src/main/java/com/google/devtools/build/lib/analysis/DependencyResolver.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.lib.analysis;

import com.google.common.collect.ArrayListMultimap;
import com.google.common.collect.ImmutableList;
import com.google.common.collect.ImmutableMap;
import com.google.common.collect.ImmutableSet;
import com.google.common.collect.ListMultimap;
import com.google.common.collect.Sets;
import com.google.devtools.build.lib.analysis.config.BuildConfiguration;
import com.google.devtools.build.lib.analysis.config.ConfigMatchingProvider;
import com.google.devtools.build.lib.cmdline.Label;
import com.google.devtools.build.lib.collect.ImmutableSortedKeyListMultimap;
import com.google.devtools.build.lib.collect.nestedset.NestedSetBuilder;
import com.google.devtools.build.lib.packages.Aspect;
import com.google.devtools.build.lib.packages.AspectClass;
import com.google.devtools.build.lib.packages.AspectDefinition;
import com.google.devtools.build.lib.packages.Attribute;
import com.google.devtools.build.lib.packages.Attribute.LateBoundDefault;
import com.google.devtools.build.lib.packages.AttributeMap;
import com.google.devtools.build.lib.packages.BuildType;
import com.google.devtools.build.lib.packages.EnvironmentGroup;
import com.google.devtools.build.lib.packages.InputFile;
import com.google.devtools.build.lib.packages.NativeAspectClass;
import com.google.devtools.build.lib.packages.NoSuchThingException;
import com.google.devtools.build.lib.packages.OutputFile;
import com.google.devtools.build.lib.packages.PackageGroup;
import com.google.devtools.build.lib.packages.Rule;
import com.google.devtools.build.lib.packages.RuleClass;
import com.google.devtools.build.lib.packages.Target;
import com.google.devtools.build.lib.syntax.EvalException;
import com.google.devtools.build.lib.syntax.EvalUtils;
import com.google.devtools.build.lib.util.Preconditions;

import java.util.ArrayList;
import java.util.Collection;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;

import javax.annotation.Nullable;

/**
 * Resolver for dependencies between configured targets.
 *
 * <p>Includes logic to derive the right configurations depending on transition type.
 */
public abstract class DependencyResolver {
  protected DependencyResolver() {
  }

  /**
   * Returns ids for dependent nodes of a given node, sorted by attribute. Note that some
   * dependencies do not have a corresponding attribute here, and we use the null attribute to
   * represent those edges. Visibility attributes are only visited if {@code visitVisibility} is
   * {@code true}.
   *
   * <p>If {@code aspect} is null, returns the dependent nodes of the configured
   * target node representing the given target and configuration, otherwise that of the aspect
   * node accompanying the aforementioned configured target node for the specified aspect.
   *
   * <p>The values are not simply labels because this also implements the first step of applying
   * configuration transitions, namely, split transitions. This needs to be done before the labels
   * are resolved because late bound attributes depend on the configuration. A good example for this
   * is @{code :cc_toolchain}.
   *
   * <p>The long-term goal is that most configuration transitions be applied here. However, in order
   * to do that, we first have to eliminate transitions that depend on the rule class of the
   * dependency.
   */
  public final ListMultimap<Attribute, Dependency> dependentNodeMap(
      TargetAndConfiguration node,
      BuildConfiguration hostConfig,
      Aspect aspect,
      ImmutableMap<Label, ConfigMatchingProvider> configConditions)
      throws EvalException, InterruptedException {
    NestedSetBuilder<Label> rootCauses = NestedSetBuilder.<Label>stableOrder();
    ListMultimap<Attribute, Dependency> outgoingEdges =
        dependentNodeMap(node, hostConfig, aspect, configConditions, rootCauses);
    if (!rootCauses.isEmpty()) {
      throw new IllegalStateException(rootCauses.build().iterator().next().toString());
    }
    return outgoingEdges;
  }

