src/main/java/com/google/devtools/build/lib/skyframe/AspectFunction.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.skyframe;

import static com.google.common.collect.ImmutableList.toImmutableList;

import com.google.common.base.Preconditions;
import com.google.common.base.Predicates;
import com.google.common.collect.ImmutableList;
import com.google.common.collect.ImmutableMap;
import com.google.common.collect.ImmutableSet;
import com.google.common.collect.Lists;
import com.google.devtools.build.lib.actions.MutableActionGraph.ActionConflictException;
import com.google.devtools.build.lib.analysis.AliasProvider;
import com.google.devtools.build.lib.analysis.AspectResolver;
import com.google.devtools.build.lib.analysis.AspectValue;
import com.google.devtools.build.lib.analysis.CachingAnalysisEnvironment;
import com.google.devtools.build.lib.analysis.CachingAnalysisEnvironment.MissingDepException;
import com.google.devtools.build.lib.analysis.ConfiguredAspect;
import com.google.devtools.build.lib.analysis.ConfiguredAspectFactory;
import com.google.devtools.build.lib.analysis.ConfiguredRuleClassProvider;
import com.google.devtools.build.lib.analysis.ConfiguredTarget;
import com.google.devtools.build.lib.analysis.ConfiguredTargetValue;
import com.google.devtools.build.lib.analysis.Dependency;
import com.google.devtools.build.lib.analysis.DependencyKey;
import com.google.devtools.build.lib.analysis.DependencyKind;
import com.google.devtools.build.lib.analysis.DuplicateException;
import com.google.devtools.build.lib.analysis.ExecGroupCollection;
import com.google.devtools.build.lib.analysis.ExecGroupCollection.InvalidExecGroupException;
import com.google.devtools.build.lib.analysis.IncompatiblePlatformProvider;
import com.google.devtools.build.lib.analysis.InconsistentAspectOrderException;
import com.google.devtools.build.lib.analysis.ResolvedToolchainContext;
import com.google.devtools.build.lib.analysis.TargetAndConfiguration;
import com.google.devtools.build.lib.analysis.ToolchainCollection;
import com.google.devtools.build.lib.analysis.config.BuildConfigurationValue;
import com.google.devtools.build.lib.analysis.config.ConfigConditions;
import com.google.devtools.build.lib.analysis.config.ConfigurationResolver;
import com.google.devtools.build.lib.analysis.config.DependencyEvaluationException;
import com.google.devtools.build.lib.analysis.config.InvalidConfigurationException;
import com.google.devtools.build.lib.analysis.config.TransitionResolver;
import com.google.devtools.build.lib.analysis.config.transitions.ConfigurationTransition;
import com.google.devtools.build.lib.analysis.config.transitions.NoTransition;
import com.google.devtools.build.lib.analysis.configuredtargets.MergedConfiguredTarget;
import com.google.devtools.build.lib.analysis.starlark.StarlarkTransition.TransitionException;
import com.google.devtools.build.lib.bugreport.BugReport;
import com.google.devtools.build.lib.causes.Cause;
import com.google.devtools.build.lib.causes.LabelCause;
import com.google.devtools.build.lib.cmdline.Label;
import com.google.devtools.build.lib.collect.nestedset.NestedSet;
import com.google.devtools.build.lib.collect.nestedset.NestedSetBuilder;
import com.google.devtools.build.lib.collect.nestedset.Order;
import com.google.devtools.build.lib.events.Event;
import com.google.devtools.build.lib.events.StoredEventHandler;
import com.google.devtools.build.lib.packages.Aspect;
import com.google.devtools.build.lib.packages.Attribute;
import com.google.devtools.build.lib.packages.BuildFileContainsErrorsException;
import com.google.devtools.build.lib.packages.NativeAspectClass;
import com.google.devtools.build.lib.packages.NoSuchPackageException;
import com.google.devtools.build.lib.packages.NoSuchTargetException;
import com.google.devtools.build.lib.packages.NoSuchThingException;
import com.google.devtools.build.lib.packages.OutputFile;
import com.google.devtools.build.lib.packages.Package;
import com.google.devtools.build.lib.packages.Rule;
import com.google.devtools.build.lib.packages.RuleClassProvider;
import com.google.devtools.build.lib.packages.StarlarkAspectClass;
import com.google.devtools.build.lib.packages.StarlarkDefinedAspect;
import com.google.devtools.build.lib.packages.Target;
import com.google.devtools.build.lib.packages.semantics.BuildLanguageOptions;
import com.google.devtools.build.lib.profiler.memory.CurrentRuleTracker;
import com.google.devtools.build.lib.skyframe.AspectKeyCreator.AspectKey;
import com.google.devtools.build.lib.skyframe.BzlLoadFunction.BzlLoadFailedException;
import com.google.devtools.build.lib.skyframe.PrerequisiteProducer.ComputedToolchainContexts;
import com.google.devtools.build.lib.skyframe.SkyframeExecutor.BuildViewProvider;
import com.google.devtools.build.lib.util.OrderedSetMultimap;
import com.google.devtools.build.skyframe.SkyFunction;
import com.google.devtools.build.skyframe.SkyFunction.Environment.SkyKeyComputeState;
import com.google.devtools.build.skyframe.SkyFunctionException;
import com.google.devtools.build.skyframe.SkyKey;
import com.google.devtools.build.skyframe.SkyValue;
import com.google.devtools.build.skyframe.SkyframeLookupResult;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Map;
import java.util.Objects;
import java.util.Set;
import javax.annotation.Nullable;
import net.starlark.java.eval.StarlarkSemantics;

