// 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 com.google.common.base.Preconditions;
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.Maps;
import com.google.common.collect.Streams;
import com.google.devtools.build.lib.actions.Actions;
import com.google.devtools.build.lib.actions.Actions.GeneratingActions;
import com.google.devtools.build.lib.actions.MutableActionGraph.ActionConflictException;
import com.google.devtools.build.lib.analysis.AspectResolver;
import com.google.devtools.build.lib.analysis.CachingAnalysisEnvironment;
import com.google.devtools.build.lib.analysis.ConfiguredAspect;
import com.google.devtools.build.lib.analysis.ConfiguredRuleClassProvider;
import com.google.devtools.build.lib.analysis.ConfiguredTarget;
import com.google.devtools.build.lib.analysis.Dependency;
import com.google.devtools.build.lib.analysis.DependencyResolver;
import com.google.devtools.build.lib.analysis.DependencyResolver.AttributeDependencyKind;
import com.google.devtools.build.lib.analysis.DependencyResolver.DependencyKind;
import com.google.devtools.build.lib.analysis.DependencyResolver.InconsistentAspectOrderException;
import com.google.devtools.build.lib.analysis.PlatformSemantics;
import com.google.devtools.build.lib.analysis.TargetAndConfiguration;
import com.google.devtools.build.lib.analysis.ToolchainContext;
import com.google.devtools.build.lib.analysis.ToolchainResolver;
import com.google.devtools.build.lib.analysis.ToolchainResolver.UnloadedToolchainContext;
import com.google.devtools.build.lib.analysis.config.BuildConfiguration;
import com.google.devtools.build.lib.analysis.config.BuildOptions;
import com.google.devtools.build.lib.analysis.config.ConfigMatchingProvider;
import com.google.devtools.build.lib.analysis.config.ConfigurationResolver;
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.configuredtargets.MergedConfiguredTarget.DuplicateException;
import com.google.devtools.build.lib.analysis.configuredtargets.RuleConfiguredTarget;
import com.google.devtools.build.lib.analysis.skylark.StarlarkTransition.TransitionException;
import com.google.devtools.build.lib.buildeventstream.BuildEventId;
import com.google.devtools.build.lib.buildeventstream.BuildEventStreamProtos.BuildEventId.ConfigurationId;
import com.google.devtools.build.lib.causes.AnalysisFailedCause;
import com.google.devtools.build.lib.causes.Cause;
import com.google.devtools.build.lib.causes.LoadingFailedCause;
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.EventKind;
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.BuildType;
import com.google.devtools.build.lib.packages.NoSuchTargetException;
import com.google.devtools.build.lib.packages.NoSuchThingException;
import com.google.devtools.build.lib.packages.NonconfigurableAttributeMapper;
import com.google.devtools.build.lib.packages.Package;
import com.google.devtools.build.lib.packages.RawAttributeMapper;
import com.google.devtools.build.lib.packages.Rule;
import com.google.devtools.build.lib.packages.RuleClass.ExecutionPlatformConstraintsAllowed;
import com.google.devtools.build.lib.packages.RuleClassProvider;
import com.google.devtools.build.lib.packages.Target;
import com.google.devtools.build.lib.packages.TargetUtils;
import com.google.devtools.build.lib.skyframe.AspectFunction.AspectCreationException;
import com.google.devtools.build.lib.skyframe.SkyframeExecutor.BuildViewProvider;
import com.google.devtools.build.lib.skyframe.serialization.autocodec.AutoCodec;
import com.google.devtools.build.lib.syntax.EvalException;
import com.google.devtools.build.lib.util.OrderedSetMultimap;
import com.google.devtools.build.skyframe.SkyFunction;
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.ValueOrException;
import java.math.BigInteger;
import java.util.ArrayList;
import java.util.Collection;
import java.util.HashSet;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;
import java.util.Objects;
import java.util.Set;
import java.util.concurrent.Semaphore;
import java.util.function.Supplier;
import java.util.stream.Collectors;
import javax.annotation.Nullable;

/**
 * SkyFunction for {@link ConfiguredTargetValue}s.
 *
 * <p>This class, together with {@link AspectFunction} drives the analysis phase. For more
 * information, see {@link com.google.devtools.build.lib.analysis.RuleConfiguredTargetFactory}.
 *
 * @see com.google.devtools.build.lib.analysis.RuleConfiguredTargetFactory
 */
public final class ConfiguredTargetFunction implements SkyFunction {
  // This construction is a bit funky, but guarantees that the Object reference here is globally
  // unique.
  static final ImmutableMap<Label, ConfigMatchingProvider> NO_CONFIG_CONDITIONS =
      ImmutableMap.<Label, ConfigMatchingProvider>of();

