Do not crash when constructing an error message for a directory sourc…

…e artifact with an owner label of `:.`.

Such a label occurs with `exports_files["."]`, however an exception is unexpected and may indicate that the filesystem is corrupted.

PiperOrigin-RevId: 392679200

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

@@ -437,8 +437,9 @@ private static final class ArtifactFunctionException extends SkyFunctionExceptio
 
   private static String constructErrorMessage(Artifact artifact, String error) {
     Label ownerLabel = artifact.getOwner();
-    if (ownerLabel == null) {
-      // Discovered inputs may not have an owner.
+    if (ownerLabel == null || ownerLabel.getName().equals(".")) {
+      // Discovered inputs may not have an owner. Directory source artifacts may be owned by a label
+      // ':.' which will crash toPathFragment below.
       return String.format("%s '%s'", error, artifact.getExecPathString());
     } else if (ownerLabel.toPathFragment().equals(artifact.getExecPath())) {
       // No additional useful information from path.