LUCENE-8459: add SearcherTaxonomyManager constructor taking already opened readers

lucene/CHANGES.txt

@@ -219,6 +219,10 @@ API Changes:
 * LUCENE-8422: Static helper functions for Matches and MatchesIterator implementations
   have been moved from Matches to MatchesUtils (Alan Woodward)
 
+* LUCENE-8459: SearcherTaxonomyManager now has a constructor taking already opened
+  IndexReaders, allowing the caller to pass a FilterDirectoryReader, for example.
+  (Mike McCandless)
+
 Bug Fixes:
 
 * LUCENE-8445: Tighten condition when two planes are identical to prevent constructing

lucene/facet/src/java/org/apache/lucene/facet/taxonomy/SearcherTaxonomyManager.java

@@ -98,6 +98,20 @@ public class SearcherTaxonomyManager extends ReferenceManager<SearcherTaxonomyMa
     taxoEpoch = -1;
   }
 
+  /**
+   * Creates this from already opened {@link IndexReader} and {@link DirectoryTaxonomyReader} instances.  Note that
+   * the incoming readers will be closed when you call {@link #close}.
+   */
+  public SearcherTaxonomyManager(IndexReader reader, DirectoryTaxonomyReader taxoReader, SearcherFactory searcherFactory) throws IOException {
+    if (searcherFactory == null) {
+      searcherFactory = new SearcherFactory();
+    }
+    this.searcherFactory = searcherFactory;
+    current = new SearcherAndTaxonomy(SearcherManager.getSearcher(searcherFactory, reader, null), taxoReader);
+    this.taxoWriter = null;
+    taxoEpoch = -1;
+  }
+
   @Override
   protected void decRef(SearcherAndTaxonomy ref) throws IOException {
     ref.searcher.getIndexReader().decRef();

lucene/facet/src/test/org/apache/lucene/facet/taxonomy/TestSearcherTaxonomyManager.java

@@ -32,14 +32,18 @@ import org.apache.lucene.facet.Facets;
 import org.apache.lucene.facet.FacetsCollector;
 import org.apache.lucene.facet.FacetsConfig;
 import org.apache.lucene.facet.taxonomy.SearcherTaxonomyManager.SearcherAndTaxonomy;
+import org.apache.lucene.facet.taxonomy.directory.DirectoryTaxonomyReader;
 import org.apache.lucene.facet.taxonomy.directory.DirectoryTaxonomyWriter;
+import org.apache.lucene.index.DirectoryReader;
 import org.apache.lucene.index.IndexNotFoundException;
+import org.apache.lucene.index.IndexReader;
 import org.apache.lucene.index.IndexWriter;
 import org.apache.lucene.index.IndexWriterConfig;
 import org.apache.lucene.index.SegmentInfos;
 import org.apache.lucene.index.TieredMergePolicy;
 import org.apache.lucene.search.MatchAllDocsQuery;
 import org.apache.lucene.search.ReferenceManager;
+import org.apache.lucene.search.SearcherFactory;
 import org.apache.lucene.store.Directory;
 import org.apache.lucene.util.IOUtils;
 import org.apache.lucene.util.TestUtil;
@@ -347,4 +351,14 @@ public class TestSearcherTaxonomyManager extends FacetTestCase {
     expectThrows(IndexNotFoundException.class, mgr::maybeRefreshBlocking);
     IOUtils.close(w, tw, mgr, indexDir, taxoDir);
   }
+
+  private SearcherTaxonomyManager getSearcherTaxonomyManager(Directory indexDir, Directory taxoDir, SearcherFactory searcherFactory) throws IOException {
+    if (random().nextBoolean()) {
+      return new SearcherTaxonomyManager(indexDir, taxoDir, searcherFactory);
+    } else {
+      IndexReader reader = DirectoryReader.open(indexDir);
+      DirectoryTaxonomyReader taxoReader = new DirectoryTaxonomyReader(taxoDir);
+      return new SearcherTaxonomyManager(reader, taxoReader, searcherFactory);
+    }
+  }
 }