Remove index setting when override value is null (#122267) (#122346)

(cherry picked from commit 2b9d7f6)

Author: lukewhiting

x-pack/plugin/migrate/src/internalClusterTest/java/org/elasticsearch/xpack/migrate/action/CreateIndexFromSourceActionIT.java

@@ -208,7 +208,7 @@ public void testSettingsNullOverride() throws Exception {
         assertTrue(destSettings.getAsBoolean(IndexMetadata.SETTING_BLOCKS_READ, false));
 
         // override null removed
-        assertNull(destSettings.get(IndexMetadata.SETTING_BLOCKS_WRITE));
+        assertThat(destSettings.keySet(), not(hasItem(IndexMetadata.SETTING_BLOCKS_WRITE)));
     }
 
     public void testRemoveIndexBlocksByDefault() throws Exception {

x-pack/plugin/migrate/src/main/java/org/elasticsearch/xpack/migrate/action/CreateIndexFromSourceTransportAction.java

@@ -89,9 +89,12 @@ protected void doExecute(Task task, CreateIndexFromSourceAction.Request request,
 
         Settings.Builder settings = Settings.builder()
             // first settings from source index
-            .put(filterSettings(sourceIndex))
-            // then override with request settings
-            .put(request.settingsOverride());
+            .put(filterSettings(sourceIndex));
+
+        if (request.settingsOverride().isEmpty() == false) {
+            applyOverrides(settings, request.settingsOverride());
+        }
+
         if (request.removeIndexBlocks()) {
             // lastly, override with settings to remove index blocks if requested
             INDEX_BLOCK_SETTINGS.forEach(settings::remove);
@@ -114,6 +117,16 @@ protected void doExecute(Task task, CreateIndexFromSourceAction.Request request,
         client.admin().indices().create(createIndexRequest, listener.map(response -> response));
     }
 
+    private void applyOverrides(Settings.Builder settings, Settings overrides) {
+        overrides.keySet().forEach(key -> {
+            if (overrides.get(key) != null) {
+                settings.put(key, overrides.get(key));
+            } else {
+                settings.remove(key);
+            }
+        });
+    }
+
     private static Map<String, Object> toMap(@Nullable MappingMetadata sourceMapping) {
         return Optional.ofNullable(sourceMapping)
             .map(MappingMetadata::source)