src/test/java/com/google/devtools/build/skyframe/EagerInvalidatorTest.java

// Copyright 2014 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.skyframe;

import static com.google.common.truth.Truth.assertThat;
import static com.google.devtools.build.skyframe.GraphTester.CONCATENATE;
import static com.google.devtools.build.skyframe.GraphTester.NODE_TYPE;
import static org.junit.Assert.fail;

import com.google.common.base.Preconditions;
import com.google.common.collect.ImmutableList;
import com.google.common.collect.Iterables;
import com.google.common.collect.Sets;
import com.google.common.eventbus.EventBus;
import com.google.common.testing.GcFinalization;
import com.google.devtools.build.lib.concurrent.AbstractQueueVisitor;
import com.google.devtools.build.lib.events.Reporter;
import com.google.devtools.build.lib.testutil.TestUtils;
import com.google.devtools.build.lib.util.Pair;
import com.google.devtools.build.skyframe.GraphTester.StringValue;
import com.google.devtools.build.skyframe.InvalidatingNodeVisitor.DeletingNodeVisitor;
import com.google.devtools.build.skyframe.InvalidatingNodeVisitor.DirtyingInvalidationState;
import com.google.devtools.build.skyframe.InvalidatingNodeVisitor.DirtyingNodeVisitor;
import com.google.devtools.build.skyframe.InvalidatingNodeVisitor.InvalidationState;
import com.google.devtools.build.skyframe.InvalidatingNodeVisitor.InvalidationType;
import com.google.devtools.build.skyframe.QueryableGraph.Reason;
import java.lang.ref.WeakReference;
import java.util.HashSet;
import java.util.Random;
import java.util.Set;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicReference;
import org.junit.After;
import org.junit.Before;
import org.junit.Test;
import org.junit.experimental.runners.Enclosed;
import org.junit.runner.RunWith;
import org.junit.runners.JUnit4;

/**
 * Tests for {@link InvalidatingNodeVisitor}.
 */
@RunWith(Enclosed.class)
public class EagerInvalidatorTest {
  protected InMemoryGraphImpl graph;
  protected GraphTester tester = new GraphTester();
  protected InvalidationState state = newInvalidationState();
  protected AtomicReference<InvalidatingNodeVisitor<?>> visitor = new AtomicReference<>();
  protected DirtyTrackingProgressReceiver progressReceiver;
  private IntVersion graphVersion = IntVersion.of(0);

  @After
  public void assertNoTrackedErrors() {
    TrackingAwaiter.INSTANCE.assertNoErrors();
  }

  // The following three methods should be abstract, but junit4 does not allow us to run inner
  // classes in an abstract outer class. Thus, we provide implementations. These methods will never
  // be run because only the inner classes, annotated with @RunWith, will actually be executed.
  EvaluationProgressReceiver.InvalidationState expectedState() {
    throw new UnsupportedOperationException();
  }

  @SuppressWarnings("unused") // Overridden by subclasses.
  void invalidate(
      InMemoryGraph graph, DirtyTrackingProgressReceiver progressReceiver, SkyKey... keys)
      throws InterruptedException {
    throw new UnsupportedOperationException();
  }

  boolean gcExpected() { throw new UnsupportedOperationException(); }

  private boolean isInvalidated(SkyKey key) {
    NodeEntry entry = graph.get(null, Reason.OTHER, key);
    if (gcExpected()) {
      return entry == null;
    } else {
      return entry == null || entry.isDirty();
    }
  }

  private void assertChanged(SkyKey key) {
    NodeEntry entry = graph.get(null, Reason.OTHER, key);
    if (gcExpected()) {
      assertThat(entry).isNull();
    } else {
      assertThat(entry.isChanged()).isTrue();
    }
  }

  private void assertDirtyAndNotChanged(SkyKey key) {
    NodeEntry entry = graph.get(null, Reason.OTHER, key);
    if (gcExpected()) {
      assertThat(entry).isNull();
    } else {
      assertThat(entry.isDirty()).isTrue();
      assertThat(entry.isChanged()).isFalse();
    }