/**
 * The Skyframe function that generates aspects.
 *
 * <p>This class, together with {@link ConfiguredTargetFunction} drives the analysis phase. For more
 * information, see {@link com.google.devtools.build.lib.analysis.RuleConfiguredTargetFactory}.
 *
 * <p>{@link AspectFunction} takes a SkyKey containing an {@link AspectKey} [a tuple of (target
 * label, configurations, aspect class and aspect parameters)], loads an {@link Aspect} from aspect
 * class and aspect parameters, gets a {@link ConfiguredTarget} for label and configurations, and
 * then creates a {@link ConfiguredAspect} for a given {@link AspectKey}.
 *
 * <p>See {@link com.google.devtools.build.lib.packages.AspectClass} documentation for an overview
 * of aspect-related classes
 *
 * @see com.google.devtools.build.lib.analysis.RuleConfiguredTargetFactory
 * @see com.google.devtools.build.lib.packages.AspectClass
 */
final class AspectFunction implements SkyFunction {
  private final BuildViewProvider buildViewProvider;
  private final RuleClassProvider ruleClassProvider;
  /**
   * Indicates whether the set of packages transitively loaded for a given {@link AspectValue} will
   * be needed later (see {@link
   * com.google.devtools.build.lib.analysis.ConfiguredObjectValue#getTransitivePackages}). If not,
   * they are not collected and stored.
   */
  private final boolean storeTransitivePackages;

  AspectFunction(
      BuildViewProvider buildViewProvider,
      RuleClassProvider ruleClassProvider,
      boolean storeTransitivePackages) {
    this.buildViewProvider = buildViewProvider;
    this.ruleClassProvider = ruleClassProvider;
    this.storeTransitivePackages = storeTransitivePackages;
  }

  static class State implements SkyKeyComputeState {
    /** Null if AspectFuncton is not storing this information. */
    @Nullable NestedSetBuilder<Package> transitivePackages;

    NestedSetBuilder<Cause> transitiveRootCauses = NestedSetBuilder.stableOrder();

    @Nullable InitialValues initialValues;

    PrerequisiteProducer.State computeDependenciesState = new PrerequisiteProducer.State();

    State(boolean storeTransitivePackages) {
      this.transitivePackages = storeTransitivePackages ? NestedSetBuilder.stableOrder() : null;
    }
  }

  private static class InitialValues {
    @Nullable private final Aspect aspect;
    @Nullable private final ConfiguredAspectFactory aspectFactory;
    @Nullable private final BuildConfigurationValue configuration;
    private final ConfiguredTarget associatedTarget;
    private final Target target;

    private InitialValues(
        @Nullable Aspect aspect,
        @Nullable ConfiguredAspectFactory aspectFactory,
        @Nullable BuildConfigurationValue configuration,
        ConfiguredTarget associatedTarget,
        Target target) {
      this.aspect = aspect;
      this.aspectFactory = aspectFactory;
      this.configuration = configuration;
      this.associatedTarget = associatedTarget;
      this.target = target;
    }
  }

  @Nullable
  @Override
  public SkyValue compute(SkyKey skyKey, Environment env)
      throws AspectFunctionException, InterruptedException {
    AspectKey key = (AspectKey) skyKey.argument();
    State state = env.getState(() -> new State(storeTransitivePackages));

    if (state.initialValues == null) {
      InitialValues initialValues = getInitialValues(key, env);
      if (initialValues == null) {
        return null;
      }
      state.initialValues = initialValues;
    }
    Aspect aspect = state.initialValues.aspect;
    ConfiguredAspectFactory aspectFactory = state.initialValues.aspectFactory;
    BuildConfigurationValue configuration = state.initialValues.configuration;
    ConfiguredTarget associatedTarget = state.initialValues.associatedTarget;
    Target target = state.initialValues.target;

    // If the target is incompatible, then there's not much to do. The intent here is to create an
    // AspectValue that doesn't trigger any of the associated target's dependencies to be evaluated
    // against this aspect.
    if (associatedTarget.get(IncompatiblePlatformProvider.PROVIDER) != null
        ||
        // Similarly, aspects that propagate into post-NoConfigTransition targets can't access
        // most flags or dependencies and are likely to be unsound. So make aspects propagating to
        // these configurations no-ops.
        (configuration != null && configuration.getOptions().hasNoConfig())) {
      return new AspectValue(
          key,
          aspect,
          target.getLocation(),
          ConfiguredAspect.forNonapplicableTarget(),
          state.transitivePackages == null ? null : state.transitivePackages.build());
    }

    if (AliasProvider.isAlias(associatedTarget)) {
      return createAliasAspect(
          env,
          new TargetAndConfiguration(target, configuration),
          aspect,
          key,
          configuration,
          associatedTarget);
    }
    // If we get here, label should match original label, and therefore the target we looked up
    // above indeed corresponds to associatedTarget.getLabel().
    Preconditions.checkState(
        associatedTarget.getOriginalLabel().equals(associatedTarget.getLabel()),
        "Non-alias %s should have matching label but found %s",
        associatedTarget.getOriginalLabel(),
        associatedTarget.getLabel());

