// 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.annotations.VisibleForTesting;
import com.google.common.base.Preconditions;
import com.google.common.base.Throwables;
import com.google.common.cache.Cache;
import com.google.common.collect.ImmutableCollection;
import com.google.common.collect.ImmutableList;
import com.google.common.collect.ImmutableMap;
import com.google.common.collect.ImmutableSet;
import com.google.common.collect.Iterables;
import com.google.common.collect.Lists;
import com.google.common.collect.Maps;
import com.google.common.collect.Sets;
import com.google.devtools.build.lib.actions.FileValue;
import com.google.devtools.build.lib.actions.InconsistentFilesystemException;
import com.google.devtools.build.lib.clock.BlazeClock;
import com.google.devtools.build.lib.cmdline.Label;
import com.google.devtools.build.lib.cmdline.LabelConstants;
import com.google.devtools.build.lib.cmdline.LabelSyntaxException;
import com.google.devtools.build.lib.cmdline.PackageIdentifier;
import com.google.devtools.build.lib.cmdline.RepositoryName;
import com.google.devtools.build.lib.collect.nestedset.NestedSet;
import com.google.devtools.build.lib.events.Event;
import com.google.devtools.build.lib.events.ExtendedEventHandler.Postable;
import com.google.devtools.build.lib.events.Location;
import com.google.devtools.build.lib.events.StoredEventHandler;
import com.google.devtools.build.lib.packages.AstParseResult;
import com.google.devtools.build.lib.packages.BuildFileContainsErrorsException;
import com.google.devtools.build.lib.packages.BuildFileNotFoundException;
import com.google.devtools.build.lib.packages.CachingPackageLocator;
import com.google.devtools.build.lib.packages.Globber;
import com.google.devtools.build.lib.packages.InvalidPackageNameException;
import com.google.devtools.build.lib.packages.NoSuchPackageException;
import com.google.devtools.build.lib.packages.Package;
import com.google.devtools.build.lib.packages.PackageFactory;
import com.google.devtools.build.lib.packages.PackageFactory.LegacyGlobber;
import com.google.devtools.build.lib.packages.RuleVisibility;
import com.google.devtools.build.lib.packages.Target;
import com.google.devtools.build.lib.packages.WorkspaceFileValue;
import com.google.devtools.build.lib.profiler.Profiler;
import com.google.devtools.build.lib.profiler.ProfilerTask;
import com.google.devtools.build.lib.profiler.SilentCloseable;
import com.google.devtools.build.lib.skyframe.GlobValue.InvalidGlobPatternException;
import com.google.devtools.build.lib.skyframe.SkylarkImportLookupFunction.SkylarkImportFailedException;
import com.google.devtools.build.lib.syntax.BuildFileAST;
import com.google.devtools.build.lib.syntax.Environment.Extension;
import com.google.devtools.build.lib.syntax.EvalException;
import com.google.devtools.build.lib.syntax.ParserInputSource;
import com.google.devtools.build.lib.syntax.SkylarkImport;
import com.google.devtools.build.lib.syntax.StarlarkSemantics;
import com.google.devtools.build.lib.syntax.Statement;
import com.google.devtools.build.lib.vfs.FileSystemUtils;
import com.google.devtools.build.lib.vfs.Path;
import com.google.devtools.build.lib.vfs.PathFragment;
import com.google.devtools.build.lib.vfs.Root;
import com.google.devtools.build.lib.vfs.RootedPath;
import com.google.devtools.build.lib.vfs.UnixGlob;
import com.google.devtools.build.skyframe.SkyFunction;
import com.google.devtools.build.skyframe.SkyFunctionException;
import com.google.devtools.build.skyframe.SkyFunctionException.Transience;
import com.google.devtools.build.skyframe.SkyKey;
import com.google.devtools.build.skyframe.SkyValue;
import com.google.devtools.build.skyframe.ValueOrException2;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Iterator;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.concurrent.atomic.AtomicInteger;
import javax.annotation.Nullable;

/**
 * A SkyFunction for {@link PackageValue}s.
 */
public class PackageFunction implements SkyFunction {

  private final PackageFactory packageFactory;
  private final CachingPackageLocator packageLocator;
  private final Cache<PackageIdentifier, LoadedPackageCacheEntry> packageFunctionCache;
  private final Cache<PackageIdentifier, AstParseResult> astCache;
  private final AtomicBoolean showLoadingProgress;
  private final AtomicInteger numPackagesLoaded;
  @Nullable private final PackageProgressReceiver packageProgress;
  private final Label preludeLabel;

  // Not final only for testing.
  @Nullable private SkylarkImportLookupFunction skylarkImportLookupFunctionForInlining;

  private final ActionOnIOExceptionReadingBuildFile actionOnIOExceptionReadingBuildFile;

  private final IncrementalityIntent incrementalityIntent;

  public PackageFunction(
      PackageFactory packageFactory,
      CachingPackageLocator pkgLocator,
      AtomicBoolean showLoadingProgress,
      Cache<PackageIdentifier, LoadedPackageCacheEntry> packageFunctionCache,
      Cache<PackageIdentifier, AstParseResult> astCache,
      AtomicInteger numPackagesLoaded,
      @Nullable SkylarkImportLookupFunction skylarkImportLookupFunctionForInlining,
      @Nullable PackageProgressReceiver packageProgress,
      ActionOnIOExceptionReadingBuildFile actionOnIOExceptionReadingBuildFile,
      IncrementalityIntent incrementalityIntent) {
    this.skylarkImportLookupFunctionForInlining = skylarkImportLookupFunctionForInlining;
    // Can be null in tests.
    this.preludeLabel = packageFactory == null
        ? null
        : packageFactory.getRuleClassProvider().getPreludeLabel();
    this.packageFactory = packageFactory;
    this.packageLocator = pkgLocator;
    this.showLoadingProgress = showLoadingProgress;
    this.packageFunctionCache = packageFunctionCache;
    this.astCache = astCache;
    this.numPackagesLoaded = numPackagesLoaded;
    this.packageProgress = packageProgress;
    this.actionOnIOExceptionReadingBuildFile = actionOnIOExceptionReadingBuildFile;
    this.incrementalityIntent = incrementalityIntent;
  }