  /**
   * Returns ids for dependent nodes of a given node, sorted by attribute. Note that some
   * dependencies do not have a corresponding attribute here, and we use the null attribute to
   * represent those edges. Visibility attributes are only visited if {@code visitVisibility} is
   * {@code true}.
   *
   * <p>If {@code aspect} is null, returns the dependent nodes of the configured
   * target node representing the given target and configuration, otherwise that of the aspect
   * node accompanying the aforementioned configured target node for the specified aspect.
   *
   * <p>The values are not simply labels because this also implements the first step of applying
   * configuration transitions, namely, split transitions. This needs to be done before the labels
   * are resolved because late bound attributes depend on the configuration. A good example for this
   * is @{code :cc_toolchain}.
   *
   * <p>The long-term goal is that most configuration transitions be applied here. However, in order
   * to do that, we first have to eliminate transitions that depend on the rule class of the
   * dependency.
   */
  public final ListMultimap<Attribute, Dependency> dependentNodeMap(
      TargetAndConfiguration node,
      BuildConfiguration hostConfig,
      Aspect aspect,
      ImmutableMap<Label, ConfigMatchingProvider> configConditions,
      NestedSetBuilder<Label> rootCauses)
      throws EvalException, InterruptedException {
    Target target = node.getTarget();
    BuildConfiguration config = node.getConfiguration();
    ListMultimap<Attribute, Dependency> outgoingEdges = ArrayListMultimap.create();
    if (target instanceof OutputFile) {
      Preconditions.checkNotNull(config);
      visitTargetVisibility(node, rootCauses, outgoingEdges.get(null));
      Rule rule = ((OutputFile) target).getGeneratingRule();
      outgoingEdges.put(null, Dependency.withConfiguration(rule.getLabel(), config));
    } else if (target instanceof InputFile) {
      visitTargetVisibility(node, rootCauses, outgoingEdges.get(null));
    } else if (target instanceof EnvironmentGroup) {
      visitTargetVisibility(node, rootCauses, outgoingEdges.get(null));
    } else if (target instanceof Rule) {
      Preconditions.checkNotNull(config);
      visitTargetVisibility(node, rootCauses, outgoingEdges.get(null));
      Rule rule = (Rule) target;
      ListMultimap<Attribute, LabelAndConfiguration> labelMap =
          resolveAttributes(
              rule,
              aspect != null ? aspect.getDefinition() : null,
              config,
              hostConfig,
              configConditions);
      visitRule(rule, aspect, labelMap, rootCauses, outgoingEdges);
    } else if (target instanceof PackageGroup) {
      visitPackageGroup(node, (PackageGroup) target, rootCauses, outgoingEdges.get(null));
    } else {
      throw new IllegalStateException(target.getLabel().toString());
    }
    return outgoingEdges;
  }

  private ListMultimap<Attribute, LabelAndConfiguration> resolveAttributes(
      Rule rule, AspectDefinition aspect, BuildConfiguration configuration,
      BuildConfiguration hostConfiguration,
      ImmutableMap<Label, ConfigMatchingProvider> configConditions)
      throws EvalException, InterruptedException {
    ConfiguredAttributeMapper attributeMap = ConfiguredAttributeMapper.of(rule, configConditions);
    attributeMap.validateAttributes();
    List<Attribute> attributes;
    if (aspect == null) {
      attributes = rule.getRuleClassObject().getAttributes();
    } else {
      attributes = new ArrayList<>();
      attributes.addAll(rule.getRuleClassObject().getAttributes());
      attributes.addAll(aspect.getAttributes().values());
    }

    ImmutableSortedKeyListMultimap.Builder<Attribute, LabelAndConfiguration> result =
        ImmutableSortedKeyListMultimap.builder();

    resolveExplicitAttributes(configuration, attributeMap, result);
    resolveImplicitAttributes(rule, configuration, attributeMap, attributes, result);
    resolveLateBoundAttributes(rule, configuration, hostConfiguration, attributeMap, attributes,
        result);

    // Add the rule's visibility labels (which may come from the rule or from package defaults).
    addExplicitDeps(result, rule, "visibility", rule.getVisibility().getDependencyLabels(),
        configuration);