    // If the incompatible flag is set, the top-level aspect should not be applied on top-level
    // targets whose rules do not advertise the aspect's required providers. The aspect should not
    // also propagate to these targets dependencies.
    StarlarkSemantics starlarkSemantics = PrecomputedValue.STARLARK_SEMANTICS.get(env);
    if (starlarkSemantics == null) {
      return null;
    }
    boolean checkRuleAdvertisedProviders =
        starlarkSemantics.getBool(
            BuildLanguageOptions.INCOMPATIBLE_TOP_LEVEL_ASPECTS_REQUIRE_PROVIDERS);
    if (checkRuleAdvertisedProviders) {
      if (target instanceof Rule) {
        if (!aspect
            .getDefinition()
            .getRequiredProviders()
            .isSatisfiedBy(((Rule) target).getRuleClassObject().getAdvertisedProviders())) {
          return new AspectValue(
              key,
              aspect,
              target.getLocation(),
              ConfiguredAspect.forNonapplicableTarget(),
              /*transitivePackages=*/ NestedSetBuilder.emptySet(Order.STABLE_ORDER));
        }
      }
    }

    ImmutableList<Aspect> topologicalAspectPath;
    if (key.getBaseKeys().isEmpty()) {
      topologicalAspectPath = ImmutableList.of(aspect);
    } else {
      LinkedHashSet<AspectKey> orderedKeys = new LinkedHashSet<>();
      collectAspectKeysInTopologicalOrder(key.getBaseKeys(), orderedKeys);
      SkyframeLookupResult aspectValues = env.getValuesAndExceptions(orderedKeys);
      if (env.valuesMissing()) {
        return null;
      }
      ImmutableList.Builder<Aspect> topologicalAspectPathBuilder =
          ImmutableList.builderWithExpectedSize(orderedKeys.size() + 1);
      for (AspectKey aspectKey : orderedKeys) {
        AspectValue aspectValue = (AspectValue) aspectValues.get(aspectKey);
        if (aspectValue == null) {
          BugReport.logUnexpected(
              "aspectValue for: '%s' was missing, this should never happen", aspectKey);
          return null;
        }
        topologicalAspectPathBuilder.add(aspectValue.getAspect());
      }
      topologicalAspectPath = topologicalAspectPathBuilder.add(aspect).build();

      List<ConfiguredAspect> directlyRequiredAspects =
          Lists.transform(
              key.getBaseKeys(), k -> ((AspectValue) aspectValues.get(k)).getConfiguredAspect());
      try {
        associatedTarget = MergedConfiguredTarget.of(associatedTarget, directlyRequiredAspects);
      } catch (DuplicateException e) {
        env.getListener().handle(Event.error(target.getLocation(), e.getMessage()));
        throw new AspectFunctionException(
            new AspectCreationException(e.getMessage(), target.getLabel(), configuration));
      }
    }

    SkyframeDependencyResolver resolver = new SkyframeDependencyResolver(env);

    TargetAndConfiguration originalTargetAndConfiguration =
        new TargetAndConfiguration(target, configuration);
    try {
      ComputedToolchainContexts computedToolchainContexts =
          getUnloadedToolchainContexts(env, key, aspect, configuration);
      if (env.valuesMissing()) {
        return null;
      }

      ToolchainCollection<UnloadedToolchainContext> unloadedToolchainContexts = null;
      ExecGroupCollection.Builder execGroupCollectionBuilder = null;
      if (computedToolchainContexts != null) {
        unloadedToolchainContexts = computedToolchainContexts.toolchainCollection;
        execGroupCollectionBuilder = computedToolchainContexts.execGroupCollectionBuilder;
      }

      // Get the configuration targets that trigger this rule's configurable attributes.
      ConfigConditions configConditions =
          PrerequisiteProducer.computeConfigConditions(
              env,
              originalTargetAndConfiguration,
              state.transitivePackages,
              unloadedToolchainContexts == null
                  ? null
                  : unloadedToolchainContexts.getTargetPlatform(),
              state.transitiveRootCauses);
      if (configConditions == null) {
        // Those targets haven't yet been resolved.
        return null;
      }

