gh-159091: Fix a data race in PyFunction_GetAnnotations()

Lib/test/test_free_threading/test_func_annotations.py

@@ -6,6 +6,11 @@
 
 from test.support import threading_helper, Py_GIL_DISABLED
 
+try:
+    import _testcapi
+except ImportError:
+    _testcapi = None
+
 threading_helper.requires_working_threading(module=True)
 
 
@@ -19,6 +24,11 @@ def get_func_annotation_dunder(f, b):
     return f.__annotations__
 
 
+def get_func_annotation_capi(f, b):
+    b.wait()
+    return _testcapi.function_get_annotations(f)
+
+
 def set_func_annotation(f, b):
     b.wait()
     f.__annotations__ = {'x': int, 'y': int, 'return': int}
@@ -65,3 +75,24 @@ def bar(x: int, y: float) -> float:
 
             # func_get_annotations returns in-place dict, so bar.__annotations__ should be modified as well
             self.assertEqual(bar.__annotations__, {'x': int, 'y': int, 'return': int})
+
+    @unittest.skipIf(_testcapi is None, "requires _testcapi")
+    def test_concurrent_capi_read_and_write(self):
+        # PyFunction_GetAnnotations() lazily computes and stores the
+        # annotations dict, so it must hold a critical section on the
+        # function object, like the __annotations__ getter and setter do.
+        for _ in range(10):
+            def foo(x: int) -> int:
+                return x + 1
+
+            with concurrent.futures.ThreadPoolExecutor(max_workers=self.NUM_THREADS) as executor:
+                b = Barrier(self.NUM_THREADS)
+                futures = {}
+                for i in range(self.NUM_THREADS):
+                    func = get_func_annotation_capi if i % 2 else set_func_annotation
+                    futures[executor.submit(func, foo, b)] = i
+                for fut in concurrent.futures.as_completed(futures):
+                    annotations = fut.result()
+                    # foo always has annotations, either computed from
+                    # foo.__annotate__ or set by set_func_annotation()
+                    self.assertIsInstance(annotations, dict)

Misc/NEWS.d/next/Core_and_Builtins/2026-10-09-17-54-30.gh-issue-159091.Kx7Qm2.rst

@@ -0,0 +1 @@
+Fix a data race between :c:func:`PyFunction_GetAnnotations` and assigning to ``__annotations__`` in the free-threaded build.

Objects/funcobject.c

@@ -1,6 +1,7 @@
 /* Function object implementation */
 
 #include "Python.h"
+#include "pycore_critical_section.h" // Py_BEGIN_CRITICAL_SECTION()
 #include "pycore_code.h"          // _PyCode_VerifyStateless()
 #include "pycore_dict.h"          // _Py_INCREF_DICT()
 #include "pycore_function.h"      // _PyFunction_Vectorcall
@@ -589,7 +590,11 @@ PyFunction_GetAnnotations(PyObject *op)
         PyErr_BadInternalCall();
         return NULL;
     }
-    return func_get_annotation_dict((PyFunctionObject *)op);
+    PyObject *annotations;
+    Py_BEGIN_CRITICAL_SECTION(op);
+    annotations = func_get_annotation_dict((PyFunctionObject *)op);
+    Py_END_CRITICAL_SECTION();
+    return annotations;
 }
 
 int