  /**
   * Exception class that signals an error during the evaluation of a dependency.
   */
  public static class DependencyEvaluationException extends Exception {
    public DependencyEvaluationException(InvalidConfigurationException cause) {
      super(cause);
    }

    public DependencyEvaluationException(ConfiguredValueCreationException cause) {
      super(cause);
    }

    public DependencyEvaluationException(InconsistentAspectOrderException cause) {
      super(cause);
    }

    public DependencyEvaluationException(TransitionException cause) {
      super(cause);
    }

    @Override
    public synchronized Exception getCause() {
      return (Exception) super.getCause();
    }
  }

  private final BuildViewProvider buildViewProvider;
  private final RuleClassProvider ruleClassProvider;
  private final Semaphore cpuBoundSemaphore;
  private final BuildOptions defaultBuildOptions;
  @Nullable private final ConfiguredTargetProgressReceiver configuredTargetProgress;
  private final Supplier<BigInteger> nonceVersion;

  /**
   * Indicates whether the set of packages transitively loaded for a given {@link
   * ConfiguredTargetValue} will be needed for package root resolution later in the build. If not,
   * they are not collected and stored.
   */
  private final boolean storeTransitivePackagesForPackageRootResolution;

  private final boolean shouldUnblockCpuWorkWhenFetchingDeps;

  ConfiguredTargetFunction(
      BuildViewProvider buildViewProvider,
      RuleClassProvider ruleClassProvider,
      Semaphore cpuBoundSemaphore,
      boolean storeTransitivePackagesForPackageRootResolution,
      boolean shouldUnblockCpuWorkWhenFetchingDeps,
      BuildOptions defaultBuildOptions,
      @Nullable ConfiguredTargetProgressReceiver configuredTargetProgress,
      Supplier<BigInteger> nonceVersion) {
    this.buildViewProvider = buildViewProvider;
    this.ruleClassProvider = ruleClassProvider;
    this.cpuBoundSemaphore = cpuBoundSemaphore;
    this.storeTransitivePackagesForPackageRootResolution =
        storeTransitivePackagesForPackageRootResolution;
    this.shouldUnblockCpuWorkWhenFetchingDeps = shouldUnblockCpuWorkWhenFetchingDeps;
    this.defaultBuildOptions = defaultBuildOptions;
    this.configuredTargetProgress = configuredTargetProgress;
    this.nonceVersion = nonceVersion;
  }

  @Override
  public SkyValue compute(SkyKey key, Environment env) throws ConfiguredTargetFunctionException,
      InterruptedException {
    if (shouldUnblockCpuWorkWhenFetchingDeps) {
      env =
          new StateInformingSkyFunctionEnvironment(
              env,
              /*preFetch=*/ cpuBoundSemaphore::release,
              /*postFetch=*/ cpuBoundSemaphore::acquire);
    }
    SkyframeBuildView view = buildViewProvider.getSkyframeBuildView();
    NestedSetBuilder<Package> transitivePackagesForPackageRootResolution =
        storeTransitivePackagesForPackageRootResolution ? NestedSetBuilder.stableOrder() : null;
    NestedSetBuilder<Cause> transitiveRootCauses = NestedSetBuilder.stableOrder();

    ConfiguredTargetKey configuredTargetKey = (ConfiguredTargetKey) key.argument();
    Label label = configuredTargetKey.getLabel();
    BuildConfiguration configuration = null;
    ImmutableSet<SkyKey> packageAndMaybeConfiguration;
    SkyKey packageKey = PackageValue.key(label.getPackageIdentifier());
    SkyKey configurationKeyMaybe = configuredTargetKey.getConfigurationKey();
    if (configurationKeyMaybe == null) {
      packageAndMaybeConfiguration = ImmutableSet.of(packageKey);
    } else {
      packageAndMaybeConfiguration = ImmutableSet.of(packageKey, configurationKeyMaybe);
    }
    Map<SkyKey, SkyValue> packageAndMaybeConfigurationValues =
        env.getValues(packageAndMaybeConfiguration);
    if (env.valuesMissing()) {
      return null;
    }
    PackageValue packageValue = (PackageValue) packageAndMaybeConfigurationValues.get(packageKey);
    if (configurationKeyMaybe != null) {
      configuration =
          ((BuildConfigurationValue) packageAndMaybeConfigurationValues.get(configurationKeyMaybe))
              .getConfiguration();
    }

