Allow float settings to be configured with other settings as default (#126751)

Relates ES-11367

Author: nicktindall

docs/changelog/126751.yaml

@@ -0,0 +1,5 @@
+pr: 126751
+summary: Allow float settings to be configured with other settings as default
+area: Infra/Settings
+type: enhancement
+issues: []

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

@@ -1367,8 +1367,16 @@ public static Setting<Float> floatSetting(String key, float defaultValue, Proper
     }
 
     public static Setting<Float> floatSetting(String key, float defaultValue, float minValue, Property... properties) {
+        return new Setting<>(key, Float.toString(defaultValue), floatParser(key, minValue, properties), properties);
+    }
+
+    public static Setting<Float> floatSetting(String key, Setting<Float> fallbackSetting, float minValue, Property... properties) {
+        return new Setting<>(key, fallbackSetting, floatParser(key, minValue, properties), properties);
+    }
+
+    private static Function<String, Float> floatParser(String key, float minValue, Property... properties) {
         final boolean isFiltered = isFiltered(properties);
-        return new Setting<>(key, Float.toString(defaultValue), (s) -> {
+        return (s) -> {
             float value = Float.parseFloat(s);
             if (value < minValue) {
                 String err = "Failed to parse value"
@@ -1380,7 +1388,7 @@ public static Setting<Float> floatSetting(String key, float defaultValue, float
                 throw new IllegalArgumentException(err);
             }
             return value;
-        }, properties);
+        };
     }
 
     private static boolean isFiltered(Property[] properties) {

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

@@ -387,6 +387,34 @@ public void testFilteredBooleanSetting() {
         assertNull(e.getCause());
     }
 
+    public void testFloatSettingWithOtherSettingAsDefault() {
+        float defaultFallbackValue = randomFloat();
+        Setting<Float> fallbackSetting = Setting.floatSetting("fallback_setting", defaultFallbackValue);
+        Setting<Float> floatSetting = Setting.floatSetting("float_setting", fallbackSetting, Float.MIN_VALUE);
+
+        // Neither float_setting nor fallback_setting specified
+        assertThat(floatSetting.get(Settings.builder().build()), equalTo(defaultFallbackValue));
+
+        // Only fallback_setting specified
+        float explicitFallbackValue = randomValueOtherThan(defaultFallbackValue, ESTestCase::randomFloat);
+        assertThat(
+            floatSetting.get(Settings.builder().put("fallback_setting", explicitFallbackValue).build()),
+            equalTo(explicitFallbackValue)
+        );
+
+        // Both float_setting and fallback_setting specified
+        float explicitFloatValue = randomValueOtherThanMany(
+            v -> v != explicitFallbackValue && v != defaultFallbackValue,
+            ESTestCase::randomFloat
+        );
+        assertThat(
+            floatSetting.get(
+                Settings.builder().put("fallback_setting", explicitFallbackValue).put("float_setting", explicitFloatValue).build()
+            ),
+            equalTo(explicitFloatValue)
+        );
+    }
+
     private enum TestEnum {
         ON,
         OFF