feat(imapx): message dedup key (Message-ID + header fallback)

internal/imapx/messagekey.go

@@ -0,0 +1,37 @@
+package imapx
+
+import (
+	"crypto/md5"
+	"fmt"
+	"strings"
+
+	"github.com/emersion/go-imap/v2"
+)
+
+func MessageKey(env *imap.Envelope, size int64) string {
+	if env != nil && env.MessageID != "" {
+		return env.MessageID
+	}
+	var b strings.Builder
+	if env != nil {
+		b.WriteString(addrList(env.From))
+		b.WriteByte('|')
+		b.WriteString(addrList(env.To))
+		b.WriteByte('|')
+		b.WriteString(env.Subject)
+		b.WriteByte('|')
+		b.WriteString(env.Date.UTC().Format("2006-01-02T15:04:05Z"))
+	}
+	b.WriteByte('|')
+	fmt.Fprintf(&b, "%d", size)
+	sum := md5.Sum([]byte(b.String()))
+	return fmt.Sprintf("h:%x", sum)
+}
+
+func addrList(addrs []imap.Address) string {
+	parts := make([]string, 0, len(addrs))
+	for _, a := range addrs {
+		parts = append(parts, a.Mailbox+"@"+a.Host)
+	}
+	return strings.Join(parts, ",")
+}