make the example works.
+	email.Epoch = func() int64 {
+		return 1645600000
+	}
+
 	var (
 		txFrom      = "Postmaster <postmaster@mail.example.com>"
 		fromAddress = []byte("Noreply <noreply@example.com>")
+		toAddresses = []byte("John <john@example.com>, Jane <jane@example.com>")
 		subject     = []byte(`Example subject`)
 		bodyText    = []byte(`Email body as plain text`)
 		bodyHtml    = []byte(`Email body as <b>HTML</b>`)
-		toAddresses = []byte("John <john@example.com>, Jane <jane@example.com>")
+		timeNowUtc  = time.Unix(email.Epoch(), 0).UTC()
+		dateNowUtc  = timeNowUtc.Format(email.DateFormat)
 
 		recipients []string
 		mboxes     []*email.Mailbox
@@ -65,14 +74,21 @@ func ExampleNewMailTx() {
 	fmt.Printf("Tx Recipients: %s\n", mailtx.Recipients)
 
 	// In order to make the example Output works, we need to replace all
-	// CRLF with LF.
+	// CRLF with LF and "date:" with the system timezone.
 	data = bytes.ReplaceAll(mailtx.Data, []byte("\r\n"), []byte("\n"))
 
+	//fmt.Printf("timeNowUtc: %s\n", timeNowUtc)
+	//fmt.Printf("dateNowUtc: %s\n", dateNowUtc)
+
+	reDate := regexp.MustCompile(`^date: Wed(.*) \+....`)
+	data = reDate.ReplaceAll(data, []byte(`date: `+dateNowUtc))
+
 	fmt.Printf("Tx Data:\n%s", data)
 	//Output:
 	//Tx From: Postmaster <postmaster@mail.example.com>
 	//Tx Recipients: [john@example.com jane@example.com]
 	//Tx Data:
+	//date: Wed, 23 Feb 2022 07:06:40 +0000
 	//from: Noreply <noreply@example.com>
 	//to: John <john@example.com>, Jane <jane@example.com>
 	//subject: Example subject