  @VisibleForTesting
  public PackageFunction(
      PackageFactory packageFactory,
      CachingPackageLocator pkgLocator,
      AtomicBoolean showLoadingProgress,
      Cache<PackageIdentifier, LoadedPackageCacheEntry> packageFunctionCache,
      Cache<PackageIdentifier, AstParseResult> astCache,
      AtomicInteger numPackagesLoaded,
      @Nullable SkylarkImportLookupFunction skylarkImportLookupFunctionForInlining) {
    this(
        packageFactory,
        pkgLocator,
        showLoadingProgress,
        packageFunctionCache,
        astCache,
        numPackagesLoaded,
        skylarkImportLookupFunctionForInlining,
        /*packageProgress=*/ null,
        ActionOnIOExceptionReadingBuildFile.UseOriginalIOException.INSTANCE,
        IncrementalityIntent.INCREMENTAL);
  }

  public void setSkylarkImportLookupFunctionForInliningForTesting(
      SkylarkImportLookupFunction skylarkImportLookupFunctionForInlining) {
    this.skylarkImportLookupFunctionForInlining = skylarkImportLookupFunctionForInlining;
  }

  /**
   * What to do when encountering an {@link IOException} trying to read the contents of a BUILD
   * file.
   *
   * <p>Any choice besides
   * {@link ActionOnIOExceptionReadingBuildFile.UseOriginalIOException#INSTANCE} is potentially
   * incrementally unsound: if the initial {@link IOException} is transient, then Blaze will
   * "incorrectly" not attempt to redo package loading for this BUILD file on incremental builds.
   *
   * <p>The fact that this behavior is configurable and potentially unsound is a concession to
   * certain desired use cases with fancy filesystems.
   */
  public interface ActionOnIOExceptionReadingBuildFile {
    /**
     * Given the {@link IOException} encountered when reading the contents of the given BUILD file,
     * returns the contents that should be used, or {@code null} if the original {@link IOException}
     * should be respected (that is, we should error-out with a package loading error).
     */
    @Nullable
    byte[] maybeGetBuildFileContentsToUse(
        PathFragment buildFilePathFragment, IOException originalExn);

    /**
     * A {@link ActionOnIOExceptionReadingBuildFile} whose {@link #maybeGetBuildFileContentsToUse}
     * has the sensible behavior of always respecting the initial {@link IOException}.
     */
    public static class UseOriginalIOException implements ActionOnIOExceptionReadingBuildFile {
      public static final UseOriginalIOException INSTANCE = new UseOriginalIOException();

      private UseOriginalIOException() {
      }

      @Override
      @Nullable
      public byte[] maybeGetBuildFileContentsToUse(
          PathFragment buildFilePathFragment, IOException originalExn) {
        return null;
      }
    }
  }

  /** An entry in {@link PackageFunction} internal cache. */
  public static class LoadedPackageCacheEntry {
    private final Package.Builder builder;
    private final Set<SkyKey> globDepKeys;
    private final long loadTimeNanos;

    private LoadedPackageCacheEntry(
        Package.Builder builder, Set<SkyKey> globDepKeys, long loadTimeNanos) {
      this.builder = builder;
      this.globDepKeys = globDepKeys;
      this.loadTimeNanos = loadTimeNanos;
    }
  }

  /**
   * A declaration to {@link PackageFunction} about how it will be used, for the sake of making
   * use-case-driven performance optimizations.
   */
  public enum IncrementalityIntent {
    /**
     * {@link PackageFunction} will be used to load packages incrementally (e.g. on both clean
     * builds and incremental builds, perhaps with cached globs). This is Bazel's normal use-case.
     */
    INCREMENTAL,

    /**
     * {@link PackageFunction} will never be used to load packages incrementally.
     *
     * <p>Do not use this unless you know what you are doing; Bazel will be intentionally
     * incrementally incorrect!
     */
    NON_INCREMENTAL
  }

  private static void maybeThrowFilesystemInconsistency(PackageIdentifier packageIdentifier,
      Exception skyframeException, boolean packageWasInError)
          throws InternalInconsistentFilesystemException {
    if (!packageWasInError) {
      throw new InternalInconsistentFilesystemException(packageIdentifier, "Encountered error '"
          + skyframeException.getMessage() + "' but didn't encounter it when doing the same thing "
          + "earlier in the build");
    }
  }

  /**
   * These deps have already been marked (see {@link SkyframeHybridGlobber}) but we need to properly
   * handle some errors that legacy package loading can't handle gracefully.
   */
  private static boolean handleGlobDepsAndPropagateFilesystemExceptions(
      PackageIdentifier packageIdentifier,
      Iterable<SkyKey> depKeys,
      Environment env,
      boolean packageWasInError)
      throws InternalInconsistentFilesystemException, InterruptedException {
    Preconditions.checkState(
        Iterables.all(depKeys, SkyFunctions.isSkyFunction(SkyFunctions.GLOB)), depKeys);
     boolean packageShouldBeInErrorFromGlobDeps = false;
    for (Map.Entry<SkyKey, ValueOrException2<IOException, BuildFileNotFoundException>> entry :
        env.getValuesOrThrow(
            depKeys, IOException.class, BuildFileNotFoundException.class).entrySet()) {
      try {
        entry.getValue().get();
      } catch (InconsistentFilesystemException e) {
        throw new InternalInconsistentFilesystemException(packageIdentifier, e);
      } catch (FileSymlinkException e) {
        // Legacy doesn't detect symlink cycles.
        packageShouldBeInErrorFromGlobDeps = true;
      } catch (IOException | BuildFileNotFoundException e) {
        maybeThrowFilesystemInconsistency(packageIdentifier, e, packageWasInError);
      }
    }
    return packageShouldBeInErrorFromGlobDeps;
  }

  /**
   * Adds a dependency on the WORKSPACE file, representing it as a special type of package.
   *
   * @throws PackageFunctionException if there is an error computing the workspace file or adding
   *     its rules to the //external package.
   */
  private SkyValue getExternalPackage(Environment env, Root packageLookupPath)
      throws PackageFunctionException, InterruptedException {
    StarlarkSemantics starlarkSemantics = PrecomputedValue.STARLARK_SEMANTICS.get(env);
    if (starlarkSemantics == null) {
      return null;
    }
    RootedPath workspacePath =
        RootedPath.toRootedPath(packageLookupPath, LabelConstants.WORKSPACE_FILE_NAME);
    SkyKey workspaceKey = ExternalPackageFunction.key(workspacePath);
    PackageValue workspace = null;
    try {
      // This may throw a NoSuchPackageException if the WORKSPACE file was malformed or had other
      // problems. Since this function can't add much context, we silently bubble it up.
      workspace =
          (PackageValue)
              env.getValueOrThrow(
                  workspaceKey,
                  IOException.class,
                  EvalException.class,
                  SkylarkImportFailedException.class);
    } catch (IOException | EvalException | SkylarkImportFailedException e) {
      throw new PackageFunctionException(
          new NoSuchPackageException(
              LabelConstants.EXTERNAL_PACKAGE_IDENTIFIER,
              "Error encountered while dealing with the WORKSPACE file: " + e.getMessage()),
          Transience.PERSISTENT);
    }
    if (workspace == null) {
      return null;
    }

