From 684f2327b000a753cc6266c0ea418d7d0f3be4eb Mon Sep 17 00:00:00 2001
From: Brij <97006829+brijkapadia@users.noreply.github.com>
Date: Thu, 23 Jul 2026 18:53:52 -0400
Subject: [PATCH 1/2] gh-154523: Fix data-race in `TextIOWrapper.detach()`

---
 Lib/test/test_free_threading/test_io.py         | 17 +++++++++++++++++
 ...26-07-23-18-53-48.gh-issue-154523.XUSRBO.rst |  2 ++
 Modules/_io/textio.c                            |  2 +-
 3 files changed, 20 insertions(+), 1 deletion(-)
 create mode 100644 Misc/NEWS.d/next/Library/2026-07-23-18-53-48.gh-issue-154523.XUSRBO.rst

diff --git a/Lib/test/test_free_threading/test_io.py b/Lib/test/test_free_threading/test_io.py
index e0bd7e211e73024..8214ae330e25fe5 100644
--- a/Lib/test/test_free_threading/test_io.py
+++ b/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])
diff --git a/Misc/NEWS.d/next/Library/2026-07-23-18-53-48.gh-issue-154523.XUSRBO.rst b/Misc/NEWS.d/next/Library/2026-07-23-18-53-48.gh-issue-154523.XUSRBO.rst
new file mode 100644
index 000000000000000..016505cf08e3dd3
--- /dev/null
+++ b/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.TextIOWrapper.detach` in
+:term:`free-threaded build`.
diff --git a/Modules/_io/textio.c b/Modules/_io/textio.c
index 5b2a20a30c28cb2..135904c3ab34b95 100644
--- a/Modules/_io/textio.c
+++ b/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;
 }

From 828618fdbbe8591fc29d47c008ddd62058e81c9b Mon Sep 17 00:00:00 2001
From: Brij <97006829+brijkapadia@users.noreply.github.com>
Date: Thu, 23 Jul 2026 19:03:09 -0400
Subject: [PATCH 2/2] Fix news

---
 .../next/Library/2026-07-23-18-53-48.gh-issue-154523.XUSRBO.rst | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Misc/NEWS.d/next/Library/2026-07-23-18-53-48.gh-issue-154523.XUSRBO.rst b/Misc/NEWS.d/next/Library/2026-07-23-18-53-48.gh-issue-154523.XUSRBO.rst
index 016505cf08e3dd3..23b25a26effb642 100644
--- a/Misc/NEWS.d/next/Library/2026-07-23-18-53-48.gh-issue-154523.XUSRBO.rst
+++ b/Misc/NEWS.d/next/Library/2026-07-23-18-53-48.gh-issue-154523.XUSRBO.rst
@@ -1,2 +1,2 @@
-Fixed data-race when calling :meth:`io.TextIOWrapper.detach` in
+Fixed data-race when calling :meth:`io.TextIOBase.detach` in
 :term:`free-threaded build`.