Stream source mailbox migration

backend/00-router.go

@@ -1552,14 +1552,26 @@ func fetchSourceFolderMessages(account Account, folder string) ([]RawMessage, er
 			return nil, err
 		}
 		defer src.Close()
-		return src.Fetch(folder)
+		return collectSourceMessages(src, folder)
 	}
 	src, err := OpenIMAPSource(account)
 	if err != nil {
 		return nil, err
 	}
 	defer src.Close()
-	return src.Fetch(folder)
+	return collectSourceMessages(src, folder)
+}
+
+func collectSourceMessages(src SourceMailbox, folder string) ([]RawMessage, error) {
+	var messages []RawMessage
+	err := src.Fetch(folder, func(m RawMessage) error {
+		messages = append(messages, m)
+		return nil
+	})
+	if err != nil {
+		return messages, err
+	}
+	return messages, nil
 }
 
 func renderInitialSourceMailboxList(account Account) string {