FileSystem.java

// Copyright 2019 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.vfs;

import static com.google.common.base.Preconditions.checkArgument;
import static java.nio.charset.StandardCharsets.ISO_8859_1;

import com.google.common.base.Preconditions;
import com.google.common.collect.Lists;
import com.google.common.hash.Funnels;
import com.google.common.io.ByteSource;
import com.google.common.io.CharStreams;
import com.google.devtools.build.lib.concurrent.ThreadSafety.ThreadSafe;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.nio.channels.SeekableByteChannel;
import java.nio.file.FileAlreadyExistsException;
import java.util.Collection;
import java.util.Iterator;
import java.util.List;
import java.util.Locale;
import javax.annotation.Nullable;

/** This interface models a file system. */
@ThreadSafe
public abstract class FileSystem {

  // The maximum number of symbolic links that may be traversed by resolveSymbolicLinks() while
  // canonicalizing a path before it gives up and throws a FileSymlinkLoopException.
  public static final int MAX_SYMLINKS = 32;

  private final DigestHashFunction digestFunction;

  public FileSystem(DigestHashFunction digestFunction) {
    this.digestFunction = Preconditions.checkNotNull(digestFunction);
  }

  public DigestHashFunction getDigestFunction() {
    return digestFunction;
  }

  /** An exception thrown when attempting to resolve an ordinary file as a symlink. */
  public static final class NotASymlinkException extends IOException {
    public NotASymlinkException(PathFragment path) {
      super(path.getPathString() + " is not a symlink");
    }

    public NotASymlinkException(PathFragment path, Throwable cause) {
      super(path.getPathString() + " is not a symlink", cause);
    }
  }

  private final Root absoluteRoot = new Root.AbsoluteRoot(this);

  /**
   * Returns an absolute path instance, given an absolute path name, without double slashes, .., or
   * . segments. While this method will normalize the path representation by creating a
   * structured/parsed representation, it will not cause any IO. (e.g., it will not resolve symbolic
   * links if it's a Unix file system.
   */
  public Path getPath(String path) {
    return Path.create(path, this);
  }

  /** Returns an absolute path instance, given an absolute path fragment. */
  public Path getPath(PathFragment pathFragment) {
    return Path.create(pathFragment, this);
  }

  final Root getAbsoluteRoot() {
    return absoluteRoot;
  }

  /**
   * Returns whether or not the FileSystem supports modifications of files and file entries.
   *
   * <p>Returns true if FileSystem supports the following:
   *
   * <ul>
   *   <li>{@link #setWritable(PathFragment, boolean)}
   *   <li>{@link #setExecutable(PathFragment, boolean)}
   * </ul>
   *
   * The above calls will result in an {@link UnsupportedOperationException} on a FileSystem where
   * this method returns {@code false}.
   */
  public abstract boolean supportsModifications(PathFragment path);

  /**
   * Returns whether or not the FileSystem supports symbolic links.
   *
   * <p>Returns true if FileSystem supports the following:
   *
   * <ul>
   *   <li>{@link #createSymbolicLink(PathFragment, PathFragment)}
   *   <li>{@link #getFileSize(PathFragment, boolean)} where {@code followSymlinks=false}
   *   <li>{@link #getLastModifiedTime(PathFragment, boolean)} where {@code followSymlinks=false}
   *   <li>{@link #readSymbolicLink(PathFragment)} where the link points to a non-existent file
   * </ul>
   *
   * The above calls may result in an {@link UnsupportedOperationException} on a FileSystem where
   * this method returns {@code false}. The implementation can try to emulate these calls at its own
   * discretion.
   */
  public abstract boolean supportsSymbolicLinksNatively(PathFragment path);

  /**
   * Returns whether or not the FileSystem supports hard links.
   *
   * <p>Returns true if FileSystem supports the following:
   *
   * <ul>
   *   <li>{@link #createFSDependentHardLink(PathFragment, PathFragment)}
   * </ul>
   *
   * The above calls may result in an {@link UnsupportedOperationException} on a FileSystem where
   * this method returns {@code false}. The implementation can try to emulate these calls at its own
   * discretion.
   */
  public abstract boolean supportsHardLinksNatively(PathFragment path);

