GET _cluster/settings with include_defaults returns the expected fall…

…back value if defined in elasticsearch.yml (elastic#110816)

* test: add unit test that reproduces elastic#110815

* fix: consider the fallback setting when getting the default value of a setting

* doc: add changelog entry

docs/changelog/110816.yaml

@@ -0,0 +1,6 @@
+pr: 110816
+summary: GET _cluster/settings with include_defaults returns the expected fallback value if defined in elasticsearch.yml
+area: Infra/Settings
+type: bug
+issues:
+ - 110815

server/src/main/java/org/elasticsearch/common/settings/Setting.java

@@ -584,7 +584,7 @@ private T get(Settings settings, boolean validate) {
  */
  public void diff(Settings.Builder builder, Settings source, Settings defaultSettings) {
  if (exists(source) == false) {
- if (exists(defaultSettings)) {
+ if (existsOrFallbackExists(defaultSettings)) {
  // If the setting is only in the defaults, use the value from the defaults
  builder.put(getKey(), getRaw(defaultSettings));
  } else {

server/src/test/java/org/elasticsearch/common/settings/ScopedSettingsTests.java

@@ -924,6 +924,26 @@ public void testDiff() throws IOException {
  assertThat(diff.getAsInt("foo.bar", null), equalTo(1));
  }
 
+ public void testDiffWithFallbackDefaultSetting() {
+ final String fallbackSettingName = "fallback";
+ final Setting<Integer> fallbackSetting = Setting.intSetting(fallbackSettingName, 1, Property.Dynamic, Property.NodeScope);
+
+ final String settingName = "setting.with.fallback";
+ final Setting<Integer> dependentSetting = new Setting<>(
+ settingName,
+ fallbackSetting,
+ (s) -> Setting.parseInt(s, 1, settingName),
+ value -> {},
+ Property.Dynamic,
+ Property.NodeScope
+ );
+
+ ClusterSettings settings = new ClusterSettings(Settings.EMPTY, new HashSet<>(Arrays.asList(fallbackSetting, dependentSetting)));
+
+ final Settings diff = settings.diff(Settings.EMPTY, Settings.builder().put(fallbackSettingName, 2).build());
+ assertThat(diff.getAsInt(settingName, null), equalTo(2));
+ }
+
  public void testDiffWithDependentSettings() {
  final String dependedSettingName = "this.setting.is.depended.on";
  Setting<Integer> dependedSetting = Setting.intSetting(dependedSettingName, 1, Property.Dynamic, Property.NodeScope);