  }

  protected InvalidationState newInvalidationState() {
    throw new UnsupportedOperationException("Sublcasses must override");
  }

  protected InvalidationType defaultInvalidationType() {
    throw new UnsupportedOperationException("Sublcasses must override");
  }

  protected boolean reverseDepsPresent() {
    throw new UnsupportedOperationException("Subclasses must override");
  }

  // Convenience method for eval-ing a single value.
  protected SkyValue eval(boolean keepGoing, SkyKey key) throws InterruptedException {
    SkyKey[] keys = { key };
    return eval(keepGoing, keys).get(key);
  }

  protected <T extends SkyValue> EvaluationResult<T> eval(boolean keepGoing, SkyKey... keys)
    throws InterruptedException {
    Reporter reporter = new Reporter(new EventBus());
    ParallelEvaluator evaluator =
        new ParallelEvaluator(
            graph,
            graphVersion,
            tester.getSkyFunctionMap(),
            reporter,
            new MemoizingEvaluator.EmittedEventState(),
            InMemoryMemoizingEvaluator.DEFAULT_STORED_EVENT_FILTER,
            ErrorInfoManager.UseChildErrorInfoIfNecessary.INSTANCE,
            keepGoing,
            new DirtyTrackingProgressReceiver(null),
            GraphInconsistencyReceiver.THROWING,
            () -> AbstractQueueVisitor.createExecutorService(200),
            new SimpleCycleDetector(),
            EvaluationVersionBehavior.MAX_CHILD_VERSIONS);
    graphVersion = graphVersion.next();
    return evaluator.eval(ImmutableList.copyOf(keys));
  }

  protected void invalidateWithoutError(DirtyTrackingProgressReceiver progressReceiver,
      SkyKey... keys) throws InterruptedException {
    invalidate(graph, progressReceiver, keys);
    assertThat(state.isEmpty()).isTrue();
  }

  protected void set(String name, String value) {
    tester.set(name, new StringValue(value));
  }

  protected SkyKey skyKey(String name) {
    return GraphTester.toSkyKeys(name).get(0);
  }

  protected void assertValueValue(String name, String expectedValue) throws InterruptedException {
    StringValue value = (StringValue) eval(false, skyKey(name));
    assertThat(value.getValue()).isEqualTo(expectedValue);
  }

  @Before
  public void setUp() throws Exception {
    progressReceiver = new DirtyTrackingProgressReceiver(null);
  }

  @Test
  public void receiverWorks() throws Exception {
    final Set<SkyKey> invalidated = Sets.newConcurrentHashSet();
    DirtyTrackingProgressReceiver receiver = new DirtyTrackingProgressReceiver(
        new EvaluationProgressReceiver.NullEvaluationProgressReceiver() {
      @Override
      public void invalidated(SkyKey skyKey, InvalidationState state) {
        Preconditions.checkState(state == expectedState());
        invalidated.add(skyKey);
      }
    });
    graph = new InMemoryGraphImpl();
    SkyKey aKey = GraphTester.nonHermeticKey("a");
    SkyKey bKey = GraphTester.nonHermeticKey("b");
    tester.set(aKey, new StringValue("a"));
    tester.set(bKey, new StringValue("b"));
    tester.getOrCreate("ab").addDependency(aKey).addDependency(bKey).setComputedValue(CONCATENATE);
    assertValueValue("ab", "ab");

    tester.set(aKey, new StringValue("c"));
    invalidateWithoutError(receiver, aKey);
    assertThat(invalidated).containsExactly(aKey, skyKey("ab"));
    assertValueValue("ab", "cb");
    tester.set(bKey, new StringValue("d"));
    invalidateWithoutError(receiver, bKey);
    assertThat(invalidated).containsExactly(aKey, skyKey("ab"), bKey);
  }

  @Test
  public void receiverIsNotifiedAboutNodesInError() throws Exception {
    final Set<SkyKey> invalidated = Sets.newConcurrentHashSet();
    DirtyTrackingProgressReceiver receiver = new DirtyTrackingProgressReceiver(
        new EvaluationProgressReceiver.NullEvaluationProgressReceiver() {
      @Override
      public void invalidated(SkyKey skyKey, InvalidationState state) {
        Preconditions.checkState(state == expectedState());
        invalidated.add(skyKey);
      }
    });

