src/main/java/com/google/devtools/build/lib/remote/PathCanonicalizer.java

// Copyright 2024 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.remote;

import static com.google.common.base.Preconditions.checkArgument;

import com.google.devtools.build.lib.vfs.FileSymlinkLoopException;
import com.google.devtools.build.lib.vfs.FileSystem;
import com.google.devtools.build.lib.vfs.PathFragment;
import java.io.IOException;
import java.util.Iterator;
import java.util.concurrent.ConcurrentHashMap;
import javax.annotation.Nullable;

/**
 * Canonicalizes paths like {@link FileSystem#resolveSymbolicLinks}, while storing the intermediate
 * results in a trie so they can be reused by future canonicalizations.
 *
 * <p>This is an implementation detail of {@link RemoteActionFileSystem}, factored out for testing.
 * Because {@link RemoteActionFileSystem} implements a union filesystem and must account for the
 * possibility of symlinks straddling the underlying filesystems, the performance of large
 * filesystem scans can be greatly improved with a custom {@link FileSystem#resolveSymbolicLinks}
 * implementation that leverages the trie to avoid repeated work.
 *
 * <p>On case-insensitive filesystems, accessing the same path through different case variations
 * will produce distinct trie entries. This could be fixed, but it's a performance rather than a
 * correctness concern, and shouldn't matter most of the time.
 *
 * <p>Thread-safe: concurrent calls to {@link #resolveSymbolicLinks} are supported. As with {@link
 * FileSystem#resolveSymbolicLinks}, the result is undefined if the filesystem is mutated
 * concurrently.
 */
final class PathCanonicalizer {

  interface Resolver {
    /**
     * Returns the result of {@link FileSystem#readSymbolicLink} if the path is a symlink, otherwise
     * null. All but the last path segment must be canonical.
     *
     * @throws IOException if the file type or symlink target path could not be determined
     */
    @Nullable
    PathFragment resolveOneLink(PathFragment path) throws IOException;
  }

  /** The trie node for a path segment. */
  private static final class Node {
    /**
     * Maps the next segment to the corresponding node. Null iff the path ending with the current
     * segment is a symlink.
     */
    @Nullable private final ConcurrentHashMap<String, Node> tab;

    /** The symlink target. Null iff the path ending with the current segment is not a symlink. */
    @Nullable private final PathFragment targetPath;

    private Node(@Nullable PathFragment targetPath) {
      this.tab = targetPath == null ? new ConcurrentHashMap<>() : null;
      this.targetPath = targetPath;
    }
  }

  private final Resolver resolver;
  private final Node root = new Node(null);

  PathCanonicalizer(Resolver resolver) {
    this.resolver = resolver;
  }

  /** Returns the root node for an absolute path. */
  private Node getRootNode(PathFragment path) {
    checkArgument(path.isAbsolute());
    // Unix has a single root. Windows has one root per drive.
    return path.getDriveStrLength() > 1
        ? root.tab.computeIfAbsent(path.getDriveStr(), unused -> new Node(null))
        : root;
  }

  /**
   * Canonicalizes a path, reusing cached information if possible.
   *
   * @param path the path to canonicalize.
   * @param maxLinks the maximum number of symlinks that can be followed in the process of
   *     canonicalizing the path.
   * @throws FileSymlinkLoopException if too many symlinks had to be followed.
   * @throws IOException if an I/O error occurs
   * @return the canonical path.
   */
  private PathFragment resolveSymbolicLinks(PathFragment path, int maxLinks) throws IOException {
    // This code is carefully written to be as fast as possible when the path is already canonical
    // and has been previously cached. Avoid making changes without benchmarking. A tree artifact
    // with hundreds of thousands of files makes for a good benchmark.

    Node node = getRootNode(path);
    Iterable<String> segments = path.segments();
    int segmentIndex = 0;

    // Loop invariants:
    // - `segmentIndex` is the index of the current `segment` relative to the start of `path`. The
    //   first segment has index 0.
    // - `path` is the absolute path to canonicalize. If `segmentIndex` > 0, `path` is already
    //    canonical up to and including `segmentIndex` - 1.
    // - `node` is the trie node corresponding to the `path` prefix ending with `segmentIndex` - 1,
    //   or to the root path when `segmentIndex` is 0.
    for (String segment : segments) {
      Node nextNode = node.tab.get(segment);
      if (nextNode == null) {
        PathFragment naivePath = path.subFragment(0, segmentIndex + 1);
        PathFragment targetPath = resolver.resolveOneLink(naivePath);
        nextNode = node.tab.computeIfAbsent(segment, unused -> new Node(targetPath));
      }

      if (nextNode.targetPath != null) {
        if (maxLinks == 0) {
          throw new FileSymlinkLoopException(path);
        }
        maxLinks--;

        // Compute the path obtained by resolving the symlink.
        // Note that path normalization already handles uplevel references.
        PathFragment newPath;
        if (nextNode.targetPath.isAbsolute()) {
          newPath = nextNode.targetPath.getRelative(path.subFragment(segmentIndex + 1));
        } else {
          newPath =
              path.subFragment(0, segmentIndex)
                  .getRelative(nextNode.targetPath)
                  .getRelative(path.subFragment(segmentIndex + 1));
        }

        // For absolute symlinks, we must start over.
        // For relative symlinks, it would have been possible to restart after the already
        // canonicalized prefix, but they're too rare to be worth optimizing for.
        return resolveSymbolicLinks(newPath, maxLinks);
      }

      node = nextNode;
      segmentIndex++;
    }

    return path;
  }

  /**
   * Canonicalizes a path, reusing cached information if possible.
   *
   * <p>See {@link FileSystem#resolveSymbolicLinks} for the full specification.
   *
   * @param path the path to canonicalize.
   * @throws FileSymlinkLoopException if too many symlinks had to be followed.
   * @throws IOException if an I/O error occurs
   * @return the canonical path.
   */
  PathFragment resolveSymbolicLinks(PathFragment path) throws IOException {
    return resolveSymbolicLinks(path, FileSystem.MAX_SYMLINKS);
  }

  /** Removes cached information for a path prefix. */
  void clearPrefix(PathFragment pathPrefix) {
    Node node = getRootNode(pathPrefix);
    Iterator<String> segments = pathPrefix.segments().iterator();
    boolean hasNext = segments.hasNext();

    while (node != null && hasNext) {
      String segment = segments.next();
      hasNext = segments.hasNext();

      if (!hasNext) {
        // Found the path prefix.
        node.tab.remove(segment);
        return;
      }

      if (node.targetPath != null) {
        // Path prefix not in trie.
        return;
      }

      node = node.tab.get(segment);
    }
  }
}