Update number of replicas when removing setting (#56723)

We previously rejected removing the number of replicas setting, which
prevents users from reverting this setting to its default the natural
way. To fix this, we put back the setting with the default value in the
cases that the user is trying to remove it. Yet, we also need to do the
work of updating the routing table and so on appropriately. This case
was missed because when the setting is being removed, we were defaulting
to -1 in this code path, which is treated as not being updated. Instead,
we must treat the case when we are removing this setting as if the
setting is being updated, too. This commit does that.

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

@@ -147,26 +147,30 @@ public ClusterState execute(ClusterState currentState) {
                             "Can't update non dynamic settings [%s] for open indices %s", skippedSettings, openIndices));
                 }
 
-                int updatedNumberOfReplicas = openSettings.getAsInt(IndexMetaData.SETTING_NUMBER_OF_REPLICAS, -1);
-                if (updatedNumberOfReplicas != -1 && preserveExisting == false) {
+                if (IndexMetaData.INDEX_NUMBER_OF_REPLICAS_SETTING.exists(openSettings)) {
+                    final int updatedNumberOfReplicas = IndexMetaData.INDEX_NUMBER_OF_REPLICAS_SETTING.get(openSettings);
+                    if (preserveExisting == false) {
+                        // Verify that this won't take us over the cluster shard limit.
+                        int totalNewShards = Arrays.stream(request.indices())
+                            .mapToInt(i -> getTotalNewShards(i, currentState, updatedNumberOfReplicas))
+                            .sum();
+                        Optional<String> error = IndicesService.checkShardLimit(totalNewShards, currentState);
+                        if (error.isPresent()) {
+                            ValidationException ex = new ValidationException();
+                            ex.addValidationError(error.get());
+                            throw ex;
+                        }
 
-                    // Verify that this won't take us over the cluster shard limit.
-                    int totalNewShards = Arrays.stream(request.indices())
-                        .mapToInt(i -> getTotalNewShards(i, currentState, updatedNumberOfReplicas))
-                        .sum();
-                    Optional<String> error = IndicesService.checkShardLimit(totalNewShards, currentState);
-                    if (error.isPresent()) {
-                        ValidationException ex = new ValidationException();
-                        ex.addValidationError(error.get());
-                        throw ex;
+                        /*
+                         * We do not update the in-sync allocation IDs as they will be removed upon the first index operation which makes
+                         * these copies stale.
+                         *
+                         * TODO: should we update the in-sync allocation IDs once the data is deleted by the node?
+                         */
+                        routingTableBuilder.updateNumberOfReplicas(updatedNumberOfReplicas, actualIndices);
+                        metaDataBuilder.updateNumberOfReplicas(updatedNumberOfReplicas, actualIndices);
+                        logger.info("updating number_of_replicas to [{}] for indices {}", updatedNumberOfReplicas, actualIndices);
                     }
-
-                    // we do *not* update the in sync allocation ids as they will be removed upon the first index
-                    // operation which make these copies stale
-                    // TODO: update the list once the data is deleted by the node?
-                    routingTableBuilder.updateNumberOfReplicas(updatedNumberOfReplicas, actualIndices);
-                    metaDataBuilder.updateNumberOfReplicas(updatedNumberOfReplicas, actualIndices);
-                    logger.info("updating number_of_replicas to [{}] for indices {}", updatedNumberOfReplicas, actualIndices);
                 }
 
                 ClusterBlocks.Builder blocks = ClusterBlocks.builder().blocks(currentState.blocks());