[3.15] gh-159739: Preserve ContextVar cache updates from finalizers (GH-160197) (#160330)

Resetting a ContextVar can finalize the replaced value, whose finalizer may set the variable again. Updating the lookup cache after releasing the old context root overwrites that newer cached value, so get() can return a stale value or use a freed object. Update the cache before the old root can be released. Backport to 3.15 and 3.14.
(cherry picked from commit f4f8b93)

Co-authored-by: Jelle Zijlstra <jelle@openai.com>

Author: miss-islington

Lib/test/test_context.py

@@ -336,6 +336,19 @@ def fun():
         contextvars.copy_context().run(fun)
         self.assertEqual(c.get(), [])
 
+    @isolated_context
+    def test_reset_with_reentrant_finalizer(self):
+        var = contextvars.ContextVar('var')
+        var.set('original')
+
+        class Reenter:
+            def __del__(self):
+                var.set('finalizer')
+
+        token = var.set(Reenter())
+        var.reset(token)
+        self.assertEqual(var.get(), 'finalizer')
+
     def test_context_copy_1(self):
         ctx1 = contextvars.Context()
         c = contextvars.ContextVar('c', default=42)

Misc/NEWS.d/next/Library/2026-10-21-21-26-10.gh-issue-159739.ghGctt.rst

@@ -0,0 +1 @@
+Fix :class:`contextvars.ContextVar` returning a stale value after a reentrant finalizer changes it during :meth:`~contextvars.ContextVar.reset`.

Python/context.c

@@ -872,13 +872,13 @@ contextvar_set(PyContextVar *var, PyObject *val)
         return -1;
     }
 
-    context_set_vars(ctx, new_vars);
-
 #ifndef Py_GIL_DISABLED
     var->var_cached = val;  /* borrow */
     var->var_cached_tsid = ts->id;
     var->var_cached_tsver = ts->context_ver;
 #endif
+    /* Releasing the old variables may run a finalizer that updates the cache. */
+    context_set_vars(ctx, new_vars);
     return 0;
 }