      OrderedSetMultimap<DependencyKind, ConfiguredTargetAndData> depValueMap;
      try {
        depValueMap =
            PrerequisiteProducer.computeDependencies(
                state.computeDependenciesState,
                state.transitivePackages,
                state.transitiveRootCauses,
                env,
                resolver,
                originalTargetAndConfiguration,
                topologicalAspectPath,
                configConditions.asProviders(),
                unloadedToolchainContexts == null
                    ? null
                    : unloadedToolchainContexts.asToolchainContexts(),
                ruleClassProvider,
                buildViewProvider.getSkyframeBuildView());
      } catch (ConfiguredValueCreationException e) {
        throw new AspectCreationException(
            e.getMessage(), key.getLabel(), configuration, e.getDetailedExitCode());
      }
      if (depValueMap == null) {
        return null;
      }
      if (!state.transitiveRootCauses.isEmpty()) {
        NestedSet<Cause> causes = state.transitiveRootCauses.build();
        throw new AspectFunctionException(
            new AspectCreationException(
                "Loading failed",
                causes,
                ConfiguredTargetFunction.getPrioritizedDetailedExitCode(causes)));
      }

      // Load the requested toolchains into the ToolchainContext, now that we have dependencies.
      ToolchainCollection<ResolvedToolchainContext> toolchainContexts = null;
      if (unloadedToolchainContexts != null) {
        String targetDescription =
            String.format(
                "aspect %s applied to %s", aspect.getDescriptor().getDescription(), target);
        ToolchainCollection.Builder<ResolvedToolchainContext> contextsBuilder =
            ToolchainCollection.builder();
        for (Map.Entry<String, UnloadedToolchainContext> unloadedContext :
            unloadedToolchainContexts.getContextMap().entrySet()) {
          Set<ConfiguredTargetAndData> toolchainDependencies =
              depValueMap.get(DependencyKind.forExecGroup(unloadedContext.getKey()));
          contextsBuilder.addContext(
              unloadedContext.getKey(),
              ResolvedToolchainContext.load(
                  unloadedContext.getValue(), targetDescription, toolchainDependencies));
        }
        toolchainContexts = contextsBuilder.build();
      }

      return createAspect(
          env,
          key,
          topologicalAspectPath,
          aspect,
          aspectFactory,
          target,
          associatedTarget,
          configuration,
          configConditions,
          toolchainContexts,
          execGroupCollectionBuilder,
          depValueMap,
          state.transitivePackages);
    } catch (DependencyEvaluationException e) {
      // TODO(bazel-team): consolidate all env.getListener().handle() calls in this method, like in
      // ConfiguredTargetFunction. This encourages clear, consistent user messages (ideally without
      // the programmer having to think about it).
      if (!e.depReportedOwnError()) {
        env.getListener().handle(Event.error(e.getLocation(), e.getMessage()));
      }
      if (e.getCause() instanceof ConfiguredValueCreationException) {
        ConfiguredValueCreationException cause = (ConfiguredValueCreationException) e.getCause();
        throw new AspectFunctionException(
            new AspectCreationException(
                cause.getMessage(), cause.getRootCauses(), cause.getDetailedExitCode()));
      } else if (e.getCause() instanceof InconsistentAspectOrderException) {
        InconsistentAspectOrderException cause = (InconsistentAspectOrderException) e.getCause();
        env.getListener().handle(Event.error(cause.getLocation(), cause.getMessage()));
        throw new AspectFunctionException(
            new AspectCreationException(cause.getMessage(), key.getLabel(), configuration));
      } else if (e.getCause() instanceof TransitionException) {
        TransitionException cause = (TransitionException) e.getCause();
        throw new AspectFunctionException(
            new AspectCreationException(cause.getMessage(), key.getLabel(), configuration));
      } else {
        // Cast to InvalidConfigurationException as a consistency check. If you add any
        // DependencyEvaluationException constructors, you may need to change this code, too.
        InvalidConfigurationException cause = (InvalidConfigurationException) e.getCause();
        throw new AspectFunctionException(
            new AspectCreationException(
                cause.getMessage(), key.getLabel(), configuration, cause.getDetailedExitCode()));
      }
    } catch (AspectCreationException e) {
      throw new AspectFunctionException(e);
    } catch (ConfiguredValueCreationException e) {
      throw new AspectFunctionException(e);
    } catch (ToolchainException e) {
      throw new AspectFunctionException(
          new AspectCreationException(
              e.getMessage(), new LabelCause(key.getLabel(), e.getDetailedExitCode())));
    }
  }

  static BzlLoadValue.Key bzlLoadKeyForStarlarkAspect(StarlarkAspectClass starlarkAspectClass) {
    Label extensionLabel = starlarkAspectClass.getExtensionLabel();
    return StarlarkBuiltinsValue.isBuiltinsRepo(extensionLabel.getRepository())
        ? BzlLoadValue.keyForBuiltins(extensionLabel)
        : BzlLoadValue.keyForBuild(extensionLabel);
  }

  @Nullable
  private static InitialValues getInitialValues(AspectKey key, Environment env)
      throws AspectFunctionException, InterruptedException {
    StarlarkAspectClass starlarkAspectClass;
    ConfiguredAspectFactory aspectFactory = null;
    Aspect aspect = null;

