Improve comments in light of unknown commit.
PiperOrigin-RevId: 679178728
Change-Id: I83ad7e7c8e7e608f5bdb2a4b3ce5b66a8144f966
diff --git a/src/main/java/com/google/devtools/build/lib/skyframe/FileFunction.java b/src/main/java/com/google/devtools/build/lib/skyframe/FileFunction.java
index 1439311..aa0df45 100644
--- a/src/main/java/com/google/devtools/build/lib/skyframe/FileFunction.java
+++ b/src/main/java/com/google/devtools/build/lib/skyframe/FileFunction.java
@@ -52,6 +52,11 @@
* <p>For an overview of the problem space and our approach, see the https://youtu.be/EoYdWmMcqDs
* talk from BazelCon 2019 (slides:
* https://docs.google.com/presentation/d/e/2PACX-1vQWq1DUhl92dDs_okNxM7Qy9zX72tp7hMsGosGxmjhBLZ5e02IJf9dySK_6lEU2j6u_NOEaUCQGxEFh/pub).
+ * [2024] N.B. The general idea of that talk is still right, but as of cl/334982640 aka commit
+ * 7598bc6 on GitHub (Oct 2020), we no longer unconditionally error out when encountering an
+ * unbounded ancestor expansion and instead leave it to consumers to decide what to do. A consumer
+ * that wants to do a recursive directory traversal starting from the path will probably want to
+ * error out, while a consumer that just wants metadata from the path probably doesn't care.
*/
public class FileFunction implements SkyFunction {
private final AtomicReference<PathPackageLocator> pkgLocator;
@@ -311,14 +316,17 @@
// chain of paths we've considered so far, and 'rootedPath' / 'path' is the proposed next path
// we consider.
//
- // Before we proceed with 'rootedPath', we need to ensure there won't be a problem. There are
- // three sorts of issues, all stemming from symlinks:
+ // There are three interesting cases to consider, all stemming from symlinks:
// (i) Symlink cycle:
// p -> p1 -> p2 -> p1
+ // This means `p` has no real path, so we error out.
// (ii) Unbounded expansion caused by a symlink to a descendant of a member of the chain:
// p -> a/b -> c/d -> a/b/e
+ // This means `p` has no real path, so we error out.
// (iii) Unbounded expansion caused by a symlink to an ancestor of a member of the chain:
// p -> a/b -> c/d -> a
+ // This is not necessarily a problem (the real path of `p` in this example is simply `a`),
+ // so we just note the unbounded ancestor expansion and let consumers decide what to do.
//
// We can detect all three of these symlink issues via inspection of the proposed new element.
// Here is our incremental algorithm:
