default void preCompactSelection(ObserverContext<? extends RegionCoprocessorEnvironment> c,
    Store store, List<? extends StoreFile> candidates, CompactionLifeCycleTracker tracker)
    throws IOException {
  }
Called prior to selecting the {@link StoreFile StoreFiles} to compact from the list of
available candidates. To alter the files used for compaction, you may mutate the passed in list
of candidates. If you remove all the candidates then the compaction will be canceled.
<p>
Supports Coprocessor 'bypass' -- 'bypass' is how this method indicates that it changed the
passed in <code>candidates</code>. If 'bypass' is set, we skip out on calling any subsequent
chained coprocessors.
@param c          the environment provided by the region server
@param store      the store where compaction is being requested
@param candidates the store files currently available for compaction
@param tracker    tracker used to track the life cycle of a compaction
hbase-server/src/main/java/org/apache/hadoop/hbase/coprocessor/RegionObserver.java