Do not add index event listener if CCR disabled (#37432)

Currently we add the CcrRestoreSourceService as a index event
listener. However, if ccr is disabled, this service is null and we
attempt to add a null listener throwing an exception. This commit only
adds the listener if ccr is enabled.

x-pack/plugin/ccr/src/main/java/org/elasticsearch/xpack/ccr/Ccr.java

@@ -306,7 +306,9 @@ public Map<String, Repository.Factory> getInternalRepositories(Environment env,
 
     @Override
     public void onIndexModule(IndexModule indexModule) {
-        indexModule.addIndexEventListener(this.restoreSourceService.get());
+        if (enabled) {
+            indexModule.addIndexEventListener(this.restoreSourceService.get());
+        }
     }
 
     protected XPackLicenseState getLicenseState() { return XPackPlugin.getSharedLicenseState(); }

x-pack/plugin/ccr/src/test/java/org/elasticsearch/xpack/ccr/CcrDisabledIT.java

@@ -0,0 +1,45 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License;
+ * you may not use this file except in compliance with the Elastic License.
+ */
+
+package org.elasticsearch.xpack.ccr;
+
+import org.elasticsearch.common.settings.Settings;
+import org.elasticsearch.plugins.Plugin;
+import org.elasticsearch.test.ESIntegTestCase;
+import org.elasticsearch.xpack.core.XPackClientPlugin;
+import org.elasticsearch.xpack.core.XPackSettings;
+
+import java.util.Collection;
+import java.util.Collections;
+
+public class CcrDisabledIT extends ESIntegTestCase {
+
+    public void testClusterCanStartWithCcrInstalledButNotEnabled() throws Exception {
+        // TODO: Assert that x-pack ccr feature is not enabled once feature functionality has been added
+        ensureGreen();
+    }
+
+    @Override
+    protected Settings nodeSettings(int nodeOrdinal) {
+        return Settings.builder().put(super.nodeSettings(nodeOrdinal)).put(XPackSettings.CCR_ENABLED_SETTING.getKey(), true)
+            .put(XPackSettings.SECURITY_ENABLED.getKey(), false).build();
+    }
+
+    @Override
+    protected Settings transportClientSettings() {
+        return Settings.builder().put(super.transportClientSettings()).put(XPackSettings.SECURITY_ENABLED.getKey(), false).build();
+    }
+
+    @Override
+    protected Collection<Class<? extends Plugin>> nodePlugins() {
+        return Collections.singletonList(LocalStateCcr.class);
+    }
+
+    @Override
+    protected Collection<Class<? extends Plugin>> transportClientPlugins() {
+        return Collections.singletonList(XPackClientPlugin.class);
+    }
+}