    Package pkg = workspace.getPackage();
    Event.replayEventsOn(env.getListener(), pkg.getEvents());
    for (Postable post : pkg.getPosts()) {
      env.getListener().post(post);
    }

    if (packageFactory != null) {
      packageFactory.afterDoneLoadingPackage(
          pkg,
          starlarkSemantics,
          // This is a lie.
          /*loadTimeNanos=*/ 0L);
    }
    return new PackageValue(pkg);
  }

  @Override
  public SkyValue compute(SkyKey key, Environment env) throws PackageFunctionException,
      InterruptedException {
    PackageIdentifier packageId = (PackageIdentifier) key.argument();

    SkyKey packageLookupKey = PackageLookupValue.key(packageId);
    PackageLookupValue packageLookupValue;
    try {
      packageLookupValue = (PackageLookupValue)
          env.getValueOrThrow(packageLookupKey, BuildFileNotFoundException.class,
              InconsistentFilesystemException.class);
    } catch (BuildFileNotFoundException e) {
      throw new PackageFunctionException(e, Transience.PERSISTENT);
    } catch (InconsistentFilesystemException e) {
      // This error is not transient from the perspective of the PackageFunction.
      throw new PackageFunctionException(
          new NoSuchPackageException(packageId, e.getMessage(), e), Transience.PERSISTENT);
    }
    if (packageLookupValue == null) {
      return null;
    }

    if (!packageLookupValue.packageExists()) {
      switch (packageLookupValue.getErrorReason()) {
        case NO_BUILD_FILE:
          throw new PackageFunctionException(
              new BuildFileNotFoundException(
                  packageId, PackageLookupFunction.explainNoBuildFileValue(packageId, env)),
              Transience.PERSISTENT);
        case DELETED_PACKAGE:
        case REPOSITORY_NOT_FOUND:
          throw new PackageFunctionException(
              new BuildFileNotFoundException(packageId, packageLookupValue.getErrorMsg()),
              Transience.PERSISTENT);
        case INVALID_PACKAGE_NAME:
          throw new PackageFunctionException(new InvalidPackageNameException(packageId,
              packageLookupValue.getErrorMsg()), Transience.PERSISTENT);
        default:
          // We should never get here.
          throw new IllegalStateException();
      }
    }

    if (packageId.equals(LabelConstants.EXTERNAL_PACKAGE_IDENTIFIER)) {
      return getExternalPackage(env, packageLookupValue.getRoot());
    }
    WorkspaceNameValue workspaceNameValue =
        (WorkspaceNameValue) env.getValue(WorkspaceNameValue.key());
    if (workspaceNameValue == null) {
      return null;
    }
    String workspaceName = workspaceNameValue.getName();

    RepositoryMappingValue repositoryMappingValue =
        (RepositoryMappingValue)
            env.getValue(RepositoryMappingValue.key(packageId.getRepository()));
    if (repositoryMappingValue == null) {
      return null;
    }
    ImmutableMap<RepositoryName, RepositoryName> repositoryMapping =
        repositoryMappingValue.getRepositoryMapping();

    RootedPath buildFileRootedPath = packageLookupValue.getRootedPath(packageId);
    FileValue buildFileValue = null;

    buildFileValue = getBuildFileValue(env, buildFileRootedPath);
    if (buildFileValue == null) {
      return null;
    }

    RuleVisibility defaultVisibility = PrecomputedValue.DEFAULT_VISIBILITY.get(env);
    if (defaultVisibility == null) {
      return null;
    }

    StarlarkSemantics starlarkSemantics = PrecomputedValue.STARLARK_SEMANTICS.get(env);
    if (starlarkSemantics == null) {
      return null;
    }

    // Load the prelude from the same repository as the package being loaded.  Can't use
    // Label.resolveRepositoryRelative because preludeLabel is in the main repository, not the
    // default one, so it is resolved to itself.
    Label pkgPreludeLabel =
        Label.createUnvalidated(
            PackageIdentifier.create(packageId.getRepository(), preludeLabel.getPackageFragment()),
            preludeLabel.getName());
    SkyKey astLookupKey = ASTFileLookupValue.key(pkgPreludeLabel);
    ASTFileLookupValue astLookupValue = null;
    try {
      astLookupValue = (ASTFileLookupValue) env.getValueOrThrow(astLookupKey,
          ErrorReadingSkylarkExtensionException.class, InconsistentFilesystemException.class);
    } catch (ErrorReadingSkylarkExtensionException | InconsistentFilesystemException e) {
      throw new PackageFunctionException(
          new NoSuchPackageException(
              packageId, "Error encountered while reading the prelude file: " + e.getMessage()),
          Transience.PERSISTENT);
    }
    if (astLookupValue == null) {
      return null;
    }
    // The prelude file doesn't have to exist. If not, we substitute an empty statement list.
    List<Statement> preludeStatements =
        astLookupValue.lookupSuccessful()
            ? astLookupValue.getAST().getStatements() : ImmutableList.<Statement>of();
    LoadedPackageCacheEntry packageCacheEntry =
        loadPackage(
            workspaceName,
            repositoryMapping,
            packageId,
            buildFileRootedPath,
            buildFileValue,
            defaultVisibility,
            starlarkSemantics,
            preludeStatements,
            packageLookupValue.getRoot(),
            env);
    if (packageCacheEntry == null) {
      return null;
    }
    Package.Builder pkgBuilder = packageCacheEntry.builder;
    try {
      pkgBuilder.buildPartial();
    } catch (NoSuchPackageException e) {
      throw new PackageFunctionException(
          e,
          e.getCause() instanceof SkyframeGlobbingIOException
              ? Transience.PERSISTENT
              : Transience.TRANSIENT);
    }
    try {
      // Since the Skyframe dependencies we request below in
      // handleGlobDepsAndPropagateFilesystemExceptions are requested independently of
      // the ones requested here in
      // handleLabelsCrossingSubpackagesAndPropagateInconsistentFilesystemExceptions, we don't
      // bother checking for missing values and instead piggyback on the env.missingValues() call
      // for the former. This avoids a Skyframe restart.
      handleLabelsCrossingSubpackagesAndPropagateInconsistentFilesystemExceptions(
          packageLookupValue.getRoot(), packageId, pkgBuilder, env);
    } catch (InternalInconsistentFilesystemException e) {
      packageFunctionCache.invalidate(packageId);
      throw new PackageFunctionException(
          e.toNoSuchPackageException(),
          e.isTransient() ? Transience.TRANSIENT : Transience.PERSISTENT);
    }
    Set<SkyKey> globKeys = packageCacheEntry.globDepKeys;
    boolean packageShouldBeConsideredInErrorFromGlobDeps;
    try {
      packageShouldBeConsideredInErrorFromGlobDeps =
          handleGlobDepsAndPropagateFilesystemExceptions(
          packageId, globKeys, env, pkgBuilder.containsErrors());
    } catch (InternalInconsistentFilesystemException e) {
      packageFunctionCache.invalidate(packageId);
      throw new PackageFunctionException(
          e.toNoSuchPackageException(),
          e.isTransient() ? Transience.TRANSIENT : Transience.PERSISTENT);
    }
    if (env.valuesMissing()) {
      return null;
    }

