// Copyright 2017 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.base.Preconditions.checkArgument;
import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.base.Preconditions.checkState;

import com.github.benmanes.caffeine.cache.Cache;
import com.github.benmanes.caffeine.cache.Caffeine;
import com.google.common.base.Throwables;
import com.google.common.collect.ImmutableCollection;
import com.google.common.collect.ImmutableList;
import com.google.common.collect.ImmutableMap;
import com.google.common.collect.ImmutableSet;
import com.google.common.collect.Iterables;
import com.google.common.collect.Sets;
import com.google.common.flogger.GoogleLogger;
import com.google.common.graph.ImmutableGraph;
import com.google.common.graph.Traverser;
import com.google.common.util.concurrent.ListenableFuture;
import com.google.devtools.build.lib.clock.BlazeClock;
import com.google.devtools.build.lib.concurrent.ComparableRunnable;
import com.google.devtools.build.lib.concurrent.QuiescingExecutor;
import com.google.devtools.build.lib.events.Event;
import com.google.devtools.build.lib.events.ExtendedEventHandler;
import com.google.devtools.build.lib.profiler.Profiler;
import com.google.devtools.build.lib.profiler.ProfilerTask;
import com.google.devtools.build.lib.supplier.InterruptibleSupplier;
import com.google.devtools.build.skyframe.EvaluationProgressReceiver.EvaluationState;
import com.google.devtools.build.skyframe.EvaluationProgressReceiver.NodeState;
import com.google.devtools.build.skyframe.NodeEntry.DependencyState;
import com.google.devtools.build.skyframe.NodeEntry.DirtyState;
import com.google.devtools.build.skyframe.NodeEntry.DirtyType;
import com.google.devtools.build.skyframe.QueryableGraph.Reason;
import com.google.devtools.build.skyframe.SkyFunction.Environment.SkyKeyComputeState;
import com.google.devtools.build.skyframe.SkyFunction.Restart;
import com.google.devtools.build.skyframe.SkyFunctionEnvironment.UndonePreviouslyRequestedDeps;
import com.google.devtools.build.skyframe.SkyFunctionException.ReifiedSkyFunctionException;
import com.google.devtools.build.skyframe.proto.GraphInconsistency.Inconsistency;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import java.io.IOException;
import java.time.Duration;
import java.util.Collection;
import java.util.List;
import java.util.Set;
import java.util.concurrent.atomic.AtomicInteger;
import javax.annotation.Nullable;

/**
 * Defines the evaluation action used in the multi-threaded Skyframe evaluation, and constructs the
 * {@link ParallelEvaluatorContext} that the actions rely on.
 *
 * <p>Evaluates a set of given functions ({@code SkyFunction}s) with arguments ({@code SkyKey}s).
 * Cycles are not allowed and are detected during the traversal.
 *
 * <p>This class implements multi-threaded evaluation. This is a fairly complex process that has
 * strong consistency requirements between the {@link ProcessableGraph}, the nodes in the graph of
 * type {@link NodeEntry}, the work queue, and the set of in-flight nodes.
 *
 * <p>The basic invariants are:
 *
 * <p>A node can be in one of three states: ready, waiting, and done. A node is ready if and only if
 * all of its dependencies have been signaled. A node is done if it has a value. It is waiting if
 * not all of its dependencies have been signaled.
 *
 * <p>A node must be in the work queue if and only if it is ready. It is an error for a node to be
 * in the work queue twice at the same time.
 *
 * <p>A node is considered in-flight if it has been created, and is not done yet. In case of an
 * interrupt, the work queue is discarded, and the in-flight set is used to remove partially
 * computed values.
 *
 * <p>Each evaluation of the graph takes place at a "version," which is currently given by a
 * non-negative {@code long}. The version can also be thought of as an "mtime." Each node in the
 * graph has a version, which is the last version at which its value changed. This version data is
 * used to avoid unnecessary re-evaluation of values. If a node is re-evaluated and found to have
 * the same data as before, its version (mtime) remains the same. If all of a node's children's have
 * the same version as before, its re-evaluation can be skipped.
 *
 * <p>This does not implement other parts of Skyframe evaluation setup and post-processing, such as
 * translating a set of requested top-level nodes into actions, or constructing an evaluation
 * result. Derived classes should do this.
 */
abstract class AbstractParallelEvaluator {
  private static final GoogleLogger logger = GoogleLogger.forEnclosingClass();

  final ProcessableGraph graph;
  final ParallelEvaluatorContext evaluatorContext;
  protected final CycleDetector cycleDetector;

  /**
   * A decreasing counter that results in FIFO priority tie-breaking for prioritization.
   *
   * <p>FIFO is friendlier to priority queues.
   */
  private final AtomicInteger nextEvaluateId = new AtomicInteger(Integer.MAX_VALUE);

  protected final Cache<SkyKey, SkyKeyComputeState> stateCache = Caffeine.newBuilder().build();

  AbstractParallelEvaluator(
      ProcessableGraph graph,
      Version graphVersion,
      Version minimalVersion,
      ImmutableMap<SkyFunctionName, SkyFunction> skyFunctions,
      ExtendedEventHandler reporter,
      EmittedEventState emittedEventState,
      EventFilter storedEventFilter,
      ErrorInfoManager errorInfoManager,
      boolean keepGoing,
      DirtyTrackingProgressReceiver progressReceiver,
      GraphInconsistencyReceiver graphInconsistencyReceiver,
      QuiescingExecutor executor,
      CycleDetector cycleDetector,
      boolean mergingSkyframeAnalysisExecutionPhases) {
    this.graph = graph;
    this.cycleDetector = cycleDetector;
    this.evaluatorContext =
        new ParallelEvaluatorContext(
            graph,
            graphVersion,
            minimalVersion,
            skyFunctions,
            reporter,
            emittedEventState,
            keepGoing,
            progressReceiver,
            storedEventFilter,
            errorInfoManager,
            graphInconsistencyReceiver,
            () -> new NodeEntryVisitor(executor, progressReceiver, Evaluate::new, stateCache),
            /* mergingSkyframeAnalysisExecutionPhases= */ mergingSkyframeAnalysisExecutionPhases,
            stateCache);
  }

  /**
   * If the entry is dirty and not already rebuilding, puts it in a state so that it can rebuild.
   */
  static void maybeMarkRebuilding(NodeEntry entry) {
    if (entry.isDirty()
        && entry.getDirtyState() != DirtyState.REBUILDING
        && entry.getDirtyState() != DirtyState.FORCED_REBUILDING) {
      entry.markRebuilding();
    }
  }

  enum DirtyOutcome {
    ALREADY_PROCESSED,
    NEEDS_EVALUATION
  }

  /**
   * An action that evaluates a value.
   *
   * <p>{@link Comparable} for use in priority queues.
   */
  private final class Evaluate implements ComparableRunnable {
    private final SkyKey skyKey;
    private final long priority;

    private Evaluate(SkyKey skyKey, int partialPriority) {
      this.skyKey = skyKey;