    SkyKey aspectPackageKey = PackageValue.key(key.getLabel().getPackageIdentifier());
    SkyKey baseConfiguredTargetKey = key.getBaseConfiguredTargetKey();
    SkyKey basePackageKey =
        PackageValue.key(key.getBaseConfiguredTargetKey().getLabel().getPackageIdentifier());
    SkyKey configurationKey = key.getConfigurationKey();
    BzlLoadValue.Key bzlLoadKey;

    if (key.getAspectClass() instanceof NativeAspectClass) {
      NativeAspectClass nativeAspectClass = (NativeAspectClass) key.getAspectClass();
      starlarkAspectClass = null;
      aspectFactory = (ConfiguredAspectFactory) nativeAspectClass;
      aspect = Aspect.forNative(nativeAspectClass, key.getParameters());
      bzlLoadKey = null;
    } else {
      Preconditions.checkState(
          key.getAspectClass() instanceof StarlarkAspectClass, "Unknown aspect class: %s", key);
      starlarkAspectClass = (StarlarkAspectClass) key.getAspectClass();
      bzlLoadKey = bzlLoadKeyForStarlarkAspect(starlarkAspectClass);
    }

    ImmutableSet.Builder<SkyKey> initialKeys = ImmutableSet.builder();
    initialKeys.add(aspectPackageKey).add(baseConfiguredTargetKey).add(basePackageKey);
    if (configurationKey != null) {
      initialKeys.add(configurationKey);
    }
    if (bzlLoadKey != null) {
      initialKeys.add(bzlLoadKey);
    }
    SkyframeLookupResult initialValues = env.getValuesAndExceptions(initialKeys.build());
    if (env.valuesMissing()) {
      return null;
    }

    if (starlarkAspectClass != null) {
      StarlarkDefinedAspect starlarkAspect;
      try {
        BzlLoadValue bzlLoadvalue;
        try {
          bzlLoadvalue =
              (BzlLoadValue) initialValues.getOrThrow(bzlLoadKey, BzlLoadFailedException.class);
          if (bzlLoadvalue == null) {
            BugReport.logUnexpected(
                "Unexpected exception with %s and AspectKey %s", bzlLoadKey, key);
            return null;
          }
        } catch (BzlLoadFailedException e) {
          throw new AspectCreationException(
              e.getMessage(), starlarkAspectClass.getExtensionLabel(), e.getDetailedExitCode());
        }
        starlarkAspect = loadAspectFromBzl(starlarkAspectClass, bzlLoadvalue);
      } catch (AspectCreationException e) {
        env.getListener().handle(Event.error(e.getMessage()));
        throw new AspectFunctionException(e);
      }
      aspectFactory = new StarlarkAspectFactory(starlarkAspect);
      aspect =
          Aspect.forStarlark(
              starlarkAspect.getAspectClass(),
              starlarkAspect.getDefinition(key.getParameters()),
              key.getParameters());
    }

    // Keep this in sync with the same code in ConfiguredTargetFunction.
    PackageValue aspectPackage = (PackageValue) initialValues.get(aspectPackageKey);
    if (aspectPackage.getPackage().containsErrors()) {
      throw new AspectFunctionException(
          new BuildFileContainsErrorsException(key.getLabel().getPackageIdentifier()));
    }

    ConfiguredTargetValue baseConfiguredTargetValue;
    try {
      baseConfiguredTargetValue =
          (ConfiguredTargetValue)
              initialValues.getOrThrow(
                  baseConfiguredTargetKey, ConfiguredValueCreationException.class);
      if (baseConfiguredTargetValue == null) {
        BugReport.logUnexpected(
            "Unexpected exception with %s and AspectKey %s", baseConfiguredTargetKey, key);
        return null;
      }
    } catch (ConfiguredValueCreationException e) {
      throw new AspectFunctionException(
          new AspectCreationException(e.getMessage(), e.getRootCauses(), e.getDetailedExitCode()));
    }

    ConfiguredTarget associatedTarget = baseConfiguredTargetValue.getConfiguredTarget();
    Preconditions.checkState(
        Objects.equals(key.getConfigurationKey(), associatedTarget.getConfigurationKey()),
        "Aspect not in same configuration as associated target: %s, %s",
        key,
        associatedTarget);

    BuildConfigurationValue configuration =
        configurationKey == null
            ? null
            : (BuildConfigurationValue) initialValues.get(configurationKey);

    PackageValue basePackage = (PackageValue) initialValues.get(basePackageKey);
    Target target;
    try {
      target = basePackage.getPackage().getTarget(associatedTarget.getOriginalLabel().getName());
    } catch (NoSuchTargetException e) {
      throw new IllegalStateException("Name already verified", e);
    }

    return new InitialValues(aspect, aspectFactory, configuration, associatedTarget, target);
  }

  /**
   * Loads a Starlark-defined aspect from an extension file.
   *
   * @throws AspectCreationException if the value loaded is not a {@link StarlarkDefinedAspect}
   */
  static StarlarkDefinedAspect loadAspectFromBzl(
      StarlarkAspectClass starlarkAspectClass, BzlLoadValue bzlLoadValue)
      throws AspectCreationException {
    Label extensionLabel = starlarkAspectClass.getExtensionLabel();
    String starlarkValueName = starlarkAspectClass.getExportedName();
    Object starlarkValue = bzlLoadValue.getModule().getGlobal(starlarkValueName);
    if (!(starlarkValue instanceof StarlarkDefinedAspect)) {
      throw new AspectCreationException(
          String.format(
              starlarkValue == null ? "%s is not exported from %s" : "%s from %s is not an aspect",
              starlarkValueName,
              extensionLabel),
          extensionLabel);
    }
    return (StarlarkDefinedAspect) starlarkValue;
  }

