Write warn log if Filter is empty; Add comments (opensearch-project#1…

…2067)

Signed-off-by: Robin Friedmann <[email protected]>

libs/core/src/main/java/org/opensearch/core/xcontent/filtering/FilterPath.java

@@ -32,6 +32,8 @@
 
 package org.opensearch.core.xcontent.filtering;
 
+import org.apache.logging.log4j.LogManager;
+import org.apache.logging.log4j.Logger;
 import org.opensearch.common.Glob;
 
 import java.util.ArrayList;
@@ -52,6 +54,7 @@ public class FilterPath {
     private final FilterPath next;
     private final boolean simpleWildcard;
     private final boolean doubleWildcard;
+    private static final Logger logger = LogManager.getLogger(FilterPath.class);
 
     protected FilterPath(String filter, String segment, FilterPath next) {
         this.filter = filter;
@@ -103,17 +106,22 @@ public static FilterPath[] compile(Set<String> filters) {
                 filter = filter.trim();
                 if (filter.length() > 0) {
                     paths.add(parse(filter));
+                } else {
+                    // write a warn log
+                    logger.warn("Filter is empty!");
                 }
             }
         }
         return paths.toArray(new FilterPath[0]);
     }
 
     private static FilterPath parse(final String filter) {
+        // Split the filter into segments using a regex that avoids splitting escaped dots.
         String[] segments = filter.split("(?<!\\\\)\\.");
         FilterPath next = EMPTY;
 
         for (int i = segments.length - 1; i >= 0; i--) {
+            // Replace escaped dots with actual dots in the current segment.
             String segment = segments[i].replaceAll("\\\\.", ".");
             next = new FilterPath(filter, segment, next);
         }