    // Given a graph consisting of two nodes, "a" and "ab" such that "ab" depends on "a",
    // And given "ab" is in error,
    graph = new InMemoryGraphImpl();
    SkyKey aKey = GraphTester.nonHermeticKey("a");
    tester.set(aKey, new StringValue("a"));
    tester.getOrCreate("ab").addDependency(aKey).setHasError(true);
    eval(false, skyKey("ab"));

    // When "a" is invalidated,
    invalidateWithoutError(receiver, aKey);

    // Then the invalidation receiver is notified of both "a" and "ab"'s invalidations.
    assertThat(invalidated).containsExactly(aKey, skyKey("ab"));

    // Note that this behavior isn't strictly required for correctness. This test is
    // meant to document current behavior and protect against programming error.
  }

  @Test
  public void invalidateValuesNotInGraph() throws Exception {
    final Set<SkyKey> invalidated = Sets.newConcurrentHashSet();
    DirtyTrackingProgressReceiver receiver = new DirtyTrackingProgressReceiver(
        new EvaluationProgressReceiver.NullEvaluationProgressReceiver() {
      @Override
      public void invalidated(SkyKey skyKey, InvalidationState state) {
        Preconditions.checkState(state == InvalidationState.DIRTY);
        invalidated.add(skyKey);
      }
    });
    graph = new InMemoryGraphImpl();
    SkyKey aKey = GraphTester.nonHermeticKey("a");
    invalidateWithoutError(receiver, aKey);
    assertThat(invalidated).isEmpty();
    tester.set(aKey, new StringValue("a"));
    StringValue value = (StringValue) eval(false, aKey);
    assertThat(value.getValue()).isEqualTo("a");
    invalidateWithoutError(receiver, GraphTester.nonHermeticKey("b"));
    assertThat(invalidated).isEmpty();
  }

  @Test
  public void invalidatedValuesAreGCedAsExpected() throws Exception {
    SkyKey key = GraphTester.nonHermeticKey("a");
    HeavyValue heavyValue = new HeavyValue();
    WeakReference<HeavyValue> weakRef = new WeakReference<>(heavyValue);
    tester.set(key, heavyValue);

    graph = new InMemoryGraphImpl();
    eval(false, key);
    invalidate(graph, new DirtyTrackingProgressReceiver(null), key);

    tester = null;
    heavyValue = null;
    if (gcExpected()) {
      GcFinalization.awaitClear(weakRef);
    } else {
      // Not a reliable check, but better than nothing.
      System.gc();
      Thread.sleep(300);
      assertThat(weakRef.get()).isNotNull();
    }
  }

  @Test
  public void reverseDepsConsistent() throws Exception {
    graph = new InMemoryGraphImpl();
    set("a", "a");
    set("b", "b");
    set("c", "c");
    SkyKey abKey = GraphTester.nonHermeticKey("ab");
    tester.getOrCreate(abKey).addDependency("a").addDependency("b").setComputedValue(CONCATENATE);
    tester.getOrCreate("bc").addDependency("b").addDependency("c").setComputedValue(CONCATENATE);
    tester
        .getOrCreate("ab_c")
        .addDependency(abKey)
        .addDependency("c")
        .setComputedValue(CONCATENATE);
    eval(false, skyKey("ab_c"), skyKey("bc"));

    assertThat(graph.get(null, Reason.OTHER, skyKey("a")).getReverseDepsForDoneEntry())
        .containsExactly(abKey);
    assertThat(graph.get(null, Reason.OTHER, skyKey("b")).getReverseDepsForDoneEntry())
        .containsExactly(abKey, skyKey("bc"));
    assertThat(graph.get(null, Reason.OTHER, skyKey("c")).getReverseDepsForDoneEntry())
        .containsExactly(skyKey("ab_c"), skyKey("bc"));

    invalidateWithoutError(new DirtyTrackingProgressReceiver(null), abKey);
    eval(false);

    // The graph values should be gone.
    assertThat(isInvalidated(abKey)).isTrue();
    assertThat(isInvalidated(skyKey("abc"))).isTrue();

