src/main/java/com/google/devtools/build/lib/skyframe/FileFunction.java - bazel - Git at Google

 // 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.base.Predicate;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.Iterables;
 import com.google.common.collect.Sets;
 import com.google.devtools.build.lib.actions.FileStateType;
 import com.google.devtools.build.lib.actions.FileStateValue;
 import com.google.devtools.build.lib.actions.FileValue;
 import com.google.devtools.build.lib.pkgcache.PathPackageLocator;
 import com.google.devtools.build.lib.util.Pair;
 import com.google.devtools.build.lib.vfs.Path;
 import com.google.devtools.build.lib.vfs.PathFragment;
 import com.google.devtools.build.lib.vfs.RootedPath;
 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 java.io.IOException;
 import java.util.ArrayList;
 import java.util.TreeSet;
 import java.util.concurrent.atomic.AtomicReference;
 import javax.annotation.Nullable;

 /**
  * A {@link SkyFunction} for {@link FileValue}s.
  *
  * <p>Most of the complexity in the implementation is associated to handling symlinks. Namely,
  * this class makes sure that {@code FileValue}s corresponding to symlinks are correctly invalidated
  * if the destination of the symlink is invalidated. Directory symlinks are also covered.
  */
 public class FileFunction implements SkyFunction {
   private final AtomicReference<PathPackageLocator> pkgLocator;
   @Nullable private final NonexistentFileReceiver nonexistentFileReceiver;

   /** Temporary interface to help track down why files are missing in some cases. */
   public interface NonexistentFileReceiver {
     void accept(
         RootedPath rootedPath,
         RootedPath realRootedPath,
         RootedPath parentRootedPath,
         FileValue parentFileValue);
   }

   public FileFunction(AtomicReference<PathPackageLocator> pkgLocator) {
     this(pkgLocator, null);
   }

   FileFunction(
       AtomicReference<PathPackageLocator> pkgLocator,
       @Nullable NonexistentFileReceiver nonexistentFileReceiver) {
     this.pkgLocator = pkgLocator;
     this.nonexistentFileReceiver = nonexistentFileReceiver;
   }

   @Override
   public FileValue compute(SkyKey skyKey, Environment env)
       throws FileFunctionException, InterruptedException {
     RootedPath rootedPath = (RootedPath) skyKey.argument();
     RootedPath realRootedPath = null;
     FileStateValue realFileStateValue = null;
     PathFragment relativePath = rootedPath.getRootRelativePath();

     // Resolve ancestor symlinks, but only if the current file is not the filesystem root (has no
     // parent) or a package path root (treated opaquely and handled by skyframe's DiffAwareness
     // interface). Note that this is the first thing we do - if an ancestor is part of a
     // symlink cycle, we want to detect that quickly as it gives a more informative error message
     // than we'd get doing bogus filesystem operations.
     if (!relativePath.equals(PathFragment.EMPTY_FRAGMENT)) {
       Pair<RootedPath, FileStateValue> resolvedState = resolveFromAncestors(rootedPath, env);
       if (resolvedState == null) {
         return null;
       }
       realRootedPath = resolvedState.getFirst();
       realFileStateValue = resolvedState.getSecond();
       if (realFileStateValue.getType() == FileStateType.NONEXISTENT) {
         return FileValue.value(
             rootedPath,
             FileStateValue.NONEXISTENT_FILE_STATE_NODE,
             realRootedPath,
             realFileStateValue);
       }
     }

     FileStateValue fileStateValue;
     if (rootedPath.equals(realRootedPath)) {
       fileStateValue = Preconditions.checkNotNull(realFileStateValue, rootedPath);
     } else {
       fileStateValue = (FileStateValue) env.getValue(FileStateValue.key(rootedPath));
       if (fileStateValue == null) {
         return null;
       }
     }

     if (realFileStateValue == null) {
       realRootedPath = rootedPath;
       realFileStateValue = fileStateValue;
     }

