From aaadd007542f242ad566528c337e757996e24fa2 Mon Sep 17 00:00:00 2001
From: Fiddle-Config Team <fiddle-config-team@google.com>
Date: Wed, 22 Oct 2025 12:30:12 -0700
Subject: [PATCH] Ensures that fdl.Config values can only be built once. This
 prevents accidental stateful coupling of config values.

If you need to perform multiple sequential build steps, you should avoid structural sharing, or explicitly copy / reconstruct the buildable.

PiperOrigin-RevId: 822695081
---
 fiddle/_src/building.py      | 11 ++++++++++-
 fiddle/_src/building_test.py |  8 +++++++-
 2 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/fiddle/_src/building.py b/fiddle/_src/building.py
index 61fc29ce..f62ac015 100644
--- a/fiddle/_src/building.py
+++ b/fiddle/_src/building.py
@@ -25,6 +25,7 @@
 from fiddle._src import daglish
 from fiddle._src import partial
 from fiddle._src import reraised_exception
+from fiddle._src.mutate_buildable import update_callable
 
 T = TypeVar('T')
 
@@ -44,7 +45,8 @@ def _in_build():
   """A context manager to ensure fdl.build is not called recursively."""
   if _state.in_build:
     raise ValueError(
-        'It is forbidden to call `fdl.build` inside another `fdl.build` call.')
+        'It is forbidden to call `fdl.build` inside another `fdl.build` call.'
+    )
   _state.in_build = True
   try:
     yield
@@ -192,4 +194,11 @@ def _build(value: Any, state: daglish.State) -> Any:
         type(buildable),
     )
 
+  # Poison the buildable to prevent future builds.
+  def _poison(*args, **kwargs):  # pylint: disable=unused-argument
+    raise ValueError('Only call fld.build() once.')
+
+  if isinstance(buildable, config_lib.Buildable):
+    update_callable(buildable, _poison)
+
   return result
diff --git a/fiddle/_src/building_test.py b/fiddle/_src/building_test.py
index 15e8cfb1..0ae51a12 100644
--- a/fiddle/_src/building_test.py
+++ b/fiddle/_src/building_test.py
@@ -14,12 +14,12 @@
 # limitations under the License.
 
 """Tests for history."""
+
 import dataclasses
 import unittest
 import warnings
 from absl import logging
 from absl.testing import absltest
-
 from fiddle._src import building
 from fiddle._src import config
 
@@ -82,6 +82,12 @@ def test_traversable_w_buildable(self):
       building.build(value)
       self.assertEmpty(log_output)
 
+  def test_build_poisoning(self):
+    foo = config.Config(Foo, 1, 2)
+    with self.assertRaises(ValueError):
+      building.build(foo)
+      building.build(foo)
+
 
 if __name__ == '__main__':
   unittest.main()