File Discovery: Remove fallback with zen discovery (#26667)

When adding file based discovery, we added a fallback when the discovery
type was set to zen (the default, so everyone got this warning). This
commit removes the fallback for 6.0. Setting file discovery should now
happen explicitly through the hosts_provider setting.

closes #26661

plugins/discovery-file/build.gradle

@@ -52,6 +52,7 @@ setupSeedNodeAndUnicastHostsFile.doLast {
 integTestCluster {
     dependsOn setupSeedNodeAndUnicastHostsFile
     clusterName = 'discovery-file-test-cluster'
+    setting 'discovery.zen.hosts_provider', 'file'
     extraConfigFile 'discovery-file/unicast_hosts.txt', srcUnicastHostsFile
 }
 

plugins/discovery-file/src/main/java/org/elasticsearch/discovery/file/FileBasedDiscoveryPlugin.java

@@ -102,20 +102,4 @@ public class FileBasedDiscoveryPlugin extends Plugin implements DiscoveryPlugin
             () -> new FileBasedUnicastHostsProvider(
                     new Environment(settings, configPath), transportService, fileBasedDiscoveryExecutorService));
     }
-
-    @Override
-    public Settings additionalSettings() {
-        // For 5.0, the hosts provider was "zen", but this was before the discovery.zen.hosts_provider
-        // setting existed. This check looks for the legacy zen, and sets the file hosts provider if not set
-        String discoveryType = DiscoveryModule.DISCOVERY_TYPE_SETTING.get(settings);
-        if (discoveryType.equals("zen")) {
-            deprecationLogger.deprecated("Using " + DiscoveryModule.DISCOVERY_TYPE_SETTING.getKey() +
-                " setting to set hosts provider is deprecated. " +
-                "Set \"" + DiscoveryModule.DISCOVERY_HOSTS_PROVIDER_SETTING.getKey() + ": file\" instead");
-            if (DiscoveryModule.DISCOVERY_HOSTS_PROVIDER_SETTING.exists(settings) == false) {
-                return Settings.builder().put(DiscoveryModule.DISCOVERY_HOSTS_PROVIDER_SETTING.getKey(), "file").build();
-            }
-        }
-        return Settings.EMPTY;
-    }
 }

plugins/discovery-file/src/test/java/org/elasticsearch/discovery/file/FileBasedDiscoveryPluginTests.java

@@ -1,47 +0,0 @@
-/*
- * Licensed to Elasticsearch under one or more contributor
- * license agreements. See the NOTICE file distributed with
- * this work for additional information regarding copyright
- * ownership. Elasticsearch licenses this file to you under
- * the Apache License, Version 2.0 (the "License"); you may
- * not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- *    http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing,
- * software distributed under the License is distributed on an
- * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
- * KIND, either express or implied.  See the License for the
- * specific language governing permissions and limitations
- * under the License.
- */
-
-package org.elasticsearch.discovery.file;
-
-import org.elasticsearch.common.settings.Settings;
-import org.elasticsearch.discovery.DiscoveryModule;
-import org.elasticsearch.test.ESTestCase;
-
-import java.io.IOException;
-import java.nio.file.Path;
-
-public class FileBasedDiscoveryPluginTests extends ESTestCase {
-
-    public void testHostsProviderBwc() {
-        FileBasedDiscoveryPlugin plugin = new FileBasedDiscoveryPlugin(Settings.EMPTY, createTempDir());
-        Settings additionalSettings = plugin.additionalSettings();
-        assertEquals("file", additionalSettings.get(DiscoveryModule.DISCOVERY_HOSTS_PROVIDER_SETTING.getKey()));
-        assertWarnings("Using discovery.type setting to set hosts provider is deprecated. " +
-                "Set \"discovery.zen.hosts_provider: file\" instead");
-    }
-
-    public void testHostsProviderExplicit() {
-        Settings settings = Settings.builder().put(DiscoveryModule.DISCOVERY_HOSTS_PROVIDER_SETTING.getKey(), "foo").build();
-        FileBasedDiscoveryPlugin plugin = new FileBasedDiscoveryPlugin(settings, createTempDir());
-        assertEquals(Settings.EMPTY, plugin.additionalSettings());
-        assertWarnings("Using discovery.type setting to set hosts provider is deprecated. " +
-                "Set \"discovery.zen.hosts_provider: file\" instead");
-    }
-
-}