Properly implement dirtiness check for unresolved symlink

This will fix warning when using `--experimental_guard_against_concurrent_changes` with unresolved symlink.

See #17162.

Closes #18588.

PiperOrigin-RevId: 538170661
Change-Id: I6c95f6281f86bf7721dc8074c8a305fe62ccca67

src/main/java/com/google/devtools/build/lib/actions/FileArtifactValue.java

@@ -735,8 +735,12 @@ public FileContentsProxy getContentsProxy() {
 
     @Override
     public boolean wasModifiedSinceDigest(Path path) {
-      // We could store an mtime but I have no clue where to get one from createFromMetadata
-      return true;
+      try {
+        var newMetadata = FileArtifactValue.createForUnresolvedSymlink(path);
+        return !Arrays.equals(digest, newMetadata.getDigest());
+      } catch (IOException e) {
+        return true;
+      }
     }
   }
 

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

@@ -252,6 +252,23 @@ public void testUptodateChangeFileToDirectory() throws Exception {
     assertThat(value.wasModifiedSinceDigest(path)).isTrue();
   }
 
+  @Test
+  public void testUptodateUnresolvedSymlink() throws Exception {
+    Path path = fs.getPath("/dir/symlink");
+    path.getParentDirectory().createDirectoryAndParents();
+    path.createSymbolicLink(PathFragment.create("target_path"));
+    FileArtifactValue value = FileArtifactValue.createForUnresolvedSymlink(path);
+
+    clock.advanceMillis(1);
+    assertThat(value.wasModifiedSinceDigest(path)).isFalse();
+
+    path.delete();
+    path.createSymbolicLink(PathFragment.create("modified_target_path"));
+
+    clock.advanceMillis(1);
+    assertThat(value.wasModifiedSinceDigest(path)).isTrue();
+  }
+
   @Test
   public void addToFingerprint_equalByDigest() throws Exception {
     FileArtifactValue value1 =