    // Add package default constraints when the rule doesn't explicitly declare them.
    //
    // Note that this can have subtle implications for constraint semantics. For example: say that
    // package defaults declare compatibility with ':foo' and rule R declares compatibility with
    // ':bar'. Does that mean that R is compatible with [':foo', ':bar'] or just [':bar']? In other
    // words, did R's author intend to add additional compatibility to the package defaults or to
    // override them? More severely, what if package defaults "restrict" support to just [':baz']?
    // Should R's declaration signify [':baz'] + ['bar'], [ORIGINAL_DEFAULTS] + ['bar'], or
    // something else?
    //
    // Rather than try to answer these questions with possibly confusing logic, we take the
    // simple approach of assigning the rule's "restriction" attribute to the rule-declared value if
    // it exists, else the package defaults value (and likewise for "compatibility"). This may not
    // always provide what users want, but it makes it easy for them to understand how rule
    // declarations and package defaults intermix (and how to refactor them to get what they want).
    //
    // An alternative model would be to apply the "rule declaration" / "rule class defaults"
    // relationship, i.e. the rule class' "compatibility" and "restriction" declarations are merged
    // to generate a set of default environments, then the rule's declarations are independently
    // processed on top of that. This protects against obscure coupling behavior between
    // declarations from wildly different places (e.g. it offers clear answers to the examples posed
    // above). But within the scope of a single package it seems better to keep the model simple and
    // make the user responsible for resolving ambiguities.
    if (!rule.isAttributeValueExplicitlySpecified(RuleClass.COMPATIBLE_ENVIRONMENT_ATTR)) {
      addExplicitDeps(result, rule, RuleClass.COMPATIBLE_ENVIRONMENT_ATTR,
          rule.getPackage().getDefaultCompatibleWith(), configuration);
    }
    if (!rule.isAttributeValueExplicitlySpecified(RuleClass.RESTRICTED_ENVIRONMENT_ATTR)) {
      addExplicitDeps(result, rule, RuleClass.RESTRICTED_ENVIRONMENT_ATTR,
          rule.getPackage().getDefaultRestrictedTo(), configuration);
    }

    return result.build();
  }

  /**
   * Adds new dependencies to the given rule under the given attribute name
   *
   * @param result the builder for the attribute --> dependency labels map
   * @param rule the rule being evaluated
   * @param attrName the name of the attribute to add dependency labels to
   * @param labels the dependencies to add
   * @param configuration the configuration to apply to those dependencies
   */
  private void addExplicitDeps(
      ImmutableSortedKeyListMultimap.Builder<Attribute, LabelAndConfiguration> result, Rule rule,
      String attrName, Iterable<Label> labels, BuildConfiguration configuration) {
    if (!rule.isAttrDefined(attrName, BuildType.LABEL_LIST)
        && !rule.isAttrDefined(attrName, BuildType.NODEP_LABEL_LIST)) {
      return;
    }
    Attribute attribute = rule.getRuleClassObject().getAttributeByName(attrName);
    for (Label label : labels) {
      // The configuration must be the configuration after the first transition step (applying
      // split configurations). The proper configuration (null) for package groups will be set
      // later.
      result.put(attribute, LabelAndConfiguration.of(label, configuration));
    }
  }

  private void resolveExplicitAttributes(final BuildConfiguration configuration,
      AttributeMap attributes,
      final ImmutableSortedKeyListMultimap.Builder<Attribute, LabelAndConfiguration> builder) {
    attributes.visitLabels(
        new AttributeMap.AcceptsLabelAttribute() {
          @Override
          public void acceptLabelAttribute(Label label, Attribute attribute) {
            if (attribute.getType() == BuildType.NODEP_LABEL || attribute.isImplicit()
                || attribute.isLateBound()) {
              return;
            }
            builder.put(attribute, LabelAndConfiguration.of(label, configuration));
          }
        });
  }

  private void resolveImplicitAttributes(Rule rule, BuildConfiguration configuration,
      AttributeMap attributeMap, Iterable<Attribute> attributes,
      ImmutableSortedKeyListMultimap.Builder<Attribute, LabelAndConfiguration> builder) {
    // Since the attributes that come from aspects do not appear in attributeMap, we have to get
    // their values from somewhere else. This incidentally means that aspects attributes are not
    // configurable. It would be nice if that wasn't the case, but we'd have to revamp how
    // attribute mapping works, which is a large chunk of work.
    Label ruleLabel = rule.getLabel();
    ImmutableSet<String> mappedAttributes = ImmutableSet.copyOf(attributeMap.getAttributeNames());
    for (Attribute attribute : attributes) {
      if (!attribute.isImplicit() || !attribute.getCondition().apply(attributeMap)) {
        continue;
      }

