Avoid calling cfg constructor in nested cells

Summary:
PACKAGE values aren't shared across cells, but we can use
get_root_config() and get_config() to determine if we are in the root cell or
not.

Reviewed By: bigfootjon

Differential Revision: D64993387

fbshipit-source-id: 240929d4bc5f87ea7e8ce1460821e3243323da6a

shim/PACKAGE

@@ -5,8 +5,11 @@
 # License, Version 2.0 found in the LICENSE-APACHE file in the root directory
 # of this source tree.
 
-load(":cfg.bzl", "get_shim_modifiers")
+load(":cfg.bzl", "SHIM_ALIASES", "set_cfg_constructor", "get_shim_modifiers")
 load("@prelude//cfg/modifier:set_cfg_modifiers.bzl", "set_cfg_modifiers")
 
+# Activate cfg modifiers from CLI / PACKAGE / targets
+set_cfg_constructor(SHIM_ALIASES)
+
 modifiers = get_shim_modifiers()
 set_cfg_modifiers(modifiers)

shim/cfg.bzl

@@ -13,10 +13,10 @@ SHIM_ALIASES = {
     "ubuntu": "shim//os/linux/distro/constraints:ubuntu",
 }
 
-CFG_CONSTRUCTOR_INITIALIZED_KEY = "buck.cfg_constructor_initialized"
-
 def set_cfg_constructor(aliases = dict()):
-    if not read_parent_package_value(CFG_CONSTRUCTOR_INITIALIZED_KEY):
+    project_root_cell = read_root_config("cell_aliases", "root")
+    current_root_cell = read_config("cell_aliases", "root")
+    if project_root_cell == current_root_cell:
         native.set_cfg_constructor(
             stage0 = cfg_constructor_pre_constraint_analysis,
             stage1 = cfg_constructor_post_constraint_analysis,
@@ -25,8 +25,6 @@ def set_cfg_constructor(aliases = dict()):
             extra_data = struct(),
         )
 
-        write_package_value(CFG_CONSTRUCTOR_INITIALIZED_KEY, True)
-
 def get_shim_modifiers():
     modifiers = []