gh-153291: Fix data race in readline.get_completer() and get_pre_input_hook() (gh-153362)

The setters store these hooks while holding the module critical section
(via set_hook's Py_XSETREF), but the getters read and Py_NewRef the same
fields without it. Annotate both getters with @critical_section, matching
the other readline functions (gh-126895).

Co-authored-by: Neil Schemenauer <nas-github@arctrix.com>

Author: harjothkhara

Lib/test/test_free_threading/test_readline.py

@@ -0,0 +1,59 @@
+import unittest
+
+from test.support import import_helper
+from test.support import threading_helper
+
+readline = import_helper.import_module("readline")
+
+
+@threading_helper.requires_working_threading()
+class TestReadlineRaces(unittest.TestCase):
+    def test_completer_delims_get_set(self):
+        def worker():
+            for _ in range(100):
+                readline.get_completer_delims()
+                readline.set_completer_delims(
+                    ' \t\n`@#%^&*()=+[{]}\\|;:\'",<>?')
+                readline.set_completer_delims(
+                    ' \t\n`@#%^&*()=+[{]}\\|;:\'",<>?')
+                readline.get_completer_delims()
+
+        threading_helper.run_concurrently(worker, nthreads=40)
+
+    # get_completer()/get_pre_input_hook() must take the module critical
+    # section like their setters do; otherwise reading and Py_NewRef-ing the
+    # stored hook races the setter replacing it (gh-153291).
+
+    def test_completer_get_set(self):
+        def setter():
+            for _ in range(1000):
+                readline.set_completer(lambda text, state: None)
+                readline.set_completer(None)
+
+        def getter():
+            for _ in range(1000):
+                readline.get_completer()
+
+        original = readline.get_completer()
+        self.addCleanup(readline.set_completer, original)
+        threading_helper.run_concurrently([setter] * 2 + [getter] * 6)
+
+    @unittest.skipUnless(hasattr(readline, "set_pre_input_hook"),
+                         "needs readline.set_pre_input_hook")
+    def test_pre_input_hook_get_set(self):
+        def setter():
+            for _ in range(1000):
+                readline.set_pre_input_hook(lambda: None)
+                readline.set_pre_input_hook(None)
+
+        def getter():
+            for _ in range(1000):
+                readline.get_pre_input_hook()
+
+        original = readline.get_pre_input_hook()
+        self.addCleanup(readline.set_pre_input_hook, original)
+        threading_helper.run_concurrently([setter] * 2 + [getter] * 6)
+
+
+if __name__ == "__main__":
+    unittest.main()

Lib/test/test_readline.py

@@ -8,10 +8,7 @@
 import sysconfig
 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
@@ -445,26 +442,5 @@ def my_hook():
         self.assertIs(readline.get_pre_input_hook(), my_hook)
 
 
-@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()

Misc/NEWS.d/next/Library/2026-07-08-00-00-00.gh-issue-153291.rdlnCS.rst

@@ -0,0 +1,4 @@
+Fix a data race in :func:`readline.get_completer` and
+:func:`readline.get_pre_input_hook` on the :term:`free-threaded <free
+threading>` build: the getters read the stored hook without the critical
+section that the corresponding setters hold.

Modules/clinic/readline.c.h

@@ -578,14 +578,15 @@ readline_set_pre_input_hook_impl(PyObject *module, PyObject *function)
 /* Get pre-input hook */
 
 /*[clinic input]
+@critical_section
 readline.get_pre_input_hook
 
 Get the current pre-input hook function.
 [clinic start generated code]*/
 
 static PyObject *
 readline_get_pre_input_hook_impl(PyObject *module)
-/*[clinic end generated code: output=ad56b77a8e8981ca input=fb1e1b1fbd94e4e5]*/
+/*[clinic end generated code: output=ad56b77a8e8981ca input=fbbf0106bd015414]*/
 {
     readlinestate *state = get_readline_state(module);
     if (state->pre_input_hook == NULL) {
@@ -886,14 +887,15 @@ readline_set_completer_impl(PyObject *module, PyObject *function)
 }
 
 /*[clinic input]
+@critical_section
 readline.get_completer
 
 Get the current completer function.
 [clinic start generated code]*/
 
 static PyObject *
 readline_get_completer_impl(PyObject *module)
-/*[clinic end generated code: output=6e6bbd8226d14475 input=6457522e56d70d13]*/
+/*[clinic end generated code: output=6e6bbd8226d14475 input=0df9ba4107115c44]*/
 {
     readlinestate *state = get_readline_state(module);
     if (state->completer == NULL) {