Let MLTQuery throw IAE when no analyzer is set

docs/changelog/124662.yaml

@@ -0,0 +1,6 @@
+pr: 124662
+summary: Let MLTQuery throw IAE when no analyzer is set
+area: Search
+type: bug
+issues:
+ - 124562

server/src/main/java/org/elasticsearch/index/query/MoreLikeThisQueryBuilder.java

@@ -911,9 +911,7 @@ protected Query doToQuery(SearchExecutionContext context) throws IOException {
         // set analyzer
         Analyzer analyzerObj = context.getIndexAnalyzers().get(analyzer);
         if (analyzerObj == null) {
-            analyzerObj = context.getIndexAnalyzer(
-                f -> { throw new UnsupportedOperationException("No analyzer configured for field " + f); }
-            );
+            analyzerObj = context.getIndexAnalyzer(f -> { throw new IllegalArgumentException("No analyzer configured for field " + f); });
         }
         mltQuery.setAnalyzer(analyzer, analyzerObj);
 

server/src/test/java/org/elasticsearch/index/query/MoreLikeThisQueryBuilderTests.java

@@ -9,6 +9,7 @@
 
 package org.elasticsearch.index.query;
 
+import org.apache.lucene.analysis.Analyzer;
 import org.apache.lucene.analysis.TokenStream;
 import org.apache.lucene.analysis.core.WhitespaceAnalyzer;
 import org.apache.lucene.analysis.tokenattributes.CharTermAttribute;
@@ -41,6 +42,7 @@
 import org.junit.Before;
 
 import java.io.IOException;
+import java.io.StringReader;
 import java.util.Arrays;
 import java.util.Collections;
 import java.util.EnumSet;
@@ -393,6 +395,19 @@ public void testItemFromXContent() throws IOException {
         assertEquals(expectedItem, newItem);
     }
 
+    public void testNonExistingAnalyzer() throws IOException {
+        MoreLikeThisQueryBuilder moreLikeThisQueryBuilder = moreLikeThisQuery(
+            new String[] { "name.first", "name.last" },
+            new String[] { "something" },
+            null
+        );
+        moreLikeThisQueryBuilder.analyzer("thisDoesntExist");
+        SearchExecutionContext searchExecutionContext = createSearchExecutionContext();
+        Query query = moreLikeThisQueryBuilder.toQuery(searchExecutionContext);
+        Analyzer analyzer = ((MoreLikeThisQuery) query).getAnalyzer();
+        assertThrows(IllegalArgumentException.class, () -> analyzer.tokenStream("thisDoesntExist", new StringReader("something")));
+    }
+
     /**
      * Check that this query is generally not cacheable, except when we fetch 0 items
      */