     ArrayList<RootedPath> symlinkChain = new ArrayList<>();
     TreeSet<Path> orderedSeenPaths = Sets.newTreeSet();
     while (realFileStateValue.getType().isSymlink()) {
       symlinkChain.add(realRootedPath);
       orderedSeenPaths.add(realRootedPath.asPath());
       Pair<RootedPath, FileStateValue> resolvedState = getSymlinkTargetRootedPath(realRootedPath,
           realFileStateValue.getSymlinkTarget(), orderedSeenPaths, symlinkChain, env);
       if (resolvedState == null) {
         return null;
       }
       realRootedPath = resolvedState.getFirst();
       realFileStateValue = resolvedState.getSecond();
     }
     return FileValue.value(rootedPath, fileStateValue, realRootedPath, realFileStateValue);
   }

   /**
    * Returns the path and file state of {@code rootedPath}, accounting for ancestor symlinks, or
    * {@code null} if there was a missing dep.
    */
   @Nullable
   private Pair<RootedPath, FileStateValue> resolveFromAncestors(
       RootedPath rootedPath, Environment env) throws InterruptedException {
     PathFragment relativePath = rootedPath.getRootRelativePath();
     RootedPath realRootedPath = rootedPath;
     FileValue parentFileValue = null;
     PathFragment parentDirectory = relativePath.getParentDirectory();
     if (parentDirectory != null) {
       RootedPath parentRootedPath = RootedPath.toRootedPath(rootedPath.getRoot(), parentDirectory);

       parentFileValue = (FileValue) env.getValue(FileValue.key(parentRootedPath));
       if (parentFileValue == null) {
         return null;
       }
       PathFragment baseName = PathFragment.create(relativePath.getBaseName());
       RootedPath parentRealRootedPath = parentFileValue.realRootedPath();
       realRootedPath =
           RootedPath.toRootedPath(
               parentRealRootedPath.getRoot(),
               parentRealRootedPath.getRootRelativePath().getRelative(baseName));

       if (!parentFileValue.exists() || !parentFileValue.isDirectory()) {
         if (nonexistentFileReceiver != null) {
           nonexistentFileReceiver.accept(
               rootedPath, realRootedPath, parentRootedPath, parentFileValue);
         }
         return Pair.of(realRootedPath, FileStateValue.NONEXISTENT_FILE_STATE_NODE);
       }
     }
     FileStateValue realFileStateValue =
         (FileStateValue)
             env.getValue(FileStateValue.key(realRootedPath));

     if (realFileStateValue == null) {
       return null;
     }
     return Pair.of(realRootedPath, realFileStateValue);
   }

