Add xpack.eql.enabled feature flag, disabled by default. Enabled only for integration tests. (#51370)

Related to https://github.com/elastic/elasticsearch/issues/49581

x-pack/plugin/eql/qa/rest/build.gradle

@@ -9,6 +9,7 @@ dependencies {
 
 testClusters.integTest {
   testDistribution = 'DEFAULT'
+  setting 'xpack.eql.enabled', 'true'
   setting 'xpack.license.self_generated.type', 'basic'
   setting 'xpack.monitoring.collection.enabled', 'true'
 }

x-pack/plugin/eql/src/main/java/org/elasticsearch/xpack/eql/plugin/EqlPlugin.java

@@ -5,12 +5,14 @@
  */
 package org.elasticsearch.xpack.eql.plugin;
 
+import org.elasticsearch.Build;
 import org.elasticsearch.action.ActionRequest;
 import org.elasticsearch.action.ActionResponse;
 import org.elasticsearch.cluster.metadata.IndexNameExpressionResolver;
 import org.elasticsearch.cluster.node.DiscoveryNodes;
 import org.elasticsearch.common.settings.ClusterSettings;
 import org.elasticsearch.common.settings.IndexScopedSettings;
+import org.elasticsearch.common.settings.Setting;
 import org.elasticsearch.common.settings.Settings;
 import org.elasticsearch.common.settings.SettingsFilter;
 import org.elasticsearch.plugins.ActionPlugin;
@@ -20,10 +22,19 @@ import org.elasticsearch.rest.RestHandler;
 import org.elasticsearch.xpack.eql.action.EqlSearchAction;
 
 import java.util.Arrays;
+import java.util.Collections;
 import java.util.List;
 import java.util.function.Supplier;
 
 public class EqlPlugin extends Plugin implements ActionPlugin {
+
+    public static final Setting<Boolean> EQL_ENABLED_SETTING = Setting.boolSetting(
+        "xpack.eql.enabled",
+        false,
+        Setting.Property.NodeScope
+    );
+
+
     @Override
     public List<ActionHandler<? extends ActionRequest, ? extends ActionResponse>> getActions() {
         return Arrays.asList(
@@ -31,6 +42,24 @@ public class EqlPlugin extends Plugin implements ActionPlugin {
         );
     }
 
+    /**
+     * The settings defined by EQL plugin.
+     *
+     * @return the settings
+     */
+    @Override
+    public List<Setting<?>> getSettings() {
+        if (isSnapshot()) {
+            return List.of(EQL_ENABLED_SETTING);
+        } else {
+            return List.of();
+        }
+    }
+
+    boolean isSnapshot() {
+        return Build.CURRENT.isSnapshot();
+    }
+
     @Override
     public List<RestHandler> getRestHandlers(Settings settings,
                                              RestController restController,
@@ -39,7 +68,11 @@ public class EqlPlugin extends Plugin implements ActionPlugin {
                                              SettingsFilter settingsFilter,
                                              IndexNameExpressionResolver indexNameExpressionResolver,
                                              Supplier<DiscoveryNodes> nodesInCluster) {
-        return Arrays.asList(
-            new RestEqlSearchAction(restController));
+
+        boolean enabled = EQL_ENABLED_SETTING.get(settings);
+        if (!enabled) {
+            return Collections.emptyList();
+        }
+        return Arrays.asList(new RestEqlSearchAction(restController));
     }
 }

x-pack/plugin/eql/src/test/java/org/elasticsearch/xpack/eql/plugin/EqlPluginTests.java

@@ -0,0 +1,38 @@
+/*
+ * 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.eql.plugin;
+
+import org.elasticsearch.test.ESTestCase;
+
+import static org.hamcrest.Matchers.hasItem;
+import static org.hamcrest.Matchers.not;
+
+public class EqlPluginTests extends ESTestCase {
+    public void testEnabledSettingRegisteredInSnapshotBuilds() {
+        final EqlPlugin plugin = new EqlPlugin() {
+
+            @Override
+            protected boolean isSnapshot() {
+                return true;
+            }
+
+        };
+        assertThat(plugin.getSettings(), hasItem(EqlPlugin.EQL_ENABLED_SETTING));
+    }
+
+    public void testEnabledSettingNotRegisteredInNonSnapshotBuilds() {
+        final EqlPlugin plugin = new EqlPlugin() {
+
+            @Override
+            protected boolean isSnapshot() {
+                return false;
+            }
+
+        };
+        assertThat(plugin.getSettings(), not(hasItem(EqlPlugin.EQL_ENABLED_SETTING)));
+    }
+}