Make dangling symlinks in external repositories work to some degree.

This seems to be the least insane approach within the following boundaries:

- Skyframe apparently doesn't allow GlobFunction to recover if FileFunction had already raised an exception that Skyframe knows about (this is somewhat surprising)
- I didn't want to change FileFunction not to throw an exception for dangling symlinks because this part of the code is scary
- I didn't want to revert to Skyframe-based symlink resolution for symlinks in immutable directories because that would be a performance hit
- I didn't want to write yet another symlink resolver and the two existing ones (FileSystem#resolveSymlinks() and and FileFunction#getSymlinkTargetRootedPath()) don't work: the former cannot resolve just one level of symlinks and the latter cannot do its job without adding Skyframe dependencies

I had to put in a placeholder value for realRootedPath when the FileValue represents a dangling symlink, because FileStateValue.create() relies on the symlink target being different than the symlink itself.


RELNOTES:

--
MOS_MIGRATED_REVID=101756189

1 files changed, 6 insertions, 1 deletions

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 ecccc075d1..32cce31b85 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
@@ -99,7 +99,12 @@ public class FileFunction implements SkyFunction {
               pkgLocator.get().getPathEntries());
           realFileStateValue = FileStateValue.create(realRootedPath, tsgm);
         } catch (IOException e) {
-          throw new FileFunctionException(e, Transience.TRANSIENT);
+          RootedPath root = RootedPath.toRootedPath(
+              rootedPath.asPath().getFileSystem().getRootDirectory(),
+              rootedPath.asPath().getFileSystem().getRootDirectory());
+          return FileValue.value(
+              rootedPath, fileStateValue,
+              root, FileStateValue.NONEXISTENT_FILE_STATE_NODE);
         } catch (InconsistentFilesystemException e) {
           throw new FileFunctionException(e, Transience.TRANSIENT);
         }