Merge remote-tracking branch 'h2database/master' into batch-append

h2/src/main/org/h2/mvstore/tx/TransactionStore.java

@@ -147,6 +147,16 @@ public class TransactionStore {
         if (!init) {
             for (String mapName : store.getMapNames()) {
                 if (mapName.startsWith(UNDO_LOG_NAME_PREFIX)) {
+                    // The following block will be executed only once
+                    // upon upgrade from older version where
+                    // undo log was persisted as a single map
+                    if (mapName.equals(UNDO_LOG_NAME_PREFIX)) {
+                        if (!store.hasData(mapName) && !store.isReadOnly()) {
+                            store.removeMap(mapName);
+                        }
+                        continue;
+                    }
+
                     boolean committed = mapName.charAt(UNDO_LOG_NAME_PREFIX.length()) == UNDO_LOG_COMMITTED;
                     if (store.hasData(mapName) || committed) {
                         int transactionId = Integer.parseInt(mapName.substring(UNDO_LOG_NAME_PREFIX.length() + 1));