[7.1.0] Optimize RemoteActionFileSystem#resolveSymbolicLinks by cachi…

…ng intermediate results in a trie. (#21333)

When scanning a filesystem tree, resolveSymbolicLinks does O(M*N) work,
where M is the number of components in a file path and N is the number
of files. This CL makes it O(M+N) instead. This makes large output tree
artifacts (~250k files) much more efficient to scan (from ~45s to ~9s in
a particular example; additional optimizations are possible and will be
made in a followup CL).

Related to #17009.

PiperOrigin-RevId: 606259673
Change-Id: Icf781a78b3271196e0029e3049d969a9e6073906

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

@@ -0,0 +1,193 @@
+// 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);
+ }
+ }
+}

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

@@ -68,7 +68,7 @@
 import javax.annotation.Nullable;
 
 /**
- * An action filesystem suitable for use when building with remote caching or execution.
+ * An action filesystem suitable for use when building with disk/remote caching or execution.
  *
  * <p>It acts as a union filesystem over three different sources:
  *
@@ -91,17 +91,21 @@
  * to an input. Most operations call resolveSymbolicLinks upfront (which is able to canonicalize
  * paths taking every source into account) and only then delegate to the underlying sources.
  *
- * <p>The implementation assumes that an action never modifies its input files, but may otherwise
- * modify any path in the output tree, including modifications that undo previous ones (e.g.,
- * writing to a path and then moving it elsewhere). No effort is made to detect irreconcilable
- * differences between sources (e.g., distinct files of the same name in two different sources).
+ * <p>The implementation assumes that an action never modifies its input paths, but may otherwise
+ * modify any path in the output tree. Concurrent operations are supported as long as they don't
+ * affect filesystem structure (i.e., create, move or delete paths). Otherwise, they might fail or
+ * produce inconsistent results. No effort is made to detect irreconcilable differences between
+ * sources, such as the same path existing in multiple underlying sources with different type or
+ * contents.
  */
-public class RemoteActionFileSystem extends AbstractFileSystemWithCustomStat {
+public class RemoteActionFileSystem extends AbstractFileSystemWithCustomStat
+ implements PathCanonicalizer.Resolver {
  private final PathFragment execRoot;
  private final PathFragment outputBase;
  private final InputMetadataProvider fileCache;
  private final ActionInputMap inputArtifactData;
  private final TreeArtifactDirectoryCache inputTreeArtifactDirectoryCache;
+ private final PathCanonicalizer pathCanonicalizer;
  private final ImmutableMap<PathFragment, Artifact> outputMapping;
  private final RemoteActionInputFetcher inputFetcher;
  private final FileSystem localFs;
@@ -241,6 +245,7 @@ public RemoteActionFileSystem(
  this.outputBase = execRoot.getRelative(checkNotNull(relativeOutputPath, "relativeOutputPath"));
  this.inputArtifactData = checkNotNull(inputArtifactData, "inputArtifactData");
  this.inputTreeArtifactDirectoryCache = new TreeArtifactDirectoryCache();
+ this.pathCanonicalizer = new PathCanonicalizer(this);
  this.outputMapping =
  stream(outputArtifacts).collect(toImmutableMap(Artifact::getExecPath, a -> a));
  this.fileCache = checkNotNull(fileCache, "fileCache");
@@ -310,6 +315,27 @@ public String getFileSystemType(PathFragment path) {
  return "remoteActionFS";
  }
 