    // TODO(ulfjack): This tries to match the logic in TransitiveTargetFunction /
    // TargetMarkerFunction. Maybe we can merge the two?
    Package pkg = packageValue.getPackage();
    Target target;
    try {
      target = pkg.getTarget(label.getName());
    } catch (NoSuchTargetException e) {
      throw new ConfiguredTargetFunctionException(
          new ConfiguredValueCreationException(e.getMessage(), label, configuration));
    }
    if (pkg.containsErrors()) {
      transitiveRootCauses.add(
          new LoadingFailedCause(label, new NoSuchTargetException(target).getMessage()));
    }
    if (transitivePackagesForPackageRootResolution != null) {
      transitivePackagesForPackageRootResolution.add(pkg);
    }
    // TODO(bazel-team): This is problematic - we create the right key, but then end up with a value
    // that doesn't match; we can even have the same value multiple times. However, I think it's
    // only triggered in tests (i.e., in normal operation, the configuration passed in is already
    // null).
    if (!target.isConfigurable()) {
      configuration = null;
    }

    // This line is only needed for accurate error messaging. Say this target has a circular
    // dependency with one of its deps. With this line, loading this target fails so Bazel
    // associates the corresponding error with this target, as expected. Without this line,
    // the first TransitiveTargetValue call happens on its dep (in trimConfigurations), so Bazel
    // associates the error with the dep, which is misleading.
    if (configuration != null
        && configuration.trimConfigurations()
        && env.getValue(TransitiveTargetKey.of(label)) == null) {
      return null;
    }

    TargetAndConfiguration ctgValue = new TargetAndConfiguration(target, configuration);

    SkyframeDependencyResolver resolver = view.createDependencyResolver(env);

    UnloadedToolchainContext unloadedToolchainContext = null;

    // TODO(janakr): this acquire() call may tie up this thread indefinitely, reducing the
    // parallelism of Skyframe. This is a strict improvement over the prior state of the code, in
    // which we ran with #processors threads, but ideally we would call #tryAcquire here, and if we
    // failed, would exit this SkyFunction and restart it when permits were available.
    cpuBoundSemaphore.acquire();
    try {
      // Get the configuration targets that trigger this rule's configurable attributes.
      ImmutableMap<Label, ConfigMatchingProvider> configConditions =
          getConfigConditions(
              ctgValue.getTarget(),
              env,
              resolver,
              ctgValue,
              transitivePackagesForPackageRootResolution,
              transitiveRootCauses);
      if (env.valuesMissing()) {
        return null;
      }
      // TODO(ulfjack): ConfiguredAttributeMapper (indirectly used from computeDependencies) isn't
      // safe to use if there are missing config conditions, so we stop here, but only if there are
      // config conditions - though note that we can't check if configConditions is non-empty - it
      // may be empty for other reasons. It would be better to continue here so that we can collect
      // more root causes during computeDependencies.
      // Note that this doesn't apply to AspectFunction, because aspects can't have configurable
      // attributes.
      if (!transitiveRootCauses.isEmpty()
          && !Objects.equals(configConditions, NO_CONFIG_CONDITIONS)) {
        throw new ConfiguredTargetFunctionException(
            new ConfiguredValueCreationException(
                "Cannot compute config conditions", configuration, transitiveRootCauses.build()));
      }

      // Determine what toolchains are needed by this target.
      if (target instanceof Rule) {
        Rule rule = ((Rule) target);
        if (rule.getRuleClassObject().supportsPlatforms()) {
          ImmutableSet<Label> requiredToolchains =
              rule.getRuleClassObject().getRequiredToolchains();

          // Collect local (target, rule) constraints for filtering out execution platforms.
          ImmutableSet<Label> execConstraintLabels = getExecutionPlatformConstraints(rule);
          unloadedToolchainContext =
              new ToolchainResolver(env, configuredTargetKey.getConfigurationKey())
                  .setTargetDescription(rule.toString())
                  .setRequiredToolchainTypes(requiredToolchains)
                  .setExecConstraintLabels(execConstraintLabels)
                  .resolve();
          if (env.valuesMissing()) {
            return null;
          }
        }
      }

      // Calculate the dependencies of this target.
      OrderedSetMultimap<DependencyKind, ConfiguredTargetAndData> depValueMap =
          computeDependencies(
              env,
              resolver,
              ctgValue,
              ImmutableList.<Aspect>of(),
              configConditions,
              unloadedToolchainContext == null
                  ? ImmutableSet.of()
                  : unloadedToolchainContext.resolvedToolchainLabels(),
              ruleClassProvider,
              view.getHostConfiguration(configuration),
              transitivePackagesForPackageRootResolution,
              transitiveRootCauses,
              defaultBuildOptions);
      if (env.valuesMissing()) {
        return null;
      }
      if (!transitiveRootCauses.isEmpty()) {
        throw new ConfiguredTargetFunctionException(
            new ConfiguredValueCreationException(
                "Analysis failed", configuration, transitiveRootCauses.build()));
      }
      Preconditions.checkNotNull(depValueMap);

