Use consistent shard map type in IndexService (#88465)

It's faster and easier to reason about if we always have
an immutable collections map here and not have the type depend
on what the last operation on the index service was.

server/src/main/java/org/elasticsearch/index/IndexService.java

@@ -96,7 +96,6 @@ import java.util.function.LongUnaryOperator;
 import java.util.function.Supplier;
 
 import static java.util.Collections.emptyMap;
-import static java.util.Collections.unmodifiableMap;
 import static org.elasticsearch.core.Strings.format;
 
 public class IndexService extends AbstractIndexComponent implements IndicesClusterStateService.AllocatedIndex<IndexShard> {
@@ -118,7 +117,7 @@ public class IndexService extends AbstractIndexComponent implements IndicesClust
     private final SimilarityService similarityService;
     private final EngineFactory engineFactory;
     private final IndexWarmer warmer;
-    private volatile Map<Integer, IndexShard> shards = emptyMap();
+    private volatile Map<Integer, IndexShard> shards = Map.of();
     private final AtomicBoolean closed = new AtomicBoolean(false);
     private final AtomicBoolean deleted = new AtomicBoolean(false);
     private final IndexSettings indexSettings;
@@ -523,16 +522,13 @@ public class IndexService extends AbstractIndexComponent implements IndicesClust
 
     @Override
     public synchronized void removeShard(int shardId, String reason) {
-        final ShardId sId = new ShardId(index(), shardId);
-        final IndexShard indexShard;
-        if (shards.containsKey(shardId) == false) {
+        final IndexShard indexShard = shards.get(shardId);
+        if (indexShard == null) {
             return;
         }
         logger.debug("[{}] closing... (reason: [{}])", shardId, reason);
-        HashMap<Integer, IndexShard> newShards = new HashMap<>(shards);
-        indexShard = newShards.remove(shardId);
-        shards = unmodifiableMap(newShards);
-        closeShard(reason, sId, indexShard, indexShard.store(), indexShard.getIndexEventListener());
+        shards = Maps.copyMapWithRemovedEntry(shards, shardId);
+        closeShard(reason, indexShard.shardId(), indexShard, indexShard.store(), indexShard.getIndexEventListener());
         logger.debug("[{}] closed (reason: [{}])", shardId, reason);
     }