  /***
   * Returns true if file path is case-sensitive on this file system. Default is true.
   */
  public abstract boolean isFilePathCaseSensitive();

  /**
   * Returns the type of the file system path belongs to.
   *
   * <p>The string returned is obtained directly from the operating system, so it's a best guess in
   * absence of a guaranteed api.
   *
   * <p>This implementation uses <code>/proc/mounts</code> to determine the file system type.
   */
  public String getFileSystemType(PathFragment path) {
    String fileSystem = "unknown";
    int bestMountPointSegmentCount = -1;
    try {
      Path canonicalPath = resolveSymbolicLinks(path);
      PathFragment mountTable = PathFragment.createAlreadyNormalized("/proc/mounts");
      try (InputStreamReader reader =
          new InputStreamReader(getInputStream(mountTable), ISO_8859_1)) {
        for (String line : CharStreams.readLines(reader)) {
          String[] words = line.split("\\s+");
          if (words.length >= 3) {
            if (!words[1].startsWith("/")) {
              continue;
            }
            PathFragment mountPoint = PathFragment.create(words[1]);
            int segmentCount = mountPoint.segmentCount();
            if (canonicalPath.startsWith(mountPoint) && segmentCount > bestMountPointSegmentCount) {
              bestMountPointSegmentCount = segmentCount;
              fileSystem = words[2];
            }
          }
        }
      }
    } catch (IOException e) {
      // pass
    }
    return fileSystem;
  }

  /**
   * Creates a directory with the name of the current path. See {@link Path#createDirectory} for
   * specification.
   */
  public abstract boolean createDirectory(PathFragment path) throws IOException;

  /**
   * Creates a writable directory at a given path or makes existing directory writable if it is
   * already present. Returns whether a new directory was created.
   *
   * <p>This method is not atomic -- concurrent modifications for the same path will result in
   * undefined behavior.
   */
  protected boolean createWritableDirectory(PathFragment path) throws IOException {
    FileStatus stat = statNullable(path, /* followSymlinks= */ false);
    if (stat == null) {
      return createDirectory(path);
    }

    if (!stat.isDirectory()) {
      throw new IOException(path + " (Not a directory)");
    }

    chmod(path, 0755);
    return false;
  }

  /**
   * Creates all directories up to the path. See {@link Path#createDirectoryAndParents} for
   * specification.
   */
  public abstract void createDirectoryAndParents(PathFragment path) throws IOException;

  /**
   * Returns the size in bytes of the file denoted by {@code path}. See {@link
   * Path#getFileSize(Symlinks)} for specification.
   *
   * <p>Note: for <@link FileSystem>s where {@link #supportsSymbolicLinksNatively(PathFragment)}
   * returns false, this method will throw an {@link UnsupportedOperationException} if {@code
   * followSymLinks=false}.
   */
  protected abstract long getFileSize(PathFragment path, boolean followSymlinks) throws IOException;

  /** Deletes the file denoted by {@code path}. See {@link Path#delete} for specification. */
  protected abstract boolean delete(PathFragment path) throws IOException;

  /**
   * Deletes all directory trees recursively beneath the given path and removes that path as well.
   *
   * @param path the directory hierarchy to remove
   * @throws IOException if the hierarchy cannot be removed successfully
   */
  protected void deleteTree(PathFragment path) throws IOException {
    deleteTreesBelow(path);
    delete(path);
  }

  /**
   * Deletes all directory trees recursively beneath the given path. Does nothing if the given path
   * is not a directory.
   *
   * <p>This generic implementation is not as efficient as it could be: for example, we issue
   * separate stats for each directory entry to determine if they are directories or not (instead of
   * reusing the information that readdir returns), and we issue separate operations to toggle
   * different permissions while they could be done at once via chmod. Subclasses can optimize this
   * by taking advantage of platform-specific features.
   *
   * @param dir the directory hierarchy to remove
   * @throws IOException if the hierarchy cannot be removed successfully
   */
  protected void deleteTreesBelow(PathFragment dir) throws IOException {
    if (isDirectory(dir, /*followSymlinks=*/ false)) {
      Collection<String> entries;
      try {
        entries = getDirectoryEntries(dir);
      } catch (IOException e) {
        // If we couldn't read the directory, it may be because it's not readable. Try granting this
        // permission and retry. If the retry fails, give up.
        setReadable(dir, true);
        setExecutable(dir, true);
        entries = getDirectoryEntries(dir);
      }

