private void tryGetNext() {
        ICachedMNode node;
        if (nodeBufferIterator.hasNext()) {
          node = nodeBufferIterator.next();

          // prevent this node being added to nodeBuffer during flush
          // unlock in PBTreeFlushExecutor
          lockManager.writeLock(node);

          // if there's flush failure, such node and ancestors will be removed from cache again by
          // #updateCacheStatusAfterFlushFailure
          nodeBuffer.remove(getCacheEntry(node));

          nextSubtree = node;
        }
      }
Collect updated storage group node.

@return null if not exist
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/schemaengine/schemaregion/mtree/impl/pbtree/memory/MemoryManager.java