   /**
    * Returns the symlink target and file state of {@code rootedPath}'s symlink to {@code
    * symlinkTarget}, accounting for ancestor symlinks, or {@code null} if there was a missing dep.
    */
   @Nullable
   private Pair<RootedPath, FileStateValue> getSymlinkTargetRootedPath(
       RootedPath rootedPath,
       PathFragment symlinkTarget,
       TreeSet<Path> orderedSeenPaths,
       Iterable<RootedPath> symlinkChain,
       Environment env)
       throws FileFunctionException, InterruptedException {
     RootedPath symlinkTargetRootedPath;
     if (symlinkTarget.isAbsolute()) {
       Path path = rootedPath.asPath().getFileSystem().getPath(symlinkTarget);
       symlinkTargetRootedPath =
           RootedPath.toRootedPathMaybeUnderRoot(path, pkgLocator.get().getPathEntries());
     } else {
       Path path = rootedPath.asPath();
       Path symlinkTargetPath;
       if (path.getParentDirectory() != null) {
         RootedPath parentRootedPath = RootedPath.toRootedPathMaybeUnderRoot(
             path.getParentDirectory(), pkgLocator.get().getPathEntries());
         FileValue parentFileValue = (FileValue) env.getValue(FileValue.key(parentRootedPath));
         if (parentFileValue == null) {
           return null;
         }
         symlinkTargetPath = parentFileValue.realRootedPath().asPath().getRelative(symlinkTarget);
       } else {
         // This means '/' is a symlink to 'symlinkTarget'.
         symlinkTargetPath = path.getRelative(symlinkTarget);
       }
       symlinkTargetRootedPath = RootedPath.toRootedPathMaybeUnderRoot(symlinkTargetPath,
           pkgLocator.get().getPathEntries());
     }
     // Suppose we have a symlink chain p -> p1 -> p2 -> ... pK. We want to determine the fully
     // resolved path, if any, of p. This entails following the chain and noticing if there's a
     // symlink issue. There three sorts of issues:
     // (i) Symlink cycle:
     //   p -> p1 -> p2 -> p1
     // (ii) Unbounded expansion caused by a symlink to a descendant of a member of the chain:
     //   p -> a/b -> c/d -> a/b/e
     // (iii) Unbounded expansion caused by a symlink to an ancestor of a member of the chain:
     //   p -> a/b -> c/d -> a
     //
     // We can detect all three of these symlink issues by following the chain and deciding if each
     // new element is problematic. Here is our incremental algorithm:
     //
     // Suppose we encounter the symlink target p and we have already encountered all the paths in P:
     //   If p is in P then we have a found a cycle (i).
     //   If p is a descendant of any path p' in P then we have unbounded expansion (ii).
     //   If p is an ancestor of any path p' in P then we have unbounded expansion (iii).
     // We can check for these cases efficiently (read: sublinear time) by finding the extremal
     // candidate p' for (ii) and (iii).
     Path symlinkTargetPath = symlinkTargetRootedPath.asPath();
     SkyKey uniquenessKey = null;
     FileSymlinkException fse = null;
     Path seenFloorPath = orderedSeenPaths.floor(symlinkTargetPath);
     Path seenCeilingPath = orderedSeenPaths.ceiling(symlinkTargetPath);
     if (orderedSeenPaths.contains(symlinkTargetPath)) {
       // 'rootedPath' is a symlink to a previous element in the symlink chain (i).
       Pair<ImmutableList<RootedPath>, ImmutableList<RootedPath>> pathAndChain =
           CycleUtils.splitIntoPathAndChain(
               isPathPredicate(symlinkTargetRootedPath.asPath()), symlinkChain);
       FileSymlinkCycleException fsce =
           new FileSymlinkCycleException(pathAndChain.getFirst(), pathAndChain.getSecond());
       uniquenessKey = FileSymlinkCycleUniquenessFunction.key(fsce.getCycle());
       fse = fsce;
     } else if (seenFloorPath != null && symlinkTargetPath.startsWith(seenFloorPath)) {
       // 'rootedPath' is a symlink to a descendant of a previous element in the symlink chain (ii).
       Pair<ImmutableList<RootedPath>, ImmutableList<RootedPath>> pathAndChain =
           CycleUtils.splitIntoPathAndChain(
               isPathPredicate(seenFloorPath),
               ImmutableList.copyOf(
                   Iterables.concat(symlinkChain, ImmutableList.of(symlinkTargetRootedPath))));
       uniquenessKey = FileSymlinkInfiniteExpansionUniquenessFunction.key(pathAndChain.getSecond());
       fse = new FileSymlinkInfiniteExpansionException(
           pathAndChain.getFirst(), pathAndChain.getSecond());
     } else if (seenCeilingPath != null && seenCeilingPath.startsWith(symlinkTargetPath)) {
       // 'rootedPath' is a symlink to an ancestor of a previous element in the symlink chain (iii).
       Pair<ImmutableList<RootedPath>, ImmutableList<RootedPath>> pathAndChain =
           CycleUtils.splitIntoPathAndChain(
               isPathPredicate(seenCeilingPath),
               ImmutableList.copyOf(
                   Iterables.concat(symlinkChain, ImmutableList.of(symlinkTargetRootedPath))));
       uniquenessKey = FileSymlinkInfiniteExpansionUniquenessFunction.key(pathAndChain.getSecond());
       fse =
           new FileSymlinkInfiniteExpansionException(
               pathAndChain.getFirst(), pathAndChain.getSecond());
     }
     if (uniquenessKey != null) {
       if (env.getValue(uniquenessKey) == null) {
         // Note that this dependency is merely to ensure that each unique symlink error gets
         // reported exactly once.
         return null;
       }
       throw new FileFunctionException(
           Preconditions.checkNotNull(fse, rootedPath), Transience.PERSISTENT);
     }

     return resolveFromAncestors(symlinkTargetRootedPath, env);
   }

   private static final Predicate<RootedPath> isPathPredicate(final Path path) {
     return new Predicate<RootedPath>() {
       @Override
       public boolean apply(RootedPath rootedPath) {
         return rootedPath.asPath().equals(path);
       }
     };
   }