    if (pkgBuilder.containsErrors() || packageShouldBeConsideredInErrorFromGlobDeps) {
      pkgBuilder.setContainsErrors();
    }
    Package pkg = pkgBuilder.finishBuild();

    Event.replayEventsOn(env.getListener(), pkgBuilder.getEvents());
    for (Postable post : pkgBuilder.getPosts()) {
      env.getListener().post(post);
    }

    // We know this SkyFunction will not be called again, so we can remove the cache entry.
    packageFunctionCache.invalidate(packageId);

    packageFactory.afterDoneLoadingPackage(pkg, starlarkSemantics, packageCacheEntry.loadTimeNanos);
    return new PackageValue(pkg);
  }

  private static FileValue getBuildFileValue(Environment env, RootedPath buildFileRootedPath)
      throws InterruptedException {
    FileValue buildFileValue;
    try {
      buildFileValue =
          (FileValue) env.getValueOrThrow(FileValue.key(buildFileRootedPath), IOException.class);
    } catch (IOException e) {
      throw new IllegalStateException("Package lookup succeeded but encountered error when "
          + "getting FileValue for BUILD file directly.", e);
    }
    if (buildFileValue == null) {
      return null;
    }
    Preconditions.checkState(buildFileValue.exists(),
        "Package lookup succeeded but BUILD file doesn't exist");
    return buildFileValue;
  }

  private static BuildFileContainsErrorsException propagateSkylarkImportFailedException(
      PackageIdentifier packageId, SkylarkImportFailedException e)
          throws BuildFileContainsErrorsException {
    Throwable rootCause = Throwables.getRootCause(e);
    throw (rootCause instanceof IOException)
        ? new BuildFileContainsErrorsException(
            packageId, e.getMessage(), (IOException) rootCause)
        : new BuildFileContainsErrorsException(packageId, e.getMessage());
  }

  /**
   * Fetch the skylark loads for this BUILD file. If any of them haven't been computed yet, returns
   * null.
   */
  @Nullable
  static SkylarkImportResult fetchImportsFromBuildFile(
      RootedPath buildFilePath,
      PackageIdentifier packageId,
      BuildFileAST buildFileAST,
      int workspaceChunk,
      Environment env,
      SkylarkImportLookupFunction skylarkImportLookupFunctionForInlining)
      throws NoSuchPackageException, InterruptedException {
    Preconditions.checkArgument(!packageId.getRepository().isDefault());

    ImmutableList<SkylarkImport> imports = buildFileAST.getImports();
    Map<String, Extension> importMap = Maps.newHashMapWithExpectedSize(imports.size());
    ImmutableList.Builder<SkylarkFileDependency> fileDependencies = ImmutableList.builder();

    // Find the labels corresponding to the load statements.
    Label labelForCurrBuildFile;
    try {
      labelForCurrBuildFile = Label.create(packageId, "BUILD");
    } catch (LabelSyntaxException e) {
      // Shouldn't happen; the Label is well-formed by construction.
      throw new IllegalStateException(e);
    }
    ImmutableMap<String, Label> importPathMap =
        SkylarkImportLookupFunction.getLabelsForLoadStatements(imports, labelForCurrBuildFile);

    // Look up and load the imports.
    ImmutableCollection<Label> importLabels = importPathMap.values();
    List<SkyKey> importLookupKeys = Lists.newArrayListWithExpectedSize(importLabels.size());
    boolean inWorkspace = buildFilePath.getRootRelativePath().getBaseName().endsWith("WORKSPACE");
    for (Label importLabel : importLabels) {
      int originalChunk =
          getOriginalWorkspaceChunk(env, buildFilePath, workspaceChunk, importLabel);
      if (inWorkspace) {
        importLookupKeys.add(
            SkylarkImportLookupValue.keyInWorkspace(importLabel, originalChunk, buildFilePath));
      } else {
        importLookupKeys.add(SkylarkImportLookupValue.key(importLabel));
      }
    }
    Map<SkyKey, SkyValue> skylarkImportMap = Maps.newHashMapWithExpectedSize(importPathMap.size());
    boolean valuesMissing = false;

    try {
      if (skylarkImportLookupFunctionForInlining == null) {
        // Not inlining
        Map<SkyKey,
            ValueOrException2<
                SkylarkImportFailedException,
                InconsistentFilesystemException>> skylarkLookupResults = env.getValuesOrThrow(
                    importLookupKeys,
                    SkylarkImportFailedException.class,
                    InconsistentFilesystemException.class);
        valuesMissing = env.valuesMissing();
        for (Map.Entry<
              SkyKey,
              ValueOrException2<
                  SkylarkImportFailedException,
                  InconsistentFilesystemException>> entry : skylarkLookupResults.entrySet()) {
          // Fetching the value will raise any deferred exceptions
          skylarkImportMap.put(entry.getKey(), entry.getValue().get());
        }
      } else {
        // Inlining calls to SkylarkImportLookupFunction
        for (SkyKey importLookupKey : importLookupKeys) {
          SkyValue skyValue =
              skylarkImportLookupFunctionForInlining.computeWithInlineCalls(importLookupKey, env);
          if (skyValue == null) {
            Preconditions.checkState(
                env.valuesMissing(), "no starlark import value for %s", importLookupKey);
            // We continue making inline calls even if some requested values are missing, to
            // maximize the number of dependent (non-inlined) SkyFunctions that are requested, thus
            // avoiding a quadratic number of restarts.
            valuesMissing = true;
          } else {
            skylarkImportMap.put(importLookupKey, skyValue);
          }
        }