      // Load the requested toolchains into the ToolchainContext, now that we have dependencies.
      ToolchainContext toolchainContext = null;
      if (unloadedToolchainContext != null) {
        toolchainContext =
            unloadedToolchainContext.load(depValueMap.get(DependencyResolver.TOOLCHAIN_DEPENDENCY));
      }

      ConfiguredTargetValue ans =
          createConfiguredTarget(
              view,
              env,
              target,
              configuration,
              configuredTargetKey,
              depValueMap,
              configConditions,
              toolchainContext,
              transitivePackagesForPackageRootResolution);
      if (configuredTargetProgress != null) {
        configuredTargetProgress.doneConfigureTarget();
      }
      return ans;
    } catch (DependencyEvaluationException e) {
      if (e.getCause() instanceof ConfiguredValueCreationException) {
        ConfiguredValueCreationException cvce = (ConfiguredValueCreationException) e.getCause();

        // Check if this is caused by an unresolved toolchain, and report it as such.
        if (unloadedToolchainContext != null) {
          UnloadedToolchainContext finalUnloadedToolchainContext = unloadedToolchainContext;
          Set<Label> toolchainDependencyErrors =
              Streams.stream(cvce.getRootCauses())
                  .map(Cause::getLabel)
                  .filter(l -> finalUnloadedToolchainContext.resolvedToolchainLabels().contains(l))
                  .collect(ImmutableSet.toImmutableSet());

          if (!toolchainDependencyErrors.isEmpty()) {
            env.getListener()
                .handle(
                    Event.error(
                        String.format(
                            "XXX - While resolving toolchains for target %s: %s",
                            target.getLabel(), e.getCause().getMessage())));
          }
        }

        throw new ConfiguredTargetFunctionException(cvce);
      } else if (e.getCause() instanceof InconsistentAspectOrderException) {
        InconsistentAspectOrderException cause = (InconsistentAspectOrderException) e.getCause();
        throw new ConfiguredTargetFunctionException(
            new ConfiguredValueCreationException(
                cause.getMessage(), target.getLabel(), configuration));
      } else if (e.getCause() instanceof InvalidConfigurationException) {
        InvalidConfigurationException cause = (InvalidConfigurationException) e.getCause();
        env.getListener().handle(Event.error(cause.getMessage()));
        throw new ConfiguredTargetFunctionException(
            new ConfiguredValueCreationException(
                cause.getMessage(), target.getLabel(), configuration));
      } else {
        // Unknown exception type.
        throw new ConfiguredTargetFunctionException(
            new ConfiguredValueCreationException(e.getMessage(), target.getLabel(), configuration));
      }
    } catch (AspectCreationException e) {
      throw new ConfiguredTargetFunctionException(
          new ConfiguredValueCreationException(
              e.getMessage(),
              configuration,
              e.getCauses()));
    } catch (ToolchainException e) {
      // We need to throw a ConfiguredValueCreationException, so either find one or make one.
      ConfiguredValueCreationException cvce;
      if (e.getCause() instanceof ConfiguredValueCreationException) {
        cvce = (ConfiguredValueCreationException) e.getCause();
      } else {
        cvce =
            new ConfiguredValueCreationException(e.getMessage(), target.getLabel(), configuration);
      }

      env.getListener()
          .handle(
              Event.error(
                  String.format(
                      "While resolving toolchains for target %s: %s",
                      target.getLabel(), e.getMessage())));
      throw new ConfiguredTargetFunctionException(cvce);
    } finally {
      cpuBoundSemaphore.release();
    }
  }

  /**
   * Returns the target-specific execution platform constraints, based on the rule definition and
   * any constraints added by the target.
   */
  private static ImmutableSet<Label> getExecutionPlatformConstraints(Rule rule) {
    NonconfigurableAttributeMapper mapper = NonconfigurableAttributeMapper.of(rule);
    ImmutableSet.Builder<Label> execConstraintLabels = new ImmutableSet.Builder<>();

    execConstraintLabels.addAll(rule.getRuleClassObject().getExecutionPlatformConstraints());

    if (rule.getRuleClassObject().executionPlatformConstraintsAllowed()
        == ExecutionPlatformConstraintsAllowed.PER_TARGET) {
      execConstraintLabels.addAll(
          mapper.get(PlatformSemantics.EXEC_COMPATIBLE_WITH_ATTR, BuildType.LABEL_LIST));
    }