      Iterator<String> iterator = entries.iterator();
      if (iterator.hasNext()) {
        PathFragment first = dir.getChild(iterator.next());
        deleteTreesBelow(first);
        try {
          // If the directory is not executable, delete(), depending on implementation, may decide
          // that the directory entry does not exist and return false without throwing.
          if (!delete(first)) {
            throw new IOException(
                "Unable to delete \"" + first + "\": directory entry does not exist");
          }
        } catch (IOException e) {
          // If we couldn't delete the first entry in a directory, it may be because the directory
          // (not the entry!) is not writable or executable. Try granting this permission and retry.
          // If the retry fails, give up. Note that we have to retry deleteTreesBelow() too in case
          // first is itself a directory; if the directory were not executable, the initial
          // first.deleteTreesBelow() call would have been a silent no-op (since first.isDirectory()
          // would have returned false) and sub-entries of first would not have been deleted.
          setWritable(dir, true);
          setExecutable(dir, true);
          deleteTreesBelow(first);
          delete(first);
        }
      }
      while (iterator.hasNext()) {
        PathFragment path = dir.getChild(iterator.next());
        deleteTreesBelow(path);
        // No need to retry here: if needed, we already unprotected the directory earlier.
        delete(path);
      }
    }
  }

  /**
   * Returns the last modification time of the file denoted by {@code path}. See {@link
   * Path#getLastModifiedTime(Symlinks)} for specification.
   *
   * <p>Note: for {@link FileSystem}s where {@link #supportsSymbolicLinksNatively(PathFragment)}
   * returns false, this method will throw an {@link UnsupportedOperationException} if {@code
   * followSymLinks=false}.
   */
  protected abstract long getLastModifiedTime(PathFragment path, boolean followSymlinks)
      throws IOException;

  /**
   * Sets the last modification time of the file denoted by {@code path}. See {@link
   * Path#setLastModifiedTime} for specification.
   */
  public abstract void setLastModifiedTime(PathFragment path, long newTime) throws IOException;

  /**
   * Returns value of the given extended attribute name or null if attribute does not exist or file
   * system does not support extended attributes.
   *
   * <p>Default implementation assumes that file system does not support extended attributes and
   * always returns null. Specific file system implementations should override this method if they
   * do provide support for extended attributes.
   *
   * @param path the file whose extended attribute is to be returned.
   * @param name the name of the extended attribute key.
   * @param followSymlinks whether to follow symlinks or not; if false, returns the xattr of the
   *     link itself, not its target.
   * @return the value of the extended attribute associated with 'path', if any, or null if no such
   *     attribute is defined (ENODATA) or file system does not support extended attributes at all.
   * @throws IOException if the call failed for any other reason.
   */
  public byte[] getxattr(PathFragment path, String name, boolean followSymlinks)
      throws IOException {
    return null;
  }

  /**
   * Gets a fast digest for the given path, or {@code null} if there isn't one available or the
   * filesystem doesn't support them. This digest should be suitable for detecting changes to the
   * file.
   */
  @Nullable
  protected byte[] getFastDigest(PathFragment path) throws IOException {
    return null;
  }

  /**
   * Returns the digest of the file denoted by the path, following symbolic links.
   *
   * <p>Subclasses may (and do) optimize this computation for a particular digest functions.
   *
   * @return a new byte array containing the file's digest
   * @throws IOException if the digest could not be computed for any reason
   */
  protected byte[] getDigest(PathFragment path, long expectedSize) throws IOException {
    var bs =
        new ByteSource() {
          @Override
          public InputStream openStream() throws IOException {
            return getInputStream(path);
          }
        };
    var hasher = digestFunction.getHashFunction().newHasher();
    var copied = bs.copyTo(Funnels.asOutputStream(hasher));
    if (expectedSize != -1 && copied != expectedSize) {
      throw new IOException(
          String.format(
              Locale.US,
              "digesting %s saw %s bytes rather than the expected %s",
              path,
              copied,
              expectedSize));
    }
    return hasher.hash().asBytes();
  }