      // LIFO could be more robust here. In the absence of other prioritization, LIFO exploits the
      // fact that later arriving requests tend to be deeper in the graph. However, LIFO is not a
      // good access pattern for priority queues as it increases reader / writer contention.
      // Measurements show FIFO is faster with existing prioritization.
      this.priority = (((long) partialPriority) << 32) | nextEvaluateId.getAndDecrement();
    }

    @Override
    public boolean isCpuHeavy() {
      return skyKey instanceof CPUHeavySkyKey;
    }

    @Override
    public int compareTo(ComparableRunnable other) {
      // Compares in reverse order so that keys with high priority are evaluated first.
      return Long.compare(((Evaluate) other).priority, priority);
    }

    /**
     * Notes the rdep from the parent to the child, and then does the appropriate thing with the
     * child or the parent, returning whether the parent has both been signalled and also is ready
     * for evaluation.
     *
     * @param childDepth this should match {@code entry.getChildDepth()} but that performs some
     *     computation and this is often called in a loop with the same {@code entry}.
     */
    @CanIgnoreReturnValue
    private boolean enqueueChild(
        SkyKey skyKey,
        NodeEntry entry,
        SkyKey child,
        NodeEntry childEntry,
        boolean depAlreadyExists,
        int childDepth,
        boolean enqueueParentIfReady,
        @Nullable SkyFunctionEnvironment environmentIfEnqueuing)
        throws InterruptedException {
      checkState(!entry.isDone(), "%s %s", skyKey, entry);
      DependencyState dependencyState;
      try {
        dependencyState =
            depAlreadyExists
                ? childEntry.checkIfDoneForDirtyReverseDep(skyKey)
                : childEntry.addReverseDepAndCheckIfDone(skyKey);
      } catch (IllegalStateException e) {
        // Add some more context regarding crashes.
        throw new IllegalStateException("child key: " + child + " error: " + e.getMessage(), e);
      }
      childEntry.updateDepthIfGreater(childDepth);
      switch (dependencyState) {
        case DONE:
          if (entry.signalDep(childEntry.getVersion(), child)) {
            if (enqueueParentIfReady) {
              evaluatorContext.getVisitor().enqueueEvaluation(skyKey, entry.getPriority(), child);
            }
            return true;
          } else {
            if (skyKey.supportsPartialReevaluation()
                && environmentIfEnqueuing != null
                && environmentIfEnqueuing.wasNewlyRequestedDepNullForPartialReevaluation(child)) {
              // If a dep was observed not-done by its parent when the parent tried to read its
              // value, but that dep is now done, then this is the only chance the parent has to be
              // signalled by that dep.
              evaluatorContext.getVisitor().enqueueEvaluation(skyKey, entry.getPriority(), child);
            }
          }
          break;
        case ALREADY_EVALUATING:
          break;
        case NEEDS_SCHEDULING:
          evaluatorContext.getVisitor().enqueueEvaluation(child, childEntry.getPriority(), null);
          break;
      }
      return false;
    }

    /**
     * Returns true if this depGroup consists of the error transience value and the error transience
     * value is newer than the entry, meaning that the entry must be re-evaluated.
     */
    private boolean invalidatedByErrorTransience(Collection<SkyKey> depGroup, NodeEntry entry)
        throws InterruptedException {
      return depGroup.size() == 1
          && depGroup.contains(ErrorTransienceValue.KEY)
          && !graph
              .get(null, Reason.OTHER, ErrorTransienceValue.KEY)
              .getVersion()
              .atMost(entry.getVersion());
    }

    private DirtyOutcome maybeHandleDirtyNode(NodeEntry nodeEntry) throws InterruptedException {
      while (nodeEntry.getDirtyState().equals(DirtyState.CHECK_DEPENDENCIES)) {
        // Evaluating a dirty node for the first time, and checking its children to see if any
        // of them have changed. Note that there must be dirty children for this to happen.

        // Check the children group by group -- we don't want to evaluate a value that is no
        // longer needed because an earlier dependency changed. For example, //foo:foo depends
        // on target //bar:bar and is built. Then foo/BUILD is modified to remove the dependence
        // on bar, and bar/BUILD is deleted. Reloading //bar:bar would incorrectly throw an
        // exception. To avoid this, we must reload foo/BUILD first, at which point we will
        // discover that it has changed, and re-evaluate target //foo:foo from scratch.
        // On the other hand, when an action requests all of its inputs, we can safely check all
        // of them in parallel on a subsequent build. So we allow checking an entire group in
        // parallel here, if the node builder requested a group last build.
        // Note: every dep returned here must either have this node re-registered for it (using
        // checkIfDoneForDirtyReverseDep) and be registered as a direct dep of this node, or have
        // its reverse dep on this node removed. Failing to do either one of these would result in
        // a graph inconsistency, where the child had a reverse dep on this node, but this node
        // had no kind of dependency on the child.
        List<SkyKey> directDepsToCheck = nodeEntry.getNextDirtyDirectDeps();

