Add meta() info to DMARC, spam, and parse failures

This may help diagnose potential issues by more clearly seeing where the
message was from, where it was intended, and what the subject looked
like.

handler/handler.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"log"
 	"net/http"
+	"strings"
 
 	"github.com/aws/aws-lambda-go/events"
 	"github.com/mbland/elistman/ops"
@@ -157,26 +158,35 @@ func newMailtoEvent(ses *events.SimpleEmailService) *mailtoEvent {
 // - https://docs.aws.amazon.com/ses/latest/dg/receiving-email-notifications-contents.html
 // - https://docs.aws.amazon.com/ses/latest/dg/receiving-email-notifications-examples.html
 func (h *Handler) handleMailtoEvent(ev *mailtoEvent) error {
-	prefix := "message " + ev.MessageId + ": "
+	prefix := "unsubscribe message " + ev.MessageId + ": "
 
 	if bounced, err := h.bounceIfDmarcFails(ev); err != nil {
-		return fmt.Errorf("%serror while bouncing: %s", prefix, err)
+		return fmt.Errorf("%sDMARC bounce fail: %s: %s", prefix, meta(ev), err)
 	} else if bounced {
-		log.Printf("%sbounced", prefix)
+		log.Printf("%sDMARC bounce: %s", prefix, meta(ev))
 	} else if isSpam(ev) {
-		log.Printf("%sspam, ignoring", prefix)
+		log.Printf("%smarked as spam, ignored: %s", prefix, meta(ev))
 	} else if op, err := parseMailtoEvent(ev, h.UnsubscribeAddr); err != nil {
-		log.Printf("%sparse error, ignoring: %s", prefix, err)
+		log.Printf("%sfailed to parse, ignoring: %s: %s", prefix, meta(ev), err)
 	} else if result, err := h.Agent.Unsubscribe(op.Email, op.Uid); err != nil {
-		return fmt.Errorf("%serror unsubscribing %s: %s", prefix, op.Email, err)
+		return fmt.Errorf("%serror: %s: %s", prefix, op.Email, err)
 	} else if result != ops.Unsubscribed {
-		log.Printf("%snot unsubscribed: %s: %s", prefix, op.Email, result)
+		log.Printf("%sfailed: %s: %s", prefix, op.Email, result)
 	} else {
-		log.Printf("%ssuccessfully unsubscribed: %s", prefix, op.Email)
+		log.Printf("%ssuccess: %s", prefix, op.Email)
 	}
 	return nil
 }
 
+func meta(ev *mailtoEvent) string {
+	return fmt.Sprintf(
+		"[From:\"%s\" To:\"%s\" Subject:\"%s\"]",
+		strings.Join(ev.From, ","),
+		strings.Join(ev.To, ","),
+		ev.Subject,
+	)
+}
+
 func (h *Handler) bounceIfDmarcFails(ev *mailtoEvent) (bool, error) {
 	return false, nil
 }