code: private Journal.JournalEntry readInternal() throws IOException {
      if (mInputStream == null) {
        return null;
      }
      JournalEntry entry = mInputStream.mReader.readEntry();
      if (entry != null) {
        return entry;
      }
      if (mInputStream.mFile.isIncompleteLog()) {
        // Incomplete logs may end early.
        return null;
      } else {
        Preconditions.checkState(mInputStream.mFile.isCompletedLog(),
            "Expected log to be either checkpoint, incomplete, or complete");
        ProcessUtils.fatalError(LOG, "Journal entry %s was truncated", mNextSequenceNumber);
        return null;
      }
    }
docs: Reads the next journal entry.
  
  @return the journal entry, null if no journal entry is found