    // The reverse deps to ab and ab_c should have been removed if reverse deps are cleared.
    Set<SkyKey> reverseDeps = new HashSet<>();
    if (reverseDepsPresent()) {
      reverseDeps.add(abKey);
    }
    assertThat(graph.get(null, Reason.OTHER, skyKey("a")).getReverseDepsForDoneEntry())
        .containsExactlyElementsIn(reverseDeps);
    reverseDeps.add(skyKey("bc"));
    assertThat(graph.get(null, Reason.OTHER, skyKey("b")).getReverseDepsForDoneEntry())
        .containsExactlyElementsIn(reverseDeps);
    reverseDeps.clear();
    if (reverseDepsPresent()) {
      reverseDeps.add(skyKey("ab_c"));
    }
    reverseDeps.add(skyKey("bc"));
    assertThat(graph.get(null, Reason.OTHER, skyKey("c")).getReverseDepsForDoneEntry())
        .containsExactlyElementsIn(reverseDeps);
  }

  @Test
  public void interruptChild() throws Exception {
    graph = new InMemoryGraphImpl();
    int numValues = 50; // More values than the invalidator has threads.
    SkyKey[] family = new SkyKey[numValues];
    SkyKey child = GraphTester.nonHermeticKey("child");
    StringValue childValue = new StringValue("child");
    tester.set(child, childValue);
    family[0] = child;
    for (int i = 1; i < numValues; i++) {
      SkyKey member = skyKey(Integer.toString(i));
      tester.getOrCreate(member).addDependency(family[i - 1]).setComputedValue(CONCATENATE);
      family[i] = member;
    }
    SkyKey parent = GraphTester.skyKey("parent");
    tester.getOrCreate(parent).addDependency(family[numValues - 1]).setComputedValue(CONCATENATE);
    eval(/*keepGoing=*/false, parent);
    final Thread mainThread = Thread.currentThread();
    final AtomicReference<SkyKey> badKey = new AtomicReference<>();
    DirtyTrackingProgressReceiver receiver =
        new DirtyTrackingProgressReceiver(
            new EvaluationProgressReceiver.NullEvaluationProgressReceiver() {
              @Override
              public void invalidated(SkyKey skyKey, InvalidationState state) {
                if (skyKey.equals(child)) {
                  // Interrupt on the very first invalidate
                  mainThread.interrupt();
                } else if (!skyKey.functionName().equals(NODE_TYPE)) {
                  // All other invalidations should have the GraphTester's key type.
                  // Exceptions thrown here may be silently dropped, so keep track of errors
                  // ourselves.
                  badKey.set(skyKey);
                }
                try {
                  assertThat(
                          visitor
                              .get()
                              .getInterruptionLatchForTestingOnly()
                              .await(2, TimeUnit.HOURS))
                      .isTrue();
                } catch (InterruptedException e) {
                  // We may well have thrown here because by the time we try to await, the main
                  // thread is already interrupted.
                }
              }
            });
    try {
      invalidateWithoutError(receiver, child);
      fail();
    } catch (InterruptedException e) {
      // Expected.
    }
    assertThat(badKey.get()).isNull();
    assertThat(state.isEmpty()).isFalse();
    final Set<SkyKey> invalidated = Sets.newConcurrentHashSet();
    assertThat(isInvalidated(parent)).isFalse();
    assertThat(graph.get(null, Reason.OTHER, parent).getValue()).isNotNull();
    receiver = new DirtyTrackingProgressReceiver(
        new EvaluationProgressReceiver.NullEvaluationProgressReceiver() {
      @Override
      public void invalidated(SkyKey skyKey, InvalidationState state) {
        invalidated.add(skyKey);
      }
    });
    invalidateWithoutError(receiver);
    assertThat(invalidated).contains(parent);
    assertThat(state.getInvalidationsForTesting()).isEmpty();

    // Regression test coverage:
    // "all pending values are marked changed on interrupt".
    assertThat(isInvalidated(child)).isTrue();
    assertChanged(child);
    for (int i = 1; i < numValues; i++) {
      assertDirtyAndNotChanged(family[i]);
    }
    assertDirtyAndNotChanged(parent);
  }

  private SkyKey[] constructLargeGraph(int size) {
    Random random = new Random(TestUtils.getRandomSeed());
    SkyKey[] values = new SkyKey[size];
    for (int i = 0; i < size; i++) {
      String iString = Integer.toString(i);
      SkyKey iKey = GraphTester.nonHermeticKey(iString);
      tester.set(iKey, new StringValue(iString));
      set(iString, iString);
      for (int j = 0; j < i; j++) {
        if (random.nextInt(3) == 0) {
          tester.getOrCreate(iKey).addDependency(GraphTester.nonHermeticKey(Integer.toString(j)));
        }
      }
      values[i] = iKey;
    }
    return values;
  }

