gh-154523: Fix data-race in TextIOWrapper.detach()

Lib/test/test_free_threading/test_io.py

@@ -204,3 +204,20 @@ def reset_worker():
                 decoder.reset()
 
         run_concurrently([decode_worker] * 2 + [reset_worker] * 2)
+
+
+class TextIOWrapperTest(TestCase):
+    def test_buffer_detach_race(self):
+        make = lambda: io.TextIOWrapper(io.BytesIO())
+        slot = [make()]
+
+        def reader():
+            for _ in range(1000):
+                slot[0].buffer
+
+        def detacher():
+            for _ in range(1000):
+                slot[0] = make()
+                slot[0].detach()
+
+        run_concurrently([reader, detacher])

Misc/NEWS.d/next/Library/2026-07-23-18-53-48.gh-issue-154523.XUSRBO.rst

@@ -0,0 +1,2 @@
+Fixed data-race when calling :meth:`io.TextIOBase.detach` in
+:term:`free-threaded build`.

Modules/_io/textio.c

@@ -1639,7 +1639,7 @@ _io_TextIOWrapper_detach_impl(textio *self)
     if (buffer == NULL) {
         return NULL;
     }
-    self->buffer = NULL;
+    FT_ATOMIC_STORE_PTR_RELAXED(self->buffer, NULL);
     self->detached = 1;
     return buffer;
 }