+ @Override
+ protected Path resolveSymbolicLinks(PathFragment path) throws IOException {
+ return getPath(pathCanonicalizer.resolveSymbolicLinks(path));
+ }
+
+ @Override
+ @Nullable
+ public PathFragment resolveOneLink(PathFragment path) throws IOException {
+ // The base implementation attempts to readSymbolicLink first and falls back to stat, but that
+ // unnecessarily allocates a NotASymlinkException in the overwhelmingly likely non-symlink case.
+ // It's more efficient to stat unconditionally.
+ //
+ // The parent path has already been canonicalized, so FOLLOW_NONE is effectively the same as
+ // FOLLOW_PARENT, but much more efficient as it doesn't call stat recursively.
+ var stat = statInternal(path, FollowMode.FOLLOW_NONE, StatSources.ALL);
+ if (stat == null) {
+ throw new FileNotFoundException(path.getPathString() + " (No such file or directory)");
+ }
+ return stat.isSymbolicLink() ? readSymbolicLink(path) : null;
+ }
+
  // Like resolveSymbolicLinks(), except that only the parent path is canonicalized.
  private PathFragment resolveSymbolicLinksForParent(PathFragment path) throws IOException {
  PathFragment parentPath = path.getParentDirectory();
@@ -328,10 +354,15 @@ protected boolean delete(PathFragment path) throws IOException {
  return false;
  }
 
+ // No action implementations call renameTo concurrently with other filesystem operations, so
+ // there's no risk of a race condition below.
+ pathCanonicalizer.clearPrefix(path);
+
  boolean deleted = localFs.getPath(path).delete();
  if (isOutput(path)) {
  deleted = remoteOutputTree.getPath(path).delete() || deleted;
  }
+
  return deleted;
  }
 
@@ -538,22 +569,6 @@ protected void createSymbolicLink(PathFragment linkPath, PathFragment targetFrag
  localFs.getPath(linkPath).createSymbolicLink(targetFragment);
  }
 
- @Nullable
- @Override
- protected PathFragment resolveOneLink(PathFragment path) throws IOException {
- // The base implementation attempts to readSymbolicLink first and falls back to stat, but that
- // unnecessarily allocates a NotASymlinkException in the overwhelmingly likely non-symlink case.
- // It's more efficient to stat unconditionally.
- //
- // The parent path has already been canonicalized, so FOLLOW_NONE is effectively the same as
- // FOLLOW_PARENT, but much more efficient as it doesn't call stat recursively.
- var stat = statInternal(path, FollowMode.FOLLOW_NONE, StatSources.ALL);
- if (stat == null) {
- throw new FileNotFoundException(path.getPathString() + " (No such file or directory)");
- }
- return stat.isSymbolicLink() ? readSymbolicLink(path) : null;
- }
-
  @Override
  protected long getLastModifiedTime(PathFragment path, boolean followSymlinks) throws IOException {
  FileStatus stat = stat(path, followSymlinks);
@@ -760,6 +775,11 @@ public void renameTo(PathFragment srcPath, PathFragment dstPath) throws IOExcept
  checkArgument(isOutput(srcPath), "srcPath must be an output path");
  checkArgument(isOutput(dstPath), "dstPath must be an output path");
 
+ // No action implementations call renameTo concurrently with other filesystem operations, so
+ // there's no risk of a race condition below.
+ pathCanonicalizer.clearPrefix(srcPath);
+ pathCanonicalizer.clearPrefix(dstPath);
+
  FileNotFoundException remoteException = null;
  try {
  remoteOutputTree.renameTo(srcPath, dstPath);

src/main/java/com/google/devtools/build/lib/vfs/FileSystem.java

@@ -40,6 +40,10 @@
 @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) {
@@ -362,7 +366,7 @@ public InputStream openStream() throws IOException {
  /**
  * 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 (the Linux kernel uses 32).
+ * 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.)
@@ -443,7 +447,8 @@ protected Path resolveSymbolicLinks(PathFragment path) throws IOException {
  return parentNode == null
  ? getPath(path) // (root)
  : getPath(
- appendSegment(resolveSymbolicLinks(parentNode).asFragment(), path.getBaseName(), 32));
+ appendSegment(
+ resolveSymbolicLinks(parentNode).asFragment(), path.getBaseName(), MAX_SYMLINKS));
  }
 
  /**

src/test/java/com/google/devtools/build/lib/remote/BUILD

@@ -133,6 +133,7 @@ java_library(
  "//src/test/java/com/google/devtools/build/lib/testutil:TestUtils",
  "//third_party:auth",
  "//third_party:caffeine",
+ "//third_party:checker_framework_annotations",
  "//third_party:error_prone_annotations",
  "//third_party:gson",
  "//third_party:guava",