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.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.pkgcache.PathPackageLocator;
 import com.google.devtools.build.lib.util.Pair;
 import com.google.devtools.build.lib.util.io.TimestampGranularityMonitor;
 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 com.google.devtools.build.skyframe.SkyValue;

 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;
   private final TimestampGranularityMonitor tsgm;
   private final ExternalFilesHelper externalFilesHelper;

   public FileFunction(AtomicReference<PathPackageLocator> pkgLocator,
       TimestampGranularityMonitor tsgm,
       ExternalFilesHelper externalFilesHelper) {
     this.pkgLocator = pkgLocator;
     this.tsgm = tsgm;
     this.externalFilesHelper = externalFilesHelper;
   }

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

     // 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();
     }

     FileStateValue fileStateValue = (FileStateValue) env.getValue(FileStateValue.key(rootedPath));
     if (fileStateValue == null) {
       return null;
     }
     if (realFileStateValue == null) {
       realFileStateValue = fileStateValue;
     }

     ArrayList<RootedPath> symlinkChain = new ArrayList<>();
     TreeSet<Path> orderedSeenPaths = Sets.newTreeSet();
     while (realFileStateValue.getType().equals(FileStateValue.Type.SYMLINK)) {
       symlinkChain.add(realRootedPath);
       orderedSeenPaths.add(realRootedPath.asPath());
       if (externalFilesHelper.shouldAssumeImmutable(realRootedPath)) {
         // If the file is assumed to be immutable, we want to resolve the symlink chain without
         // adding dependencies since we don't care about incremental correctness.
         try {
           Path realPath = rootedPath.asPath().resolveSymbolicLinks();
           realRootedPath = RootedPath.toRootedPathMaybeUnderRoot(realPath,
               pkgLocator.get().getPathEntries());
           realFileStateValue = FileStateValue.create(realRootedPath, tsgm);
         } catch (IOException e) {
           RootedPath root = RootedPath.toRootedPath(
               rootedPath.asPath().getFileSystem().getRootDirectory(),
               rootedPath.asPath().getFileSystem().getRootDirectory());
           return FileValue.value(
               rootedPath, fileStateValue,
               root, FileStateValue.NONEXISTENT_FILE_STATE_NODE);
         } catch (InconsistentFilesystemException e) {
           throw new FileFunctionException(e, Transience.TRANSIENT);
         }
       } else {
         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 FileFunctionException {
     PathFragment relativePath = rootedPath.getRelativePath();
     RootedPath realRootedPath = rootedPath;
     FileValue parentFileValue = null;
     // We only resolve ancestors if the file is not assumed to be immutable (handling ancestors
     // would be too aggressive).
     if (!externalFilesHelper.shouldAssumeImmutable(rootedPath)
         && !relativePath.equals(PathFragment.EMPTY_FRAGMENT)) {
       RootedPath parentRootedPath = RootedPath.toRootedPath(rootedPath.getRoot(),
           relativePath.getParentDirectory());
       parentFileValue = (FileValue) env.getValue(FileValue.key(parentRootedPath));
       if (parentFileValue == null) {
         return null;
       }
       PathFragment baseName = new PathFragment(relativePath.getBaseName());
       RootedPath parentRealRootedPath = parentFileValue.realRootedPath();
       realRootedPath = RootedPath.toRootedPath(parentRealRootedPath.getRoot(),
           parentRealRootedPath.getRelativePath().getRelative(baseName));
       if (!parentFileValue.exists()) {
         return Pair.of(realRootedPath, FileStateValue.NONEXISTENT_FILE_STATE_NODE);
       }
     }
     FileStateValue realFileStateValue =
         (FileStateValue) env.getValue(FileStateValue.key(realRootedPath));
     if (realFileStateValue == null) {
       return null;
     }
     if (realFileStateValue.getType() != FileStateValue.Type.NONEXISTENT
         && parentFileValue != null && !parentFileValue.isDirectory()) {
       String type = realFileStateValue.getType().toString().toLowerCase();
       String message = type + " " + rootedPath.asPath() + " exists but its parent "
           + "path " + parentFileValue.realRootedPath().asPath() + " isn't an existing directory.";
       throw new FileFunctionException(new InconsistentFilesystemException(message),
           Transience.TRANSIENT);
     }
     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 {
     RootedPath symlinkTargetRootedPath;
     if (symlinkTarget.isAbsolute()) {
       Path path = rootedPath.asPath().getFileSystem().getRootDirectory().getRelative(
           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());
     }
     Path symlinkTargetPath = symlinkTargetRootedPath.asPath();
     Path existingFloorPath = orderedSeenPaths.floor(symlinkTargetPath);
     // Here is a brief argument that the following logic is correct.
     //
     // Any path 'p' in the symlink chain that is no larger than 'symlinkTargetPath' is one of:
     //   (i)   'symlinkTargetPath'
     //   (ii)   a smaller sibling 's' of 'symlinkTargetPath' or a sibling of an ancestor of
     //         'symlinkTargetPath'
     //   (iii)  an ancestor 'a' of 'symlinkTargetPath'
     //   (iv) something else (e.g. a smaller sibling of an ancestor of 'symlinkTargetPath')
     // If the largest 'p' is 'symlinkTarget' itself then 'existingFloorPath' will be that and we
     // have found cycle. Otherwise, if there is such a 's' then 'existingFloorPath' will be the
     // largest one. But the presence of any such 's' in the symlink chain implies an infinite
     // expansion, which we would have already noticed. On the other hand, if there is such an 'a'
     // then 'existingFloorPath' will be the largest one that and we definitely have found an
     // infinite symlink expansion. Otherwise, if there is no such 'a', then the presence of
     // 'symlinkTargetPath' doesn't create an infinite symlink expansion.
     if (existingFloorPath != null && symlinkTargetPath.startsWith(existingFloorPath)) {
       SkyKey uniquenessKey;
       FileSymlinkException fse;
       if (symlinkTargetPath.equals(existingFloorPath)) {
         Pair<ImmutableList<RootedPath>, ImmutableList<RootedPath>> pathAndChain =
             CycleUtils.splitIntoPathAndChain(
                 isPathPredicate(symlinkTargetRootedPath.asPath()), symlinkChain);
         FileSymlinkCycleException fsce =
             new FileSymlinkCycleException(pathAndChain.getFirst(), pathAndChain.getSecond());
         uniquenessKey = FileSymlinkCycleUniquenessValue.key(fsce.getCycle());
         fse = fsce;
       } else {
         Pair<ImmutableList<RootedPath>, ImmutableList<RootedPath>> pathAndChain =
             CycleUtils.splitIntoPathAndChain(
                 isPathPredicate(existingFloorPath),
                 ImmutableList.copyOf(
                     Iterables.concat(symlinkChain, ImmutableList.of(symlinkTargetRootedPath))));
         uniquenessKey = FileSymlinkInfiniteExpansionUniquenessValue.key(pathAndChain.getSecond());
         fse = new FileSymlinkInfiniteExpansionException(
             pathAndChain.getFirst(), pathAndChain.getSecond());
       }
       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(fse);
     }
     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(InconsistentFilesystemException e, Transience transience) {
       super(e, transience);
     }