      }
    } catch (SkylarkImportFailedException e) {
      throw propagateSkylarkImportFailedException(packageId, e);
    } catch (InconsistentFilesystemException e) {
      throw new NoSuchPackageException(packageId, e.getMessage(), e);
    }

    if (valuesMissing) {
      // Some imports are unavailable.
      return null;
    }

    // Process the loaded imports.
    for (Map.Entry<String, Label> importEntry : importPathMap.entrySet()) {
      String importString = importEntry.getKey();
      Label importLabel = importEntry.getValue();

      int originalChunk =
          getOriginalWorkspaceChunk(env, buildFilePath, workspaceChunk, importLabel);
      SkyKey keyForLabel;
      if (inWorkspace) {
        keyForLabel =
            SkylarkImportLookupValue.keyInWorkspace(importLabel, originalChunk, buildFilePath);
      } else {
        keyForLabel = SkylarkImportLookupValue.key(importLabel);
      }
      SkylarkImportLookupValue importLookupValue =
          (SkylarkImportLookupValue) skylarkImportMap.get(keyForLabel);
      importMap.put(importString, importLookupValue.getEnvironmentExtension());
      fileDependencies.add(importLookupValue.getDependency());
    }

    return new SkylarkImportResult(importMap, transitiveClosureOfLabels(fileDependencies.build()));
  }

  private static int getOriginalWorkspaceChunk(
      Environment env, RootedPath workspacePath, int workspaceChunk, Label importLabel)
      throws InterruptedException {
    if (workspaceChunk < 1) {
      return workspaceChunk;
    }
    // If we got here, we are already computing workspaceChunk "workspaceChunk", and so we know
    // that the value for "workspaceChunk-1" has already been computed so we don't need to check
    // for nullness
    SkyKey workspaceFileKey = WorkspaceFileValue.key(workspacePath, workspaceChunk - 1);
    WorkspaceFileValue workspaceFileValue = (WorkspaceFileValue) env.getValue(workspaceFileKey);
    ImmutableMap<String, Integer> importToChunkMap = workspaceFileValue.getImportToChunkMap();
    String importString = importLabel.toString();
    return importToChunkMap.getOrDefault(importString, workspaceChunk);
  }

  private static ImmutableList<Label> transitiveClosureOfLabels(
      ImmutableList<SkylarkFileDependency> immediateDeps) {
    Set<Label> transitiveClosure = Sets.newHashSet();
    transitiveClosureOfLabels(immediateDeps, transitiveClosure);
    return ImmutableList.copyOf(transitiveClosure);
  }

  private static void transitiveClosureOfLabels(
      ImmutableList<SkylarkFileDependency> immediateDeps, Set<Label> transitiveClosure) {
    for (SkylarkFileDependency dep : immediateDeps) {
      if (transitiveClosure.add(dep.getLabel())) {
        transitiveClosureOfLabels(dep.getDependencies(), transitiveClosure);
      }
    }
  }

  @Nullable
  @Override
  public String extractTag(SkyKey skyKey) {
    return null;
  }

  private static void handleLabelsCrossingSubpackagesAndPropagateInconsistentFilesystemExceptions(
      Root pkgRoot, PackageIdentifier pkgId, Package.Builder pkgBuilder, Environment env)
      throws InternalInconsistentFilesystemException, InterruptedException {
    PathFragment pkgDir = pkgId.getPackageFragment();
    Set<SkyKey> containingPkgLookupKeys = Sets.newHashSet();
    Map<Target, SkyKey> targetToKey = new HashMap<>();
    for (Target target : pkgBuilder.getTargets()) {
      PathFragment dir = Label.getContainingDirectory(target.getLabel());
      if (dir.equals(pkgDir)) {
        continue;
      }
      PackageIdentifier dirId = PackageIdentifier.create(pkgId.getRepository(), dir);
      SkyKey key = ContainingPackageLookupValue.key(dirId);
      targetToKey.put(target, key);
      containingPkgLookupKeys.add(key);
    }
    Map<SkyKey, ValueOrException2<BuildFileNotFoundException, InconsistentFilesystemException>>
        containingPkgLookupValues =
            env.getValuesOrThrow(
                containingPkgLookupKeys,
                BuildFileNotFoundException.class,
                InconsistentFilesystemException.class);
    if (env.valuesMissing() || containingPkgLookupKeys.isEmpty()) {
      return;
    }
    for (Iterator<Target> iterator = pkgBuilder.getTargets().iterator(); iterator.hasNext(); ) {
      Target target = iterator.next();
      SkyKey key = targetToKey.get(target);
      if (!containingPkgLookupValues.containsKey(key)) {
        continue;
      }
      ContainingPackageLookupValue containingPackageLookupValue =
          getContainingPkgLookupValueAndPropagateInconsistentFilesystemExceptions(
              pkgId, containingPkgLookupValues.get(key), env);
      if (maybeAddEventAboutLabelCrossingSubpackage(
          pkgBuilder,
          pkgRoot,
          target.getLabel(),
          target.getLocation(),
          containingPackageLookupValue)) {
        iterator.remove();
        pkgBuilder.setContainsErrors();
      }
    }
  }

