[Backport 2.x] [bug fix] validate lower bound for top n size

plugins/query-insights/src/main/java/org/opensearch/plugin/insights/core/service/TopQueriesService.java

@@ -138,17 +138,15 @@ public int getTopNSize() {
      * @param size the wanted top N size
      */
     public void validateTopNSize(final int size) {
-        if (size > QueryInsightsSettings.MAX_N_SIZE) {
+        if (size < 1 || size > QueryInsightsSettings.MAX_N_SIZE) {
             throw new IllegalArgumentException(
                 "Top N size setting for ["
                     + metricType
                     + "]"
-                    + " should be smaller than max top N size ["
+                    + " should be between 1 and "
                     + QueryInsightsSettings.MAX_N_SIZE
-                    + "was ("
+                    + ", was ("
                     + size
-                    + " > "
-                    + QueryInsightsSettings.MAX_N_SIZE
                     + ")"
             );
         }

plugins/query-insights/src/test/java/org/opensearch/plugin/insights/core/service/TopQueriesServiceTests.java

@@ -78,6 +78,10 @@ public void testValidateTopNSize() {
         assertThrows(IllegalArgumentException.class, () -> { topQueriesService.validateTopNSize(QueryInsightsSettings.MAX_N_SIZE + 1); });
     }
 
+    public void testValidateNegativeTopNSize() {
+        assertThrows(IllegalArgumentException.class, () -> { topQueriesService.validateTopNSize(-1); });
+    }
+
     public void testGetTopQueriesWhenNotEnabled() {
         topQueriesService.setEnabled(false);
         assertThrows(IllegalArgumentException.class, () -> { topQueriesService.getTopQueriesRecords(false); });