     public FileFunctionException(FileSymlinkException e) {
       super(e, Transience.PERSISTENT);
     }

     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.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.pkgcache.PathPackageLocator;
	import com.google.devtools.build.lib.util.Pair;
	import com.google.devtools.build.lib.util.io.TimestampGranularityMonitor;
	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 com.google.devtools.build.skyframe.SkyValue;

	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;
	private final TimestampGranularityMonitor tsgm;
	private final ExternalFilesHelper externalFilesHelper;

	public FileFunction(AtomicReference<PathPackageLocator> pkgLocator,
	TimestampGranularityMonitor tsgm,
	ExternalFilesHelper externalFilesHelper) {
	this.pkgLocator = pkgLocator;
	this.tsgm = tsgm;
	this.externalFilesHelper = externalFilesHelper;
	}

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

	// 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();
	}

	FileStateValue fileStateValue = (FileStateValue) env.getValue(FileStateValue.key(rootedPath));
	if (fileStateValue == null) {
	return null;
	}
	if (realFileStateValue == null) {
	realFileStateValue = fileStateValue;
	}

	ArrayList<RootedPath> symlinkChain = new ArrayList<>();
	TreeSet<Path> orderedSeenPaths = Sets.newTreeSet();
	while (realFileStateValue.getType().equals(FileStateValue.Type.SYMLINK)) {
	symlinkChain.add(realRootedPath);
	orderedSeenPaths.add(realRootedPath.asPath());
	if (externalFilesHelper.shouldAssumeImmutable(realRootedPath)) {
	// If the file is assumed to be immutable, we want to resolve the symlink chain without
	// adding dependencies since we don't care about incremental correctness.
	try {
	Path realPath = rootedPath.asPath().resolveSymbolicLinks();
	realRootedPath = RootedPath.toRootedPathMaybeUnderRoot(realPath,
	pkgLocator.get().getPathEntries());
	realFileStateValue = FileStateValue.create(realRootedPath, tsgm);
	} catch (IOException e) {
	RootedPath root = RootedPath.toRootedPath(
	rootedPath.asPath().getFileSystem().getRootDirectory(),
	rootedPath.asPath().getFileSystem().getRootDirectory());
	return FileValue.value(
	rootedPath, fileStateValue,
	root, FileStateValue.NONEXISTENT_FILE_STATE_NODE);
	} catch (InconsistentFilesystemException e) {
	throw new FileFunctionException(e, Transience.TRANSIENT);
	}
	} else {
	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 FileFunctionException {
	PathFragment relativePath = rootedPath.getRelativePath();
	RootedPath realRootedPath = rootedPath;
	FileValue parentFileValue = null;
	// We only resolve ancestors if the file is not assumed to be immutable (handling ancestors
	// would be too aggressive).
	if (!externalFilesHelper.shouldAssumeImmutable(rootedPath)
	&& !relativePath.equals(PathFragment.EMPTY_FRAGMENT)) {
	RootedPath parentRootedPath = RootedPath.toRootedPath(rootedPath.getRoot(),
	relativePath.getParentDirectory());
	parentFileValue = (FileValue) env.getValue(FileValue.key(parentRootedPath));
	if (parentFileValue == null) {
	return null;
	}
	PathFragment baseName = new PathFragment(relativePath.getBaseName());
	RootedPath parentRealRootedPath = parentFileValue.realRootedPath();
	realRootedPath = RootedPath.toRootedPath(parentRealRootedPath.getRoot(),
	parentRealRootedPath.getRelativePath().getRelative(baseName));
	if (!parentFileValue.exists()) {
	return Pair.of(realRootedPath, FileStateValue.NONEXISTENT_FILE_STATE_NODE);
	}
	}
	FileStateValue realFileStateValue =
