src/main/java/com/google/devtools/build/lib/skyframe/ExternalFilesHelper.java

// Copyright 2014 Google Inc. 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.annotations.VisibleForTesting;
import com.google.common.collect.ImmutableSet;
import com.google.devtools.build.lib.pkgcache.PathPackageLocator;
import com.google.devtools.build.lib.vfs.Path;
import com.google.devtools.build.lib.vfs.RootedPath;
import com.google.devtools.build.skyframe.SkyFunction;

import java.util.Set;
import java.util.concurrent.atomic.AtomicReference;

/** Common utilities for dealing with files outside the package roots. */
class ExternalFilesHelper {

  private final AtomicReference<PathPackageLocator> pkgLocator;
  private final Set<Path> immutableDirs;
  private final boolean errorOnExternalFiles;

  @VisibleForTesting
  ExternalFilesHelper(AtomicReference<PathPackageLocator> pkgLocator) {
    this(pkgLocator, ImmutableSet.<Path>of(), /*errorOnExternalFiles=*/false);
  }

  @VisibleForTesting
  ExternalFilesHelper(AtomicReference<PathPackageLocator> pkgLocator,
      boolean errorOnExternalFiles) {
    this(pkgLocator, ImmutableSet.<Path>of(), errorOnExternalFiles);
  }

  /**
   * @param pkgLocator an {@link AtomicReference} to a {@link PathPackageLocator} used to
   *    determine what files are internal
   * @param immutableDirs directories whose contents may be considered unchangeable
   * @param errorOnExternalFiles whether or not to allow references to files outside of
   *    the directories provided by pkgLocator or immutableDirs. See
   *    {@link #maybeHandleExternalFile(RootedPath, SkyFunction.Environment)} for more details.
   */
  ExternalFilesHelper(AtomicReference<PathPackageLocator> pkgLocator, Set<Path> immutableDirs,
      boolean errorOnExternalFiles) {
    this.pkgLocator = pkgLocator;
    this.immutableDirs = immutableDirs;
    this.errorOnExternalFiles = errorOnExternalFiles;
  }

  private enum FileType {
    // A file inside the package roots.
    INTERNAL_FILE,

    // A file outside the package roots that we may pretend is immutable.
    EXTERNAL_IMMUTABLE_FILE,

    // A file outside the package roots about which we may make no other assumptions.
    EXTERNAL_MUTABLE_FILE,
  }

  private FileType getFileType(RootedPath rootedPath) {
    // TODO(bazel-team): This is inefficient when there are a lot of package roots or there are a
    // lot of immutable directories. Consider either explicitly preventing this case or using a more
    // efficient approach here (e.g. use a trie for determing if a file is under an immutable
    // directory).
    if (!pkgLocator.get().getPathEntries().contains(rootedPath.getRoot())) {
      Path path = rootedPath.asPath();
      for (Path immutableDir : immutableDirs) {
        if (path.startsWith(immutableDir)) {
          return FileType.EXTERNAL_IMMUTABLE_FILE;
        }
      }
      return FileType.EXTERNAL_MUTABLE_FILE;
    }
    return FileType.INTERNAL_FILE;
  }

  public boolean shouldAssumeImmutable(RootedPath rootedPath) {
    return getFileType(rootedPath) == FileType.EXTERNAL_IMMUTABLE_FILE;
  }

  /**
   * Potentially adds a dependency on build_id to env if this instance is configured
   * with errorOnExternalFiles=false and rootedPath is an external mutable file.
   * If errorOnExternalFiles=true and rootedPath is an external mutable file then
   * a FileOutsidePackageRootsException is thrown. This method is a no-op for any
   * rootedPaths that fall within the known package roots.
   *
   * @param rootedPath
   * @param env
   * @throws FileOutsidePackageRootsException
   */
  public void maybeHandleExternalFile(RootedPath rootedPath, SkyFunction.Environment env)
      throws FileOutsidePackageRootsException {
    if (getFileType(rootedPath) == FileType.EXTERNAL_MUTABLE_FILE) {
      if (!errorOnExternalFiles) {
        // For files outside the package roots that are not assumed to be immutable, add a
        // dependency on the build_id. This is sufficient for correctness; all other files
        // will be handled by diff awareness of their respective package path, but these
        // files need to be addressed separately.
        //
        // Using the build_id here seems to introduce a performance concern because the upward
        // transitive closure of these external files will get eagerly invalidated on each
        // incremental build (e.g. if every file had a transitive dependency on the filesystem root,
        // then we'd have a big performance problem). But this a non-issue by design:
        // - We don't add a dependency on the parent directory at the package root boundary, so the
        // only transitive dependencies from files inside the package roots to external files are
        // through symlinks. So the upwards transitive closure of external files is small.
        // - The only way external source files get into the skyframe graph in the first place is
        // through symlinks outside the package roots, which we neither want to encourage nor
        // optimize for since it is not common. So the set of external files is small.
        //
        // The above reasoning doesn't hold for bazel, because external repositories
        // (e.g. new_local_repository) cause lots of external symlinks to be present in the build.
        // So bazel pretends that these external repositories are immutable to avoid the performance
        // penalty described above.
        PrecomputedValue.dependOnBuildId(env);
      } else {
        throw new FileOutsidePackageRootsException(rootedPath);
      }
    }
  }
}