src/test/java/com/google/devtools/build/lib/util/LatestObjectMetricExporterTest.java

// Copyright 2023 The Bazel Authors. All rights reserved.
//
// Licensed 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 specific language governing permissions and
// limitations under the License.

package com.google.devtools.build.lib.util;

import static com.google.common.truth.Truth.assertThat;

import java.util.concurrent.atomic.AtomicReference;
import java.util.function.Supplier;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.junit.runners.JUnit4;

/** Unit test for {@link LatestObjectMetricExporter}. */
@RunWith(JUnit4.class)
public class LatestObjectMetricExporterTest {

  @Test
  public void weakReferencesAreGarbageCollected() {
    // Create an exporter with the given strength and whose registration stores the Supplier<Object>
    // in an AtomicReference.
    LatestObjectMetricExporter.Strength strength = LatestObjectMetricExporter.Strength.WEAK;
    AtomicReference<Supplier<Object>> registeredSupplierRef = new AtomicReference<>(null);
    LatestObjectMetricExporter.CallbackRegistration<Object> registration =
        registeredSupplierRef::set;
    LatestObjectMetricExporter<Object> exporter =
        new LatestObjectMetricExporter<>(strength, registration);
    assertThat(registeredSupplierRef.get()).isNull();
    // Set up three Objects to serve as dummy "latest objects" to pass through the exporter.
    Object first = new Object();
    Object second = new Object();
    Object third = new Object();
    // Set the first value, at which point the registration will run and we will get the Supplier
    // that tells us the currently exported object.
    exporter.setLatestInstance(first);
    Supplier<Object> latestObjectSupplier = registeredSupplierRef.get();
    assertThat(latestObjectSupplier).isNotNull();
    assertThat(latestObjectSupplier.get()).isSameInstanceAs(first);

    // Remove only reference to the latest object and run the GC. The supplier should start
    // producing null, not `first`.
    first = null;
    Runtime runtime = Runtime.getRuntime();
    runtime.gc();
    assertThat(latestObjectSupplier.get()).isNull();

    // Remove only reference to the latest object but don't run the GC. The supplier should still
    // return `second` until we change the latest inatance to `third`, at which point GC has no
    // observable effect..
    exporter.setLatestInstance(second);
    assertThat(latestObjectSupplier.get()).isSameInstanceAs(second);
    second = null;
    exporter.setLatestInstance(third);
    assertThat(latestObjectSupplier.get()).isSameInstanceAs(third);
    runtime.gc();
    assertThat(latestObjectSupplier.get()).isSameInstanceAs(third);

    // Repeat the first assertion: removing the reference and GCing will cause the Supplier
    // to produce null, not `third`.
    third = null;
    runtime.gc();
    assertThat(latestObjectSupplier.get()).isNull();
  }
}