        if (invalidatedByErrorTransience(directDepsToCheck, nodeEntry)) {
          // If this dep is the ErrorTransienceValue and the ErrorTransienceValue has been
          // updated then we need to force a rebuild. We would like to just signal the entry as
          // usual, but we can't, because then the ErrorTransienceValue would remain as a dep,
          // which would be incorrect if, for instance, the value re-evaluated to a non-error.
          nodeEntry.forceRebuild();
          graph.get(skyKey, Reason.RDEP_REMOVAL, ErrorTransienceValue.KEY).removeReverseDep(skyKey);
          return DirtyOutcome.NEEDS_EVALUATION;
        }
        NodeBatch entriesToCheck = null;
        if (!evaluatorContext.keepGoing()) {
          // This check ensures that we maintain the invariant that if a node with an error is
          // reached during a no-keep-going build, none of its currently building parents
          // finishes building. If the child isn't done building yet, it will detect on its own
          // that it has an error (see the VERIFIED_CLEAN case below). On the other hand, if it
          // is done, then it is the parent's responsibility to notice that, which we do here.
          // We check the deps for errors so that we don't continue building this node if it has
          // a child error.
          entriesToCheck = graph.getBatch(skyKey, Reason.OTHER, directDepsToCheck);
          for (SkyKey keyToCheck : directDepsToCheck) {
            NodeEntry nodeEntryToCheck = entriesToCheck.get(keyToCheck);
            SkyValue valueMaybeWithMetadata = nodeEntryToCheck.getValueMaybeWithMetadata();
            if (valueMaybeWithMetadata == null) {
              continue;
            }
            ErrorInfo maybeErrorInfo = ValueWithMetadata.getMaybeErrorInfo(valueMaybeWithMetadata);
            if (maybeErrorInfo == null) {
              continue;
            }
            // This child has an error. We add a dep from this node to it and throw an exception
            // coming from it.
            nodeEntry.addSingletonTemporaryDirectDep(keyToCheck);
            nodeEntryToCheck.checkIfDoneForDirtyReverseDep(skyKey);
            // Perform the necessary bookkeeping for any deps that are not being used.
            for (SkyKey depKey : directDepsToCheck) {
              if (!depKey.equals(keyToCheck)) {
                entriesToCheck.get(depKey).removeReverseDep(skyKey);
              }
            }
            if (!evaluatorContext.getVisitor().preventNewEvaluations()) {
              // An error was already thrown in the evaluator. Don't do anything here.
              return DirtyOutcome.ALREADY_PROCESSED;
            }
            throw SchedulerException.ofError(maybeErrorInfo, keyToCheck, ImmutableSet.of(skyKey));
          }
        }
        // It is safe to add these deps back to the node -- even if one of them has changed, the
        // contract of pruning is that the node will request these deps again when it rebuilds.
        // We must add these deps before enqueuing them, so that the node knows that it depends
        // on them. If one of these deps is the error transience node, the check we did above
        // in #invalidatedByErrorTransience means that the error transience node is not newer
        // than this node, so we are going to mark it clean (since the error transience node is
        // always the last dep).
        nodeEntry.addTemporaryDirectDepGroup(directDepsToCheck);
        DepsReport depsReport = graph.analyzeDepsDoneness(skyKey, directDepsToCheck);
        Collection<SkyKey> unknownStatusDeps =
            depsReport.hasInformation() ? depsReport : directDepsToCheck;
        boolean needsScheduling = false;
        for (int i = 0; i < directDepsToCheck.size() - unknownStatusDeps.size(); i++) {
          // Since all of these nodes were done at an earlier version than this one, we may safely
          // signal with the minimal version, since they cannot trigger a re-evaluation.
          needsScheduling = nodeEntry.signalDep(Version.minimal(), /* childForDebugging= */ null);
        }
        if (needsScheduling) {
          checkState(
              unknownStatusDeps.isEmpty(),
              "Ready without all deps checked? %s %s %s",
              skyKey,
              nodeEntry,
              unknownStatusDeps);
          continue;
        }
        if (entriesToCheck == null || depsReport.hasInformation()) {
          entriesToCheck = graph.getBatch(skyKey, Reason.ENQUEUING_CHILD, unknownStatusDeps);
        }
        boolean parentIsSignalledAndReady =
            handleKnownChildrenForDirtyNode(
                unknownStatusDeps,
                entriesToCheck,
                nodeEntry,
                /* enqueueParentIfReady= */ false,
                /* environmentIfEnqueuing= */ null);
        if (!parentIsSignalledAndReady
            || evaluatorContext.getVisitor().shouldPreventNewEvaluations()) {
          return DirtyOutcome.ALREADY_PROCESSED;
        }
        // If we're here, then we may proceed to the rest of the method and continue processing
        // the node intra-thread. This is a performance optimization: By not enqueuing the node,
        // we avoid contention on the queue data structure (between concurrent threads
        // enqueueing and dequeueing), and we also save wall time since the node gets processed
        // now rather than at some point in the future.
      }
      switch (nodeEntry.getDirtyState()) {
        case VERIFIED_CLEAN:
          // No child has a changed value. This node can be marked done and its parents signaled
          // without any re-evaluation.
          NodeEntry.NodeValueAndRdepsToSignal nodeValueAndRdeps = nodeEntry.markClean();
          Set<SkyKey> rDepsToSignal = nodeValueAndRdeps.getRdepsToSignal();
          // Make sure to replay events once change-pruned
          replay(ValueWithMetadata.wrapWithMetadata(nodeValueAndRdeps.getValue()));
          // Tell the receiver that the value was not actually changed this run.
          evaluatorContext
              .getProgressReceiver()
              .evaluated(
                  skyKey,
                  /* newValue= */ null,
                  /* newError= */ null,
                  new EvaluationSuccessStateSupplier(nodeEntry),
                  EvaluationState.CLEAN,
                  /* directDeps= */ null);
          if (!evaluatorContext.keepGoing() && nodeEntry.getErrorInfo() != null) {
            if (!evaluatorContext.getVisitor().preventNewEvaluations()) {
              return DirtyOutcome.ALREADY_PROCESSED;
            }
            throw SchedulerException.ofError(nodeEntry.getErrorInfo(), skyKey, rDepsToSignal);
          }
          evaluatorContext.signalParentsAndEnqueueIfReady(
              skyKey, rDepsToSignal, nodeEntry.getVersion());
          return DirtyOutcome.ALREADY_PROCESSED;
        case NEEDS_REBUILDING:
          nodeEntry.markRebuilding();
          return DirtyOutcome.NEEDS_EVALUATION;
        case NEEDS_FORCED_REBUILDING:
          nodeEntry.forceRebuild();
          return DirtyOutcome.NEEDS_EVALUATION;
        case REBUILDING:
        case FORCED_REBUILDING:
          return DirtyOutcome.NEEDS_EVALUATION;
        default:
          throw new IllegalStateException("key: " + skyKey + ", entry: " + nodeEntry);
      }
    }

    /** Returns whether the parent has both been signalled and also is ready for evaluation. */
    @CanIgnoreReturnValue
    private boolean handleKnownChildrenForDirtyNode(
        Collection<SkyKey> knownChildren,
        NodeBatch oldChildren,
        NodeEntry nodeEntry,
        boolean enqueueParentIfReady,
        @Nullable SkyFunctionEnvironment environmentIfEnqueuing)
        throws InterruptedException {
      boolean parentIsSignalledAndReady = false;
      int childDepth = nodeEntry.getChildDepth();
      for (SkyKey directDep : knownChildren) {
        NodeEntry directDepEntry =
            checkNotNull(
                oldChildren.get(directDep),
                "Dirty parent had missing child (child=%s, parent=%s %s)",
                directDep,
                skyKey,
                nodeEntry);
        parentIsSignalledAndReady |=
            enqueueChild(
                skyKey,
                nodeEntry,
                directDep,
                directDepEntry,
                /* depAlreadyExists= */ true,
                childDepth,
                enqueueParentIfReady,
                environmentIfEnqueuing);
      }
      return parentIsSignalledAndReady;
    }

