From 247d38669d0abc815d13817c47b6bcb58bb127b8 Mon Sep 17 00:00:00 2001
From: Thomas Kowalski <thom.kowa@gmail.com>
Date: Tue, 2 Jun 2026 16:26:37 +0200
Subject: [PATCH 1/3] misc: add news entry

---
 .../next/Library/2026-06-02-14-22-05.gh-issue-150791.aQ7rNp.rst  | 1 +
 1 file changed, 1 insertion(+)
 create mode 100644 Misc/NEWS.d/next/Library/2026-06-02-14-22-05.gh-issue-150791.aQ7rNp.rst

diff --git a/Misc/NEWS.d/next/Library/2026-06-02-14-22-05.gh-issue-150791.aQ7rNp.rst b/Misc/NEWS.d/next/Library/2026-06-02-14-22-05.gh-issue-150791.aQ7rNp.rst
new file mode 100644
index 00000000000000..b69c3d221bf530
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-06-02-14-22-05.gh-issue-150791.aQ7rNp.rst
@@ -0,0 +1 @@
+Fix a data race in :func:`itertools.groupby` on free-threaded builds where concurrent calls to :func:`next` could corrupt the iterator's internal state.

From 54fc409769da00a42522628bda1c7521414a7a3b Mon Sep 17 00:00:00 2001
From: Thomas Kowalski <thom.kowa@gmail.com>
Date: Tue, 2 Jun 2026 16:27:13 +0200
Subject: [PATCH 2/3] test: add regression test

---
 Lib/test/test_itertools.py | 37 +++++++++++++++++++++++++++++++++++++
 1 file changed, 37 insertions(+)

diff --git a/Lib/test/test_itertools.py b/Lib/test/test_itertools.py
index cf579d4da4e0df..1c3cc96ac48a64 100644
--- a/Lib/test/test_itertools.py
+++ b/Lib/test/test_itertools.py
@@ -786,6 +786,43 @@ def keyfunc(element):
         items = list(grouper_iter)
         self.assertEqual(len(items), 1)
 
+    @threading_helper.requires_working_threading()
+    def test_groupby_concurrent_next_does_not_crash(self):
+        # regression test for gh-150791
+        # Concurrent next calls on a shared groupby object should
+        # not race / corrupt state.
+        class K:
+            __slots__ = ("v",)
+            def __init__(self, v):
+                self.v = v
+            def __eq__(self, other):
+                if not isinstance(other, K):
+                    return NotImplemented
+                return self.v == other.v
+            def __hash__(self):
+                return hash(self.v)
+
+        keys = [K(i) for i in range(5_000)]
+        g = itertools.groupby(keys)
+        errors = []
+
+        def consume():
+            try:
+                while True:
+                    _, _ = next(g)
+            except StopIteration:
+                pass
+            except Exception as e:
+                errors.append(e)
+
+        threads = [threading.Thread(target=consume) for _ in range(8)]
+        for t in threads:
+            t.start()
+        for t in threads:
+            t.join()
+
+        self.assertEqual(errors, [])  # must pass with ThreadSanitizer
+
     def test_filter(self):
         self.assertEqual(list(filter(isEven, range(6))), [0,2,4])
         self.assertEqual(list(filter(None, [0,1,0,2,0])), [1,2])

From ba8c0cd0f064bc4bbe294a6a34faa5117d2e37e9 Mon Sep 17 00:00:00 2001
From: Thomas Kowalski <thom.kowa@gmail.com>
Date: Tue, 2 Jun 2026 16:27:59 +0200
Subject: [PATCH 3/3] fix: add critical section for grouper_next

---
 Modules/itertoolsmodule.c | 24 ++++++++++++++++++++++--
 1 file changed, 22 insertions(+), 2 deletions(-)

diff --git a/Modules/itertoolsmodule.c b/Modules/itertoolsmodule.c
index 68ac810eaad237..a21466a892c593 100644
--- a/Modules/itertoolsmodule.c
+++ b/Modules/itertoolsmodule.c
@@ -529,7 +529,7 @@ groupby_step(groupbyobject *gbo)
 }
 
 static PyObject *
-groupby_next(PyObject *op)
+groupby_next_lock_held(PyObject *op)
 {
     PyObject *grouper;
     groupbyobject *gbo = groupbyobject_CAST(op);
@@ -574,6 +574,16 @@ groupby_next(PyObject *op)
     return _PyTuple_FromPairSteal(Py_NewRef(gbo->currkey), grouper);
 }
 
+static PyObject *
+groupby_next(PyObject *op)
+{
+    PyObject *result;
+    Py_BEGIN_CRITICAL_SECTION(op);
+    result = groupby_next_lock_held(op);
+    Py_END_CRITICAL_SECTION()
+    return result;
+}
+
 static PyType_Slot groupby_slots[] = {
     {Py_tp_dealloc, groupby_dealloc},
     {Py_tp_getattro, PyObject_GenericGetAttr},
@@ -659,7 +669,7 @@ _grouper_traverse(PyObject *op, visitproc visit, void *arg)
 }
 
 static PyObject *
-_grouper_next(PyObject *op)
+_grouper_next_lock_held(PyObject *op)
 {
     _grouperobject *igo = _grouperobject_CAST(op);
     groupbyobject *gbo = groupbyobject_CAST(igo->parent);
@@ -695,6 +705,16 @@ _grouper_next(PyObject *op)
     return r;
 }
 
+static PyObject *
+_grouper_next(PyObject *op)
+{
+    PyObject *result;
+    Py_BEGIN_CRITICAL_SECTION(_grouperobject_CAST(op)->parent);
+    result = _grouper_next_lock_held(op);
+    Py_END_CRITICAL_SECTION()
+    return result;
+}
+
 static PyType_Slot _grouper_slots[] = {
     {Py_tp_dealloc, _grouper_dealloc},
     {Py_tp_getattro, PyObject_GenericGetAttr},