[3.13] gh-128714: Fix function object races in __annotate__, __annotations__ and __type_params__ in free-threading build (GH-129016) (#129729)

* gh-128714: Fix function object races in `__annotate__`, `__annotations__` and `__type_params__` in  free-threading build (#129016)

(cherry picked from commit 55f17b77c3)

---------

Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>

Lib/test/test_free_threading/test_func_annotations.py

@@ -0,0 +1,67 @@
+import concurrent.futures
+import unittest
+import inspect
+from threading import Thread, Barrier
+from unittest import TestCase
+
+from test.support import threading_helper, Py_GIL_DISABLED
+
+threading_helper.requires_working_threading(module=True)
+
+
+def get_func_annotation(f, b):
+    b.wait()
+    return inspect.get_annotations(f)
+
+
+def get_func_annotation_dunder(f, b):
+    b.wait()
+    return f.__annotations__
+
+
+def set_func_annotation(f, b):
+    b.wait()
+    f.__annotations__ = {'x': int, 'y': int, 'return': int}
+    return f.__annotations__
+
+
+@unittest.skipUnless(Py_GIL_DISABLED, "Enable only in FT build")
+class TestFTFuncAnnotations(TestCase):
+    NUM_THREADS = 8
+
+    def test_concurrent_read(self):
+        def f(x: int) -> int:
+            return x + 1
+
+        for _ in range(100):
+            with concurrent.futures.ThreadPoolExecutor(max_workers=self.NUM_THREADS) as executor:
+                b = Barrier(self.NUM_THREADS)
+                futures = {executor.submit(get_func_annotation, f, b): i for i in range(self.NUM_THREADS)}
+                for fut in concurrent.futures.as_completed(futures):
+                    annotate = fut.result()
+                    self.assertIsNotNone(annotate)
+                    self.assertEqual(annotate, {'x': int, 'return': int})
+
+            with concurrent.futures.ThreadPoolExecutor(max_workers=self.NUM_THREADS) as executor:
+                b = Barrier(self.NUM_THREADS)
+                futures = {executor.submit(get_func_annotation_dunder, f, b): i for i in range(self.NUM_THREADS)}
+                for fut in concurrent.futures.as_completed(futures):
+                    annotate = fut.result()
+                    self.assertIsNotNone(annotate)
+                    self.assertEqual(annotate, {'x': int, 'return': int})
+
+    def test_concurrent_write(self):
+        def bar(x: int, y: float) -> float:
+            return y ** x
+
+        for _ in range(100):
+            with concurrent.futures.ThreadPoolExecutor(max_workers=self.NUM_THREADS) as executor:
+                b = Barrier(self.NUM_THREADS)
+                futures = {executor.submit(set_func_annotation, bar, b): i for i in range(self.NUM_THREADS)}
+                for fut in concurrent.futures.as_completed(futures):
+                    annotate = fut.result()
+                    self.assertIsNotNone(annotate)
+                    self.assertEqual(annotate, {'x': int, 'y': int, 'return': int})
+
+            # 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})