  @Nullable
  private static ContainingPackageLookupValue
      getContainingPkgLookupValueAndPropagateInconsistentFilesystemExceptions(
          PackageIdentifier packageIdentifier,
          ValueOrException2<BuildFileNotFoundException, InconsistentFilesystemException>
              containingPkgLookupValueOrException,
          Environment env)
          throws InternalInconsistentFilesystemException {
    try {
      return (ContainingPackageLookupValue) containingPkgLookupValueOrException.get();
    } catch (BuildFileNotFoundException e) {
      env.getListener().handle(Event.error(null, e.getMessage()));
      return null;
    } catch (InconsistentFilesystemException e) {
      throw new InternalInconsistentFilesystemException(packageIdentifier, e);
    }
  }

  private static boolean maybeAddEventAboutLabelCrossingSubpackage(
      Package.Builder pkgBuilder,
      Root pkgRoot,
      Label label,
      @Nullable Location location,
      @Nullable ContainingPackageLookupValue containingPkgLookupValue) {
    if (containingPkgLookupValue == null) {
      return true;
    }
    if (!containingPkgLookupValue.hasContainingPackage()) {
      // The missing package here is a problem, but it's not an error from the perspective of
      // PackageFunction.
      return false;
    }
    PackageIdentifier containingPkg = containingPkgLookupValue.getContainingPackageName();
    if (containingPkg.equals(label.getPackageIdentifier())) {
      // The label does not cross a subpackage boundary.
      return false;
    }
    if (!containingPkg.getSourceRoot().startsWith(
        label.getPackageIdentifier().getSourceRoot())) {
      // This label is referencing an imaginary package, because the containing package should
      // extend the label's package: if the label is //a/b:c/d, the containing package could be
      // //a/b/c or //a/b, but should never be //a. Usually such errors will be caught earlier, but
      // in some exceptional cases (such as a Python-aware BUILD file catching its own io
      // exceptions), it reaches here, and we tolerate it.
      return false;
    }
    String message = ContainingPackageLookupValue.getErrorMessageForLabelCrossingPackageBoundary(
        pkgRoot, label, containingPkgLookupValue);
    pkgBuilder.addEvent(Event.error(location, message));
    return true;
  }

  private interface GlobberWithSkyframeGlobDeps extends Globber {
    Set<SkyKey> getGlobDepsRequested();
  }

  private static class LegacyGlobberWithNoGlobDeps implements GlobberWithSkyframeGlobDeps {
    private final LegacyGlobber delegate;

    private LegacyGlobberWithNoGlobDeps(LegacyGlobber delegate) {
      this.delegate = delegate;
    }

    @Override
    public Set<SkyKey> getGlobDepsRequested() {
      return ImmutableSet.of();
    }

    @Override
    public Token runAsync(
        List<String> includes, List<String> excludes, boolean excludeDirs, boolean allowEmpty)
        throws BadGlobException, InterruptedException {
      return delegate.runAsync(includes, excludes, excludeDirs, allowEmpty);
    }

    @Override
    public List<String> fetch(Token token)
        throws BadGlobException, IOException, InterruptedException {
      return delegate.fetch(token);
    }

    @Override
    public void onInterrupt() {
      delegate.onInterrupt();
    }

    @Override
    public void onCompletion() {
      delegate.onCompletion();
    }
  }

  /**
   * A {@link Globber} implemented on top of skyframe that falls back to a
   * {@link PackageFactory.LegacyGlobber} on a skyframe cache-miss. This way we don't require a
   * skyframe restart after a call to {@link Globber#runAsync} and before/during a call to
   * {@link Globber#fetch}.
   *
   * <p>There are three advantages to this hybrid approach over the more obvious approach of solely
   * using a {@link PackageFactory.LegacyGlobber}:
   * <ul>
   * <li>We trivially have the proper Skyframe {@link GlobValue} deps, whereas we would need to
   * request them after-the-fact if we solely used a {@link PackageFactory.LegacyGlobber}.
   * <li>We don't need to re-evaluate globs whose expression hasn't changed (e.g. in the common case
   * of a BUILD file edit that doesn't change a glob expression), whereas legacy package loading
   * with a {@link PackageFactory.LegacyGlobber} would naively re-evaluate globs when re-evaluating
   * the BUILD file.
   * <li>We don't need to re-evaluate invalidated globs *twice* (the single re-evaluation via our
   * GlobValue deps is sufficient and optimal). See above for why the second evaluation would
   * happen.
   * </ul>
   */
  private static class SkyframeHybridGlobber implements GlobberWithSkyframeGlobDeps {
    private final PackageIdentifier packageId;
    private final Root packageRoot;
    private final Environment env;
    private final LegacyGlobber legacyGlobber;
    private final Set<SkyKey> globDepsRequested = Sets.newConcurrentHashSet();

    private SkyframeHybridGlobber(
        PackageIdentifier packageId,
        Root packageRoot,
        Environment env,
        LegacyGlobber legacyGlobber) {
      this.packageId = packageId;
      this.packageRoot = packageRoot;
      this.env = env;
      this.legacyGlobber = legacyGlobber;
    }

    @Override
    public Set<SkyKey> getGlobDepsRequested() {
      return ImmutableSet.copyOf(globDepsRequested);
    }

    private SkyKey getGlobKey(String pattern, boolean excludeDirs) throws BadGlobException {
      try {
        return GlobValue.key(packageId, packageRoot, pattern, excludeDirs,
              PathFragment.EMPTY_FRAGMENT);
      } catch (InvalidGlobPatternException e) {
        throw new BadGlobException(e.getMessage());
      }
    }

    @Override
    public Token runAsync(
        List<String> includes, List<String> excludes, boolean excludeDirs, boolean allowEmpty)
        throws BadGlobException, InterruptedException {
      LinkedHashSet<SkyKey> globKeys = Sets.newLinkedHashSetWithExpectedSize(includes.size());
      Map<SkyKey, String> globKeyToPatternMap = Maps.newHashMapWithExpectedSize(includes.size());

      for (String pattern : includes) {
        SkyKey globKey = getGlobKey(pattern, excludeDirs);
        globKeys.add(globKey);
        globKeyToPatternMap.put(globKey, pattern);
      }

      globDepsRequested.addAll(globKeys);

      Map<SkyKey, ValueOrException2<IOException, BuildFileNotFoundException>> globValueMap =
          env.getValuesOrThrow(globKeys, IOException.class, BuildFileNotFoundException.class);

