Simplify initialization of Settings (#10194) (#10317)

There was a weird circular class-loading dependency between Settings and
Settings.Builder that could result in ClassLoader deadlock if multiple
threads trigger class-loading of Settings at the same time.


(cherry picked from commit d3bf230)

Signed-off-by: Michael Froh <froh@amazon.com>
Signed-off-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Signed-off-by: Daniel (dB.) Doubrovkine <dblock@amazon.com>
Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: Daniel (dB.) Doubrovkine <dblock@amazon.com>

CHANGELOG.md

@@ -57,6 +57,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Fix concurrent search NPE when track_total_hits, terminate_after and size=0 are used ([#10082](https://github.com/opensearch-project/OpenSearch/pull/10082))
 - Fix remove ingest processor handing ignore_missing parameter not correctly ([10089](https://github.com/opensearch-project/OpenSearch/pull/10089))
 - Fix registration and initialization of multiple extensions ([10256](https://github.com/opensearch-project/OpenSearch/pull/10256))
+- Fix circular dependency in Settings initialization ([10194](https://github.com/opensearch-project/OpenSearch/pull/10194))
 
 ### Security
 

server/src/main/java/org/opensearch/common/settings/Settings.java

@@ -101,7 +101,7 @@
 @PublicApi(since = "1.0.0")
 public final class Settings implements ToXContentFragment {
 
-    public static final Settings EMPTY = new Builder().build();
+    public static final Settings EMPTY = new Settings(Collections.emptyMap(), null);
 
     /** The raw settings from the full key to raw string value. */
     private final Map<String, Object> settings;
@@ -757,7 +757,7 @@ public Set<String> keySet() {
     @PublicApi(since = "1.0.0")
     public static class Builder {
 
-        public static final Settings EMPTY_SETTINGS = new Builder().build();
+        public static final Settings EMPTY_SETTINGS = Settings.EMPTY;
 
         // we use a sorted map for consistent serialization when using getAsMap()
         private final Map<String, Object> map = new TreeMap<>();