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

   /**
    * Used to declare all the exception types that can be wrapped in the exception thrown by
    * {@link FileFunction#compute}.
    */
   private static final class FileFunctionException extends SkyFunctionException {
     public FileFunctionException(IOException e, Transience transience) {
       super(e, transience);
     }
   }
 }
	// 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.base.Predicate;
	import com.google.common.collect.ImmutableList;
	import com.google.common.collect.Iterables;
	import com.google.common.collect.Sets;
	import com.google.devtools.build.lib.actions.FileStateType;
	import com.google.devtools.build.lib.actions.FileStateValue;
	import com.google.devtools.build.lib.actions.FileValue;
	import com.google.devtools.build.lib.pkgcache.PathPackageLocator;
	import com.google.devtools.build.lib.util.Pair;
	import com.google.devtools.build.lib.vfs.Path;
	import com.google.devtools.build.lib.vfs.PathFragment;
	import com.google.devtools.build.lib.vfs.RootedPath;
	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 java.io.IOException;
	import java.util.ArrayList;
	import java.util.TreeSet;
	import java.util.concurrent.atomic.AtomicReference;
	import javax.annotation.Nullable;

	/**
	* A {@link SkyFunction} for {@link FileValue}s.
	*
	* <p>Most of the complexity in the implementation is associated to handling symlinks. Namely,
	* this class makes sure that {@code FileValue}s corresponding to symlinks are correctly invalidated
	* if the destination of the symlink is invalidated. Directory symlinks are also covered.
	*/
	public class FileFunction implements SkyFunction {
	private final AtomicReference<PathPackageLocator> pkgLocator;
	@Nullable private final NonexistentFileReceiver nonexistentFileReceiver;

	/** Temporary interface to help track down why files are missing in some cases. */
	public interface NonexistentFileReceiver {
	void accept(
	RootedPath rootedPath,
	RootedPath realRootedPath,
	RootedPath parentRootedPath,
	FileValue parentFileValue);
	}

	public FileFunction(AtomicReference<PathPackageLocator> pkgLocator) {
	this(pkgLocator, null);
	}

	FileFunction(
	AtomicReference<PathPackageLocator> pkgLocator,
	@Nullable NonexistentFileReceiver nonexistentFileReceiver) {
	this.pkgLocator = pkgLocator;
	this.nonexistentFileReceiver = nonexistentFileReceiver;
	}

	@Override
	public FileValue compute(SkyKey skyKey, Environment env)
	throws FileFunctionException, InterruptedException {
	RootedPath rootedPath = (RootedPath) skyKey.argument();
	RootedPath realRootedPath = null;
	FileStateValue realFileStateValue = null;
	PathFragment relativePath = rootedPath.getRootRelativePath();

	// Resolve ancestor symlinks, but only if the current file is not the filesystem root (has no
	// parent) or a package path root (treated opaquely and handled by skyframe's DiffAwareness
	// interface). Note that this is the first thing we do - if an ancestor is part of a
	// symlink cycle, we want to detect that quickly as it gives a more informative error message
	// than we'd get doing bogus filesystem operations.
	if (!relativePath.equals(PathFragment.EMPTY_FRAGMENT)) {
	Pair<RootedPath, FileStateValue> resolvedState = resolveFromAncestors(rootedPath, env);
	if (resolvedState == null) {
	return null;
	}
	realRootedPath = resolvedState.getFirst();
	realFileStateValue = resolvedState.getSecond();
	if (realFileStateValue.getType() == FileStateType.NONEXISTENT) {
	return FileValue.value(
	rootedPath,
	FileStateValue.NONEXISTENT_FILE_STATE_NODE,
	realRootedPath,
	realFileStateValue);
	}
	}

	FileStateValue fileStateValue;
	if (rootedPath.equals(realRootedPath)) {
	fileStateValue = Preconditions.checkNotNull(realFileStateValue, rootedPath);
	} else {
	fileStateValue = (FileStateValue) env.getValue(FileStateValue.key(rootedPath));
	if (fileStateValue == null) {
	return null;
	}
	}

	if (realFileStateValue == null) {
	realRootedPath = rootedPath;
	realFileStateValue = fileStateValue;
	}

