Fix wrong default value when setting index.number_of_routing_shards to null on index creation

CHANGELOG.md

@@ -80,6 +80,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Fix warnings from SLF4J on startup when repository-s3 is installed ([#16194](https://github.com/opensearch-project/OpenSearch/pull/16194))
 - Fix protobuf-java leak through client library dependencies ([#16254](https://github.com/opensearch-project/OpenSearch/pull/16254))
 - Fix multi-search with template doesn't return status code ([#16265](https://github.com/opensearch-project/OpenSearch/pull/16265))
+- Fix wrong default value when setting `index.number_of_routing_shards` to null on index creation ([#16331](https://github.com/opensearch-project/OpenSearch/pull/16331))
 
 ### Security
 

rest-api-spec/src/main/resources/rest-api-spec/test/indices.create/10_basic.yml

@@ -112,3 +112,33 @@
             properties:
               "":
                 type:     keyword
+
+---
+"Create index with setting index.number_of_routing_shards to null":
+  - skip:
+      version: " - 2.99.99"
+      reason: "fixed in 3.0.0"
+  - do:
+      indices.create:
+        index: test_index
+        body:
+          settings:
+            number_of_routing_shards: null
+  - do:
+      cluster.state:
+        metric: [ metadata ]
+        index: test_index
+  - match : { metadata.indices.test_index.routing_num_shards: 1024 }
+
+  - do:
+      indices.create:
+        index: test_index1
+        body:
+          settings:
+            number_of_routing_shards: null
+            number_of_shards: 3
+  - do:
+      cluster.state:
+        metric: [ metadata ]
+        index: test_index1
+  - match : { metadata.indices.test_index1.routing_num_shards: 768 }

server/src/main/java/org/opensearch/cluster/metadata/MetadataCreateIndexService.java

@@ -1210,7 +1210,7 @@ static int getIndexNumberOfRoutingShards(Settings indexSettings, @Nullable Index
             // in this case we either have no index to recover from or
             // we have a source index with 1 shard and without an explicit split factor
             // or one that is valid in that case we can split into whatever and auto-generate a new factor.
-            if (IndexMetadata.INDEX_NUMBER_OF_ROUTING_SHARDS_SETTING.exists(indexSettings)) {
+            if (indexSettings.get(IndexMetadata.INDEX_NUMBER_OF_ROUTING_SHARDS_SETTING.getKey()) != null) {
                 routingNumShards = IndexMetadata.INDEX_NUMBER_OF_ROUTING_SHARDS_SETTING.get(indexSettings);
             } else {
                 routingNumShards = calculateNumRoutingShards(numTargetShards, indexVersionCreated);

server/src/test/java/org/opensearch/cluster/metadata/MetadataCreateIndexServiceTests.java

@@ -1857,6 +1857,24 @@ public void testGetIndexNumberOfRoutingShardsYieldsSourceNumberOfShards() {
         assertThat(targetRoutingNumberOfShards, is(6));
     }
 
+    public void testGetIndexNumberOfRoutingShardsWhenExplicitlySetToNull() {
+        String nullValue = null;
+        Settings indexSettings = Settings.builder()
+            .put("index.version.created", Version.CURRENT)
+            .put(INDEX_NUMBER_OF_ROUTING_SHARDS_SETTING.getKey(), nullValue)
+            .build();
+        int targetRoutingNumberOfShards = getIndexNumberOfRoutingShards(indexSettings, null);
+        assertThat(targetRoutingNumberOfShards, is(1024));
+
+        indexSettings = Settings.builder()
+            .put("index.version.created", Version.CURRENT)
+            .put(INDEX_NUMBER_OF_SHARDS_SETTING.getKey(), 3)
+            .put(INDEX_NUMBER_OF_ROUTING_SHARDS_SETTING.getKey(), nullValue)
+            .build();
+        targetRoutingNumberOfShards = getIndexNumberOfRoutingShards(indexSettings, null);
+        assertThat(targetRoutingNumberOfShards, is(768));
+    }
+
     public void testSoftDeletesDisabledIsRejected() {
         final IllegalArgumentException error = expectThrows(IllegalArgumentException.class, () -> {
             request = new CreateIndexClusterStateUpdateRequest("create index", "test", "test");