	(FileStateValue) env.getValue(FileStateValue.key(realRootedPath));
	if (realFileStateValue == null) {
	return null;
	}
	if (realFileStateValue.getType() != FileStateValue.Type.NONEXISTENT
	&& parentFileValue != null && !parentFileValue.isDirectory()) {
	String type = realFileStateValue.getType().toString().toLowerCase();
	String message = type + " " + rootedPath.asPath() + " exists but its parent "
	+ "path " + parentFileValue.realRootedPath().asPath() + " isn't an existing directory.";
	throw new FileFunctionException(new InconsistentFilesystemException(message),
	Transience.TRANSIENT);
	}
	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 {
	RootedPath symlinkTargetRootedPath;
	if (symlinkTarget.isAbsolute()) {
	Path path = rootedPath.asPath().getFileSystem().getRootDirectory().getRelative(
	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());
	}
	Path symlinkTargetPath = symlinkTargetRootedPath.asPath();
	Path existingFloorPath = orderedSeenPaths.floor(symlinkTargetPath);
	// Here is a brief argument that the following logic is correct.
	//
	// Any path 'p' in the symlink chain that is no larger than 'symlinkTargetPath' is one of:
	// (i) 'symlinkTargetPath'
	// (ii) a smaller sibling 's' of 'symlinkTargetPath' or a sibling of an ancestor of
	// 'symlinkTargetPath'
	// (iii) an ancestor 'a' of 'symlinkTargetPath'
	// (iv) something else (e.g. a smaller sibling of an ancestor of 'symlinkTargetPath')
	// If the largest 'p' is 'symlinkTarget' itself then 'existingFloorPath' will be that and we
	// have found cycle. Otherwise, if there is such a 's' then 'existingFloorPath' will be the
	// largest one. But the presence of any such 's' in the symlink chain implies an infinite
	// expansion, which we would have already noticed. On the other hand, if there is such an 'a'
	// then 'existingFloorPath' will be the largest one that and we definitely have found an
	// infinite symlink expansion. Otherwise, if there is no such 'a', then the presence of
	// 'symlinkTargetPath' doesn't create an infinite symlink expansion.
	if (existingFloorPath != null && symlinkTargetPath.startsWith(existingFloorPath)) {
	SkyKey uniquenessKey;
	FileSymlinkException fse;
	if (symlinkTargetPath.equals(existingFloorPath)) {
	Pair<ImmutableList<RootedPath>, ImmutableList<RootedPath>> pathAndChain =
	CycleUtils.splitIntoPathAndChain(
	isPathPredicate(symlinkTargetRootedPath.asPath()), symlinkChain);
	FileSymlinkCycleException fsce =
	new FileSymlinkCycleException(pathAndChain.getFirst(), pathAndChain.getSecond());
	uniquenessKey = FileSymlinkCycleUniquenessValue.key(fsce.getCycle());
	fse = fsce;
	} else {
	Pair<ImmutableList<RootedPath>, ImmutableList<RootedPath>> pathAndChain =
	CycleUtils.splitIntoPathAndChain(
	isPathPredicate(existingFloorPath),
	ImmutableList.copyOf(
	Iterables.concat(symlinkChain, ImmutableList.of(symlinkTargetRootedPath))));
	uniquenessKey = FileSymlinkInfiniteExpansionUniquenessValue.key(pathAndChain.getSecond());
	fse = new FileSymlinkInfiniteExpansionException(
	pathAndChain.getFirst(), pathAndChain.getSecond());
	}
	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(fse);
	}
	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(InconsistentFilesystemException e, Transience transience) {
	super(e, transience);
	}

	public FileFunctionException(FileSymlinkException e) {
	super(e, Transience.PERSISTENT);
	}

	public FileFunctionException(IOException e, Transience transience) {
	super(e, transience);
	}
	}
	}