      if (attribute.getType() == BuildType.LABEL) {
        Label label = mappedAttributes.contains(attribute.getName())
            ? attributeMap.get(attribute.getName(), BuildType.LABEL)
            : BuildType.LABEL.cast(attribute.getDefaultValue(rule));

        if (label != null) {
          label = ruleLabel.resolveRepositoryRelative(label);
          builder.put(attribute, LabelAndConfiguration.of(label, configuration));
        }
      } else if (attribute.getType() == BuildType.LABEL_LIST) {
        List<Label> labelList;
        if (mappedAttributes.contains(attribute.getName())) {
          labelList = new ArrayList<>();
          for (Label label : attributeMap.get(attribute.getName(), BuildType.LABEL_LIST)) {
            labelList.add(label);
          }
        } else {
          labelList = BuildType.LABEL_LIST.cast(attribute.getDefaultValue(rule));
        }

        for (Label label : labelList) {
          label = ruleLabel.resolveRepositoryRelative(label);
          builder.put(attribute, LabelAndConfiguration.of(label, configuration));
        }
      }
    }
  }

  private void resolveLateBoundAttributes(
      Rule rule,
      BuildConfiguration configuration,
      BuildConfiguration hostConfiguration,
      AttributeMap attributeMap,
      Iterable<Attribute> attributes,
      ImmutableSortedKeyListMultimap.Builder<Attribute, LabelAndConfiguration> builder)
      throws EvalException, InterruptedException {
    for (Attribute attribute : attributes) {
      if (!attribute.isLateBound() || !attribute.getCondition().apply(attributeMap)) {
        continue;
      }

      List<BuildConfiguration> actualConfigurations = ImmutableList.of(configuration);
      if (attribute.hasSplitConfigurationTransition()) {
        Preconditions.checkState(attribute.getConfigurator() == null);
        // TODO(bazel-team): This ends up applying the split transition twice, both here and in the
        // visitRule method below - this is not currently a problem, because the configuration graph
        // never contains nested split transitions, so the second application is idempotent.
        actualConfigurations =
            configuration.getSplitConfigurations(attribute.getSplitTransition(rule));
      }

      for (BuildConfiguration actualConfig : actualConfigurations) {
        @SuppressWarnings("unchecked")
        LateBoundDefault<BuildConfiguration> lateBoundDefault =
            (LateBoundDefault<BuildConfiguration>) attribute.getLateBoundDefault();
        if (lateBoundDefault.useHostConfiguration()) {
          actualConfig = hostConfiguration;
        }
        // TODO(bazel-team): This might be too expensive - can we cache this somehow?
        if (!lateBoundDefault.getRequiredConfigurationFragments().isEmpty()) {
          if (!actualConfig.hasAllFragments(lateBoundDefault.getRequiredConfigurationFragments())) {
            continue;
          }
        }

        // TODO(bazel-team): We should check if the implementation tries to access an undeclared
        // fragment.
        Object actualValue = lateBoundDefault.resolve(rule, attributeMap, actualConfig);
        if (EvalUtils.isNullOrNone(actualValue)) {
          continue;
        }
        try {
          if (attribute.getType() == BuildType.LABEL) {
            Label label = rule.getLabel().resolveRepositoryRelative(
                BuildType.LABEL.cast(actualValue));
            builder.put(attribute, LabelAndConfiguration.of(label, actualConfig));
          } else if (attribute.getType() == BuildType.LABEL_LIST) {
            for (Label label : BuildType.LABEL_LIST.cast(actualValue)) {
              builder.put(attribute, LabelAndConfiguration.of(
                  rule.getLabel().resolveRepositoryRelative(label),
                  actualConfig));
            }
          } else {
            throw new IllegalStateException(
                String.format(
                    "Late bound attribute '%s' is not a label or a label list",
                    attribute.getName()));
          }
        } catch (ClassCastException e) { // From either of the cast calls above.
          throw new EvalException(
              rule.getLocation(),
              String.format(
                  "When computing the default value of %s, expected '%s', got '%s'",
                  attribute.getName(),
                  attribute.getType(),
                  EvalUtils.getDataTypeName(actualValue, true)));
        }
      }
    }
  }

