Invalidate parent directories (#5000)

### Problem

Watchman `4.9.0` does not send events for a parent when a child is created or deleted, which causes issues like #4662.

### Solution

Revert to explicitly invalidating the parent directory when a child has changed. More precision is possible here, but it's possible that re-gaining the precision by using the `notify` crate would be preferable (see #4999).

### Result

Fixes #4662.

src/python/pants/engine/scheduler.py

@@ -414,13 +414,12 @@ def root_entries(self, execution_request):
             self._execution_request, execution_request))
       return self._scheduler.root_entries(execution_request)
 
-  def invalidate_files(self, filenames):
+  def invalidate_files(self, direct_filenames):
     """Calls `Graph.invalidate_files()` against an internal product Graph instance."""
-    # NB: Watchman will never trigger an invalidation event for the root directory that
-    # is being watched. Instead, we treat any invalidation of a path directly in the
-    # root directory as an invalidation of the root.
-    if any(os.path.dirname(f) in ('', '.') for f in filenames):
-      filenames = tuple(filenames) + ('', '.')
+    # NB: Watchman no longer triggers events when children are created/deleted under a directory,
+    # so we always need to invalidate the direct parent as well.
+    filenames = set(direct_filenames)
+    filenames.update(os.path.dirname(f) for f in direct_filenames)
     with self._product_graph_lock:
       invalidated = self._scheduler.invalidate(filenames)
       logger.debug('invalidated %d nodes for: %s', invalidated, filenames)

tests/python/pants_test/engine/legacy/test_graph.py

@@ -126,10 +126,7 @@ def test_invalidate_fsnode_incremental(self):
 
       # Invalidate the '3rdparty/python' DirectoryListing, the `3rdparty` DirectoryListing,
       # and then the root DirectoryListing by "touching" files/dirs.
-      # NB: Invalidation of entries in the root directory is special: because Watchman will
-      # never trigger an event for the root itself, we treat changes to files in the root
-      # directory as events for the root.
-      for filename in ('3rdparty/python/BUILD', '3rdparty/python', 'non_existing_file'):
+      for filename in ('3rdparty/python/BUILD', '3rdparty/jvm', 'non_existing_file'):
         invalidated_count = scheduler.invalidate_files([filename])
         self.assertGreater(invalidated_count,
                            0,