  /** Returns a subset of {@code nodes} that are still valid and so can be invalidated. */
  private Set<Pair<SkyKey, InvalidationType>> getValuesToInvalidate(SkyKey[] nodes) {
    Set<Pair<SkyKey, InvalidationType>> result = new HashSet<>();
    Random random = new Random(TestUtils.getRandomSeed());
    for (SkyKey node : nodes) {
      if (!isInvalidated(node)) {
        if (result.isEmpty() || random.nextInt(3) == 0) {
          // Add at least one node, if we can.
          result.add(Pair.of(node, defaultInvalidationType()));
        }
      }
    }
    return result;
  }

  @Test
  public void interruptThreadInReceiver() throws Exception {
    Random random = new Random(TestUtils.getRandomSeed());
    int graphSize = 1000;
    int tries = 5;
    graph = new InMemoryGraphImpl();
    SkyKey[] values = constructLargeGraph(graphSize);
    eval(/*keepGoing=*/false, values);
    final Thread mainThread = Thread.currentThread();
    for (int run = 0; run < tries; run++) {
      Set<Pair<SkyKey, InvalidationType>> valuesToInvalidate = getValuesToInvalidate(values);
      // Find how many invalidations will actually be enqueued for invalidation in the first round,
      // so that we can interrupt before all of them are done.
      int validValuesToDo =
          Sets.difference(valuesToInvalidate, state.getInvalidationsForTesting()).size();
      for (Pair<SkyKey, InvalidationType> pair : state.getInvalidationsForTesting()) {
        if (!isInvalidated(pair.first)) {
          validValuesToDo++;
        }
      }
      int countDownStart = validValuesToDo > 0 ? random.nextInt(validValuesToDo) : 0;
      final CountDownLatch countDownToInterrupt = new CountDownLatch(countDownStart);
      final DirtyTrackingProgressReceiver receiver =
          new DirtyTrackingProgressReceiver(
              new EvaluationProgressReceiver.NullEvaluationProgressReceiver() {
            @Override
            public void invalidated(SkyKey skyKey, InvalidationState state) {
              countDownToInterrupt.countDown();
              if (countDownToInterrupt.getCount() == 0) {
                mainThread.interrupt();
                // Wait for the main thread to be interrupted uninterruptibly, because the main
                // thread is going to interrupt us, and we don't want to get into an interrupt
                // fight. Only if we get interrupted without the main thread also being interrupted
                // will this throw an InterruptedException.
                TrackingAwaiter.INSTANCE.awaitLatchAndTrackExceptions(
                    visitor.get().getInterruptionLatchForTestingOnly(),
                    "Main thread was not interrupted");
              }
            }
          });
      try {
        invalidate(
            graph,
            receiver,
            Sets.newHashSet(Iterables.transform(valuesToInvalidate, pair -> pair.first))
                .toArray(new SkyKey[0]));
        assertThat(state.getInvalidationsForTesting()).isEmpty();
      } catch (InterruptedException e) {
        // Expected.
      }
      if (state.isEmpty()) {
        // Ran out of values to invalidate.
        break;
      }
    }

    eval(/*keepGoing=*/false, values);
  }

  protected void setupInvalidatableGraph() throws Exception {
    graph = new InMemoryGraphImpl();
    SkyKey aKey = GraphTester.nonHermeticKey("a");
    tester.set(aKey, new StringValue("a"));
    set("b", "b");
    tester.getOrCreate("ab").addDependency(aKey).addDependency("b").setComputedValue(CONCATENATE);
    assertValueValue("ab", "ab");
    tester.set(aKey, new StringValue("c"));
  }

  private static class HeavyValue implements SkyValue {
  }

  /**
   * Test suite for the deleting invalidator.
   */
  @RunWith(JUnit4.class)
  public static class DeletingInvalidatorTest extends EagerInvalidatorTest {
    @Override
    protected void invalidate(
        InMemoryGraph graph, DirtyTrackingProgressReceiver progressReceiver, SkyKey... keys)
        throws InterruptedException {
      Iterable<SkyKey> diff = ImmutableList.copyOf(keys);
      DeletingNodeVisitor deletingNodeVisitor =
          EagerInvalidator.createDeletingVisitorIfNeeded(
              graph, diff, new DirtyTrackingProgressReceiver(progressReceiver), state, true);
      if (deletingNodeVisitor != null) {
        visitor.set(deletingNodeVisitor);
        deletingNodeVisitor.run();
      }
    }