	ArrayList<RootedPath> symlinkChain = new ArrayList<>();
	TreeSet<Path> orderedSeenPaths = Sets.newTreeSet();
	while (realFileStateValue.getType().isSymlink()) {
	symlinkChain.add(realRootedPath);
	orderedSeenPaths.add(realRootedPath.asPath());
	Pair<RootedPath, FileStateValue> resolvedState = getSymlinkTargetRootedPath(realRootedPath,
	realFileStateValue.getSymlinkTarget(), orderedSeenPaths, symlinkChain, env);
	if (resolvedState == null) {
	return null;
	}
	realRootedPath = resolvedState.getFirst();
	realFileStateValue = resolvedState.getSecond();
	}
	return FileValue.value(rootedPath, fileStateValue, realRootedPath, realFileStateValue);
	}

	/**
	* Returns the path and file state of {@code rootedPath}, accounting for ancestor symlinks, or
	* {@code null} if there was a missing dep.
	*/
	@Nullable
	private Pair<RootedPath, FileStateValue> resolveFromAncestors(
	RootedPath rootedPath, Environment env) throws InterruptedException {
	PathFragment relativePath = rootedPath.getRootRelativePath();
	RootedPath realRootedPath = rootedPath;
	FileValue parentFileValue = null;
	PathFragment parentDirectory = relativePath.getParentDirectory();
	if (parentDirectory != null) {
	RootedPath parentRootedPath = RootedPath.toRootedPath(rootedPath.getRoot(), parentDirectory);

	parentFileValue = (FileValue) env.getValue(FileValue.key(parentRootedPath));
	if (parentFileValue == null) {
	return null;
	}
	PathFragment baseName = PathFragment.create(relativePath.getBaseName());
	RootedPath parentRealRootedPath = parentFileValue.realRootedPath();
	realRootedPath =
	RootedPath.toRootedPath(
	parentRealRootedPath.getRoot(),
	parentRealRootedPath.getRootRelativePath().getRelative(baseName));

	if (!parentFileValue.exists() \|\| !parentFileValue.isDirectory()) {
	if (nonexistentFileReceiver != null) {
	nonexistentFileReceiver.accept(
	rootedPath, realRootedPath, parentRootedPath, parentFileValue);
	}
	return Pair.of(realRootedPath, FileStateValue.NONEXISTENT_FILE_STATE_NODE);
	}
	}
	FileStateValue realFileStateValue =
	(FileStateValue)
	env.getValue(FileStateValue.key(realRootedPath));

	if (realFileStateValue == null) {
	return null;
	}
	return Pair.of(realRootedPath, realFileStateValue);
	}