  @Nullable
  private static ComputedToolchainContexts getUnloadedToolchainContexts(
      Environment env,
      AspectKey key,
      Aspect aspect,
      @Nullable BuildConfigurationValue configuration)
      throws InterruptedException, AspectCreationException {
    if (configuration == null) {
      // Configuration can be null in the case of aspects applied to input files. In this case,
      // there are no chances of toolchains being used, so skip it.
      return null;
    }

    ImmutableMap<String, Attribute> aspectAttributes = aspect.getDefinition().getAttributes();

    boolean useAutoExecGroups;
    if (aspectAttributes.containsKey("$use_auto_exec_groups")) {
      useAutoExecGroups =
          (boolean) aspectAttributes.get("$use_auto_exec_groups").getDefaultValueUnchecked();
    } else {
      useAutoExecGroups = configuration.useAutoExecGroups();
    }

    // Determine what toolchains are needed by this target.
    try {
      return PrerequisiteProducer.computeUnloadedToolchainContexts(
          env,
          key.getLabel(),
          !configuration.getOptions().hasNoConfig(),
          Predicates.alwaysFalse(),
          useAutoExecGroups,
          configuration.getKey(),
          aspect.getDefinition().getToolchainTypes(),
          aspect.getDefinition().execCompatibleWith(),
          aspect.getDefinition().execGroups(),
          null);
    } catch (ToolchainException e) {
      // TODO(katre): better error handling
      throw new AspectCreationException(
          e.getMessage(), new LabelCause(key.getLabel(), e.getDetailedExitCode()));
    }
  }

  /**
   * Collects {@link AspectKey} dependencies by performing a postorder traversal over {@link
   * AspectKey#getBaseKeys}.
   *
   * <p>The resulting set of {@code orderedKeys} is topologically ordered: each aspect key appears
   * after all of its dependencies.
   */
  private static void collectAspectKeysInTopologicalOrder(
      List<AspectKey> baseKeys, LinkedHashSet<AspectKey> orderedKeys) {
    for (AspectKey key : baseKeys) {
      if (!orderedKeys.contains(key)) {
        collectAspectKeysInTopologicalOrder(key.getBaseKeys(), orderedKeys);
        orderedKeys.add(key);
      }
    }
  }

  /**
   * Computes the given aspectKey of an alias-like target, by depending on the corresponding key of
   * the next target in the alias chain (if there are more), or the "real" configured target.
   */
  @Nullable
  private AspectValue createAliasAspect(
      Environment env,
      TargetAndConfiguration originalTarget,
      Aspect aspect,
      AspectKey originalKey,
      BuildConfigurationValue aspectConfiguration,
      ConfiguredTarget configuredTarget)
      throws AspectFunctionException, InterruptedException {
    ImmutableList<Label> aliasChain =
        configuredTarget.getProvider(AliasProvider.class).getAliasChain();
    // Find the next alias in the chain: either the next alias (if there are two) or the name of
    // the real configured target.
    Label aliasedLabel = aliasChain.size() > 1 ? aliasChain.get(1) : configuredTarget.getLabel();

    NestedSetBuilder<Package> transitivePackages =
        storeTransitivePackages ? NestedSetBuilder.stableOrder() : null;
    NestedSetBuilder<Cause> transitiveRootCauses = NestedSetBuilder.stableOrder();

    // Compute the Dependency from originalTarget to aliasedLabel
    Dependency dep;
    try {
      ComputedToolchainContexts computedToolchainContexts =
          getUnloadedToolchainContexts(env, originalKey, aspect, originalTarget.getConfiguration());
      if (env.valuesMissing()) {
        return null;
      }

      ToolchainCollection<UnloadedToolchainContext> unloadedToolchainContexts =
          computedToolchainContexts.toolchainCollection;

      // See comment in compute() above for why we pair target with aspectConfiguration here
      TargetAndConfiguration originalTargetAndAspectConfiguration =
          new TargetAndConfiguration(originalTarget.getTarget(), aspectConfiguration);

      // Get the configuration targets that trigger this rule's configurable attributes.
      ConfigConditions configConditions =
          PrerequisiteProducer.computeConfigConditions(
              env,
              originalTargetAndAspectConfiguration,
              transitivePackages,
              unloadedToolchainContexts == null
                  ? null
                  : unloadedToolchainContexts.getTargetPlatform(),
              transitiveRootCauses);
      if (configConditions == null) {
        // Those targets haven't yet been resolved.
        return null;
      }

