Calculate routing num shards correctly during reshard

server/src/main/java/org/elasticsearch/cluster/metadata/IndexMetadata.java

@@ -1998,19 +1998,16 @@ public Builder numberOfShards(int numberOfShards) {
 
         /**
          * Builder to create IndexMetadata that has an increased shard count (used for re-shard).
-         * The new shard count must be a multiple of the original shardcount.
+         * The new shard count must be a multiple of the original shardcount as well as a factor
+         * of routingNumShards.
          * We do not support shrinking the shard count.
-         * @param shardCount   updated shardCount
-         *
-         * TODO: Check if this.version needs to be incremented
+         * @param targetShardCount   target shard count after resharding
          */
-        public Builder reshardAddShards(int shardCount) {
-            // Assert routingNumShards is null ?
-            // Assert numberOfShards > 0
-            if (shardCount % numberOfShards() != 0) {
+        public Builder reshardAddShards(int targetShardCount) {
+            if (targetShardCount % numberOfShards() != 0) {
                 throw new IllegalArgumentException(
                     "New shard count ["
-                        + shardCount
+                        + targetShardCount
                         + "] should be a multiple"
                         + " of current shard count ["
                         + numberOfShards()
@@ -2019,13 +2016,13 @@ public Builder reshardAddShards(int shardCount) {
                         + "]"
                 );
             }
-            IndexVersion indexVersionCreated = indexCreatedVersion(settings);
-            settings = Settings.builder().put(settings).put(SETTING_NUMBER_OF_SHARDS, shardCount).build();
-            var newPrimaryTerms = new long[shardCount];
+            final int sourceNumShards = numberOfShards();
+            settings = Settings.builder().put(settings).put(SETTING_NUMBER_OF_SHARDS, targetShardCount).build();
+            var newPrimaryTerms = new long[targetShardCount];
             Arrays.fill(newPrimaryTerms, this.primaryTerms.length, newPrimaryTerms.length, SequenceNumbers.UNASSIGNED_PRIMARY_TERM);
             System.arraycopy(primaryTerms, 0, newPrimaryTerms, 0, this.primaryTerms.length);
             primaryTerms = newPrimaryTerms;
-            routingNumShards = MetadataCreateIndexService.calculateNumRoutingShards(shardCount, indexVersionCreated);
+            routingNumShards = MetadataCreateIndexService.getIndexNumberOfRoutingShards(settings, sourceNumShards, this.routingNumShards);
             return this;
         }
 
@@ -3034,7 +3031,7 @@ public static ShardId selectCloneShard(int shardId, IndexMetadata sourceIndexMet
         return new ShardId(sourceIndexMetadata.getIndex(), shardId);
     }
 
-    private static void assertSplitMetadata(int numSourceShards, int numTargetShards, IndexMetadata sourceIndexMetadata) {
+    public static void assertSplitMetadata(int numSourceShards, int numTargetShards, IndexMetadata sourceIndexMetadata) {
         if (numSourceShards > numTargetShards) {
             throw new IllegalArgumentException(
                 "the number of source shards ["

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

@@ -1282,10 +1282,23 @@ private static void validateSoftDeleteSettings(Settings indexSettings) {
      * it will return the value configured for that index.
      */
     static int getIndexNumberOfRoutingShards(Settings indexSettings, @Nullable IndexMetadata sourceMetadata) {
+        final int routingNumShards = getIndexNumberOfRoutingShards(
+            indexSettings,
+            sourceMetadata == null ? 1 : sourceMetadata.getNumberOfShards(),
+            sourceMetadata == null ? 0 : sourceMetadata.getRoutingNumShards()
+        );
+        return routingNumShards;
+    }
+
+    /**
+     * Calculates the number of routing shards based on the configured value in indexSettings or if recovering from another index
+     * it will return the value configured for that index.
+     */
+    static int getIndexNumberOfRoutingShards(Settings indexSettings, final int sourceNumShards, final int sourceRoutingNumShards) {
         final int numTargetShards = INDEX_NUMBER_OF_SHARDS_SETTING.get(indexSettings);
         final IndexVersion indexVersionCreated = IndexMetadata.SETTING_INDEX_VERSION_CREATED.get(indexSettings);
         final int routingNumShards;
-        if (sourceMetadata == null || sourceMetadata.getNumberOfShards() == 1) {
+        if (sourceNumShards == 1) {
             // 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.
@@ -1299,7 +1312,7 @@ static int getIndexNumberOfRoutingShards(Settings indexSettings, @Nullable Index
         } else {
             assert IndexMetadata.INDEX_NUMBER_OF_ROUTING_SHARDS_SETTING.exists(indexSettings) == false
                 : "index.number_of_routing_shards should not be present on the target index on resize";
-            routingNumShards = sourceMetadata.getRoutingNumShards();
+            routingNumShards = sourceRoutingNumShards;
         }
         return routingNumShards;
     }