  /**
   * Converts the given multimap of attributes to labels into a multi map of attributes to
   * {@link Dependency} objects using the proper configuration transition for each attribute.
   *
   * @throws IllegalArgumentException if the {@code node} does not refer to a {@link Rule} instance
   */
  public final Collection<Dependency> resolveRuleLabels(
      TargetAndConfiguration node, ListMultimap<Attribute,
      LabelAndConfiguration> labelMap, NestedSetBuilder<Label> rootCauses) {
    Preconditions.checkArgument(node.getTarget() instanceof Rule);
    Rule rule = (Rule) node.getTarget();
    ListMultimap<Attribute, Dependency> outgoingEdges = ArrayListMultimap.create();
    visitRule(rule, labelMap, rootCauses, outgoingEdges);
    return outgoingEdges.values();
  }

  private void visitPackageGroup(TargetAndConfiguration node, PackageGroup packageGroup,
      NestedSetBuilder<Label> rootCauses, Collection<Dependency> outgoingEdges) {
    for (Label label : packageGroup.getIncludes()) {
      Target target = getTarget(packageGroup, label, rootCauses);
      if (target == null) {
        continue;
      }
      if (!(target instanceof PackageGroup)) {
        // Note that this error could also be caught in PackageGroupConfiguredTarget, but since
        // these have the null configuration, visiting the corresponding target would trigger an
        // analysis of a rule with a null configuration, which doesn't work.
        invalidPackageGroupReferenceHook(node, label);
        continue;
      }

      outgoingEdges.add(Dependency.withNullConfiguration(label));
    }
  }

  private ImmutableSet<AspectDescriptor> requiredAspects(
      Aspect aspect, Attribute attribute, final Target target, Rule originalRule) {
    if (!(target instanceof Rule)) {
      return ImmutableSet.of();
    }

    Iterable<Aspect> aspectCandidates = extractAspectCandidates(aspect, attribute, originalRule);
    RuleClass ruleClass = ((Rule) target).getRuleClassObject();
    ImmutableSet.Builder<AspectDescriptor> result = ImmutableSet.builder();
    for (Aspect aspectCandidate : aspectCandidates) {
      if (ruleClass.canHaveAnyProvider() || Sets.difference(
              aspectCandidate.getDefinition().getRequiredProviders(),
              ruleClass.getAdvertisedProviders())
          .isEmpty()) {
        result.add(
            new AspectDescriptor(
                aspectCandidate.getAspectClass(),
                aspectCandidate.getParameters()));
      }
    }
    return result.build();
  }

  private static Iterable<Aspect> extractAspectCandidates(
      Aspect aspect, Attribute attribute, Rule originalRule) {
    // The order of this set will be deterministic. This is necessary because this order eventually
    // influences the order in which aspects are merged into the main configured target, which in
    // turn influences which aspect takes precedence if two emit the same provider (maybe this
    // should be an error)
    Set<Aspect> aspectCandidates = new LinkedHashSet<>();
    aspectCandidates.addAll(attribute.getAspects(originalRule));
    if (aspect != null) {
      for (AspectClass aspectClass :
          aspect.getDefinition().getAttributeAspects().get(attribute.getName())) {
        if (aspectClass.equals(aspect.getAspectClass())) {
          aspectCandidates.add(aspect);
        } else if (aspectClass instanceof NativeAspectClass) {
          aspectCandidates.add(
              Aspect.forNative((NativeAspectClass) aspectClass, aspect.getParameters()));
        } else {
          // If we ever want to support this specifying arbitrary aspects for Skylark aspects,
          // we will need to do a Skyframe call here to load an up-to-date definition.
          throw new IllegalStateException(
              "Skylark aspect classes sending different aspects along attributes is not supported");
        }
      }
    }
    return aspectCandidates;
  }

  private void visitRule(Rule rule,
      ListMultimap<Attribute, LabelAndConfiguration> labelMap, NestedSetBuilder<Label> rootCauses,
      ListMultimap<Attribute, Dependency> outgoingEdges) {
    visitRule(rule, /*aspect=*/ null, labelMap, rootCauses, outgoingEdges);
  }

  private void visitRule(
      Rule rule,
      Aspect aspect,
      ListMultimap<Attribute, LabelAndConfiguration> labelMap,
      NestedSetBuilder<Label> rootCauses,
      ListMultimap<Attribute, Dependency> outgoingEdges) {
    Preconditions.checkNotNull(labelMap);
    for (Map.Entry<Attribute, Collection<LabelAndConfiguration>> entry :
        labelMap.asMap().entrySet()) {
      Attribute attribute = entry.getKey();
      for (LabelAndConfiguration dep : entry.getValue()) {
        Label label = dep.getLabel();
        BuildConfiguration config = dep.getConfiguration();

