Introduce a Few Settings Singleton Instances (#78897) (#78908)

This is mostly motivated by the ILM steps changes, that showed up as hot in
profiling since instantiating a `Settings` instance turns out to be somewhat expensive.
Also cleans up a couple of other spots and some duplication as well.

Author: original-brownbear

client/rest-high-level/src/main/java/org/elasticsearch/client/indices/CreateIndexRequest.java

@@ -36,8 +36,6 @@
 import java.util.Objects;
 import java.util.Set;
 
-import static org.elasticsearch.common.settings.Settings.Builder.EMPTY_SETTINGS;
-
 /**
  * A request to create an index.
  */
@@ -47,7 +45,7 @@ public class CreateIndexRequest extends TimedRequest implements Validatable, ToX
     static final ParseField ALIASES = new ParseField("aliases");
 
     private final String index;
-    private Settings settings = EMPTY_SETTINGS;
+    private Settings settings = Settings.EMPTY;
 
     private BytesReference mappings;
     private XContentType mappingsXContentType;

client/rest-high-level/src/main/java/org/elasticsearch/client/indices/IndexTemplateMetadata.java

@@ -178,7 +178,7 @@ public static class Builder {
 
         private List<String> indexPatterns;
 
-        private Settings settings = Settings.Builder.EMPTY_SETTINGS;
+        private Settings settings = Settings.EMPTY;
 
         private MappingMetadata mappings;
 

client/rest-high-level/src/main/java/org/elasticsearch/client/indices/PutIndexTemplateRequest.java

@@ -40,7 +40,6 @@
 import java.util.stream.Collectors;
 
 import static org.elasticsearch.action.ValidateActions.addValidationError;
-import static org.elasticsearch.common.settings.Settings.Builder.EMPTY_SETTINGS;
 
 /**
  * A request to create an index template.
@@ -66,7 +65,7 @@ public class PutIndexTemplateRequest extends MasterNodeRequest<PutIndexTemplateR
 
     private boolean create;
 
-    private Settings settings = EMPTY_SETTINGS;
+    private Settings settings = Settings.EMPTY;
 
     private BytesReference mappings = null;
 

server/src/internalClusterTest/java/org/elasticsearch/recovery/SimpleRecoveryIT.java

@@ -29,7 +29,7 @@ public Settings indexSettings() {
     }
 
     protected Settings recoverySettings() {
-        return Settings.Builder.EMPTY_SETTINGS;
+        return Settings.EMPTY;
     }
 
     @Override

server/src/main/java/org/elasticsearch/action/admin/cluster/repositories/put/PutRepositoryRequest.java

@@ -21,7 +21,6 @@
 import java.util.Map;
 
 import static org.elasticsearch.action.ValidateActions.addValidationError;
-import static org.elasticsearch.common.settings.Settings.Builder.EMPTY_SETTINGS;
 import static org.elasticsearch.common.settings.Settings.readSettingsFromStream;
 import static org.elasticsearch.common.settings.Settings.writeSettingsToStream;
 
@@ -39,7 +38,7 @@ public class PutRepositoryRequest extends AcknowledgedRequest<PutRepositoryReque
 
     private boolean verify = true;
 
-    private Settings settings = EMPTY_SETTINGS;
+    private Settings settings = Settings.EMPTY;
 
     public PutRepositoryRequest(StreamInput in) throws IOException {
         super(in);

server/src/main/java/org/elasticsearch/action/admin/cluster/settings/ClusterUpdateSettingsRequest.java

@@ -26,7 +26,6 @@
 import static org.elasticsearch.action.ValidateActions.addValidationError;
 import static org.elasticsearch.common.settings.Settings.readSettingsFromStream;
 import static org.elasticsearch.common.settings.Settings.writeSettingsToStream;
-import static org.elasticsearch.common.settings.Settings.Builder.EMPTY_SETTINGS;
 
 /**
  * Request for an update cluster settings action
@@ -44,8 +43,8 @@ public class ClusterUpdateSettingsRequest extends AcknowledgedRequest<ClusterUpd
         PARSER.declareObject((r, t) -> r.transientSettings = t, (p, c) -> Settings.fromXContent(p), TRANSIENT);
     }
 