      Target aliasedTarget = getTargetFromLabel(env, aliasedLabel);
      if (aliasedTarget == null) {
        return null;
      }
      ConfigurationTransition transition =
          TransitionResolver.evaluateTransition(
              aspectConfiguration,
              NoTransition.INSTANCE,
              aliasedTarget,
              ((ConfiguredRuleClassProvider) ruleClassProvider).getTrimmingTransitionFactory());

      // Use ConfigurationResolver to apply any configuration transitions on the alias edge.
      // This is a shortened/simplified variant of ConfiguredTargetFunction.computeDependencies
      // for just the one special attribute we care about here.
      DependencyKey depKey =
          DependencyKey.builder().setLabel(aliasedLabel).setTransition(transition).build();
      DependencyKind depKind =
          DependencyKind.AttributeDependencyKind.forRule(
              getAttributeContainingAlias(originalTarget.getTarget()));
      ConfigurationResolver resolver =
          new ConfigurationResolver(
              env,
              originalTargetAndAspectConfiguration,
              configConditions.asProviders(),
              buildViewProvider.getSkyframeBuildView().getStarlarkTransitionCache());
      ImmutableList<Dependency> deps =
          resolver.resolveConfiguration(depKind, depKey, env.getListener());
      if (deps == null) {
        return null;
      }
      // Actual should resolve to exactly one dependency
      Preconditions.checkState(
          deps.size() == 1, "Unexpected split in alias %s: %s", originalTarget.getLabel(), deps);
      dep = deps.get(0);
    } catch (NoSuchPackageException | NoSuchTargetException e) {
      throw new AspectFunctionException(e);
    } catch (ConfiguredValueCreationException e) {
      throw new AspectFunctionException(e);
    } catch (AspectCreationException e) {
      throw new AspectFunctionException(e);
    }

    if (!transitiveRootCauses.isEmpty()) {
      NestedSet<Cause> causes = transitiveRootCauses.build();
      throw new AspectFunctionException(
          new AspectCreationException(
              "Loading failed",
              causes,
              ConfiguredTargetFunction.getPrioritizedDetailedExitCode(causes)));
    }

    // Now that we have a Dependency, we can compute the aliased key and depend on it
    AspectKey actualKey = buildAliasAspectKey(originalKey, aliasedLabel, dep);
    return createAliasAspect(
        env, originalTarget.getTarget(), originalKey, aspect, actualKey, transitivePackages);
  }

  @Nullable
  private static AspectValue createAliasAspect(
      Environment env,
      Target originalTarget,
      AspectKey originalKey,
      Aspect aspect,
      AspectKey depKey,
      @Nullable NestedSetBuilder<Package> transitivePackages)
      throws InterruptedException {
    // Compute the AspectValue of the target the alias refers to (which can itself be either an
    // alias or a real target)
    AspectValue real = (AspectValue) env.getValue(depKey);
    if (env.valuesMissing()) {
      return null;
    }

    NestedSet<Package> finalTransitivePackages = null;
    if (transitivePackages != null) {
      finalTransitivePackages =
          transitivePackages
              .addTransitive(Preconditions.checkNotNull(real.getTransitivePackages()))
              .add(originalTarget.getPackage())
              .build();
    }
    return new AspectValue(
        originalKey,
        aspect,
        originalTarget.getLocation(),
        ConfiguredAspect.forAlias(real.getConfiguredAspect()),
        finalTransitivePackages);
  }

  @Nullable
  private static Target getTargetFromLabel(Environment env, Label aliasLabel)
      throws InterruptedException, NoSuchPackageException, NoSuchTargetException {
    SkyValue val =
        env.getValueOrThrow(
            PackageValue.key(aliasLabel.getPackageIdentifier()), NoSuchPackageException.class);
    if (val == null) {
      return null;
    }

    Package pkg = ((PackageValue) val).getPackage();
    return pkg.getTarget(aliasLabel.getName());
  }

  private static AspectKey buildAliasAspectKey(
      AspectKey originalKey, Label aliasLabel, Dependency dep) {
    ImmutableList<AspectKey> aliasedBaseKeys =
        originalKey.getBaseKeys().stream()
            .map(baseKey -> buildAliasAspectKey(baseKey, aliasLabel, dep))
            .collect(toImmutableList());
    return AspectKeyCreator.createAspectKey(
        originalKey.getAspectDescriptor(),
        aliasedBaseKeys,
        ConfiguredTargetKey.builder()
            .setLabel(aliasLabel)
            .setConfiguration(dep.getConfiguration())
            .build());
  }

  /**
   * Given an alias-like target, returns the attribute containing the "actual", by looking for
   * attribute names used in known alias rules (Alias, Bind, LateBoundAlias, XcodeConfigAlias).
   *
   * <p>Alias and Bind rules use "actual", which will be by far the most common match here. It'll
   * likely be rare that aspects need to traverse across other alias-like rules.
   */
  // TODO(lberki,kmb): try to avoid this, maybe by recording the attribute name in AliasProvider
  private static Attribute getAttributeContainingAlias(Target originalTarget) {
    Attribute aliasAttr = null;
    for (Attribute attr : originalTarget.getAssociatedRule().getAttributes()) {
      switch (attr.getName()) {
        case "actual": // alias and bind rules
        case ":alias": // LateBoundAlias-derived rules
        case ":xcode_config": // xcode_config_alias rule
          Preconditions.checkState(
              aliasAttr == null,
              "Found multiple candidate attributes %s and %s in %s",
              aliasAttr,
              attr,
              originalTarget);
          aliasAttr = attr;
          break;
        default:
          break;
      }
    }
    Preconditions.checkState(
        aliasAttr != null, "Attribute containing alias not found in %s", originalTarget);
    return aliasAttr;
  }

