Fix validators.disabled() to save/restore state on nesting (#1513)

* Fix validators.disabled() to save/restore previous state on nesting

* Add news fragment

* Add test

* Add versionchanged tag

---------

Co-authored-by: Hynek Schlawack <hs@ox.cx>

Author: bysiber

changelog.d/1513.change.md

@@ -0,0 +1 @@
+The `attrs.validators.disabled()` contextmanager can now be nested.

src/attr/validators.py

@@ -79,12 +79,14 @@ def disabled():
         This context manager is not thread-safe!
 
     .. versionadded:: 21.3.0
+    .. versionchanged:: 26.1.0 The contextmanager is nestable.
     """
+    prev = get_run_validators()
     set_run_validators(False)
     try:
         yield
     finally:
-        set_run_validators(True)
+        set_run_validators(prev)
 
 
 @attrs(repr=False, slots=True, unsafe_hash=True)

tests/test_validators.py

@@ -97,6 +97,22 @@ def test_disabled_ctx_with_errors(self):
 
         assert _config._run_validators is True
 
+    def test_disabled_ctx_nested(self):
+        """
+        Nested contextmanagers restore correct state.
+        """
+        assert _config._run_validators is True
+
+        with validator_module.disabled():
+            assert _config._run_validators is False
+
+            with validator_module.disabled():
+                assert _config._run_validators is False
+
+            assert _config._run_validators is False
+
+        assert _config._run_validators is True
+
 
 class TestInstanceOf:
     """