-    private Settings transientSettings = EMPTY_SETTINGS;
-    private Settings persistentSettings = EMPTY_SETTINGS;
+    private Settings transientSettings = Settings.EMPTY;
+    private Settings persistentSettings = Settings.EMPTY;
 
     public ClusterUpdateSettingsRequest(StreamInput in) throws IOException {
         super(in);

server/src/main/java/org/elasticsearch/action/admin/cluster/snapshots/create/CreateSnapshotRequest.java

@@ -33,7 +33,6 @@
 
 import static org.elasticsearch.action.ValidateActions.addValidationError;
 import static org.elasticsearch.common.Strings.EMPTY_ARRAY;
-import static org.elasticsearch.common.settings.Settings.Builder.EMPTY_SETTINGS;
 import static org.elasticsearch.common.settings.Settings.readSettingsFromStream;
 import static org.elasticsearch.common.settings.Settings.writeSettingsToStream;
 import static org.elasticsearch.common.xcontent.support.XContentMapValues.nodeBooleanValue;
@@ -70,7 +69,7 @@ public class CreateSnapshotRequest extends MasterNodeRequest<CreateSnapshotReque
 
     private boolean partial = false;
 
-    private Settings settings = EMPTY_SETTINGS;
+    private Settings settings = Settings.EMPTY;
 
     private boolean includeGlobalState = true;
 

server/src/main/java/org/elasticsearch/action/admin/cluster/snapshots/restore/RestoreSnapshotRequest.java

@@ -31,7 +31,6 @@
 import java.util.Objects;
 
 import static org.elasticsearch.action.ValidateActions.addValidationError;
-import static org.elasticsearch.common.settings.Settings.Builder.EMPTY_SETTINGS;
 import static org.elasticsearch.common.settings.Settings.readSettingsFromStream;
 import static org.elasticsearch.common.settings.Settings.writeSettingsToStream;
 import static org.elasticsearch.common.xcontent.support.XContentMapValues.nodeBooleanValue;
@@ -55,7 +54,7 @@ public class RestoreSnapshotRequest extends MasterNodeRequest<RestoreSnapshotReq
     private boolean includeGlobalState = false;
     private boolean partial = false;
     private boolean includeAliases = true;
-    private Settings indexSettings = EMPTY_SETTINGS;
+    private Settings indexSettings = Settings.EMPTY;
     private String[] ignoreIndexSettings = Strings.EMPTY_ARRAY;
 
     // This field does not get serialised (except toString for debugging purpose) because it is always set locally by authz

server/src/main/java/org/elasticsearch/action/admin/indices/create/CreateIndexClusterStateUpdateRequest.java

@@ -38,7 +38,7 @@ public class CreateIndexClusterStateUpdateRequest extends ClusterStateUpdateRequ
     private boolean copySettings;
     private SystemDataStreamDescriptor systemDataStreamDescriptor;
 
-    private Settings settings = Settings.Builder.EMPTY_SETTINGS;
+    private Settings settings = Settings.EMPTY;
 
     private final Map<String, String> mappings = new HashMap<>();
 

server/src/main/java/org/elasticsearch/action/admin/indices/create/CreateIndexRequest.java

@@ -45,7 +45,6 @@
 import java.util.Set;
 
 import static org.elasticsearch.action.ValidateActions.addValidationError;
-import static org.elasticsearch.common.settings.Settings.Builder.EMPTY_SETTINGS;
 import static org.elasticsearch.common.settings.Settings.readSettingsFromStream;
 import static org.elasticsearch.common.settings.Settings.writeSettingsToStream;
 
@@ -68,7 +67,7 @@ public class CreateIndexRequest extends AcknowledgedRequest<CreateIndexRequest>
 
     private String index;
 
-    private Settings settings = EMPTY_SETTINGS;
+    private Settings settings = Settings.EMPTY;
 
     private final Map<String, String> mappings = new HashMap<>();
 
@@ -127,7 +126,7 @@ public CreateIndexRequest() {
      * @param index the name of the index
      */
     public CreateIndexRequest(String index) {
-        this(index, EMPTY_SETTINGS);
+        this(index, Settings.EMPTY);
     }
 
     /**