gh-126895: Fix readline module in free-threaded build (#131208)

The underlying readline library is not thread-safe so this adds `@critical_section` to functions that use it.

Author: tom-pytel

Diff for: Lib/test/test_readline.py

@@ -6,7 +6,10 @@
 import sys
 import tempfile
 import textwrap
+import threading
 import unittest
+from test import support
+from test.support import threading_helper
 from test.support import verbose
 from test.support.import_helper import import_module
 from test.support.os_helper import unlink, temp_dir, TESTFN
@@ -403,5 +406,26 @@ def test_write_read_limited_history(self):
         # See TestHistoryManipulation for the full test.
 
 
+@unittest.skipUnless(support.Py_GIL_DISABLED, 'these tests can only possibly fail with GIL disabled')
+class FreeThreadingTest(unittest.TestCase):
+    @threading_helper.reap_threads
+    @threading_helper.requires_working_threading()
+    def test_free_threading(self):
+        def completer_delims(b):
+            b.wait()
+            for _ in range(100):
+                readline.get_completer_delims()
+                readline.set_completer_delims(' \t\n`@#%^&*()=+[{]}\\|;:\'",<>?')
+                readline.set_completer_delims(' \t\n`@#%^&*()=+[{]}\\|;:\'",<>?')
+                readline.get_completer_delims()
+
+        count   = 40
+        barrier = threading.Barrier(count)
+        threads = [threading.Thread(target=completer_delims, args=(barrier,)) for _ in range(count)]
+
+        with threading_helper.start_threads(threads):
+            pass
+
+
 if __name__ == "__main__":
     unittest.main()

Diff for: Misc/NEWS.d/next/Library/2025-03-13-19-54-57.gh-issue-126895.eJP9l0.rst

@@ -0,0 +1 @@
+Fix :mod:`readline` in :term:`free-threaded <free threading>` build.