[3.14] gh-134908: Protect textiowrapper_iternext with critical section (gh-134910) (gh-135039)

The `textiowrapper_iternext` function called `_textiowrapper_writeflush`, but did not
use a critical section, making it racy in free-threaded builds.
(cherry picked from commit 44fb7c361c)

Co-authored-by: Duane Griffin <duaneg@dghda.com>

Lib/test/test_io.py

@@ -1062,6 +1062,37 @@ def flush(self):
         # Silence destructor error
         R.flush = lambda self: None
 
+    @threading_helper.requires_working_threading()
+    def test_write_readline_races(self):
+        # gh-134908: Concurrent iteration over a file caused races
+        thread_count = 2
+        write_count = 100
+        read_count = 100
+
+        def writer(file, barrier):
+            barrier.wait()
+            for _ in range(write_count):
+                file.write("x")
+
+        def reader(file, barrier):
+            barrier.wait()
+            for _ in range(read_count):
+                for line in file:
+                    self.assertEqual(line, "")
+
+        with self.open(os_helper.TESTFN, "w+") as f:
+            barrier = threading.Barrier(thread_count + 1)
+            reader = threading.Thread(target=reader, args=(f, barrier))
+            writers = [threading.Thread(target=writer, args=(f, barrier))
+                       for _ in range(thread_count)]
+            with threading_helper.catch_threading_exception() as cm:
+                with threading_helper.start_threads(writers + [reader]):
+                    pass
+                self.assertIsNone(cm.exc_type)
+
+        self.assertEqual(os.stat(os_helper.TESTFN).st_size,
+                         write_count * thread_count)
+
 
 class CIOTest(IOTest):