code: public Log commit(long index) {
      assertIsOpen();
      if (index > 0) {
        assertValidIndex(index);
        segments.commitIndex(index);
        if (storage.flushOnCommit()) {
          segments.currentSegment().flush();
        }
      }
      return this;
    }
docs: Commits entries up to the given index to the log.
  
  @param index The index up to which to commit entries.
  @return The log.
  @throws IllegalStateException If the log is not open.