void checkForErrors() throws IOException {
      Throwable thrown = this.thrown.get();
      if (thrown == null) {
        return;
      }
      this.thrown.set(null);
      if (thrown instanceof IOException) {
        throw new IOException(thrown);
      } else {
        throw new RuntimeException(thrown);
      }
    }
Check for errors in the writer threads. If any is found, rethrow it.
hbase-server/src/main/java/org/apache/hadoop/hbase/wal/WALSplitter.java