      // For each missing glob, evaluate it asynchronously via the delegate.
      Collection<SkyKey> missingKeys = getMissingKeys(globKeys, globValueMap);
      List<String> globsToDelegate = new ArrayList<>(missingKeys.size());
      for (SkyKey missingKey : missingKeys) {
        String missingPattern = globKeyToPatternMap.get(missingKey);
        if (missingPattern != null) {
          globsToDelegate.add(missingPattern);
          globKeys.remove(missingKey);
        }
      }
      Token legacyIncludesToken =
          globsToDelegate.isEmpty()
              ? null
              : legacyGlobber.runAsync(
                  globsToDelegate, ImmutableList.of(), excludeDirs, allowEmpty);
      return new HybridToken(globValueMap, globKeys, legacyIncludesToken, excludes, allowEmpty);
    }

    private Collection<SkyKey> getMissingKeys(Collection<SkyKey> globKeys,
        Map<SkyKey, ValueOrException2<IOException, BuildFileNotFoundException>> globValueMap) {
      List<SkyKey> missingKeys = new ArrayList<>(globKeys.size());
      for (SkyKey globKey : globKeys) {
        ValueOrException2<IOException, BuildFileNotFoundException> valueOrException =
            globValueMap.get(globKey);
        if (valueOrException == null) {
          missingKeys.add(globKey);
        }
        try {
          if (valueOrException.get() == null) {
            missingKeys.add(globKey);
          }
        } catch (IOException | BuildFileNotFoundException doesntMatter) {
          continue;
        }
      }
      return missingKeys;
    }

    @Override
    public List<String> fetch(Token token)
        throws BadGlobException, IOException, InterruptedException {
      HybridToken hybridToken = (HybridToken) token;
      return hybridToken.resolve(legacyGlobber);
    }

    @Override
    public void onInterrupt() {
      legacyGlobber.onInterrupt();
    }

    @Override
    public void onCompletion() {
      legacyGlobber.onCompletion();
    }

    /**
     * A {@link Globber.Token} that encapsulates the result of a single {@link Globber#runAsync}
     * call via the fetching of some globs from skyframe, and some other globs via a{@link
     * PackageFactory.LegacyGlobber}. 'exclude' patterns are evaluated using {@link
     * UnixGlob#removeExcludes} after merging legacy and skyframe glob results in {@link #resolve}.
     */
    private static class HybridToken extends Globber.Token {
      // The result of the Skyframe lookup for all the needed glob patterns.
      private final Map<SkyKey, ValueOrException2<IOException, BuildFileNotFoundException>>
          globValueMap;
      // The skyframe keys corresponding to the 'includes' patterns fetched from Skyframe
      // (this is includes_sky above).
      private final Iterable<SkyKey> includesGlobKeys;
      // A token for computing legacy globs.
      @Nullable private final Token legacyIncludesToken;

      private final List<String> excludes;

      private final boolean allowEmpty;

      private HybridToken(
          Map<SkyKey, ValueOrException2<IOException, BuildFileNotFoundException>> globValueMap,
          Iterable<SkyKey> includesGlobKeys,
          @Nullable Token delegateIncludesToken,
          List<String> excludes,
          boolean allowEmpty) {
        this.globValueMap = globValueMap;
        this.includesGlobKeys = includesGlobKeys;
        this.legacyIncludesToken = delegateIncludesToken;
        this.excludes = excludes;
        this.allowEmpty = allowEmpty;
      }

      private List<String> resolve(Globber delegate)
          throws BadGlobException, IOException, InterruptedException {
        HashSet<String> matches = new HashSet<>();
        for (SkyKey includeGlobKey : includesGlobKeys) {
          // TODO(bazel-team): NestedSet expansion here is suboptimal.
          boolean foundMatch = false;
          for (PathFragment match : getGlobMatches(includeGlobKey, globValueMap)) {
            matches.add(match.getPathString());
            foundMatch = true;
          }
          if (!allowEmpty && !foundMatch) {
            throw new BadGlobException(
                "glob pattern '"
                    + ((GlobDescriptor) includeGlobKey.argument()).getPattern()
                    + "' didn't match anything, but allow_empty is set to False.");
          }
        }
        if (legacyIncludesToken != null) {
          matches.addAll(delegate.fetch(legacyIncludesToken));
        }
        UnixGlob.removeExcludes(matches, excludes);
        List<String> result = new ArrayList<>(matches);
        // Skyframe glob results are unsorted. And we used a LegacyGlobber that doesn't sort.
        // Therefore, we want to unconditionally sort here.
        Collections.sort(result);

        if (!allowEmpty && result.isEmpty()) {
          throw new BadGlobException(
              "all files in the glob have been excluded, but allow_empty is set to False.");
        }
        return result;
      }

      private static NestedSet<PathFragment> getGlobMatches(
          SkyKey globKey,
          Map<SkyKey, ValueOrException2<IOException, BuildFileNotFoundException>> globValueMap)
          throws SkyframeGlobbingIOException {
        ValueOrException2<IOException, BuildFileNotFoundException> valueOrException =
            Preconditions.checkNotNull(
                globValueMap.get(globKey), "%s should not be missing", globKey);
        try {
          return Preconditions.checkNotNull(
                  (GlobValue) valueOrException.get(), "%s should not be missing", globKey)
              .getMatches();
        } catch (BuildFileNotFoundException e) {
          // Legacy package loading is only able to handle an IOException, so a rethrow here is the
          // best we can do.
          throw new SkyframeGlobbingIOException(e);
        } catch (IOException e) {
          throw new SkyframeGlobbingIOException(e);
        }
      }
    }
  }

  private static class SkyframeGlobbingIOException extends IOException {
    private SkyframeGlobbingIOException(Exception cause) {
      super(cause);
    }
  }

  private GlobberWithSkyframeGlobDeps makeGlobber(
      Path buildFilePath,
      PackageIdentifier packageId,
      Root packageRoot,
      SkyFunction.Environment env) {
    LegacyGlobber legacyGlobber = packageFactory.createLegacyGlobber(
        buildFilePath.getParentDirectory(), packageId, packageLocator);
    switch (incrementalityIntent) {
      case INCREMENTAL:
        return new SkyframeHybridGlobber(packageId, packageRoot, env, legacyGlobber);
      case NON_INCREMENTAL:
        // Skyframe globbing is only useful for incremental correctness and performance. The
        // first time Bazel loads a package ever, Skyframe globbing is actually pure overhead
        // (SkyframeHybridGlobber will make full use of LegacyGlobber).
        return new LegacyGlobberWithNoGlobDeps(legacyGlobber);
      default:
        throw new IllegalStateException(incrementalityIntent.toString());
    }
  }

