Add unit test for renaming the setting reindex.remote.allowlist

Signed-off-by: Tianli Feng <ftianli@amazon.com>

modules/reindex/src/test/java/org/opensearch/index/reindex/ReindexRenamingSettingTests.java

@@ -0,0 +1,63 @@
+/*
+ * SPDX-License-Identifier: Apache-2.0
+ *
+ * The OpenSearch Contributors require contributions made to
+ * this file be licensed under the Apache-2.0 license or a
+ * compatible open source license.
+ */
+
+package org.opensearch.index.reindex;
+
+import org.junit.Before;
+import org.opensearch.common.settings.Setting;
+import org.opensearch.common.settings.Settings;
+import org.opensearch.test.OpenSearchTestCase;
+
+import java.util.Arrays;
+import java.util.List;
+
+/**
+ * A unit test to validate the former name of the setting 'reindex.remote.allowlist' still take effect, 
+ * after it is deprecated, so that the backwards compatibility is maintained.
+ * The test can be removed along with removing support of the deprecated setting.
+ */
+public class ReindexRenamingSettingTests extends OpenSearchTestCase {
+    ReindexPlugin plugin;
+
+    @Before
+    public void setup() {
+        this.plugin = new ReindexPlugin();
+    }
+
+    public void testReindexSettingsExist() {
+        List<Setting<?>> settings = plugin.getSettings();
+        assertTrue("Both 'reindex.remote.allowlist' and its predecessor should exist in settings of Reindex plugin",
+                settings.containsAll(
+                        Arrays.asList(
+                                TransportReindexAction.REMOTE_CLUSTER_WHITELIST,
+                                TransportReindexAction.REMOTE_CLUSTER_ALLOWLIST
+                        )
+                )
+        );
+    }
+
+    public void testSettingFallback() {
+        assertEquals(
+                TransportReindexAction.REMOTE_CLUSTER_ALLOWLIST.get(Settings.EMPTY),
+                TransportReindexAction.REMOTE_CLUSTER_WHITELIST.get(Settings.EMPTY)
+        );
+    }
+
+    public void testSettingGetValue() {
+        Settings settings = Settings.builder().put("reindex.remote.allowlist", "127.0.0.1:*").build();
+        System.out.println(TransportReindexAction.REMOTE_CLUSTER_ALLOWLIST.get(settings));
+        assertEquals(TransportReindexAction.REMOTE_CLUSTER_ALLOWLIST.get(settings), Arrays.asList("127.0.0.1:*"));
+        assertEquals(TransportReindexAction.REMOTE_CLUSTER_WHITELIST.get(settings), Arrays.asList());
+    }
+
+    public void testSettingGetValueWithFallback() {
+        Settings settings = Settings.builder().put("reindex.remote.whitelist", "127.0.0.1:*").build();
+        assertEquals(TransportReindexAction.REMOTE_CLUSTER_ALLOWLIST.get(settings), Arrays.asList("127.0.0.1:*"));
+        assertSettingDeprecationsAndWarnings(new Setting<?>[]{TransportReindexAction.REMOTE_CLUSTER_WHITELIST});
+    }
+}