feat(mail): RFC822 message parsing (headers, body, attachments)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/mail/message.go

@@ -0,0 +1,98 @@
+// Package mail provides IMAP reading and RFC822 message parsing.
+package mail
+
+import (
+	"bytes"
+	"io"
+	"strings"
+
+	"github.com/emersion/go-message/mail"
+)
+
+type Header struct {
+	UID            uint32
+	From           string
+	To             string
+	Subject        string
+	Date           string
+	MessageID      string
+	HasAttachments bool
+}
+
+type Attachment struct {
+	Name    string
+	Size    int
+	MIME    string
+	Content []byte
+}
+
+type Message struct {
+	Header      Header
+	BodyText    string
+	Attachments []Attachment
+}
+
+func readHeader(mr *mail.Reader, uid uint32) Header {
+	h := Header{UID: uid}
+	hd := mr.Header
+	h.Subject, _ = hd.Subject()
+	if addrs, err := hd.AddressList("From"); err == nil && len(addrs) > 0 {
+		h.From = addrs[0].String()
+	}
+	if addrs, err := hd.AddressList("To"); err == nil && len(addrs) > 0 {
+		h.To = addrs[0].String()
+	}
+	if d, err := hd.Date(); err == nil {
+		h.Date = d.UTC().Format("Mon, 02 Jan 2006 15:04:05 -0700")
+	}
+	if msgID, err := hd.MessageID(); err == nil {
+		h.MessageID = msgID
+	} else {
+		h.MessageID = strings.Trim(hd.Get("Message-Id"), "<> ")
+	}
+	return h
+}
+
+// ParseMessage decodes the full message including attachment contents.
+func ParseMessage(uid uint32, raw []byte) (Message, error) {
+	mr, err := mail.CreateReader(bytes.NewReader(raw))
+	if err != nil {
+		return Message{}, err
+	}
+	m := Message{Header: readHeader(mr, uid)}
+	for {
+		part, err := mr.NextPart()
+		if err == io.EOF {
+			break
+		}
+		if err != nil {
+			return Message{}, err
+		}
+		switch hdr := part.Header.(type) {
+		case *mail.InlineHeader:
+			ct, _, _ := hdr.ContentType()
+			if strings.HasPrefix(ct, "text/plain") && m.BodyText == "" {
+				b, _ := io.ReadAll(part.Body)
+				m.BodyText = string(b)
+			}
+		case *mail.AttachmentHeader:
+			name, _ := hdr.Filename()
+			ct, _, _ := hdr.ContentType()
+			b, _ := io.ReadAll(part.Body)
+			m.Attachments = append(m.Attachments, Attachment{
+				Name: name, Size: len(b), MIME: ct, Content: b,
+			})
+		}
+	}
+	m.Header.HasAttachments = len(m.Attachments) > 0
+	return m, nil
+}
+
+// ParseHeaderOnly decodes headers and detects attachments without keeping bodies.
+func ParseHeaderOnly(uid uint32, raw []byte) (Header, error) {
+	m, err := ParseMessage(uid, raw)
+	if err != nil {
+		return Header{}, err
+	}
+	return m.Header, nil
+}

internal/mail/message_test.go

@@ -0,0 +1,70 @@
+package mail
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func loadFixture(t *testing.T, name string) []byte {
+	t.Helper()
+	b, err := os.ReadFile(filepath.Join("testdata", name))
+	if err != nil {
+		t.Fatalf("read fixture: %v", err)
+	}
+	return b
+}
+
+func TestParseMessage(t *testing.T) {
+	raw := loadFixture(t, "with_attachment.eml")
+	m, err := ParseMessage(42, raw)
+	if err != nil {
+		t.Fatalf("ParseMessage: %v", err)
+	}
+	if m.Header.UID != 42 {
+		t.Fatalf("uid: %d", m.Header.UID)
+	}
+	if m.Header.Subject != "Your Invoice #5" {
+		t.Fatalf("subject: %q", m.Header.Subject)
+	}
+	if m.Header.From != `"Bob" <bob@trusted.com>` && m.Header.From != "Bob <bob@trusted.com>" {
+		t.Fatalf("from: %q", m.Header.From)
+	}
+	if m.Header.MessageID != "abc123@trusted.com" && m.Header.MessageID != "<abc123@trusted.com>" {
+		t.Fatalf("message-id: %q", m.Header.MessageID)
+	}
+	if want := "Hello, here is your invoice."; !contains(m.BodyText, want) {
+		t.Fatalf("body=%q want contains %q", m.BodyText, want)
+	}
+	if !m.Header.HasAttachments {
+		t.Fatal("HasAttachments should be true")
+	}
+	if len(m.Attachments) != 1 || m.Attachments[0].Name != "invoice.txt" {
+		t.Fatalf("attachments: %+v", m.Attachments)
+	}
+	if !contains(string(m.Attachments[0].Content), "LINE-ITEM 1") {
+		t.Fatalf("attachment content: %q", m.Attachments[0].Content)
+	}
+}
+
+func TestParseHeaderOnly(t *testing.T) {
+	raw := loadFixture(t, "with_attachment.eml")
+	h, err := ParseHeaderOnly(7, raw)
+	if err != nil {
+		t.Fatalf("ParseHeaderOnly: %v", err)
+	}
+	if h.Subject != "Your Invoice #5" || !h.HasAttachments {
+		t.Fatalf("header: %+v", h)
+	}
+}
+
+func contains(s, sub string) bool {
+	return len(s) >= len(sub) && (func() bool {
+		for i := 0; i+len(sub) <= len(s); i++ {
+			if s[i:i+len(sub)] == sub {
+				return true
+			}
+		}
+		return false
+	}())
+}

internal/mail/testdata/with_attachment.eml

@@ -0,0 +1,18 @@
+From: "Bob" <bob@trusted.com>
+To: me@example.com
+Subject: Your Invoice #5
+Date: Sat, 20 Jun 2026 10:00:00 +0000
+Message-ID: <abc123@trusted.com>
+MIME-Version: 1.0
+Content-Type: multipart/mixed; boundary="BOUNDARY"
+
+--BOUNDARY
+Content-Type: text/plain; charset=utf-8
+
+Hello, here is your invoice.
+--BOUNDARY
+Content-Type: text/plain; charset=utf-8
+Content-Disposition: attachment; filename="invoice.txt"
+
+LINE-ITEM 1: 100.00
+--BOUNDARY--