  /**
   * Constructs a {@link Package} object for the given package. Note that the returned package may
   * be in error.
   *
   * <p>May return null if the computation has to be restarted.
   *
   * <p>Exactly one of {@code replacementContents} and {@code buildFileValue} will be non-{@code
   * null}. The former indicates that we have a faux BUILD file with the given contents and the
   * latter indicates that we have a legitimate BUILD file and should actually read its contents.
   */
  @Nullable
  private LoadedPackageCacheEntry loadPackage(
      String workspaceName,
      ImmutableMap<RepositoryName, RepositoryName> repositoryMapping,
      PackageIdentifier packageId,
      RootedPath buildFilePath,
      @Nullable FileValue buildFileValue,
      RuleVisibility defaultVisibility,
      StarlarkSemantics starlarkSemantics,
      List<Statement> preludeStatements,
      Root packageRoot,
      Environment env)
      throws InterruptedException, PackageFunctionException {
    LoadedPackageCacheEntry packageCacheEntry = packageFunctionCache.getIfPresent(packageId);
    if (packageCacheEntry == null) {
      if (packageProgress != null) {
        packageProgress.startReadPackage(packageId);
      }
      try (SilentCloseable c =
          Profiler.instance().profile(ProfilerTask.CREATE_PACKAGE, packageId.toString())) {
        AstParseResult astParseResult = astCache.getIfPresent(packageId);
        Path inputFile = buildFilePath.asPath();
        if (astParseResult == null) {
          if (showLoadingProgress.get()) {
            env.getListener().handle(Event.progress("Loading package: " + packageId));
          }
          ParserInputSource input;
          Preconditions.checkNotNull(buildFileValue, packageId);
          byte[] buildFileBytes = null;
          try {
            buildFileBytes =
                buildFileValue.isSpecialFile()
                    ? FileSystemUtils.readContent(inputFile)
                    : FileSystemUtils.readWithKnownFileSize(inputFile, buildFileValue.getSize());
          } catch (IOException e) {
            buildFileBytes =
                actionOnIOExceptionReadingBuildFile.maybeGetBuildFileContentsToUse(
                    inputFile.asFragment(), e);
            if (buildFileBytes == null) {
              // Note that we did the work that led to this IOException, so we should
              // conservatively report this error as transient.
              throw new PackageFunctionException(
                  new BuildFileContainsErrorsException(packageId, e.getMessage(), e),
                  Transience.TRANSIENT);
            }
            // If control flow reaches here, we're in territory that is deliberately unsound.
            // See the javadoc for ActionOnIOExceptionReadingBuildFile.
          }
          input =
              ParserInputSource.create(
                  FileSystemUtils.convertFromLatin1(buildFileBytes), inputFile.asFragment());
          StoredEventHandler astParsingEventHandler = new StoredEventHandler();
          BuildFileAST ast =
              PackageFactory.parseBuildFile(
                  packageId, input, preludeStatements, repositoryMapping, astParsingEventHandler);
          astParseResult = new AstParseResult(ast, astParsingEventHandler);
          astCache.put(packageId, astParseResult);
        }
        SkylarkImportResult importResult;
        try {
          importResult =
              fetchImportsFromBuildFile(
                  buildFilePath,
                  packageId,
                  astParseResult.ast,
                  /* workspaceChunk = */ -1,
                  env,
                  skylarkImportLookupFunctionForInlining);
        } catch (NoSuchPackageException e) {
          throw new PackageFunctionException(e, Transience.PERSISTENT);
        } catch (InterruptedException e) {
          astCache.invalidate(packageId);
          throw e;
        }
        if (importResult == null) {
          return null;
        }
        astCache.invalidate(packageId);
        GlobberWithSkyframeGlobDeps globberWithSkyframeGlobDeps =
            makeGlobber(inputFile, packageId, packageRoot, env);
        long startTimeNanos = BlazeClock.nanoTime();
        Package.Builder pkgBuilder =
            packageFactory.createPackageFromAst(
                workspaceName,
                repositoryMapping,
                packageId,
                buildFilePath,
                astParseResult,
                importResult.importMap,
                importResult.fileDependencies,
                defaultVisibility,
                starlarkSemantics,
                globberWithSkyframeGlobDeps);
        long loadTimeNanos = Math.max(BlazeClock.nanoTime() - startTimeNanos, 0L);
        packageCacheEntry = new LoadedPackageCacheEntry(
            pkgBuilder,
            globberWithSkyframeGlobDeps.getGlobDepsRequested(),
            loadTimeNanos);
        numPackagesLoaded.incrementAndGet();
        if (packageProgress != null) {
          packageProgress.doneReadPackage(packageId);
        }
        packageFunctionCache.put(packageId, packageCacheEntry);
      }
    }
    return packageCacheEntry;
  }

  private static class InternalInconsistentFilesystemException extends Exception {
    private boolean isTransient;

    private PackageIdentifier packageIdentifier;

    /**
     * Used to represent a filesystem inconsistency discovered outside the
     * {@link PackageFunction}.
     */
    public InternalInconsistentFilesystemException(PackageIdentifier packageIdentifier,
        InconsistentFilesystemException e) {
      super(e.getMessage(), e);
      this.packageIdentifier = packageIdentifier;
      // This is not a transient error from the perspective of the PackageFunction.
      this.isTransient = false;
    }

    /** Used to represent a filesystem inconsistency discovered by the {@link PackageFunction}. */
    public InternalInconsistentFilesystemException(PackageIdentifier packageIdentifier,
        String inconsistencyMessage) {
      this(packageIdentifier, new InconsistentFilesystemException(inconsistencyMessage));
      this.isTransient = true;
    }

    public boolean isTransient() {
      return isTransient;
    }

    private NoSuchPackageException toNoSuchPackageException() {
      return new NoSuchPackageException(
          packageIdentifier, this.getMessage(), (Exception) this.getCause());
    }
  }

  /**
   * Used to declare all the exception types that can be wrapped in the exception thrown by
   * {@link PackageFunction#compute}.
   */
  static class PackageFunctionException extends SkyFunctionException {
    public PackageFunctionException(NoSuchPackageException e, Transience transience) {
      super(e, transience);
    }
  }

  /** A simple value class to store the result of the Skylark imports.*/
  static final class SkylarkImportResult {
    final Map<String, Extension> importMap;
    final ImmutableList<Label> fileDependencies;
    private SkylarkImportResult(
        Map<String, Extension> importMap,
        ImmutableList<Label> fileDependencies) {
      this.importMap = importMap;
      this.fileDependencies = fileDependencies;
    }
  }
}