Fix: visit of inner query for FunctionScoreQueryBuilder

CHANGELOG.md

@@ -66,6 +66,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Bound the size of cache in deprecation logger ([16702](https://github.com/opensearch-project/OpenSearch/issues/16702))
 - Ensure consistency of system flag on IndexMetadata after diff is applied ([#16644](https://github.com/opensearch-project/OpenSearch/pull/16644))
 - Skip remote-repositories validations for node-joins when RepositoriesService is not in sync with cluster-state ([#16763](https://github.com/opensearch-project/OpenSearch/pull/16763))
+- Fix visit of inner query for FunctionScoreQueryBuilder ([#16776](https://github.com/opensearch-project/OpenSearch/pull/16776))
 
 ### Security
 

server/src/main/java/org/opensearch/index/query/functionscore/FunctionScoreQueryBuilder.java

@@ -32,6 +32,7 @@
 
 package org.opensearch.index.query.functionscore;
 
+import org.apache.lucene.search.BooleanClause;
 import org.apache.lucene.search.MatchAllDocsQuery;
 import org.apache.lucene.search.Query;
 import org.opensearch.common.Nullable;
@@ -52,6 +53,7 @@
 import org.opensearch.index.query.MatchAllQueryBuilder;
 import org.opensearch.index.query.MatchNoneQueryBuilder;
 import org.opensearch.index.query.QueryBuilder;
+import org.opensearch.index.query.QueryBuilderVisitor;
 import org.opensearch.index.query.QueryRewriteContext;
 import org.opensearch.index.query.QueryShardContext;
 
@@ -704,4 +706,12 @@ private static String parseFiltersAndFunctions(
         }
         return currentFieldName;
     }
+
+    @Override
+    public void visit(QueryBuilderVisitor visitor) {
+        visitor.accept(this);
+        if (query != null) {
+            visitor.getChildVisitor(BooleanClause.Occur.MUST).accept(query);
+        }
+    }
 }

server/src/test/java/org/opensearch/index/query/functionscore/FunctionScoreQueryBuilderTests.java

@@ -75,6 +75,7 @@
 import org.hamcrest.Matcher;
 
 import java.io.IOException;
+import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collection;
 import java.util.Collections;
@@ -938,4 +939,15 @@ public void testMustRewrite() throws IOException {
         e = expectThrows(IllegalStateException.class, () -> functionQueryBuilder2.toQuery(context));
         assertEquals("Rewrite first", e.getMessage());
     }
+
+    public void testVisit() {
+        TermQueryBuilder termQueryBuilder = new TermQueryBuilder("unmapped_field", "foo");
+        FunctionScoreQueryBuilder builder = new FunctionScoreQueryBuilder(termQueryBuilder);
+
+        List<QueryBuilder> visitedQueries = new ArrayList<>();
+        builder.visit(createTestVisitor(visitedQueries));
+
+        assertEquals(2, visitedQueries.size());
+        assertTrue(visitedQueries.contains(termQueryBuilder));
+    }
 }