gh-145713: fix: make bytearray.resize thread-safe

Lib/test/test_bytes.py

@@ -2908,6 +2908,22 @@ def check(funcs, it):
             check([iter_next] + [iter_reduce] * 10, iter(ba))  # for tsan
             check([iter_next] + [iter_setstate] * 10, iter(ba))  # for tsan
 
+    @unittest.skipUnless(support.Py_GIL_DISABLED, 'this test can only possibly fail with GIL disabled')
+    @threading_helper.reap_threads
+    @threading_helper.requires_working_threading()
+    def test_free_threading_bytearray_resize(self):
+        def resize_stress(ba):
+            for _ in range(100_000):
+                try:
+                    ba.resize(10_000)
+                    ba.resize(1)
+                except (BufferError, ValueError):
+                    pass
+
+        ba = bytearray(100)
+        threads = [threading.Thread(target=resize_stress, args=(ba,)) for _ in range(4)]
+        with threading_helper.start_threads(threads):
+            pass
 
 if __name__ == "__main__":
     unittest.main()

Misc/NEWS.d/next/Core_and_Builtins/2026-03-09-00-00-00.gh-issue-145713.KR6azvzI.rst

@@ -0,0 +1,3 @@
+Make :meth:`bytearray.resize` thread-safe in the free-threaded build by
+using a critical section and calling the lock-held variant of the resize
+function.

Objects/bytearrayobject.c

@@ -1506,6 +1506,7 @@ bytearray_removesuffix_impl(PyByteArrayObject *self, Py_buffer *suffix)
 
 
 /*[clinic input]
+@critical_section
 bytearray.resize
     size: Py_ssize_t
         New size to resize to.
@@ -1515,10 +1516,10 @@ Resize the internal buffer of bytearray to len.
 
 static PyObject *
 bytearray_resize_impl(PyByteArrayObject *self, Py_ssize_t size)
-/*[clinic end generated code: output=f73524922990b2d9 input=6c9a260ca7f72071]*/
+/*[clinic end generated code: output=f73524922990b2d9 input=116046316a2b5cfc]*/
 {
     Py_ssize_t start_size = PyByteArray_GET_SIZE(self);
-    int result = PyByteArray_Resize((PyObject *)self, size);
+    int result = bytearray_resize_lock_held((PyObject *)self, size);
     if (result < 0) {
         return NULL;
     }