Add BytesRefIterator to TermInSetQuery

Addresses apache#13804

TermInSetQuery used to have an accessor to its terms that was removed in apache#12173
to protect leaking internal encoding details. This introduces an accessor to the
term data in the query that doesn't expose internal but merely allows iterating
over the decoded BytesRef, making inspection of the querys content possible again.

lucene/core/src/java/org/apache/lucene/search/TermInSetQuery.java

@@ -27,13 +27,7 @@
 import org.apache.lucene.index.Term;
 import org.apache.lucene.index.Terms;
 import org.apache.lucene.index.TermsEnum;
-import org.apache.lucene.util.Accountable;
-import org.apache.lucene.util.AttributeSource;
-import org.apache.lucene.util.BytesRef;
-import org.apache.lucene.util.BytesRefBuilder;
-import org.apache.lucene.util.BytesRefComparator;
-import org.apache.lucene.util.RamUsageEstimator;
-import org.apache.lucene.util.StringSorter;
+import org.apache.lucene.util.*;
 import org.apache.lucene.util.automaton.Automata;
 import org.apache.lucene.util.automaton.Automaton;
 import org.apache.lucene.util.automaton.ByteRunAutomaton;
@@ -141,6 +135,11 @@ public long getTermsCount() {
     return termData.size();
   }
 
+  public BytesRefIterator getBytesRefIterator() {
+    final TermIterator iterator = this.termData.iterator();
+    return () -> iterator.next();
+  }
+
   @Override
   public void visit(QueryVisitor visitor) {
     if (visitor.acceptField(field) == false) {

lucene/core/src/test/org/apache/lucene/search/TestTermInSetQuery.java

@@ -52,6 +52,7 @@
 import org.apache.lucene.tests.util.RamUsageTester;
 import org.apache.lucene.tests.util.TestUtil;
 import org.apache.lucene.util.BytesRef;
+import org.apache.lucene.util.BytesRefIterator;
 import org.apache.lucene.util.IOUtils;
 import org.apache.lucene.util.automaton.ByteRunAutomaton;
 
@@ -527,4 +528,19 @@ public void consumeTermsMatching(
           }
         });
   }
+
+  public void testTermsIterator() throws IOException {
+    TermInSetQuery empty = new TermInSetQuery("field", Collections.emptyList());
+    BytesRefIterator it = empty.getBytesRefIterator();
+    assertNull(it.next());
+
+    TermInSetQuery query =
+        new TermInSetQuery(
+            "field", List.of(newBytesRef("term1"), newBytesRef("term2"), newBytesRef("term3")));
+    it = query.getBytesRefIterator();
+    assertEquals(newBytesRef("term1"), it.next());
+    assertEquals(newBytesRef("term2"), it.next());
+    assertEquals(newBytesRef("term3"), it.next());
+    assertNull(it.next());
+  }
 }