  /**
   * Returns true if "path" denotes an existing symbolic link. See {@link Path#isSymbolicLink} for
   * specification.
   */
  protected abstract boolean isSymbolicLink(PathFragment path);

  /**
   * Appends a single regular path segment 'child' to 'dir', recursively resolving symbolic links in
   * 'child'. 'dir' must be canonical. 'maxLinks' is the maximum number of symbolic links that may
   * be traversed before it gives up.
   *
   * <p>(This method does not need to be synchronized; but the result may be stale in the case of
   * concurrent modification.)
   *
   * @throws IOException if 'dir' is not an existing directory; or if stat(child) fails for any
   *     reason, or if 'child' is a symlink and readlink(child) fails for any reason (e.g. ENOENT,
   *     EACCES), or if the chain of symbolic links exceeds 'maxLinks'.
   */
  protected final PathFragment appendSegment(PathFragment dir, String child, int maxLinks)
      throws IOException {
    PathFragment naive = dir.getChild(child);

    PathFragment linkTarget = resolveOneLink(naive);
    if (linkTarget == null) {
      return naive; // regular file or directory
    }

    if (maxLinks-- == 0) {
      throw new FileSymlinkLoopException(naive);
    }
    if (linkTarget.isAbsolute()) {
      dir = PathFragment.createAlreadyNormalized(linkTarget.getDriveStr());
    }
    for (String name : linkTarget.segments()) {
      if (name.equals(".") || name.isEmpty()) {
        // no-op
      } else if (name.equals("..")) {
        PathFragment parent = dir.getParentDirectory();
        // root's parent is root, when canonicalizing, so this is a no-op.
        if (parent != null) { dir = parent; }
      } else {
        dir = appendSegment(dir, name, maxLinks);
      }
    }
    return dir;
  }

  /**
   * Helper method of {@link #resolveSymbolicLinks(PathFragment)}. This method encapsulates the I/O
   * component of a full canonicalization operation. Subclasses can (and do) provide more efficient
   * implementations.
   *
   * <p>(This method does not need to be synchronized; but the result may be stale in the case of
   * concurrent modification.)
   *
   * @param path a path, of which all but the last segment is guaranteed to be canonical
   * @return {@link #readSymbolicLink} iff path is a symlink or null iff path exists but is not a
   *     symlink
   * @throws IOException if the file did not exist, or a parent directory could not be searched
   */
  @Nullable
  protected PathFragment resolveOneLink(PathFragment path) throws IOException {
    try {
      return readSymbolicLink(path);
    } catch (NotASymlinkException e) {
      // Not a symbolic link.  Check it exists.

      // (A simple call to lstat would replace all of this.)
      if (!exists(path, false)) {
        throw new FileNotFoundException(path + " (No such file or directory)");
      }

      // TODO(bazel-team): (2009) ideally, throw ENOTDIR if dir is not a dir, but that
      // would require twice as many stats, or a much more convoluted
      // implementation (like glibc's canonicalize.c).

      return null; //  exists.
    }
  }

  /**
   * Returns the canonical path for the given path, which must be absolute. See {@link
   * Path#resolveSymbolicLinks} for specification.
   */
  protected Path resolveSymbolicLinks(PathFragment path) throws IOException {
    checkArgument(path.isAbsolute());
    PathFragment parentNode = path.getParentDirectory();
    return parentNode == null
        ? getPath(path) // (root)
        : getPath(
            appendSegment(
                resolveSymbolicLinks(parentNode).asFragment(), path.getBaseName(), MAX_SYMLINKS));
  }

  /**
   * Returns the status of a file. See {@link Path#stat(Symlinks)} for specification.
   *
   * <p>The default implementation of this method is a "lazy" one, based on other accessor methods
   * such as {@link #isFile}, etc. Subclasses may provide more efficient specializations. However,
   * we still try to follow Unix-like semantics of failing fast in case of non-existent files (or in
   * case of permission issues).
   */
  protected FileStatus stat(PathFragment path, boolean followSymlinks) throws IOException {
    FileStatus status = new FileStatus() {
      volatile Boolean isFile;
      volatile Boolean isDirectory;
      volatile Boolean isSymbolicLink;
      volatile Boolean isSpecial;
      volatile long size = -1;
      volatile long mtime = -1;