  @Nullable
  private AspectValue createAspect(
      Environment env,
      AspectKey key,
      ImmutableList<Aspect> topologicalAspectPath,
      Aspect aspect,
      ConfiguredAspectFactory aspectFactory,
      Target associatedTarget,
      ConfiguredTarget associatedConfiguredTarget,
      BuildConfigurationValue configuration,
      ConfigConditions configConditions,
      @Nullable ToolchainCollection<ResolvedToolchainContext> toolchainContexts,
      @Nullable ExecGroupCollection.Builder execGroupCollectionBuilder,
      OrderedSetMultimap<DependencyKind, ConfiguredTargetAndData> directDeps,
      @Nullable NestedSetBuilder<Package> transitivePackages)
      throws AspectFunctionException, InterruptedException {
    // Should be successfully evaluated and cached from the loading phase.
    StarlarkBuiltinsValue starlarkBuiltinsValue =
        (StarlarkBuiltinsValue) env.getValue(StarlarkBuiltinsValue.key());
    if (env.valuesMissing()) {
      return null;
    }

    SkyframeBuildView view = buildViewProvider.getSkyframeBuildView();

    StoredEventHandler events = new StoredEventHandler();
    CachingAnalysisEnvironment analysisEnvironment =
        view.createAnalysisEnvironment(key, events, env, configuration, starlarkBuiltinsValue);

    ConfiguredAspect configuredAspect;
    if (aspect.getDefinition().applyToGeneratingRules() && associatedTarget instanceof OutputFile) {
      OutputFile outputFile = (OutputFile) associatedTarget;
      Label label = outputFile.getGeneratingRule().getLabel();
      return createAliasAspect(
          env, associatedTarget, key, aspect, key.withLabel(label), transitivePackages);
    } else if (AspectResolver.aspectMatchesConfiguredTarget(
        associatedConfiguredTarget, associatedTarget instanceof Rule, aspect)) {
      try {
        CurrentRuleTracker.beginConfiguredAspect(aspect.getAspectClass());
        configuredAspect =
            view.getConfiguredTargetFactory()
                .createAspect(
                    analysisEnvironment,
                    associatedTarget,
                    associatedConfiguredTarget,
                    topologicalAspectPath,
                    aspectFactory,
                    aspect,
                    directDeps,
                    configConditions,
                    toolchainContexts,
                    execGroupCollectionBuilder,
                    configuration,
                    transitivePackages == null ? null : transitivePackages.build(),
                    key);
      } catch (MissingDepException e) {
        Preconditions.checkState(env.valuesMissing());
        return null;
      } catch (ActionConflictException e) {
        throw new AspectFunctionException(e);
      } catch (InvalidExecGroupException e) {
        throw new AspectFunctionException(e);
      } finally {
        CurrentRuleTracker.endConfiguredAspect();
      }
    } else {
      configuredAspect = ConfiguredAspect.forNonapplicableTarget();
    }

    events.replayOn(env.getListener());
    if (events.hasErrors()) {
      analysisEnvironment.disable(associatedTarget);
      String msg = "Analysis of target '" + associatedTarget.getLabel() + "' failed";
      throw new AspectFunctionException(
          new AspectCreationException(msg, key.getLabel(), configuration));
    }
    Preconditions.checkState(!analysisEnvironment.hasErrors(),
        "Analysis environment hasError() but no errors reported");

    if (env.valuesMissing()) {
      return null;
    }

    analysisEnvironment.disable(associatedTarget);
    Preconditions.checkNotNull(configuredAspect);

    return new AspectValue(
        key,
        aspect,
        associatedTarget.getLocation(),
        configuredAspect,
        transitivePackages == null ? null : transitivePackages.build());
  }

  @Override
  public String extractTag(SkyKey skyKey) {
    AspectKey aspectKey = (AspectKey) skyKey.argument();
    return Label.print(aspectKey.getLabel());
  }

  /** Used to indicate errors during the computation of an {@link AspectValue}. */
  public static final class AspectFunctionException extends SkyFunctionException {
    public AspectFunctionException(NoSuchThingException e) {
      super(e, Transience.PERSISTENT);
    }

    public AspectFunctionException(AspectCreationException e) {
      super(e, Transience.PERSISTENT);
    }

    public AspectFunctionException(ConfiguredValueCreationException e) {
      super(e, Transience.PERSISTENT);
    }

    public AspectFunctionException(InvalidExecGroupException e) {
      super(e, Transience.PERSISTENT);
    }

    public AspectFunctionException(ActionConflictException cause) {
      super(cause, Transience.PERSISTENT);
    }
  }
}