HSEARCH-4253 Align IndexSettings#merge on AbstractTypeMapping#merge

It's easier to implement AbstractTypeMapping#merge if we consider `this`
takes precedence over the argument to `merge()`; for
`IndexSettings#merge`, both approaches are equally easy to implement.

So, let's be consistent and consider `this` takes precedence over the
argument to `merge()` in both cases.

backend/elasticsearch/src/main/java/org/hibernate/search/backend/elasticsearch/document/model/lowlevel/impl/LowLevelIndexMetadataBuilder.java

@@ -93,10 +93,17 @@ private IndexSettings buildSettings() {
 			getAnalysis( settings ).setCharFilters( analysisDefinitionRegistry.getCharFilterDefinitions() );
 		}
 
-		// if customSettings are present, merge them with the ones created by Search
-		settings.merge( customIndexSettings );
+		if ( customIndexSettings == null ) {
+			return settings;
+		}
 
-		return settings;
+		// If customIndexSettings are present, merge them with the ones created by Search.
+		// Avoid side effects: we copy the settings before modifying them.
+		IndexSettings customIndexSettingsCopy =
+				GsonUtils.deepCopy( gsonProvider.getGsonNoSerializeNulls(), IndexSettings.class, customIndexSettings );
+		// The custom settings take precedence over the Hibernate Search ones.
+		customIndexSettingsCopy.merge( settings );
+		return customIndexSettingsCopy;
 	}
 
 	private RootTypeMapping buildMapping() {

backend/elasticsearch/src/main/java/org/hibernate/search/backend/elasticsearch/lowlevel/index/settings/impl/Analysis.java

@@ -105,39 +105,41 @@ public String toString() {
 	}
 
 	/**
-	 * Merge this analysis with another one.
-	 * Any conflict of definition will be solved in favour of the other analysis.
-	 * {@link #extraAttributes} will be always overridden.
+	 * Merge these (custom) analysis settings with analysis settings generated by Hibernate Search.
+	 * <p>
+	 * Any conflict of definition will be solved in favor of {@code this}.
 	 *
-	 * @param overridingAnalysis The other analysis definition
+	 * @param other The other analysis settings
 	 */
-	public void merge(Analysis overridingAnalysis) {
-		if ( overridingAnalysis == null ) {
+	public void merge(Analysis other) {
+		if ( other == null ) {
 			// nothing to do
 			return;
 		}
 
-		analyzers = merge( analyzers, overridingAnalysis.analyzers );
-		normalizers = merge( normalizers, overridingAnalysis.normalizers );
-		tokenizers = merge( tokenizers, overridingAnalysis.tokenizers );
-		tokenFilters = merge( tokenFilters, overridingAnalysis.tokenFilters );
-		charFilters = merge( charFilters, overridingAnalysis.charFilters );
+		analyzers = merge( analyzers, other.analyzers );
+		normalizers = merge( normalizers, other.normalizers );
+		tokenizers = merge( tokenizers, other.tokenizers );
+		tokenFilters = merge( tokenFilters, other.tokenFilters );
+		charFilters = merge( charFilters, other.charFilters );
 
-		extraAttributes = overridingAnalysis.extraAttributes;
+		if ( extraAttributes == null ) {
+			extraAttributes = other.extraAttributes;
+		}
 	}
 
-	private static <K> Map<String, K> merge(Map<String, K> originalImmutableMap, Map<String, K> overridingMap) {
-		if ( overridingMap == null || overridingMap.isEmpty() ) {
+	private static <K> Map<String, K> merge(Map<String, K> originalImmutableMap, Map<String, K> otherMap) {
+		if ( otherMap == null || otherMap.isEmpty() ) {
 			return originalImmutableMap;
 		}
 
 		if ( originalImmutableMap == null || originalImmutableMap.isEmpty() ) {
-			return overridingMap;
+			return otherMap;
 		}
 
 		// let's merge
-		HashMap<String, K> result = new HashMap<>( originalImmutableMap );
-		result.putAll( overridingMap );
+		HashMap<String, K> result = new HashMap<>( otherMap );
+		result.putAll( originalImmutableMap );
 		return result;
 	}
 }

backend/elasticsearch/src/main/java/org/hibernate/search/backend/elasticsearch/lowlevel/index/settings/impl/IndexSettings.java

@@ -67,30 +67,32 @@ public String toString() {
 	}
 
 	/**
-	 * Merge these settings with other settings.
-	 * Any conflict of definition will be solved in favour of the other settings.
-	 * {@link #extraAttributes} will be always overridden.
+	 * Merge these (custom) settings with settings generated by Hibernate Search.
+	 * <p>
+	 * Any conflict of definition will be solved in favor of {@code this}.
 	 *
-	 * @param overridingIndexSettings The other index settings
+	 * @param other The other index settings
 	 */
-	public void merge(IndexSettings overridingIndexSettings) {
-		if ( overridingIndexSettings == null ) {
+	public void merge(IndexSettings other) {
+		if ( other == null ) {
 			// nothing to do
 			return;
 		}
 
 		if ( analysis == null ) {
-			analysis = overridingIndexSettings.analysis;
+			analysis = other.analysis;
 		}
 		else {
-			analysis.merge( overridingIndexSettings.analysis );
+			analysis.merge( other.analysis );
 		}
 
-		if ( overridingIndexSettings.maxResultWindow != null ) {
-			maxResultWindow = overridingIndexSettings.maxResultWindow;
+		if ( maxResultWindow == null ) {
+			maxResultWindow = other.maxResultWindow;
 		}
 
-		extraAttributes = overridingIndexSettings.extraAttributes;
+		if ( extraAttributes == null ) {
+			extraAttributes = other.extraAttributes;
+		}
 	}
 
 	/**