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   * Licensed to the Apache Software Foundation (ASF) under one
   * or more contributor license agreements.  See the NOTICE file
   * distributed with this work for additional information
   * regarding copyright ownership.  The ASF licenses this file
   * to you under the Apache License, Version 2.0 (the
   * "License"); you may not use this file except in compliance
   * with the License.  You may obtain a copy of the License at
   *
   *   http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing,
   * software distributed under the License is distributed on an
   * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
   * KIND, either express or implied.  See the License for the
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   * under the License.    
   */
  
  package org.apache.bifromq.basekv.localengine.rocksdb.metrics;
  
  import static org.awaitility.Awaitility.await;
  import static org.testng.Assert.assertFalse;
  
  import org.apache.bifromq.basekv.localengine.MockableTest;
  import org.apache.bifromq.basekv.localengine.metrics.KVSpaceMeters;
  import io.micrometer.core.instrument.Counter;
  import io.micrometer.core.instrument.Gauge;
  import io.micrometer.core.instrument.Metrics;
  import io.micrometer.core.instrument.Tags;
  import io.micrometer.core.instrument.Timer;
  import java.lang.ref.WeakReference;
  import java.util.concurrent.atomic.AtomicInteger;
  import lombok.extern.slf4j.Slf4j;
  import org.testng.annotations.Test;
  
  @Slf4j
  public class KVSpaceMetersTest extends MockableTest {
      @Test
      public void removeGaugeWhenNoRef() {
          AtomicInteger gaugeCounter = new AtomicInteger();
          Gauge gauge = KVSpaceMeters.getGauge("testSpace", RocksDBKVSpaceMetric.TotalTombstoneRangesGauge,
              gaugeCounter::get,
              Tags.of(new String[0]));
          String gaugeName = gauge.getId().getName();
          WeakReference<Gauge> weakRef = new WeakReference<>(gauge);
          assertFalse(Metrics.globalRegistry.find(gaugeName).gauges().isEmpty());
          gauge = null;
          await().forever().until(() -> {
              System.gc();
              return weakRef.get() == null && Metrics.globalRegistry.find(gaugeName).gauges().isEmpty();
          });
      }
  
      @Test
      public void removeTimerWhenNoRef() {
          Timer timer = KVSpaceMeters.getTimer("testSpace", RocksDBKVSpaceMetric.FlushTimer, Tags.of(new String[0]));
          String timerName = timer.getId().getName();
          WeakReference<Timer> weakRef = new WeakReference<>(timer);
          assertFalse(Metrics.globalRegistry.find(timerName).timers().isEmpty());
          timer = null;
          await().forever().until(() -> {
              System.gc();
              return weakRef.get() == null && Metrics.globalRegistry.find(timerName).timers().isEmpty();
          });
      }
  
      @Test
      public void removeCounterWhenNoRef() {
          Counter counter =
              KVSpaceMeters.getCounter("testSpace", RocksDBKVSpaceMetric.CompactionCounter, Tags.of(new String[0]));
          String timerName = counter.getId().getName();
          WeakReference<Counter> weakRef = new WeakReference<>(counter);
          assertFalse(Metrics.globalRegistry.find(timerName).counters().isEmpty());
          counter = null;
          await().forever().until(() -> {
              System.gc();
              return weakRef.get() == null && Metrics.globalRegistry.find(timerName).counters().isEmpty();
          });
      }
  }
  