1 files changed, 97 insertions, 0 deletions
diff --git a/src/main/java/com/google/devtools/build/lib/skyframe/ExternalFilesHelper.java b/src/main/java/com/google/devtools/build/lib/skyframe/ExternalFilesHelper.java
new file mode 100644
index 0000000000..ce858de326
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+++ b/src/main/java/com/google/devtools/build/lib/skyframe/ExternalFilesHelper.java
@@ -0,0 +1,97 @@
+// 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.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;
+
+  ExternalFilesHelper(AtomicReference<PathPackageLocator> pkgLocator) {
+    this(pkgLocator, ImmutableSet.<Path>of());
+  }
+
+  ExternalFilesHelper(AtomicReference<PathPackageLocator> pkgLocator, Set<Path> immutableDirs) {
+    this.pkgLocator = pkgLocator;
+    this.immutableDirs = immutableDirs;
+  }
+
+  private enum FileType {
+    // A file inside the package roots.
+    INTERNAL_FILE,
+
+    // A file outside the package roots that we may pretends 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;
+  }
+
+  public void maybeAddDepOnBuildId(RootedPath rootedPath, SkyFunction.Environment env) {
+   if (getFileType(rootedPath) == FileType.EXTERNAL_MUTABLE_FILE) {
+      // 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);
+    }
+  }
+}