    return execConstraintLabels.build();
  }

  /**
   * Computes the direct dependencies of a node in the configured target graph (a configured target
   * or an aspects).
   *
   * <p>Returns null if Skyframe hasn't evaluated the required dependencies yet. In this case, the
   * caller should also return null to Skyframe.
   *
   * @param env the Skyframe environment
   * @param resolver the dependency resolver
   * @param ctgValue the label and the configuration of the node
   * @param aspects
   * @param configConditions the configuration conditions for evaluating the attributes of the node
   * @param toolchainLabels labels of required toolchain dependencies
   * @param ruleClassProvider rule class provider for determining the right configuration fragments
   *     to apply to deps
   * @param hostConfiguration the host configuration. There's a noticeable performance hit from
   *     instantiating this on demand for every dependency that wants it, so it's best to compute
   *     the host configuration as early as possible and pass this reference to all consumers
   * @param defaultBuildOptions the default build options provided by the server; these are used to
   *     create diffs for {@link BuildConfigurationValue.Key}s to prevent storing the entire
   *     BuildOptions object.
   */
  @Nullable
  static OrderedSetMultimap<DependencyKind, ConfiguredTargetAndData> computeDependencies(
      Environment env,
      SkyframeDependencyResolver resolver,
      TargetAndConfiguration ctgValue,
      Iterable<Aspect> aspects,
      ImmutableMap<Label, ConfigMatchingProvider> configConditions,
      ImmutableSet<Label> toolchainLabels,
      RuleClassProvider ruleClassProvider,
      BuildConfiguration hostConfiguration,
      @Nullable NestedSetBuilder<Package> transitivePackagesForPackageRootResolution,
      NestedSetBuilder<Cause> transitiveRootCauses,
      BuildOptions defaultBuildOptions)
      throws DependencyEvaluationException, ConfiguredTargetFunctionException,
          AspectCreationException, InterruptedException {
    // Create the map from attributes to set of (target, configuration) pairs.
    OrderedSetMultimap<DependencyKind, Dependency> depValueNames;
    try {
      depValueNames =
          resolver.dependentNodeMap(
              ctgValue,
              hostConfiguration,
              aspects,
              configConditions,
              toolchainLabels,
              transitiveRootCauses,
              ((ConfiguredRuleClassProvider) ruleClassProvider).getTrimmingTransitionFactory());
    } catch (EvalException e) {
      // EvalException can only be thrown by computed Skylark attributes in the current rule.
      env.getListener().handle(Event.error(e.getLocation(), e.getMessage()));
      throw new DependencyEvaluationException(
          new ConfiguredValueCreationException(
              e.print(), ctgValue.getLabel(), ctgValue.getConfiguration()));
    } catch (InconsistentAspectOrderException e) {
      env.getListener().handle(Event.error(e.getLocation(), e.getMessage()));
      throw new DependencyEvaluationException(e);
    }

    // Trim each dep's configuration so it only includes the fragments needed by its transitive
    // closure.
    if (ctgValue.getConfiguration() != null) {
      depValueNames =
          ConfigurationResolver.resolveConfigurations(
              env,
              ctgValue,
              depValueNames,
              hostConfiguration,
              ruleClassProvider,
              defaultBuildOptions);
    }

    // Return early in case packages were not loaded yet. In theory, we could start configuring
    // dependent targets in loaded packages. However, that creates an artificial sync boundary
    // between loading all dependent packages (fast) and configuring some dependent targets (can
    // have a long tail).
    if (env.valuesMissing()) {
      return null;
    }

    // Resolve configured target dependencies and handle errors.
    Map<SkyKey, ConfiguredTargetAndData> depValues =
        resolveConfiguredTargetDependencies(
            env,
            ctgValue,
            depValueNames.values(),
            transitivePackagesForPackageRootResolution,
            transitiveRootCauses);
    if (depValues == null) {
      return null;
    }

    // Resolve required aspects.
    OrderedSetMultimap<Dependency, ConfiguredAspect> depAspects =
        AspectResolver.resolveAspectDependencies(
            env, depValues, depValueNames.values(), transitivePackagesForPackageRootResolution);
    if (depAspects == null) {
      return null;
    }

    // Merge the dependent configured targets and aspects into a single map.
    try {
      return AspectResolver.mergeAspects(depValueNames, depValues, depAspects);
    } catch (DuplicateException e) {
      env.getListener().handle(
          Event.error(ctgValue.getTarget().getLocation(), e.getMessage()));

      throw new ConfiguredTargetFunctionException(
          new ConfiguredValueCreationException(
              e.getMessage(), ctgValue.getLabel(), ctgValue.getConfiguration()));
    }
  }

  /**
   * Returns the set of {@link ConfigMatchingProvider}s that key the configurable attributes used by
   * this rule.
   *
   * <p>>If the configured targets supplying those providers aren't yet resolved by the dependency
   * resolver, returns null.
   */
  @Nullable
  static ImmutableMap<Label, ConfigMatchingProvider> getConfigConditions(
      Target target,
      Environment env,
      SkyframeDependencyResolver resolver,
      TargetAndConfiguration ctgValue,
      @Nullable NestedSetBuilder<Package> transitivePackagesForPackageRootResolution,
      NestedSetBuilder<Cause> transitiveRootCauses)
      throws DependencyEvaluationException, InterruptedException {
    if (!(target instanceof Rule)) {
      return NO_CONFIG_CONDITIONS;
    }

