public ScanInfo preStoreScannerOpen(HStore store, Scan scan) throws IOException {
    if (coprocEnvironments.isEmpty()) return store.getScanInfo();
    CustomizedScanInfoBuilder builder = new CustomizedScanInfoBuilder(store.getScanInfo(), scan);
    execOperation(new RegionObserverOperationWithoutResult() {
      @Override
      public void call(RegionObserver observer) throws IOException {
        observer.preStoreScannerOpen(this, store, builder);
      }
    });
    return builder.build();
  }
Called before open store scanner for user scan.
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/RegionCoprocessorHost.java