@Override
        public StoreFileReader call(RegionObserver observer) throws IOException {
          return observer.preStoreFileReaderOpen(this, fs, p, in, size, cacheConf, r, getResult());
        }
@param fs   fileystem to read from
@param p    path to the file
@param in   {@link FSDataInputStreamWrapper}
@param size Full size of the file
@param r    original reference file. This will be not null only when reading a split file.
@return a Reader instance to use instead of the base reader if overriding default behavior,
        null otherwise
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/RegionCoprocessorHost.java