private static boolean shouldFollowSymlinksWhenTraversing(Dirents dirents) {
   for (Dirent dirent : dirents) {
     // This is a specical sentinel file whose existence tells Blaze not to follow symlinks when
     // recursively traversing through this directory.
     //
     // This admittedly ugly feature is used to support workspaces with directories with weird
     // symlink structures that aren't intended to be consumed by Blaze.
     if (dirent.getName().equals(SENTINEL_FILE_NAME_FOR_NOT_TRAVERSING_SYMLINKS)) {
       return false;
     }
   }
   return true;
 }
  /**
   * List the directory and create {@code SkyKey}s to request contents of its children recursively.
   *
   * <p>The returned keys are of type {@link SkyFunctions#RECURSIVE_FILESYSTEM_TRAVERSAL}.
   */
  private static Collection<SkyKey> createRecursiveTraversalKeys(
      Environment env, TraversalRequest traversal) throws MissingDepException {
    // Use the traversal's path, even if it's a symlink. The contents of the directory, as listed
    // in the result, must be relative to it.
    DirectoryListingValue dirListing =
        (DirectoryListingValue)
            getDependentSkyValue(env, DirectoryListingValue.key(traversal.path));

    List<SkyKey> result = new ArrayList<>();
    for (Dirent dirent : dirListing.getDirents()) {
      RootedPath childPath =
          RootedPath.toRootedPath(
              traversal.path.getRoot(),
              traversal.path.getRelativePath().getRelative(dirent.getName()));
      TraversalRequest childTraversal = traversal.forChildEntry(childPath);
      result.add(RecursiveFilesystemTraversalValue.key(childTraversal));
    }
    return result;
  }
  /**
   * Looks in the directory specified by {@code recursivePkgKey} for a package, does some work as
   * specified by {@link Visitor} if such a package exists, then recursively does work in each
   * non-excluded subdirectory as specified by {@link #getSkyKeyForSubdirectory}, and finally
   * aggregates the {@link Visitor} value along with values from each subdirectory as specified by
   * {@link #aggregateWithSubdirectorySkyValues}, and returns that aggregation.
   *
   * <p>Returns null if {@code env.valuesMissing()} is true, checked after each call to one of
   * {@link RecursiveDirectoryTraversalFunction}'s abstract methods that were given {@code env}.
   * (And after each of {@code visitDirectory}'s own uses of {@code env}, of course.)
   */
  TReturn visitDirectory(RecursivePkgKey recursivePkgKey, Environment env) {
    RootedPath rootedPath = recursivePkgKey.getRootedPath();
    ImmutableSet<PathFragment> excludedPaths = recursivePkgKey.getExcludedPaths();
    Path root = rootedPath.getRoot();
    PathFragment rootRelativePath = rootedPath.getRelativePath();

    SkyKey fileKey = FileValue.key(rootedPath);
    FileValue fileValue;
    try {
      fileValue =
          (FileValue)
              env.getValueOrThrow(
                  fileKey,
                  InconsistentFilesystemException.class,
                  FileSymlinkException.class,
                  IOException.class);
    } catch (InconsistentFilesystemException | FileSymlinkException | IOException e) {
      return reportErrorAndReturn(
          "Failed to get information about path", e, rootRelativePath, env.getListener());
    }
    if (fileValue == null) {
      return null;
    }

    if (!fileValue.isDirectory()) {
      return getEmptyReturn();
    }

    PackageIdentifier packageId =
        PackageIdentifier.create(recursivePkgKey.getRepository(), rootRelativePath);

    if ((packageId.getRepository().isDefault() || packageId.getRepository().isMain())
        && fileValue.isSymlink()
        && fileValue
            .getUnresolvedLinkTarget()
            .startsWith(directories.getOutputBase().asFragment())) {
      // Symlinks back to the output base are not traversed so that we avoid convenience symlinks.
      // Note that it's not enough to just check for the convenience symlinks themselves, because
      // if the value of --symlink_prefix changes, the old symlinks are left in place. This
      // algorithm also covers more creative use cases where people create convenience symlinks
      // somewhere in the directory tree manually.
      return getEmptyReturn();
    }

    SkyKey pkgLookupKey = PackageLookupValue.key(packageId);
    SkyKey dirListingKey = DirectoryListingValue.key(rootedPath);
    Map<
            SkyKey,
            ValueOrException4<
                NoSuchPackageException,
                InconsistentFilesystemException,
                FileSymlinkException,
                IOException>>
        pkgLookupAndDirectoryListingDeps =
            env.getValuesOrThrow(
                ImmutableList.of(pkgLookupKey, dirListingKey),
                NoSuchPackageException.class,
                InconsistentFilesystemException.class,
                FileSymlinkException.class,
                IOException.class);
    if (env.valuesMissing()) {
      return null;
    }
    PackageLookupValue pkgLookupValue;
    try {
      pkgLookupValue =
          (PackageLookupValue)
              Preconditions.checkNotNull(
                  pkgLookupAndDirectoryListingDeps.get(pkgLookupKey).get(),
                  "%s %s",
                  recursivePkgKey,
                  pkgLookupKey);
    } catch (NoSuchPackageException | InconsistentFilesystemException e) {
      return reportErrorAndReturn("Failed to load package", e, rootRelativePath, env.getListener());
    } catch (IOException | FileSymlinkException e) {
      throw new IllegalStateException(e);
    }

    TVisitor visitor = getInitialVisitor();

