Make BundledFileSystem#getDigest more useful.

See the added comment for motivation.

PiperOrigin-RevId: 438895870

src/main/java/com/google/devtools/build/lib/analysis/ConfiguredRuleClassProvider.java

@@ -106,26 +106,34 @@ default ImmutableList<RuleSet> requires() {
 
  /** An InMemoryFileSystem for bundled builtins .bzl files. */
  public static class BundledFileSystem extends InMemoryFileSystem {
-
- private static final byte[] EMPTY_DIGEST = new byte[0];
-
  public BundledFileSystem() {
  super(DigestHashFunction.SHA256);
  }
 
- // Bundled files are guaranteed to not change throughout the lifetime of the Bazel server, so it
- // is permissible to use a fake digest. This helps avoid peculiarities in the interaction of
- // InMemoryFileSystem and Skyframe. See cl/354809138 for further discussion, including of
- // possible (but unlikely) future caveats of this approach.
+ // Pretend the digest of a bundled file is uniquely determined by its name, not its contents.
+ //
+ // The contents bundled files are guaranteed to not change throughout the lifetime of the Bazel
+ // server, we do not need to detect changes to a bundled file's contents. Not needing to worry
+ // about get the actual digest and detect changes to that digest helps avoid peculiarities in
+ // the interaction of InMemoryFileSystem and Skyframe. See cl/354809138 for further discussion,
+ // including of possible (but unlikely) future caveats of this approach.
+ //
+ // On the other hand, we do need to want different bundled files to have different digests. That
+ // way the bzl environment hashes for bzl rule classes defined in two different bundled files
+ // are guaranteed to be different, even if their set of transitive load statements is the same.
+ // This is important because it's possible for bzl rule classes defined in different files to
+ // have the same name string, and various part of Blaze rely on the pair of
+ // "rule class name string" and "bzl environment hash" to uniquely identify a bzl rule class.
+ // See b/226379109 for details.
 
  @Override
  protected synchronized byte[] getFastDigest(PathFragment path) {
- return EMPTY_DIGEST;
+ return getDigest(path);
  }
 
  @Override
- protected synchronized byte[] getDigest(PathFragment path) throws IOException {
- return EMPTY_DIGEST;
+ protected synchronized byte[] getDigest(PathFragment path) {
+ return getDigestFunction().getHashFunction().hashString(path.toString(), UTF_8).asBytes();
  }
  }