Add Validation for maxQueryTerms to be greater than 0 for MoreLikeThisQuery (#49966)

Adds validation for maxQueryTerms to be greater than 0 for MoreLikeThisQuery 
and MoreLikeThisQueryBuilder.

Closes #49927

server/src/main/java/org/elasticsearch/common/lucene/search/MoreLikeThisQuery.java

@@ -308,6 +308,9 @@ public class MoreLikeThisQuery extends Query {
     }
 
     public void setMaxQueryTerms(int maxQueryTerms) {
+        if (maxQueryTerms <= 0) {
+            throw new IllegalArgumentException("requires 'maxQueryTerms' to be greater than 0");
+        }
         this.maxQueryTerms = maxQueryTerms;
     }
 

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

@@ -577,6 +577,9 @@ public class MoreLikeThisQueryBuilder extends AbstractQueryBuilder<MoreLikeThisQ
      * Defaults to {@code 25}.
      */
     public MoreLikeThisQueryBuilder maxQueryTerms(int maxQueryTerms) {
+        if (maxQueryTerms <= 0) {
+            throw new IllegalArgumentException("requires 'maxQueryTerms' to be greater than 0");
+        }
         this.maxQueryTerms = maxQueryTerms;
         return this;
     }

server/src/test/java/org/elasticsearch/common/lucene/search/morelikethis/MoreLikeThisQueryTests.java

@@ -33,6 +33,7 @@ import org.elasticsearch.common.lucene.Lucene;
 import org.elasticsearch.common.lucene.search.MoreLikeThisQuery;
 import org.elasticsearch.test.ESTestCase;
 
+import static org.hamcrest.Matchers.containsString;
 import static org.hamcrest.Matchers.equalTo;
 
 public class MoreLikeThisQueryTests extends ESTestCase {
@@ -64,4 +65,15 @@ public class MoreLikeThisQueryTests extends ESTestCase {
         reader.close();
         indexWriter.close();
     }
+
+    public void testValidateMaxQueryTerms() {
+        IllegalArgumentException e1 = expectThrows(IllegalArgumentException.class,
+            () ->  new MoreLikeThisQuery("lucene", new String[]{"text"}, Lucene.STANDARD_ANALYZER).setMaxQueryTerms(0));
+        assertThat(e1.getMessage(), containsString("requires 'maxQueryTerms' to be greater than 0"));
+
+        IllegalArgumentException e2 = expectThrows(IllegalArgumentException.class,
+            () -> new MoreLikeThisQuery("lucene", new String[]{"text"}, Lucene.STANDARD_ANALYZER).setMaxQueryTerms(-3));
+        assertThat(e2.getMessage(), containsString("requires 'maxQueryTerms' to be greater than 0"));
+    }
+
 }

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

@@ -163,7 +163,7 @@ public class MoreLikeThisQueryBuilderTests extends AbstractQueryTestCase<MoreLik
             queryBuilder.unlike(randomUnlikeItems);
         }
         if (randomBoolean()) {
-            queryBuilder.maxQueryTerms(randomInt(25));
+            queryBuilder.maxQueryTerms(randomIntBetween(1, 25));
         }
         if (randomBoolean()) {
             queryBuilder.minTermFreq(randomInt(5));
@@ -340,6 +340,16 @@ public class MoreLikeThisQueryBuilderTests extends AbstractQueryTestCase<MoreLik
         assertThat(mltQuery.getMaxQueryTerms(), equalTo(12));
     }
 
+    public void testValidateMaxQueryTerms() {
+        IllegalArgumentException e1 = expectThrows(IllegalArgumentException.class,
+            () -> new MoreLikeThisQueryBuilder(new String[]{"name.first", "name.last"}, new String[]{"something"}, null).maxQueryTerms(0));
+        assertThat(e1.getMessage(), containsString("requires 'maxQueryTerms' to be greater than 0"));
+
+        IllegalArgumentException e2 = expectThrows(IllegalArgumentException.class,
+            () -> new MoreLikeThisQueryBuilder(new String[]{"name.first", "name.last"}, new String[]{"something"}, null).maxQueryTerms(-3));
+        assertThat(e2.getMessage(), containsString("requires 'maxQueryTerms' to be greater than 0"));
+    }
+
     public void testItemSerialization() throws IOException {
         Item expectedItem = generateRandomItem();
         BytesStreamOutput output = new BytesStreamOutput();