      @Override
      public boolean isFile() {
        if (isFile == null) { isFile = FileSystem.this.isFile(path, followSymlinks); }
        return isFile;
      }

      @Override
      public boolean isDirectory() {
        if (isDirectory == null) {
          isDirectory = FileSystem.this.isDirectory(path, followSymlinks);
        }
        return isDirectory;
      }

      @Override
      public boolean isSymbolicLink() {
        if (isSymbolicLink == null)  { isSymbolicLink = FileSystem.this.isSymbolicLink(path); }
        return isSymbolicLink;
      }

      @Override
      public boolean isSpecialFile() {
        if (isSpecial == null)  { isSpecial = FileSystem.this.isSpecialFile(path, followSymlinks); }
        return isSpecial;
      }

      @Override
      public long getSize() throws IOException {
        if (size == -1) { size = getFileSize(path, followSymlinks); }
        return size;
      }

      @Override
      public long getLastModifiedTime() throws IOException {
        if (mtime == -1) { mtime = FileSystem.this.getLastModifiedTime(path, followSymlinks); }
        return mtime;
      }

      @Override
      public long getLastChangeTime() {
        throw new UnsupportedOperationException();
      }

      @Override
      public long getNodeId() {
        throw new UnsupportedOperationException();
      }
    };

    // Fail fast in case if some operations will actually fail, since stat() call sometimes used
    // to verify file existence as well. We will use getLastModifiedTime() method for that purpose.
    status.getLastModifiedTime();

    return status;
  }

  /** Like stat(), but returns null on failures instead of throwing. */
  @Nullable
  protected FileStatus statNullable(PathFragment path, boolean followSymlinks) {
    try {
      return stat(path, followSymlinks);
    } catch (IOException e) {
      return null;
    }
  }

  /**
   * Like {@link #stat}, but returns null if the file is not found (corresponding to {@code ENOENT}
   * or {@code ENOTDIR} in Unix's stat(2) function) instead of throwing. Note that this
   * implementation does <i>not</i> successfully catch {@code ENOTDIR} exceptions. If the
   * instantiated filesystem can catch such errors, it should override this method to do so.
   */
  @Nullable
  protected FileStatus statIfFound(PathFragment path, boolean followSymlinks) throws IOException {
    try {
      return stat(path, followSymlinks);
    } catch (FileNotFoundException e) {
      return null;
    }
  }

  /**
   * Returns true iff {@code path} denotes an existing directory. See {@link
   * Path#isDirectory(Symlinks)} for specification.
   */
  protected abstract boolean isDirectory(PathFragment path, boolean followSymlinks);

  /**
   * Returns true iff {@code path} denotes an existing regular or special file. See {@link
   * Path#isFile(Symlinks)} for specification.
   */
  protected abstract boolean isFile(PathFragment path, boolean followSymlinks);

  /**
   * Returns true iff {@code path} denotes a special file. See {@link Path#isSpecialFile(Symlinks)}
   * for specification.
   */
  protected abstract boolean isSpecialFile(PathFragment path, boolean followSymlinks);

  /**
   * Creates a symbolic link. See {@link Path#createSymbolicLink(Path)} for specification.
   *
   * <p>Note: for {@link FileSystem}s where {@link #supportsSymbolicLinksNatively(PathFragment)}
   * returns false, this method will throw an {@link UnsupportedOperationException}
   */
  protected abstract void createSymbolicLink(PathFragment linkPath, PathFragment targetFragment)
      throws IOException;

  /**
   * Returns the target of a symbolic link. See {@link Path#readSymbolicLink} for specification.
   *
   * <p>Note: for {@link FileSystem}s where {@link #supportsSymbolicLinksNatively(PathFragment)}
   * returns false, this method will throw an {@link UnsupportedOperationException} if the link
   * points to a non-existent file.
   *
   * @throws NotASymlinkException if the current path is not a symbolic link
   * @throws IOException if the contents of the link could not be read for any reason.
   */
  protected abstract PathFragment readSymbolicLink(PathFragment path) throws IOException;

