Add test coverage for tree artifacts containing directory symlinks.

We have been bitten twice by this use case. See bazelbuild#9054 and bazelbuild#11813.

Fixes bazelbuild#9058.

RELNOTES: None.
PiperOrigin-RevId: 322590818

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

@@ -79,9 +79,7 @@ private ActionExecutionValue(
       }
       for (Map.Entry<TreeFileArtifact, FileArtifactValue> file :
           treeArtifact.getChildValues().entrySet()) {
-        // We should only have RegularFileValue instances in here, but apparently tree artifacts
-        // sometimes store their own root directory in here. Sad.
-        // https://github.com/bazelbuild/bazel/issues/9058
+        // Tree artifacts can contain symlinks to directories, which don't have a digest.
         if (file.getValue().getType().isFile()) {
           Preconditions.checkNotNull(
               file.getValue().getDigest(),

src/test/java/com/google/devtools/build/lib/skyframe/TreeArtifactBuildTest.java

@@ -833,6 +833,28 @@ public void emptyInputAndOutputTreeArtifactInActionTemplate() throws Exception {
     assertThat(artifact2.getPath().getDirectoryEntries()).isEmpty();
   }
 
+  // This happens in the wild. See https://github.com/bazelbuild/bazel/issues/11813.
+  @Test
+  public void treeArtifactContainsSymlinkToDirectory() throws Exception {
+    SpecialArtifact treeArtifact = createTreeArtifact("tree");
+    registerAction(
+        new SimpleTestAction(/*output=*/ treeArtifact) {
+          @Override
+          void run(ActionExecutionContext context) throws IOException {
+            PathFragment subdir = PathFragment.create("subdir");
+            touchFile(treeArtifact.getPath().getRelative(subdir).getRelative("file"));
+            treeArtifact.getPath().getRelative("link").createSymbolicLink(subdir);
+          }
+        });
+
+    TreeArtifactValue tree = buildArtifact(treeArtifact);
+
+    assertThat(tree.getChildren())
+        .containsExactly(
+            TreeFileArtifact.createTreeOutput(treeArtifact, "subdir/file"),
+            TreeFileArtifact.createTreeOutput(treeArtifact, "link"));
+  }
+
   private abstract static class SimpleTestAction extends TestAction {
     private final Button button;