    DirectoryListingValue dirListingValue;
    try {
      dirListingValue =
          (DirectoryListingValue)
              Preconditions.checkNotNull(
                  pkgLookupAndDirectoryListingDeps.get(dirListingKey).get(),
                  "%s %s",
                  recursivePkgKey,
                  dirListingKey);
    } catch (InconsistentFilesystemException | IOException e) {
      return reportErrorAndReturn(
          "Failed to list directory contents", e, rootRelativePath, env.getListener());
    } catch (FileSymlinkException e) {
      // DirectoryListingFunction only throws FileSymlinkCycleException when FileFunction throws it,
      // but FileFunction was evaluated for rootedPath above, and didn't throw there. It shouldn't
      // be able to avoid throwing there but throw here.
      throw new IllegalStateException(
          "Symlink cycle found after not being found for \"" + rootedPath + "\"");
    } catch (NoSuchPackageException e) {
      throw new IllegalStateException(e);
    }

    boolean followSymlinks = shouldFollowSymlinksWhenTraversing(dirListingValue.getDirents());
    List<SkyKey> childDeps = new ArrayList<>();
    for (Dirent dirent : dirListingValue.getDirents()) {
      Type type = dirent.getType();
      if (type != Type.DIRECTORY
          && (type != Type.SYMLINK || (type == Type.SYMLINK && !followSymlinks))) {
        // Non-directories can never host packages. Symlinks to non-directories are weeded out at
        // the next level of recursion when we check if its FileValue is a directory. This is slower
        // if there are a lot of symlinks in the tree, but faster if there are only a few, which is
        // the case most of the time.
        //
        // We are not afraid of weird symlink structure here: both cyclical ones and ones that give
        // rise to infinite directory trees are diagnosed by FileValue.
        continue;
      }
      String basename = dirent.getName();
      if (rootRelativePath.equals(PathFragment.EMPTY_FRAGMENT)
          && PathPackageLocator.DEFAULT_TOP_LEVEL_EXCLUDES.contains(basename)) {
        continue;
      }
      PathFragment subdirectory = rootRelativePath.getRelative(basename);

      // If this subdirectory is one of the excluded paths, don't recurse into it.
      if (excludedPaths.contains(subdirectory)) {
        continue;
      }

      // If we have an excluded path that isn't below this subdirectory, we shouldn't pass that
      // excluded path to our evaluation of the subdirectory, because the exclusion can't
      // possibly match anything beneath the subdirectory.
      //
      // For example, if we're currently evaluating directory "a", are looking at its subdirectory
      // "a/b", and we have an excluded path "a/c/d", there's no need to pass the excluded path
      // "a/c/d" to our evaluation of "a/b".
      //
      // This strategy should help to get more skyframe sharing. Consider the example above. A
      // subsequent request of "a/b/...", without any excluded paths, will be a cache hit.
      //
      // TODO(bazel-team): Replace the excludedPaths set with a trie or a SortedSet for better
      // efficiency.
      ImmutableSet<PathFragment> excludedSubdirectoriesBeneathThisSubdirectory =
          PathFragment.filterPathsStartingWith(excludedPaths, subdirectory);
      RootedPath subdirectoryRootedPath = RootedPath.toRootedPath(root, subdirectory);
      childDeps.add(
          getSkyKeyForSubdirectory(
              recursivePkgKey.getRepository(),
              subdirectoryRootedPath,
              excludedSubdirectoriesBeneathThisSubdirectory));
    }

    Map<SkyKey, SkyValue> subdirectorySkyValues;
    if (pkgLookupValue.packageExists() && pkgLookupValue.getRoot().equals(root)) {
      SkyKey packageKey = PackageValue.key(packageId);
      Map<SkyKey, ValueOrException<NoSuchPackageException>> dependentSkyValues =
          env.getValuesOrThrow(
              Iterables.concat(childDeps, ImmutableList.of(packageKey)),
              NoSuchPackageException.class);
      if (env.valuesMissing()) {
        return null;
      }
      Package pkg = null;
      try {
        PackageValue pkgValue = (PackageValue) dependentSkyValues.get(packageKey).get();
        if (pkgValue == null) {
          return null;
        }
        pkg = pkgValue.getPackage();
        if (pkg.containsErrors()) {
          env.getListener()
              .handle(Event.error("package contains errors: " + rootRelativePath.getPathString()));
        }
      } catch (NoSuchPackageException e) {
        // The package had errors, but don't fail-fast as there might be subpackages below the
        // current directory.
        env.getListener()
            .handle(Event.error("package contains errors: " + rootRelativePath.getPathString()));
      }
      if (pkg != null) {
        visitor.visitPackageValue(pkg, env);
        if (env.valuesMissing()) {
          return null;
        }
      }
      ImmutableMap.Builder<SkyKey, SkyValue> subdirectoryBuilder = ImmutableMap.builder();
      for (Map.Entry<SkyKey, ValueOrException<NoSuchPackageException>> entry :
          Maps.filterKeys(dependentSkyValues, Predicates.not(Predicates.equalTo(packageKey)))
              .entrySet()) {
        try {
          subdirectoryBuilder.put(entry.getKey(), entry.getValue().get());
        } catch (NoSuchPackageException e) {
          // ignored.
        }
      }
      subdirectorySkyValues = subdirectoryBuilder.build();
    } else {
      subdirectorySkyValues = env.getValues(childDeps);
    }
    if (env.valuesMissing()) {
      return null;
    }
    return aggregateWithSubdirectorySkyValues(visitor, subdirectorySkyValues);
  }