    @Override
    public void run() {
      SkyFunctionEnvironment env = null;
      try {
        NodeEntry nodeEntry = graph.get(null, Reason.EVALUATION, skyKey);
        if (nodeEntry == null || nodeEntry.isDone() || !nodeEntry.isReadyToEvaluate()) {
          checkState(skyKey.supportsPartialReevaluation(), "%s %s", skyKey, nodeEntry);
          evaluatorContext.getProgressReceiver().removeFromInflight(skyKey);
          return;
        }
        try {
          evaluatorContext.getProgressReceiver().stateStarting(skyKey, NodeState.CHECK_DIRTY);
          if (maybeHandleDirtyNode(nodeEntry) == DirtyOutcome.ALREADY_PROCESSED) {
            return;
          }
        } finally {
          evaluatorContext.getProgressReceiver().stateEnding(skyKey, NodeState.CHECK_DIRTY);
        }

        ImmutableSet<SkyKey> oldDeps = nodeEntry.getAllRemainingDirtyDirectDeps();
        try {
          evaluatorContext
              .getProgressReceiver()
              .stateStarting(skyKey, NodeState.INITIALIZING_ENVIRONMENT);
          env =
              SkyFunctionEnvironment.create(
                  skyKey, nodeEntry.getTemporaryDirectDeps(), oldDeps, evaluatorContext);
        } catch (UndonePreviouslyRequestedDeps undonePreviouslyRequestedDeps) {
          handleUndonePreviouslyRequestedDep(nodeEntry);
          return;
        } finally {
          evaluatorContext
              .getProgressReceiver()
              .stateEnding(skyKey, NodeState.INITIALIZING_ENVIRONMENT);
        }
        SkyFunctionName functionName = skyKey.functionName();
        SkyFunction skyFunction =
            checkNotNull(
                evaluatorContext.getSkyFunctions().get(functionName),
                "Unable to find SkyFunction '%s' for node with key %s, %s",
                functionName,
                skyKey,
                nodeEntry);

        SkyValue value = null;
        long startTimeNanos = BlazeClock.instance().nanoTime();
        try {
          try {
            evaluatorContext.getProgressReceiver().stateStarting(skyKey, NodeState.COMPUTE);
            value = skyFunction.compute(skyKey, env);
          } finally {
            evaluatorContext.getProgressReceiver().stateEnding(skyKey, NodeState.COMPUTE);
            long elapsedTimeNanos = BlazeClock.instance().nanoTime() - startTimeNanos;
            if (elapsedTimeNanos > 0) {
              Profiler.instance()
                  .logSimpleTaskDuration(
                      startTimeNanos,
                      Duration.ofNanos(elapsedTimeNanos),
                      ProfilerTask.SKYFUNCTION,
                      skyKey.functionName().getName());
            }
            nodeEntry.incrementEvaluationCount();
          }
        } catch (final SkyFunctionException builderException) {
          // TODO(b/261604460): invalidating the state cache here appears to be load-bearing for
          // error propagation. It ought to be allowed to invalidate it only after the following
          // early return checks pass, but something is misusing the state cache, and moving it
          // causes tests to fail.
          stateCache.invalidate(skyKey);

          // In keep-going mode, we do not let SkyFunctions complete with a thrown error if they
          // have missing deps. Instead, we wait until their deps are done and restart the
          // SkyFunction, so we can have a definitive error and definitive graph structure, thus
          // avoiding non-determinism. It's completely reasonable for SkyFunctions to throw eagerly
          // because they do not know if they are in keep-going mode.
          if (!evaluatorContext.keepGoing() || !env.valuesMissing()) {
            if (nodeEntry.hasUnsignaledDeps()) {
              // This is a partial reevaluation. It is not safe to set the error because a dep may
              // yet signal this node. We return (without preventing new evaluations) so that any
              // not-yet-complete deps can complete and signal this node.
              return;
            }

            if (maybeHandleRegisteringNewlyDiscoveredDepsForDoneEntry(
                skyKey, nodeEntry, oldDeps, env, evaluatorContext.keepGoing())) {
              // A newly requested dep transitioned from done to dirty before this node finished.
              // It is not safe to set the error because the now-dirty dep has not signaled this
              // node. We return (without preventing new evaluations) so that the dep can complete
              // and signal this node.
              return;
            }

            try {
              env.ensurePreviouslyRequestedDepsFetched();
            } catch (UndonePreviouslyRequestedDeps e) {
              handleUndonePreviouslyRequestedDep(nodeEntry);
              return;
            }

            boolean shouldFailFast =
                !evaluatorContext.keepGoing() || builderException.isCatastrophic();
            if (shouldFailFast) {
              // After we commit this error to the graph but before the doMutatingEvaluation call
              // completes with the error there is a race-like opportunity for the error to be used,
              // either by an in-flight computation or by a future computation.
              if (!evaluatorContext.getVisitor().preventNewEvaluations()) {
                // This is not the first error encountered, so we ignore it so that we can terminate
                // with the first error.
                return;
              } else {
                logger.atWarning().withCause(builderException).log(
                    "Aborting evaluation while evaluating %s", skyKey);
              }
            }
            ReifiedSkyFunctionException reifiedBuilderException =
                new ReifiedSkyFunctionException(builderException);
            boolean isTransitivelyTransient =
                reifiedBuilderException.isTransient()
                    || env.isAnyDirectDepErrorTransitivelyTransient()
                    || env.isAnyNewlyRequestedDepErrorTransitivelyTransient();
            ErrorInfo errorInfo =
                evaluatorContext
                    .getErrorInfoManager()
                    .fromException(skyKey, reifiedBuilderException, isTransitivelyTransient);
            env.setError(nodeEntry, errorInfo);
            Set<SkyKey> rdepsToBubbleUpTo = env.commitAndGetParents(nodeEntry);
            if (shouldFailFast) {
              evaluatorContext.signalParentsOnAbort(
                  skyKey, rdepsToBubbleUpTo, nodeEntry.getVersion());
              throw SchedulerException.ofError(errorInfo, skyKey, rdepsToBubbleUpTo);
            }
            evaluatorContext.signalParentsAndEnqueueIfReady(
                skyKey, rdepsToBubbleUpTo, nodeEntry.getVersion());
            return;
          }
        } catch (RuntimeException re) {
          // Programmer error (most likely NPE or a failed precondition in a SkyFunction). Output
          // some context together with the exception.
          String msg = prepareCrashMessage(skyKey, nodeEntry.getInProgressReverseDeps());
          RuntimeException ex = new RuntimeException(msg, re);
          evaluatorContext.getVisitor().noteCrash(ex);
          throw ex;
        } finally {
          env.doneBuilding();
        }

        if (value instanceof Restart) {
          if (nodeEntry.hasUnsignaledDeps()) {
            // This is a partial reevaluation. It is not safe to reset the node because a dep may
            // be racing to signal it.
            return;
          }
          dirtyRewindGraphAndResetEntry(skyKey, nodeEntry, (Restart) value);
          stateCache.invalidate(skyKey);
          cancelExternalDeps(env);
          evaluatorContext.getVisitor().enqueueEvaluation(skyKey, nodeEntry.getPriority(), null);
          return;
        }