  /**
   * Returns the target of a symbolic link, under the assumption that the given path is indeed a
   * symbolic link (this assumption permits efficient implementations). See {@link
   * Path#readSymbolicLinkUnchecked} for specification.
   *
   * @throws IOException if the contents of the link could not be read for any reason.
   */
  protected PathFragment readSymbolicLinkUnchecked(PathFragment path) throws IOException {
    return readSymbolicLink(path);
  }

  /** Returns true iff this path denotes an existing file of any kind. Follows symbolic links. */
  public boolean exists(PathFragment path) {
    return exists(path, true);
  }

  /**
   * Returns true iff {@code path} denotes an existing file of any kind. See {@link
   * Path#exists(Symlinks)} for specification.
   */
  protected abstract boolean exists(PathFragment path, boolean followSymlinks);

  /**
   * Returns a collection containing the names of all entities within the directory denoted by the
   * {@code path}. Symlinks are followed when resolving the directory whose entries are to be read.
   *
   * @throws IOException if there was an error reading the directory entries
   */
  protected abstract Collection<String> getDirectoryEntries(PathFragment path) throws IOException;

  protected static Dirent.Type direntFromStat(FileStatus stat) {
    if (stat == null) {
      return Dirent.Type.UNKNOWN;
    } else if (stat.isSpecialFile()) {
      return Dirent.Type.UNKNOWN;
    } else if (stat.isFile()) {
      return Dirent.Type.FILE;
    } else if (stat.isDirectory()) {
      return Dirent.Type.DIRECTORY;
    } else if (stat.isSymbolicLink()) {
      return Dirent.Type.SYMLINK;
    } else {
      return Dirent.Type.UNKNOWN;
    }
  }

  /**
   * Returns a Dirents structure, listing the names of all entries within the directory {@code
   * path}, plus their types (file, directory, other).
   *
   * @param followSymlinks whether to follow symlinks when determining the file types of individual
   *     directory entries. No matter the value of this parameter, symlinks are followed when
   *     resolving the directory whose entries are to be read.
   * @throws IOException if there was an error reading the directory entries
   */
  protected Collection<Dirent> readdir(PathFragment path, boolean followSymlinks)
      throws IOException {
    Collection<String> children = getDirectoryEntries(path);
    List<Dirent> dirents = Lists.newArrayListWithCapacity(children.size());
    for (String child : children) {
      PathFragment childPath = path.getChild(child);
      Dirent.Type type = direntFromStat(statNullable(childPath, followSymlinks));
      dirents.add(new Dirent(child, type));
    }
    return dirents;
  }

  /**
   * Returns true iff the file represented by {@code path} is readable.
   *
   * @throws IOException if there was an error reading the file's metadata
   */
  protected abstract boolean isReadable(PathFragment path) throws IOException;

  /**
   * Sets the file to readable (if the argument is true) or non-readable (if the argument is false)
   *
   * <p>Note: for {@link FileSystem}s where {@link #supportsModifications(PathFragment)} returns
   * false or which do not support unreadable files, this method will throw an {@link
   * UnsupportedOperationException}.
   *
   * @throws IOException if there was an error reading or writing the file's metadata
   */
  protected abstract void setReadable(PathFragment path, boolean readable) throws IOException;

  /**
   * Returns true iff the file represented by {@code path} is writable.
   *
   * @throws IOException if there was an error reading the file's metadata
   */
  protected abstract boolean isWritable(PathFragment path) throws IOException;

  /**
   * Sets the file to writable (if the argument is true) or non-writable (if the argument is false)
   *
   * <p>Note: for {@link FileSystem}s where {@link #supportsModifications(PathFragment)} returns
   * false, this method will throw an {@link UnsupportedOperationException}.
   *
   * @throws IOException if there was an error reading or writing the file's metadata
   */
  public abstract void setWritable(PathFragment path, boolean writable) throws IOException;

  /**
   * Returns true iff the file represented by the path is executable.
   *
   * @throws IOException if there was an error reading the file's metadata
   */
  protected abstract boolean isExecutable(PathFragment path) throws IOException;