        Target toTarget = getTarget(rule, label, rootCauses);
        if (toTarget == null) {
          continue;
        }
        BuildConfiguration.TransitionApplier transitionApplier = config.getTransitionApplier();
        if (config.useDynamicConfigurations() && config.isHostConfiguration()
            && !BuildConfiguration.usesNullConfiguration(toTarget)) {
          // This condition is needed because resolveLateBoundAttributes may switch config to
          // the host configuration, which is the only case DependencyResolver applies a
          // configuration transition outside of this method. We need to reflect that
          // transition in the results of this method, but config.evaluateTransition is hard-set
          // to return a NONE transition when the input is a host config. Since the outside
          // caller originally passed the *original* value of config (before the possible
          // switch), it can mistakenly interpret the result as a NONE transition from the
          // original value of config. This condition fixes that. Another fix would be to have
          // config.evaluateTransition return a HOST transition when the input config is a host,
          // but since this blemish is specific to DependencyResolver it seems best to keep the
          // fix here.
          // TODO(bazel-team): eliminate this special case by passing transitionApplier to
          // resolveLateBoundAttributes, so that method uses the same interface for transitions.
          transitionApplier.applyTransition(Attribute.ConfigurationTransition.HOST);
        } else {
          config.evaluateTransition(rule, attribute, toTarget, transitionApplier);
        }
        for (Dependency dependency :
            transitionApplier.getDependencies(
                label, requiredAspects(aspect, attribute, toTarget, rule))) {
          outgoingEdges.put(
              entry.getKey(),
              dependency);
        }
      }
    }
  }

  private void visitTargetVisibility(TargetAndConfiguration node,
      NestedSetBuilder<Label> rootCauses, Collection<Dependency> outgoingEdges) {
    Target target = node.getTarget();
    for (Label label : target.getVisibility().getDependencyLabels()) {
      Target visibilityTarget = getTarget(target, label, rootCauses);
      if (visibilityTarget == null) {
        continue;
      }
      if (!(visibilityTarget instanceof PackageGroup)) {
        // Note that this error could also be caught in
        // AbstractConfiguredTarget.convertVisibility(), but we have an
        // opportunity here to avoid dependency cycles that result from
        // the visibility attribute of a rule referring to a rule that
        // depends on it (instead of its package)
        invalidVisibilityReferenceHook(node, label);
        continue;
      }

      // Visibility always has null configuration
      outgoingEdges.add(Dependency.withNullConfiguration(label));
    }
  }

  /**
   * Hook for the error case when an invalid visibility reference is found.
   *
   * @param node the node with the visibility attribute
   * @param label the invalid visibility reference
   */
  protected abstract void invalidVisibilityReferenceHook(TargetAndConfiguration node, Label label);

  /**
   * Hook for the error case when an invalid package group reference is found.
   *
   * @param node the package group node with the includes attribute
   * @param label the invalid reference
   */
  protected abstract void invalidPackageGroupReferenceHook(TargetAndConfiguration node,
      Label label);

  /**
   * Hook for the error case where a dependency is missing.
   *
   * @param from the target referencing the missing target
   * @param to the missing target
   * @param e the exception that was thrown, e.g., by {@link #getTarget}
   */
  protected abstract void missingEdgeHook(Target from, Label to, NoSuchThingException e);

  /**
   * Returns the target by the given label.
   *
   * <p>Returns null if the target is not ready to be returned at this moment. If getTarget returns
   * null once or more during a {@link #dependentNodeMap} call, the results of that call will be
   * incomplete. For use within Skyframe, where several iterations may be needed to discover
   * all dependencies.
   */
  @Nullable
  protected abstract Target getTarget(Target from, Label label, NestedSetBuilder<Label> rootCauses);
}