        Set<SkyKey> newDeps = env.getNewlyRequestedDeps();
        if (value != null) {
          if (nodeEntry.hasUnsignaledDeps()) {
            // This is a partial reevaluation. It is not safe to set the value because a dep may be
            // racing to signal this node.
            return;
          }

          try {
            env.ensurePreviouslyRequestedDepsFetched();
          } catch (UndonePreviouslyRequestedDeps e) {
            handleUndonePreviouslyRequestedDep(nodeEntry);
            return;
          }

          checkState(
              !env.valuesMissing(),
              "Evaluation of %s returned non-null value but requested dependencies that weren't "
                  + "computed yet (one of %s), NodeEntry: %s",
              skyKey,
              newDeps,
              nodeEntry);

          stateCache.invalidate(skyKey);
          try {
            evaluatorContext.getProgressReceiver().stateStarting(skyKey, NodeState.COMMIT);
            if (maybeHandleRegisteringNewlyDiscoveredDepsForDoneEntry(
                skyKey, nodeEntry, oldDeps, env, evaluatorContext.keepGoing())) {
              // A newly requested dep transitioned from done to dirty before this node finished.
              // This node will be signalled again, and so we should return.
              return;
            }
            env.setValue(value);
            Set<SkyKey> reverseDeps = env.commitAndGetParents(nodeEntry);
            evaluatorContext.signalParentsAndEnqueueIfReady(
                skyKey, reverseDeps, nodeEntry.getVersion());
          } finally {
            evaluatorContext.getProgressReceiver().stateEnding(skyKey, NodeState.COMMIT);
          }
          return;
        }

        SkyKey childErrorKey = env.getDepErrorKey();
        if (childErrorKey != null) {
          checkState(!evaluatorContext.keepGoing(), "%s %s %s", skyKey, nodeEntry, childErrorKey);
          // We encountered a child error in noKeepGoing mode, so we want to fail fast. But we first
          // need to add the edge between the current node and the child error it requested so that
          // error bubbling can occur. Note that this edge will subsequently be removed during graph
          // cleaning (since the current node will never be committed to the graph).
          NodeEntry childErrorEntry =
              checkNotNull(
                  graph.get(skyKey, Reason.OTHER, childErrorKey),
                  "skyKey: %s, nodeEntry: %s childErrorKey: %s",
                  skyKey,
                  nodeEntry,
                  childErrorKey);
          if (newDeps.contains(childErrorKey)) {
            // Add this dep if it was just requested. In certain rare race conditions (see
            // MemoizingEvaluatorTest.cachedErrorCausesRestart) this dep may have already been
            // requested.
            nodeEntry.addSingletonTemporaryDirectDep(childErrorKey);
            DependencyState childErrorState;
            if (oldDeps.contains(childErrorKey)) {
              childErrorState = childErrorEntry.checkIfDoneForDirtyReverseDep(skyKey);
            } else {
              childErrorState = childErrorEntry.addReverseDepAndCheckIfDone(skyKey);
            }
            if (childErrorState != DependencyState.DONE) {
              // The child in error may have transitioned from done to dirty between when this node
              // discovered the error and now. Notify the graph inconsistency receiver so that we
              // can crash if that's unexpected.
              // We don't enqueue the child, even if it returns NEEDS_SCHEDULING, because we are
              // about to shut down evaluation.
              evaluatorContext
                  .getGraphInconsistencyReceiver()
                  .noteInconsistencyAndMaybeThrow(
                      skyKey,
                      ImmutableList.of(childErrorKey),
                      Inconsistency.BUILDING_PARENT_FOUND_UNDONE_CHILD);
            }
          }
          SkyValue childErrorInfoMaybe =
              checkNotNull(
                  env.maybeGetValueFromErrorOrDeps(childErrorKey),
                  "dep error found but then lost while building: %s %s",
                  skyKey,
                  childErrorKey);
          ErrorInfo childErrorInfo =
              checkNotNull(
                  ValueWithMetadata.getMaybeErrorInfo(childErrorInfoMaybe),
                  "dep error found but then wasn't an error while building: %s %s %s",
                  skyKey,
                  childErrorKey,
                  childErrorInfoMaybe);
          evaluatorContext.getVisitor().preventNewEvaluations();
          // TODO(b/166268889): Remove when fixed.
          if (childErrorInfo.getException() instanceof IOException) {
            logger.atInfo().withCause(childErrorInfo.getException()).log(
                "Child %s with IOException forced abort of %s", childErrorKey, skyKey);
          }
          throw SchedulerException.ofError(childErrorInfo, childErrorKey, ImmutableSet.of(skyKey));
        }

        // TODO(bazel-team): This code is not safe to interrupt, because we would lose the state in
        // newDirectDeps.

        // TODO(bazel-team): An ill-behaved SkyFunction can throw us into an infinite loop where we
        // add more dependencies on every run. [skyframe-core]

        // Add all the newly requested dependencies to the temporary direct deps. Note that
        // newDirectDeps does not contain any elements in common with the already existing temporary
        // direct deps. uniqueNewDeps will be the set of unique keys contained in newDirectDeps.
        env.addTemporaryDirectDepsTo(nodeEntry);

        List<ListenableFuture<?>> externalDeps = env.externalDeps;
        // If the key does not support partial reevaluation and there were no newly requested
        // dependencies, then at least one of them was in error or there is a bug in the SkyFunction
        // implementation. The environment has collected its errors, so we just order it to be
        // built.
        if (newDeps.isEmpty() && externalDeps == null && !skyKey.supportsPartialReevaluation()) {
          checkState(
              !env.getChildErrorInfos().isEmpty(),
              "Evaluation of SkyKey failed and no dependencies were requested: %s %s",
              skyKey,
              nodeEntry);
          checkState(
              evaluatorContext.keepGoing(),
              "nokeep_going evaluation should have failed on first child error: %s %s %s",
              skyKey,
              nodeEntry,
              env.getChildErrorInfos());
          // If the child error was catastrophic, committing this parent to the graph is not
          // necessary, but since we don't do error bubbling in catastrophes, it doesn't violate any
          // invariants either.
          Set<SkyKey> reverseDeps = env.commitAndGetParents(nodeEntry);
          evaluatorContext.signalParentsAndEnqueueIfReady(
              skyKey, reverseDeps, nodeEntry.getVersion());
          return;
        }

