Add additional logging for #43034

It's unclear why sometimes the shard is not flushed on closing

Author: ywelsch

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

@@ -471,6 +471,8 @@ private void closeShard(String reason, ShardId sId, IndexShard indexShard, Store
                     try {
                         // only flush we are we closed (closed index or shutdown) and if we are not deleted
                         final boolean flushEngine = deleted.get() == false && closed.get();
+                        logger.trace("[{}] closing shard (flushEngine: {}, deleted: {}, closed: {})", shardId, flushEngine, deleted.get(),
+                            closed.get());
                         indexShard.close(reason, flushEngine);
                     } catch (Exception e) {
                         logger.debug(() -> new ParameterizedMessage("[{}] failed to close index shard", shardId), e);

server/src/main/java/org/elasticsearch/index/engine/Engine.java

@@ -1716,6 +1716,8 @@ public void flushAndClose() throws IOException {
                     close(); // double close is not a problem
                 }
             }
+        } else {
+            logger.trace("skipping flushAndClose as already closed");
         }
         awaitPendingClose();
     }