feat(plugin/archive): allow custom mail id

plugin/archive/archive.go

@@ -89,8 +89,13 @@ func New(s Store, opts ...Option) postdog.Plugin {
 				errMsg = sendError.Error()
 			}
 
+			id := MailIDFromContext(ctx)
+			if id == uuid.Nil {
+				id = uuid.New()
+			}
+
 			m := ExpandMail(pm).
-				WithID(uuid.New()).
+				WithID(id).
 				WithSendError(errMsg).
 				WithSendTime(sentAt)
 

plugin/archive/context.go

@@ -0,0 +1,36 @@
+package archive
+
+import (
+	"context"
+
+	"github.com/google/uuid"
+)
+
+const (
+	ctxMailID = ctxKey("mail_id")
+)
+
+type ctxKey string
+
+// WithMailID returns a new Context that carries the given UUID. Mails that are
+// archived with that Context will use that UUID as their UUID when stored in a
+// database.
+func WithMailID(ctx context.Context, id uuid.UUID) context.Context {
+	return context.WithValue(ctx, ctxMailID, id)
+}
+
+// MailIDFromContext returns the mail UUID from the given Context, or uuid.Nil
+// if the Context has no mail UUID.
+func MailIDFromContext(ctx context.Context) uuid.UUID {
+	val := ctx.Value(ctxMailID)
+	if val == nil {
+		return uuid.Nil
+	}
+
+	id, ok := val.(uuid.UUID)
+	if !ok {
+		return uuid.Nil
+	}
+
+	return id
+}