        // If there are external deps, we register that fact on the NodeEntry before we enqueue
        // child nodes in order to prevent the current node from being re-enqueued between here and
        // the call to registerExternalDeps below.
        if (externalDeps != null) {
          nodeEntry.addExternalDep();
        }

        // We want to split apart the dependencies that existed for this node the last time we did
        // an evaluation and those that were introduced in this evaluation. To be clear, the prefix
        // "newDeps" refers to newly discovered this time around after a SkyFunction#compute call
        // and not to be confused with the oldDeps variable which refers to the last evaluation,
        // i.e. a prior call to ParallelEvaluator#eval.
        Collection<SkyKey> newDepsThatWerentInTheLastEvaluation;
        ImmutableList<SkyKey> newDepsThatWereInTheLastEvaluation;
        if (oldDeps.isEmpty()) {
          // When there are no old deps (clean evaluations), avoid set views which have O(n) size.
          newDepsThatWerentInTheLastEvaluation = newDeps;
          newDepsThatWereInTheLastEvaluation = ImmutableList.of();
        } else {
          newDepsThatWerentInTheLastEvaluation =
              ImmutableList.copyOf(Sets.difference(newDeps, oldDeps));
          newDepsThatWereInTheLastEvaluation =
              ImmutableList.copyOf(Sets.intersection(newDeps, oldDeps));
        }

        InterruptibleSupplier<NodeBatch> newDepsThatWerentInTheLastEvaluationNodes =
            graph.createIfAbsentBatchAsync(
                skyKey, Reason.RDEP_ADDITION, newDepsThatWerentInTheLastEvaluation);
        handleKnownChildrenForDirtyNode(
            newDepsThatWereInTheLastEvaluation,
            graph.getBatch(skyKey, Reason.ENQUEUING_CHILD, newDepsThatWereInTheLastEvaluation),
            nodeEntry,
            /* enqueueParentIfReady= */ true,
            env);

        // Due to multi-threading, this can potentially cause the current node to be re-enqueued if
        // all 'new' children of this node are already done. Therefore, there should not be any code
        // after this loop, as it would potentially race with the re-evaluation in another thread.
        NodeBatch newNodes = newDepsThatWerentInTheLastEvaluationNodes.get();
        int childDepth = nodeEntry.getChildDepth();
        for (SkyKey newDirectDep : newDepsThatWerentInTheLastEvaluation) {
          enqueueChild(
              skyKey,
              nodeEntry,
              newDirectDep,
              newNodes.get(newDirectDep),
              /* depAlreadyExists= */ false,
              childDepth,
              /* enqueueParentIfReady= */ true,
              env);
        }
        if (externalDeps != null) {
          // This can cause the current node to be re-enqueued if all futures are already done.
          // This is an exception to the rule above that there must not be code below the for
          // loop. It is safe because we call nodeEntry.addExternalDep above, which prevents
          // re-enqueueing of the current node in the above loop if externalDeps != null.
          evaluatorContext
              .getVisitor()
              .registerExternalDeps(skyKey, nodeEntry, externalDeps, nodeEntry.getPriority());
        }
        // Do not put any code here! Any code here can race with a re-evaluation of this same node
        // in another thread.
      } catch (InterruptedException ie) {
        // The current thread can be interrupted at various places during evaluation or while
        // committing the result in this method. Since we only register the future(s) with the
        // underlying AbstractQueueVisitor in the registerExternalDeps call above, we have to make
        // sure that any known futures are correctly canceled if we do not reach that call. Note
        // that it is safe to cancel a future multiple times.
        cancelExternalDeps(env);
        // InterruptedException cannot be thrown by Runnable.run, so we must wrap it.
        // Interrupts can be caught by both the Evaluator and the AbstractQueueVisitor.
        // The former will unwrap the IE and propagate it as is; the latter will throw a new IE.
        throw SchedulerException.ofInterruption(ie, skyKey);
      }
    }

    private void handleUndonePreviouslyRequestedDep(NodeEntry nodeEntry) {
      // If a previously requested dep is no longer done, restart this node from scratch.
      stateCache.invalidate(skyKey);
      resetEntry(skyKey, nodeEntry);
      evaluatorContext.getVisitor().enqueueEvaluation(skyKey, nodeEntry.getPriority(), null);
    }

    private void cancelExternalDeps(SkyFunctionEnvironment env) {
      if (env != null && env.externalDeps != null) {
        for (ListenableFuture<?> future : env.externalDeps) {
          future.cancel(/* mayInterruptIfRunning= */ true);
        }
      }
    }

    private String prepareCrashMessage(SkyKey skyKey, Iterable<SkyKey> reverseDeps) {
      StringBuilder reverseDepDump = new StringBuilder();
      for (SkyKey key : reverseDeps) {
        if (reverseDepDump.length() > MAX_REVERSEDEP_DUMP_LENGTH) {
          reverseDepDump.append(", ...");
          break;
        }
        if (reverseDepDump.length() > 0) {
          reverseDepDump.append(", ");
        }
        reverseDepDump.append("'");
        reverseDepDump.append(key);
        reverseDepDump.append("'");
      }

      return String.format(
          "Unrecoverable error while evaluating node '%s' (requested by nodes %s)",
          skyKey, reverseDepDump);
    }

    private static final int MAX_REVERSEDEP_DUMP_LENGTH = 1000;
  }

  protected void replay(ValueWithMetadata valueWithMetadata) {
    // Replaying actions is done on a small number of nodes, but potentially over a large dependency
    // graph. Under those conditions, using the regular NestedSet flattening with .toList() is more
    // efficient than using NestedSetVisitor's custom traversal logic.
    evaluatorContext
        .getReplayingNestedSetEventVisitor()
        .visit(valueWithMetadata.getTransitiveEvents().toList());
  }