	/**
	* Returns the symlink target and file state of {@code rootedPath}'s symlink to {@code
	* symlinkTarget}, accounting for ancestor symlinks, or {@code null} if there was a missing dep.
	*/
	@Nullable
	private Pair<RootedPath, FileStateValue> getSymlinkTargetRootedPath(
	RootedPath rootedPath,
	PathFragment symlinkTarget,
	TreeSet<Path> orderedSeenPaths,
	Iterable<RootedPath> symlinkChain,
	Environment env)
	throws FileFunctionException, InterruptedException {
	RootedPath symlinkTargetRootedPath;
	if (symlinkTarget.isAbsolute()) {
	Path path = rootedPath.asPath().getFileSystem().getPath(symlinkTarget);
	symlinkTargetRootedPath =
	RootedPath.toRootedPathMaybeUnderRoot(path, pkgLocator.get().getPathEntries());
	} else {
	Path path = rootedPath.asPath();
	Path symlinkTargetPath;
	if (path.getParentDirectory() != null) {
	RootedPath parentRootedPath = RootedPath.toRootedPathMaybeUnderRoot(
	path.getParentDirectory(), pkgLocator.get().getPathEntries());
	FileValue parentFileValue = (FileValue) env.getValue(FileValue.key(parentRootedPath));
	if (parentFileValue == null) {
	return null;
	}
	symlinkTargetPath = parentFileValue.realRootedPath().asPath().getRelative(symlinkTarget);
	} else {
	// This means '/' is a symlink to 'symlinkTarget'.
	symlinkTargetPath = path.getRelative(symlinkTarget);
	}
	symlinkTargetRootedPath = RootedPath.toRootedPathMaybeUnderRoot(symlinkTargetPath,
	pkgLocator.get().getPathEntries());
	}
	// Suppose we have a symlink chain p -> p1 -> p2 -> ... pK. We want to determine the fully
	// resolved path, if any, of p. This entails following the chain and noticing if there's a
	// symlink issue. There three sorts of issues:
	// (i) Symlink cycle:
	// p -> p1 -> p2 -> p1
	// (ii) Unbounded expansion caused by a symlink to a descendant of a member of the chain:
	// p -> a/b -> c/d -> a/b/e
	// (iii) Unbounded expansion caused by a symlink to an ancestor of a member of the chain:
	// p -> a/b -> c/d -> a
	//
	// We can detect all three of these symlink issues by following the chain and deciding if each
	// new element is problematic. Here is our incremental algorithm:
	//
	// Suppose we encounter the symlink target p and we have already encountered all the paths in P:
	// If p is in P then we have a found a cycle (i).
	// If p is a descendant of any path p' in P then we have unbounded expansion (ii).
	// If p is an ancestor of any path p' in P then we have unbounded expansion (iii).
	// We can check for these cases efficiently (read: sublinear time) by finding the extremal
	// candidate p' for (ii) and (iii).
	Path symlinkTargetPath = symlinkTargetRootedPath.asPath();
	SkyKey uniquenessKey = null;
	FileSymlinkException fse = null;
	Path seenFloorPath = orderedSeenPaths.floor(symlinkTargetPath);
	Path seenCeilingPath = orderedSeenPaths.ceiling(symlinkTargetPath);
	if (orderedSeenPaths.contains(symlinkTargetPath)) {
	// 'rootedPath' is a symlink to a previous element in the symlink chain (i).
	Pair<ImmutableList<RootedPath>, ImmutableList<RootedPath>> pathAndChain =
	CycleUtils.splitIntoPathAndChain(
	isPathPredicate(symlinkTargetRootedPath.asPath()), symlinkChain);
	FileSymlinkCycleException fsce =
	new FileSymlinkCycleException(pathAndChain.getFirst(), pathAndChain.getSecond());
	uniquenessKey = FileSymlinkCycleUniquenessFunction.key(fsce.getCycle());
	fse = fsce;
	} else if (seenFloorPath != null && symlinkTargetPath.startsWith(seenFloorPath)) {
	// 'rootedPath' is a symlink to a descendant of a previous element in the symlink chain (ii).
	Pair<ImmutableList<RootedPath>, ImmutableList<RootedPath>> pathAndChain =
	CycleUtils.splitIntoPathAndChain(
	isPathPredicate(seenFloorPath),
	ImmutableList.copyOf(
	Iterables.concat(symlinkChain, ImmutableList.of(symlinkTargetRootedPath))));
	uniquenessKey = FileSymlinkInfiniteExpansionUniquenessFunction.key(pathAndChain.getSecond());
	fse = new FileSymlinkInfiniteExpansionException(
	pathAndChain.getFirst(), pathAndChain.getSecond());
	} else if (seenCeilingPath != null && seenCeilingPath.startsWith(symlinkTargetPath)) {
	// 'rootedPath' is a symlink to an ancestor of a previous element in the symlink chain (iii).
	Pair<ImmutableList<RootedPath>, ImmutableList<RootedPath>> pathAndChain =
	CycleUtils.splitIntoPathAndChain(
	isPathPredicate(seenCeilingPath),
	ImmutableList.copyOf(
	Iterables.concat(symlinkChain, ImmutableList.of(symlinkTargetRootedPath))));
	uniquenessKey = FileSymlinkInfiniteExpansionUniquenessFunction.key(pathAndChain.getSecond());
	fse =
	new FileSymlinkInfiniteExpansionException(
	pathAndChain.getFirst(), pathAndChain.getSecond());
	}
	if (uniquenessKey != null) {
	if (env.getValue(uniquenessKey) == null) {
	// Note that this dependency is merely to ensure that each unique symlink error gets
	// reported exactly once.
	return null;
	}
	throw new FileFunctionException(
	Preconditions.checkNotNull(fse, rootedPath), Transience.PERSISTENT);
	}

	return resolveFromAncestors(symlinkTargetRootedPath, env);
	}

	private static final Predicate<RootedPath> isPathPredicate(final Path path) {
	return new Predicate<RootedPath>() {
	@Override
	public boolean apply(RootedPath rootedPath) {
	return rootedPath.asPath().equals(path);
	}
	};
	}

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

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