gh-134908: protect textiowrapper_iternext with critical section

Lib/test/test_io.py

@@ -1062,6 +1062,34 @@ def flush(self):
         # Silence destructor error
         R.flush = lambda self: None
 
+    @threading_helper.requires_working_threading()
+    def test_write_readline_races(self):
+        N=2
+        COUNT=100
+
+        def writer(file, barrier):
+            barrier.wait()
+            for _ in range(COUNT):
+                f.write("x")
+
+        def reader(file, stopping):
+            while not stopping.is_set():
+                for line in file:
+                    assert line == ""
+
+        stopping = threading.Event()
+        with self.open(os_helper.TESTFN, "w+") as f:
+            barrier = threading.Barrier(N)
+            reader = threading.Thread(target=reader, args=(f, stopping))
+            writers = [threading.Thread(target=writer, args=(f, barrier))
+                       for _ in range(N)]
+            reader.start()
+            with threading_helper.start_threads(writers):
+                pass
+            stopping.set()
+            reader.join()
+        assert(os.stat(os_helper.TESTFN).st_size == COUNT * N)
+
 
 class CIOTest(IOTest):
 

Misc/NEWS.d/next/Core_and_Builtins/2025-05-30-15-56-19.gh-issue-134908.3a7PxM.rst

@@ -0,0 +1 @@
+Protect ``textiowrapper_iternext`` with a critical section.

Modules/_io/textio.c

@@ -1578,6 +1578,8 @@ _io_TextIOWrapper_detach_impl(textio *self)
 static int
 _textiowrapper_writeflush(textio *self)
 {
+    _Py_CRITICAL_SECTION_ASSERT_OBJECT_LOCKED(self);
+
     if (self->pending_bytes == NULL)
         return 0;
 
@@ -3173,7 +3175,7 @@ _io_TextIOWrapper_close_impl(textio *self)
 }
 
 static PyObject *
-textiowrapper_iternext(PyObject *op)
+textiowrapper_iternext_locked(PyObject *op)
 {
     PyObject *line;
     textio *self = textio_CAST(op);
@@ -3210,6 +3212,16 @@ textiowrapper_iternext(PyObject *op)
     return line;
 }
 
+static PyObject *
+textiowrapper_iternext(PyObject *op)
+{
+    PyObject *result;
+    Py_BEGIN_CRITICAL_SECTION(op);
+    result = textiowrapper_iternext_locked(op);
+    Py_END_CRITICAL_SECTION();
+    return result;
+}
+
 /*[clinic input]
 @critical_section
 @getter