  /**
   * Resets {@code entry}, and the other nodes specified by {@code restart.rewindGraph()} will be
   * marked changed via postorder DFS.
   *
   * <p>{@code restart.rewindGraph()} must be empty or must contain {@code key}.
   *
   * <p>TODO(b/123993876): this should verify that edges in rewindGraph correspond to deps in the
   * Skyframe graph. Will require a safe way of requesting deps for nodes which may not be done.
   */
  // Nodes must be marked changed via postorder DFS. To see why, suppose we have this graph:
  //
  //   FailedNode   SomeOtherRdepOfR1
  //       |       /
  //       |  -----
  //       | /
  //       R1
  //       |
  //       R2
  //
  // Suppose FailedNode (FN) fails and requires that R1 and R2 must be dirtied and run again.
  // Suppose they aren't dirtied via postorder DFS, so R1 is dirtied first.
  //
  // Then, the evaluation thread working on dirtying these nodes is suspended.
  //
  // On a separate evaluation thread, SomeOtherRdepOfR1 requests R1. R1 is scheduled for evaluation,
  // checks its dep R2, and because R2 is done, R1 completes without scheduling R2 for evaluation.
  //
  // Then, the evaluation thread working on dirtying these nodes continues its work. It dirties
  // R2 and schedules FN for evaluation.
  //
  // When FN next evaluates, it requests R1, and because R1 is done, R2 is not scheduled for
  // evaluation, contrary to FN's expectations.
  private void dirtyRewindGraphAndResetEntry(SkyKey key, NodeEntry entry, Restart restart)
      throws InterruptedException {
    ImmutableGraph<SkyKey> rewindGraph = restart.rewindGraph();
    if (rewindGraph.nodes().isEmpty()) {
      resetEntry(key, entry);
      return;
    }
    checkArgument(
        rewindGraph.nodes().contains(key),
        "rewindGraph must contain the key for the failed evaluation if it's not empty. key: %s, "
            + "rewindGraph: %s",
        key,
        rewindGraph);

    ImmutableList.Builder<SkyKey> builder = ImmutableList.builder();
    for (SkyKey k : Traverser.forGraph(rewindGraph).depthFirstPostOrder(key)) {
      if (!k.equals(key)) {
        builder.add(k);
      }
    }
    ImmutableList<SkyKey> childrenToRestart = builder.build();
    if (!childrenToRestart.isEmpty()) {
      evaluatorContext
          .getGraphInconsistencyReceiver()
          .noteInconsistencyAndMaybeThrow(
              key, childrenToRestart, Inconsistency.PARENT_FORCE_REBUILD_OF_CHILD);

      NodeBatch children =
          evaluatorContext.getGraph().getBatch(key, Reason.REWINDING, childrenToRestart);

      for (SkyKey childToRestart : childrenToRestart) {
        NodeEntry childEntry =
            checkNotNull(
                children.get(childToRestart),
                "Missing child for rewinding: %s (parent=%s)",
                childToRestart,
                key);

        // Nodes are marked "force-rebuild" to ensure that they run, and to allow them to evaluate
        // to a different value than before, even if their versions remain the same.
        if (childEntry.markDirty(DirtyType.FORCE_REBUILD) != null) {
          evaluatorContext
              .getProgressReceiver()
              .invalidated(childToRestart, EvaluationProgressReceiver.InvalidationState.DIRTY);
        }
      }
    }

    // TODO(b/19539699): rdeps of children have to be handled here. If the graph does not keep
    // edges, nothing has to be done, since there are no reverse deps to keep consistent. If the
    // graph keeps edges, it's a harder problem. The reverse deps could just be removed, but in the
    // case that this node is dirty, the deps shouldn't be removed, they should just be transformed
    // back to "known reverse deps" from "reverse deps declared during this evaluation" (the inverse
    // of NodeEntry#checkIfDoneForDirtyReverseDep). Such a method doesn't currently exist, but
    // could.
    resetEntry(key, entry);
  }

  private void resetEntry(SkyKey key, NodeEntry entry) {
    evaluatorContext
        .getGraphInconsistencyReceiver()
        .noteInconsistencyAndMaybeThrow(key, /* otherKeys= */ null, Inconsistency.RESET_REQUESTED);
    entry.resetForRestartFromScratch();
  }

  void propagateEvaluatorContextCrashIfAny() {
    if (!evaluatorContext.getVisitor().getCrashes().isEmpty()) {
      evaluatorContext
          .getReporter()
          .handle(Event.error("Crashes detected: " + evaluatorContext.getVisitor().getCrashes()));
      throw checkNotNull(Iterables.getFirst(evaluatorContext.getVisitor().getCrashes(), null));
    }
  }

  static void propagateInterruption(SchedulerException e) throws InterruptedException {
    boolean mustThrowInterrupt = Thread.interrupted();
    Throwables.propagateIfPossible(e.getCause(), InterruptedException.class);
    if (mustThrowInterrupt) {
      // As per the contract of AbstractQueueVisitor#work, if an unchecked exception is thrown and
      // the build is interrupted, the thrown exception is what will be rethrown. Since the user
      // presumably wanted to interrupt the build, we ignore the thrown SchedulerException (which
      // doesn't indicate a programming bug) and throw an InterruptedException.
      throw new InterruptedException();
    }
  }

  /**
   * Add any newly discovered deps that were registered during the run of a SkyFunction that
   * finished by returning a value or throwing an error. SkyFunctions may throw errors even if all
   * their deps were not provided -- we trust that a SkyFunction might know it should throw an error
   * even if not all of its requested deps are done. However, that means we're assuming the
   * SkyFunction would throw that same error if all of its requested deps were done. Unfortunately,
   * there is no way to enforce that condition.
   *
   * <p>Returns {@code true} if any newly discovered dep is dirty when this node registers itself as
   * an rdep and if one of those dirty deps will schedule this node for evaluation.
   *
   * <p>This can happen if a newly discovered dep transitions from done to dirty between when this
   * node's evaluation accessed the dep's value and here. Adding this node as an rdep of that dep
   * (or checking that this node is an rdep of that dep) will cause this node to be signalled when
   * that dep completes.
   *
   * <p>If this returns {@code true}, this node should not actually finish, and this evaluation
   * attempt should make no changes to the node after this method returns, because a completing dep
   * may schedule a new evaluation attempt at any time.
   */
  private boolean maybeHandleRegisteringNewlyDiscoveredDepsForDoneEntry(
      SkyKey skyKey,
      NodeEntry entry,
      ImmutableSet<SkyKey> oldDeps,
      SkyFunctionEnvironment env,
      boolean keepGoing)
      throws InterruptedException {
    // We don't expect any unfinished deps in a keep-going build.
    if (!keepGoing) {
      env.removeUndoneNewlyRequestedDeps();
    }

    Set<SkyKey> newDeps = env.getNewlyRequestedDeps();
    if (newDeps.isEmpty()) {
      return false;
    }

    env.addTemporaryDirectDepsTo(entry);
    Collection<SkyKey> newlyAddedNewDeps;
    ImmutableCollection<SkyKey> previouslyRegisteredNewDeps;
    if (oldDeps.isEmpty()) {
      newlyAddedNewDeps = newDeps;
      previouslyRegisteredNewDeps = ImmutableSet.of();
    } else {
      newlyAddedNewDeps = ImmutableList.copyOf(Sets.difference(newDeps, oldDeps));
      previouslyRegisteredNewDeps = ImmutableList.copyOf(Sets.intersection(newDeps, oldDeps));
    }