    Map<Label, ConfigMatchingProvider> configConditions = new LinkedHashMap<>();

    // Collect the labels of the configured targets we need to resolve.
    OrderedSetMultimap<DependencyKind, Label> configLabelMap = OrderedSetMultimap.create();
    RawAttributeMapper attributeMap = RawAttributeMapper.of(((Rule) target));
    for (Attribute a : ((Rule) target).getAttributes()) {
      for (Label configLabel : attributeMap.getConfigurabilityKeys(a.getName(), a.getType())) {
        if (!BuildType.Selector.isReservedLabel(configLabel)) {
          configLabelMap.put(
              AttributeDependencyKind.forRule(a),
              target.getLabel().resolveRepositoryRelative(configLabel));
        }
      }
    }
    if (configLabelMap.isEmpty()) {
      return NO_CONFIG_CONDITIONS;
    }

    Map<Label, Target> configurabilityTargets =
        resolver.getTargets(configLabelMap, target, transitiveRootCauses);
    if (configurabilityTargets == null) {
      return null;
    }

    // Collect the actual deps, hard-coded to the current configuration (since by definition config
    // conditions evaluate over the current target's configuration).
    ImmutableList.Builder<Dependency> depsBuilder = ImmutableList.builder();
    for (Label configurabilityLabel : configLabelMap.values()) {
      Target configurabilityTarget = configurabilityTargets.get(configurabilityLabel);
      Dependency configurabilityDependency;
      if (TransitionResolver.usesNullConfiguration(configurabilityTarget)) {
        // This is kinda awful: select conditions should be config_setting rules, but we don't know
        // if they are one until after we analyzed them. However, we need to figure out which
        // configuration we analyze them in and input files and package groups are supposed to have
        // the null configuration.
        //
        // We can't check for the Target being a config_setting because configurability targets can
        // also be aliases pointing to config_settig rules.
        configurabilityDependency = Dependency.withNullConfiguration(configurabilityLabel);
      } else {
        configurabilityDependency =
            Dependency.withConfiguration(configurabilityLabel, ctgValue.getConfiguration());
      }
      depsBuilder.add(configurabilityDependency);
    }

    ImmutableList<Dependency> configConditionDeps = depsBuilder.build();

    Map<SkyKey, ConfiguredTargetAndData> configValues =
        resolveConfiguredTargetDependencies(
            env,
            ctgValue,
            configConditionDeps,
            transitivePackagesForPackageRootResolution,
            transitiveRootCauses);
    if (configValues == null) {
      return null;
    }

    // Get the configured targets as ConfigMatchingProvider interfaces.
    for (Dependency entry : configConditionDeps) {
      SkyKey baseKey = ConfiguredTargetValue.key(entry.getLabel(), entry.getConfiguration());
      ConfiguredTarget value = configValues.get(baseKey).getConfiguredTarget();
      // The code above guarantees that value is non-null here and since the rule is a
      // config_setting, provider must also be non-null.
      ConfigMatchingProvider provider = value.getProvider(ConfigMatchingProvider.class);
      if (provider != null) {
        configConditions.put(entry.getLabel(), provider);
      } else {
        // Not a valid provider for configuration conditions.
        String message =
            entry.getLabel() + " is not a valid configuration key for " + target.getLabel();
        env.getListener().handle(Event.error(TargetUtils.getLocationMaybe(target), message));
        throw new DependencyEvaluationException(
            new ConfiguredValueCreationException(
                message, ctgValue.getLabel(), ctgValue.getConfiguration()));
      }
    }

