Introduce RestHandler.Wrapper to help with delegate implementations (#1004) (#1031)

Signed-off-by: Vlad Rozov <vrozov@users.noreply.github.com>

Author: vrozov

server/src/main/java/org/opensearch/rest/BaseRestHandler.java

@@ -50,6 +50,7 @@
 import java.util.HashSet;
 import java.util.List;
 import java.util.Locale;
+import java.util.Objects;
 import java.util.Set;
 import java.util.SortedSet;
 import java.util.TreeSet;
@@ -209,7 +210,7 @@ public static class Wrapper extends BaseRestHandler {
         protected final BaseRestHandler delegate;
 
         public Wrapper(BaseRestHandler delegate) {
-            this.delegate = delegate;
+            this.delegate = Objects.requireNonNull(delegate, "BaseRestHandler delegate can not be null");
         }
 
         @Override
@@ -256,5 +257,10 @@ public boolean supportsContentStream() {
         public boolean allowsUnsafeBuffers() {
             return delegate.allowsUnsafeBuffers();
         }
+
+        @Override
+        public boolean allowSystemIndexAccessByDefault() {
+            return delegate.allowSystemIndexAccessByDefault();
+        }
     }
 }

server/src/main/java/org/opensearch/rest/RestHandler.java

@@ -38,6 +38,7 @@
 
 import java.util.Collections;
 import java.util.List;
+import java.util.Objects;
 import java.util.stream.Collectors;
 
 /**
@@ -113,6 +114,63 @@ default boolean allowSystemIndexAccessByDefault() {
         return false;
     }
 
+    static RestHandler wrapper(RestHandler delegate) {
+        return new Wrapper(delegate);
+    }
+
+    class Wrapper implements RestHandler {
+        private final RestHandler delegate;
+
+        public Wrapper(RestHandler delegate) {
+            this.delegate = Objects.requireNonNull(delegate, "RestHandler delegate can not be null");
+        }
+
+        @Override
+        public String toString() {
+            return delegate.toString();
+        }
+
+        @Override
+        public void handleRequest(RestRequest request, RestChannel channel, NodeClient client) throws Exception {
+            delegate.handleRequest(request, channel, client);
+        }
+
+        @Override
+        public boolean canTripCircuitBreaker() {
+            return delegate.canTripCircuitBreaker();
+        }
+
+        @Override
+        public boolean supportsContentStream() {
+            return delegate.supportsContentStream();
+        }
+
+        @Override
+        public boolean allowsUnsafeBuffers() {
+            return delegate.allowsUnsafeBuffers();
+        }
+
+        @Override
+        public List<Route> routes() {
+            return delegate.routes();
+        }
+
+        @Override
+        public List<DeprecatedRoute> deprecatedRoutes() {
+            return delegate.deprecatedRoutes();
+        }
+
+        @Override
+        public List<ReplacedRoute> replacedRoutes() {
+            return delegate.replacedRoutes();
+        }
+
+        @Override
+        public boolean allowSystemIndexAccessByDefault() {
+            return delegate.allowSystemIndexAccessByDefault();
+        }
+    }
+
     class Route {
 
         private final String path;