    InterruptibleSupplier<NodeBatch> newlyAddedNewDepNodes =
        graph.getBatchAsync(skyKey, Reason.RDEP_ADDITION, newlyAddedNewDeps);

    // Dep entries in the following two loops may not be done, but they must be present. In a
    // keep-going build, we normally expect all deps to be done. In a non-keep-going build, if
    // env.newlyRequestedDeps contained a key for a node that wasn't done, then it would have been
    // removed via removeUndoneNewlyRequestedDeps() just above this loop. However, with
    // intra-evaluation dirtying, a dep may not be done.
    boolean dirtyDepFound = false;
    boolean selfSignalled = false;

    NodeBatch previouslyRegisteredEntries =
        graph.getBatch(skyKey, Reason.SIGNAL_DEP, previouslyRegisteredNewDeps);
    for (SkyKey newDep : previouslyRegisteredNewDeps) {
      // We choose not to use `getOrRecreateDepEntry(...)` due to there is no use case where nodes
      // are expected to be missing on incremental builds (which this loop is specific to).
      NodeEntry depEntry =
          checkNotNull(
              previouslyRegisteredEntries.get(newDep),
              "Missing already declared dep %s (parent=%s)",
              newDep,
              skyKey);
      DependencyState triState = depEntry.checkIfDoneForDirtyReverseDep(skyKey);
      switch (maybeHandleUndoneDepForDoneEntry(entry, depEntry, triState, skyKey, newDep)) {
        case DEP_DONE_SELF_SIGNALLED:
          selfSignalled = true;
          break;
        case DEP_DONE_SELF_NOT_SIGNALLED:
          break;
        case DEP_NOT_DONE:
          dirtyDepFound = true;
          break;
      }
    }

    for (SkyKey newDep : newlyAddedNewDeps) {
      NodeEntry depEntry =
          getOrRecreateDepEntry(newDep, newlyAddedNewDepNodes.get(), skyKey, Reason.RDEP_ADDITION);

      DependencyState triState = depEntry.addReverseDepAndCheckIfDone(skyKey);
      switch (maybeHandleUndoneDepForDoneEntry(entry, depEntry, triState, skyKey, newDep)) {
        case DEP_DONE_SELF_SIGNALLED:
          selfSignalled = true;
          break;
        case DEP_DONE_SELF_NOT_SIGNALLED:
          break;
        case DEP_NOT_DONE:
          dirtyDepFound = true;
          break;
      }
    }

    checkState(
        selfSignalled || dirtyDepFound,
        "%s %s %s %s",
        skyKey,
        entry,
        newlyAddedNewDeps,
        previouslyRegisteredNewDeps);

    return !selfSignalled;
  }

  /**
   * Returns a {@link NodeEntry} for {@code depKey}.
   *
   * <p>If {@code depKey} is present in {@code depEntries}, its corresponding entry is returned.
   * Otherwise, if the evaluator permits {@link Inconsistency#ALREADY_DECLARED_CHILD_MISSING}, the
   * entry will be recreated.
   */
  private NodeEntry getOrRecreateDepEntry(
      SkyKey depKey, NodeBatch depEntries, SkyKey requestor, Reason reason)
      throws InterruptedException {
    NodeEntry depEntry = depEntries.get(depKey);
    if (depEntry != null) {
      return depEntry;
    }

    ImmutableList<SkyKey> missing = ImmutableList.of(depKey);
    evaluatorContext
        .getGraphInconsistencyReceiver()
        .noteInconsistencyAndMaybeThrow(
            requestor, missing, Inconsistency.ALREADY_DECLARED_CHILD_MISSING);
    return checkNotNull(graph.createIfAbsentBatch(requestor, reason, missing).get(depKey), depKey);
  }

  private enum MaybeHandleUndoneDepResult {
    DEP_DONE_SELF_SIGNALLED,
    DEP_DONE_SELF_NOT_SIGNALLED,
    DEP_NOT_DONE
  }

  /**
   * Returns {@link MaybeHandleUndoneDepResult#DEP_NOT_DONE} if {@code depEntry} was not done.
   * Notifies the {@link GraphInconsistencyReceiver} if so. Schedules {@code depEntry} for
   * evaluation if necessary.
   *
   * <p>If {@code depEntry} was done, then this calls {@code entry.signalDep}.
   *
   * <p>If the call to {@code #signalDep} returns false, this returns {@link
   * MaybeHandleUndoneDepResult#DEP_DONE_SELF_NOT_SIGNALLED}.
   *
   * <p>If the call to {@code #signalDep} returns true, this returns {@link
   * MaybeHandleUndoneDepResult#DEP_DONE_SELF_SIGNALLED}. This will happen for the last new dep if
   * all of them were done. It can also happen if some new deps weren't done but they all signal
   * {@code entry} before {@link #maybeHandleRegisteringNewlyDiscoveredDepsForDoneEntry} finishes
   * checking deps.
   */
  private MaybeHandleUndoneDepResult maybeHandleUndoneDepForDoneEntry(
      NodeEntry entry, NodeEntry depEntry, DependencyState triState, SkyKey skyKey, SkyKey depKey) {
    if (triState == DependencyState.DONE) {
      return entry.signalDep(depEntry.getVersion(), depKey)
          ? MaybeHandleUndoneDepResult.DEP_DONE_SELF_SIGNALLED
          : MaybeHandleUndoneDepResult.DEP_DONE_SELF_NOT_SIGNALLED;
    }
    // The dep may have transitioned from done to dirty between when this node read its value and
    // now. Notify the graph inconsistency receiver so that we can crash if that's unexpected. We
    // schedule the dep if it needs scheduling, because nothing else can if we don't.
    evaluatorContext
        .getGraphInconsistencyReceiver()
        .noteInconsistencyAndMaybeThrow(
            skyKey, ImmutableList.of(depKey), Inconsistency.BUILDING_PARENT_FOUND_UNDONE_CHILD);
    if (triState == DependencyState.NEEDS_SCHEDULING) {
      depEntry.updateDepthIfGreater(entry.getChildDepth());
      evaluatorContext.getVisitor().enqueueEvaluation(depKey, depEntry.getPriority(), null);
    }
    return MaybeHandleUndoneDepResult.DEP_NOT_DONE;
  }

  /**
   * Return true if the entry does not need to be re-evaluated this build. The entry will need to be
   * re-evaluated if it is not done, but also if it was not completely evaluated last build and this
   * build is keepGoing.
   */
  static boolean isDoneForBuild(@Nullable NodeEntry entry) {
    return entry != null && entry.isDone();
  }
}