    return ImmutableMap.copyOf(configConditions);
  }

  /**
   * Resolves the targets referenced in depValueNames and returns their {@link
   * ConfiguredTargetAndData} instances.
   *
   * <p>Returns null if not all instances are available yet.
   */
  @Nullable
  private static Map<SkyKey, ConfiguredTargetAndData> resolveConfiguredTargetDependencies(
      Environment env,
      TargetAndConfiguration ctgValue,
      Collection<Dependency> deps,
      @Nullable NestedSetBuilder<Package> transitivePackagesForPackageRootResolution,
      NestedSetBuilder<Cause> transitiveRootCauses)
      throws DependencyEvaluationException, InterruptedException {
    boolean missedValues = env.valuesMissing();
    String failWithMessage = null;
    // Naively we would like to just fetch all requested ConfiguredTargets, together with their
    // Packages. However, some ConfiguredTargets are AliasConfiguredTargets, which means that their
    // associated Targets (and therefore associated Packages) don't correspond to their own Labels.
    // We don't know the associated Package until we fetch the ConfiguredTarget. Therefore, we have
    // to do a potential second pass, in which we fetch all the Packages for AliasConfiguredTargets.
    Iterable<SkyKey> depKeys =
        Iterables.concat(
            Iterables.transform(
                deps,
                input -> ConfiguredTargetValue.key(input.getLabel(), input.getConfiguration())),
            Iterables.transform(
                deps, input -> PackageValue.key(input.getLabel().getPackageIdentifier())));
    Map<SkyKey, ValueOrException<ConfiguredValueCreationException>> depValuesOrExceptions =
            env.getValuesOrThrow(depKeys, ConfiguredValueCreationException.class);
    Map<SkyKey, ConfiguredTargetAndData> result = Maps.newHashMapWithExpectedSize(deps.size());
    Set<SkyKey> aliasPackagesToFetch = new HashSet<>();
    List<Dependency> aliasDepsToRedo = new ArrayList<>();
    Map<SkyKey, SkyValue> aliasPackageValues = null;
    Collection<Dependency> depsToProcess = deps;
    for (int i = 0; i < 2; i++) {
      for (Dependency dep : depsToProcess) {
        SkyKey key = ConfiguredTargetValue.key(dep.getLabel(), dep.getConfiguration());
        try {
          ConfiguredTargetValue depValue =
              (ConfiguredTargetValue) depValuesOrExceptions.get(key).get();

          if (depValue == null) {
            missedValues = true;
          } else {
            ConfiguredTarget depCt = depValue.getConfiguredTarget();
            Label depLabel = depCt.getLabel();
            SkyKey packageKey = PackageValue.key(depLabel.getPackageIdentifier());
            PackageValue pkgValue;
            if (i == 0) {
              ValueOrException<ConfiguredValueCreationException> packageResult =
                  depValuesOrExceptions.get(packageKey);
              if (packageResult == null) {
                aliasPackagesToFetch.add(packageKey);
                aliasDepsToRedo.add(dep);
                continue;
              } else {
                pkgValue =
                    Preconditions.checkNotNull(
                        (PackageValue) packageResult.get(),
                        "Package should have been loaded during dep resolution: %s",
                        dep);
              }
            } else {
              // We were doing AliasConfiguredTarget mop-up.
              pkgValue = (PackageValue) aliasPackageValues.get(packageKey);
              if (pkgValue == null) {
                // This is unexpected: on the second iteration, all packages should be present,
                // since the configured targets that depend on them are present. But since that is
                // not a guarantee Skyframe makes, we tolerate their absence.
                missedValues = true;
                continue;
              }
            }
            try {
              result.put(
                  key,
                  new ConfiguredTargetAndData(
                      depValue.getConfiguredTarget(),
                      pkgValue.getPackage().getTarget(depLabel.getName()),
                      dep.getConfiguration()));
            } catch (NoSuchTargetException e) {
              throw new IllegalStateException("Target already verified for " + dep, e);
            }
            if (transitivePackagesForPackageRootResolution != null) {
              transitivePackagesForPackageRootResolution.addTransitive(
                  depValue.getTransitivePackagesForPackageRootResolution());
            }
          }
        } catch (ConfiguredValueCreationException e) {
          transitiveRootCauses.addTransitive(e.rootCauses);
          failWithMessage = e.getMessage();
        }
      }
      if (aliasDepsToRedo.isEmpty()) {
        break;
      }
      aliasPackageValues = env.getValues(aliasPackagesToFetch);
      depsToProcess = aliasDepsToRedo;
    }
    if (missedValues) {
      return null;
    } else if (failWithMessage != null) {
      throw new DependencyEvaluationException(
          new ConfiguredValueCreationException(
              failWithMessage, ctgValue.getConfiguration(), transitiveRootCauses.build()));
    } else {
      return result;
    }
  }

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

  @Nullable
  private ConfiguredTargetValue createConfiguredTarget(
      SkyframeBuildView view,
      Environment env,
      Target target,
      BuildConfiguration configuration,
      ConfiguredTargetKey configuredTargetKey,
      OrderedSetMultimap<DependencyKind, ConfiguredTargetAndData> depValueMap,
      ImmutableMap<Label, ConfigMatchingProvider> configConditions,
      @Nullable ToolchainContext toolchainContext,
      @Nullable NestedSetBuilder<Package> transitivePackagesForPackageRootResolution)
      throws ConfiguredTargetFunctionException, InterruptedException {
    StoredEventHandler events = new StoredEventHandler();
    CachingAnalysisEnvironment analysisEnvironment =
        view.createAnalysisEnvironment(
            ConfiguredTargetKey.of(target.getLabel(), configuration),
            false,
            events,
            env,
            configuration);
    if (env.valuesMissing()) {
      return null;
    }