    @Override
    EvaluationProgressReceiver.InvalidationState expectedState() {
      return EvaluationProgressReceiver.InvalidationState.DELETED;
    }

    @Override
    boolean gcExpected() {
      return true;
    }

    @Override
    protected InvalidationState newInvalidationState() {
      return new InvalidatingNodeVisitor.DeletingInvalidationState();
    }

    @Override
    protected InvalidationType defaultInvalidationType() {
      return InvalidationType.DELETED;
    }

    @Override
    protected boolean reverseDepsPresent() {
      return false;
    }

    @Test
    public void dirtyTrackingProgressReceiverWorksWithDeletingInvalidator() throws Exception {
      setupInvalidatableGraph();
      DirtyTrackingProgressReceiver receiver = new DirtyTrackingProgressReceiver(
          new EvaluationProgressReceiver.NullEvaluationProgressReceiver());

      // Dirty the node, and ensure that the tracker is aware of it:
      ImmutableList<SkyKey> diff = ImmutableList.of(GraphTester.nonHermeticKey("a"));
      InvalidationState state1 = new DirtyingInvalidationState();
      Preconditions.checkNotNull(
              EagerInvalidator.createInvalidatingVisitorIfNeeded(graph, diff, receiver, state1))
          .run();
      assertThat(receiver.getUnenqueuedDirtyKeys()).containsExactly(diff.get(0), skyKey("ab"));

      // Delete the node, and ensure that the tracker is no longer tracking it:
      Preconditions.checkNotNull(EagerInvalidator.createDeletingVisitorIfNeeded(graph, diff,
          receiver, state, true)).run();
      assertThat(receiver.getUnenqueuedDirtyKeys()).isEmpty();
    }
  }

  /**
   * Test suite for the dirtying invalidator.
   */
  @RunWith(JUnit4.class)
  public static class DirtyingInvalidatorTest extends EagerInvalidatorTest {
    @Override
    protected void invalidate(
        InMemoryGraph graph, DirtyTrackingProgressReceiver progressReceiver, SkyKey... keys)
        throws InterruptedException {
      Iterable<SkyKey> diff = ImmutableList.copyOf(keys);
      DirtyingNodeVisitor dirtyingNodeVisitor =
          EagerInvalidator.createInvalidatingVisitorIfNeeded(graph, diff, progressReceiver, state);
      if (dirtyingNodeVisitor != null) {
        visitor.set(dirtyingNodeVisitor);
        dirtyingNodeVisitor.run();
      }
    }

    @Override
    EvaluationProgressReceiver.InvalidationState expectedState() {
      return EvaluationProgressReceiver.InvalidationState.DIRTY;
    }

    @Override
    boolean gcExpected() {
      return false;
    }

    @Override
    protected InvalidationState newInvalidationState() {
      return new DirtyingInvalidationState();
    }

    @Override
    protected InvalidationType defaultInvalidationType() {
      return InvalidationType.CHANGED;
    }

    @Override
    protected boolean reverseDepsPresent() {
      return true;
    }

    @Test
    public void dirtyTrackingProgressReceiverWorksWithDirtyingInvalidator() throws Exception {
      setupInvalidatableGraph();
      DirtyTrackingProgressReceiver receiver = new DirtyTrackingProgressReceiver(
          new EvaluationProgressReceiver.NullEvaluationProgressReceiver());

      // Dirty the node, and ensure that the tracker is aware of it:
      invalidate(graph, receiver, GraphTester.nonHermeticKey("a"));
      assertThat(receiver.getUnenqueuedDirtyKeys()).hasSize(2);
    }
  }
}