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

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).

Author: harjothkhara

Lib/test/test_readline.py

@@ -431,6 +431,22 @@ def my_hook():
         readline.set_pre_input_hook(my_hook)
         self.assertIs(readline.get_pre_input_hook(), my_hook)
 
+    def test_get_completer(self):
+        # Save and restore the original completer to avoid side effects
+        original_completer = readline.get_completer()
+        self.addCleanup(readline.set_completer, original_completer)
+
+        # Test that get_completer returns None when no completer is set
+        readline.set_completer(None)
+        self.assertIsNone(readline.get_completer())
+
+        # Set a completer and verify we can retrieve it
+        def my_completer(text, state):
+            return None
+
+        readline.set_completer(my_completer)
+        self.assertIs(readline.get_completer(), my_completer)
+
 
 @unittest.skipUnless(support.Py_GIL_DISABLED, 'these tests can only possibly fail with GIL disabled')
 class FreeThreadingTest(unittest.TestCase):
@@ -452,6 +468,44 @@ def completer_delims(b):
         with threading_helper.start_threads(threads):
             pass
 
+    @threading_helper.reap_threads
+    @threading_helper.requires_working_threading()
+    def test_free_threading_get_completer(self):
+        def completer(b):
+            b.wait()
+            for _ in range(100):
+                readline.get_completer()
+                readline.set_completer(lambda text, state: None)
+                readline.set_completer(None)
+                readline.get_completer()
+
+        count   = 40
+        barrier = threading.Barrier(count)
+        threads = [threading.Thread(target=completer, args=(barrier,)) for _ in range(count)]
+
+        with threading_helper.start_threads(threads):
+            pass
+
+    @unittest.skipUnless(hasattr(readline, "get_pre_input_hook"),
+                         "get_pre_input_hook not available")
+    @threading_helper.reap_threads
+    @threading_helper.requires_working_threading()
+    def test_free_threading_get_pre_input_hook(self):
+        def pre_input_hook(b):
+            b.wait()
+            for _ in range(100):
+                readline.get_pre_input_hook()
+                readline.set_pre_input_hook(lambda: None)
+                readline.set_pre_input_hook(None)
+                readline.get_pre_input_hook()
+
+        count   = 40
+        barrier = threading.Barrier(count)
+        threads = [threading.Thread(target=pre_input_hook, 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) {