    Preconditions.checkNotNull(depValueMap);
    ConfiguredTarget configuredTarget;
    try {
      configuredTarget =
          view.createConfiguredTarget(
              target,
              configuration,
              analysisEnvironment,
              configuredTargetKey,
              depValueMap,
              configConditions,
              toolchainContext);
    } catch (ActionConflictException e) {
      throw new ConfiguredTargetFunctionException(e);
    }

    events.replayOn(env.getListener());
    if (events.hasErrors()) {
      analysisEnvironment.disable(target);
      NestedSet<Cause> rootCauses = NestedSetBuilder.wrap(
          Order.STABLE_ORDER,
          events.getEvents().stream()
              .filter((event) -> event.getKind() == EventKind.ERROR)
              .map((event) ->
                  new AnalysisFailedCause(
                      target.getLabel(),
                      ConfiguredValueCreationException.toId(configuration),
                      event.getMessage()))
              .collect(Collectors.toList()));
      throw new ConfiguredTargetFunctionException(
          new ConfiguredValueCreationException(
              "Analysis of target '" + target.getLabel() + "' failed; build aborted",
              configuration,
              rootCauses));
    }
    Preconditions.checkState(!analysisEnvironment.hasErrors(),
        "Analysis environment hasError() but no errors reported");
    if (env.valuesMissing()) {
      return null;
    }

    analysisEnvironment.disable(target);
    Preconditions.checkNotNull(configuredTarget, target);

    if (configuredTarget instanceof RuleConfiguredTarget) {
      RuleConfiguredTarget ruleConfiguredTarget = (RuleConfiguredTarget) configuredTarget;
      return new RuleConfiguredTargetValue(
          ruleConfiguredTarget,
          transitivePackagesForPackageRootResolution == null
              ? null
              : transitivePackagesForPackageRootResolution.build(),
          nonceVersion.get());
    } else {
      GeneratingActions generatingActions;
      // Check for conflicting actions within this configured target (that indicates a bug in the
      // rule implementation).
      try {
        generatingActions =
            Actions.filterSharedActionsAndThrowActionConflict(
                analysisEnvironment.getActionKeyContext(),
                analysisEnvironment.getRegisteredActions());
      } catch (ActionConflictException e) {
        throw new ConfiguredTargetFunctionException(e);
      }
      return new NonRuleConfiguredTargetValue(
          configuredTarget,
          generatingActions,
          transitivePackagesForPackageRootResolution == null
              ? null
              : transitivePackagesForPackageRootResolution.build(),
          nonceVersion.get());
    }
  }

  /**
   * An exception indicating that there was a problem during the construction of a
   * ConfiguredTargetValue.
   */
  @AutoCodec
  public static final class ConfiguredValueCreationException extends Exception {
    private static ConfigurationId toId(BuildConfiguration config) {
      return config == null ? null : config.getEventId().asStreamProto().getConfiguration();
    }

    @Nullable private final BuildEventId configuration;
    private final NestedSet<Cause> rootCauses;

    @AutoCodec.VisibleForSerialization
    @AutoCodec.Instantiator
    ConfiguredValueCreationException(
        String message,
        @Nullable BuildEventId configuration,
        NestedSet<Cause> rootCauses) {
      super(message);
      this.rootCauses = rootCauses;
      this.configuration = configuration;
    }

    private ConfiguredValueCreationException(
        String message, Label currentTarget, @Nullable BuildConfiguration configuration) {
      this(
          message,
          configuration == null ? null : configuration.getEventId(),
          NestedSetBuilder.<Cause>stableOrder()
              .add(new AnalysisFailedCause(currentTarget, toId(configuration), message))
              .build());
    }

    private ConfiguredValueCreationException(
        String message, @Nullable BuildConfiguration configuration, NestedSet<Cause> rootCauses) {
      this(message, configuration == null ? null : configuration.getEventId(), rootCauses);
    }

    public NestedSet<Cause> getRootCauses() {
      return rootCauses;
    }

    @Nullable public BuildEventId getConfiguration() {
      return configuration;
    }
  }

  /**
   * Used to declare all the exception types that can be wrapped in the exception thrown by
   * {@link ConfiguredTargetFunction#compute}.
   */
  public static final class ConfiguredTargetFunctionException extends SkyFunctionException {
    public ConfiguredTargetFunctionException(NoSuchThingException e) {
      super(e, Transience.PERSISTENT);
    }

    private ConfiguredTargetFunctionException(ConfiguredValueCreationException e) {
      super(e, Transience.PERSISTENT);
    }

    private ConfiguredTargetFunctionException(ActionConflictException e) {
      super(e, Transience.PERSISTENT);
    }
  }
}