  /**
   * Sets the file to executable, if the argument is true. It is currently not supported to unset
   * the executable status of a file, so {code executable=false} yields an {@link
   * UnsupportedOperationException}.
   *
   * <p>Note: for {@link FileSystem}s where {@link #supportsModifications(PathFragment)} returns
   * false, this method will throw an {@link UnsupportedOperationException}.
   *
   * @throws IOException if there was an error reading or writing the file's metadata
   */
  protected abstract void setExecutable(PathFragment path, boolean executable) throws IOException;

  /**
   * Sets the file permissions. If permission changes on this {@link FileSystem} are slow (e.g. one
   * syscall per change), this method should aim to be faster than setting each permission
   * individually. If this {@link FileSystem} does not support group or others permissions, those
   * bits will be ignored.
   *
   * <p>Note: for {@link FileSystem}s where {@link #supportsModifications(PathFragment)} returns
   * false, this method will throw an {@link UnsupportedOperationException}.
   *
   * @throws IOException if there was an error reading or writing the file's metadata
   */
  protected void chmod(PathFragment path, int mode) throws IOException {
    setReadable(path, (mode & 0400) != 0);
    setWritable(path, (mode & 0200) != 0);
    setExecutable(path, (mode & 0100) != 0);
  }

  /**
   * Creates an InputStream accessing the file denoted by the path.
   *
   * @throws IOException if there was an error opening the file for reading
   */
  protected abstract InputStream getInputStream(PathFragment path) throws IOException;

  /**
   * Returns a {@link SeekableByteChannel} for writing to a file at provided path.
   *
   * <p>Truncates the target file, therefore it cannot be used to read already existing files.
   */
  protected abstract SeekableByteChannel createReadWriteByteChannel(PathFragment path)
      throws IOException;

  /**
   * Creates an OutputStream accessing the file denoted by path.
   *
   * @throws IOException if there was an error opening the file for writing
   */
  protected final OutputStream getOutputStream(PathFragment path) throws IOException {
    return getOutputStream(path, /* append= */ false);
  }

  /**
   * Creates an OutputStream accessing the file denoted by path.
   *
   * @param append whether to open the output stream in append mode
   * @throws IOException if there was an error opening the file for writing
   */
  protected final OutputStream getOutputStream(PathFragment path, boolean append)
      throws IOException {
    return getOutputStream(path, append, /* internal= */ false);
  }

  /**
   * Creates an OutputStream accessing the file denoted by path.
   *
   * @param append whether to open the output stream in append mode
   * @param internal whether the file is a Bazel internal file
   * @throws IOException if there was an error opening the file for writing
   */
  protected abstract OutputStream getOutputStream(
      PathFragment path, boolean append, boolean internal) throws IOException;

  /**
   * Renames the file denoted by "sourceNode" to the location "targetNode". See {@link
   * Path#renameTo} for specification.
   */
  public abstract void renameTo(PathFragment sourcePath, PathFragment targetPath)
      throws IOException;

  /**
   * Create a new hard link file at "linkPath" for file at "originalPath".
   *
   * @param linkPath The path of the new link file to be created
   * @param originalPath The path of the original file
   * @throws IOException if the original file does not exist or the link file already exists
   */
  protected void createHardLink(PathFragment linkPath, PathFragment originalPath)
      throws IOException {

    if (!exists(originalPath)) {
      throw new FileNotFoundException(
          "File \""
              + originalPath.getBaseName()
              + "\" linked from \""
              + linkPath.getBaseName()
              + "\" does not exist");
    }

    if (exists(linkPath)) {
      throw new FileAlreadyExistsException(
          "New link file \"" + linkPath.getBaseName() + "\" already exists");
    }

    createFSDependentHardLink(linkPath, originalPath);
  }

  /**
   * Create a new hard link file at "linkPath" for file at "originalPath".
   *
   * @param linkPath The path of the new link file to be created
   * @param originalPath The path of the original file
   * @throws IOException if there was an I/O error
   */
  protected abstract void createFSDependentHardLink(
      PathFragment linkPath, PathFragment originalPath) throws IOException;

  /**
   * Prefetch all directories and symlinks within the package rooted at "path". Enter at most
   * "maxDirs" total directories. Specializations for high-latency remote filesystems may wish to
   * implement this in order to warm the filesystem's internal caches.
   */
  protected void prefetchPackageAsync(PathFragment path, int maxDirs) {}

}