gh-154431: Eagerly initialize the audit hooks list

Lib/test/audit-tests.py

@@ -88,6 +88,31 @@ def test_basic():
         assertEqual(hook.seen[0][1], (1, 2, 3))
 
 
+def test_concurrent_add_hook():
+    from test.support import threading_helper
+
+    # gh-154431: Race the first hook registrations against audit events.
+    seen = []
+
+    def add_hook(index):
+        def hook(event, args):
+            if event == "test.concurrent_add_hook":
+                seen.append(index)
+
+        sys.addaudithook(hook)
+
+    def audit():
+        for _ in range(2000):
+            sys.audit("test.concurrent_audit")
+
+    workers = [lambda i=i: add_hook(i) for i in range(24)]
+    workers.extend([audit] * 8)
+    threading_helper.run_concurrently(workers)
+
+    sys.audit("test.concurrent_add_hook")
+    assertEqual(sorted(seen), list(range(24)))
+
+
 def test_block_add_hook():
     # Raising an exception should prevent a new hook from being added,
     # but will not propagate out.

Lib/test/test_audit.py

@@ -7,6 +7,7 @@
 from test import support
 from test.support import import_helper
 from test.support import os_helper
+from test.support import threading_helper
 
 
 if not hasattr(sys, "addaudithook") or not hasattr(sys, "audit"):
@@ -52,6 +53,10 @@ def run_python(self, *args, expect_stderr=False):
     def test_basic(self):
         self.do_test("test_basic")
 
+    @threading_helper.requires_working_threading()
+    def test_concurrent_add_hook(self):
+        self.do_test("test_concurrent_add_hook")
+
     def test_block_add_hook(self):
         self.do_test("test_block_add_hook")
 

Misc/NEWS.d/next/Core_and_Builtins/2026-10-20-21-58-53.gh-issue-154431.pR4x6T.rst

@@ -0,0 +1,2 @@
+Fix a data race when :func:`sys.addaudithook` is called concurrently in the
+free-threaded build.

Python/sysmodule.c

@@ -239,8 +239,12 @@ should_audit(PyInterpreterState *interp)
     if (!interp) {
         return 0;
     }
+    // interp->audit_hooks can only ever be NULL very early during initialization
+    // or very late during finalization.
+    int interp_has_audit_hooks = (interp->audit_hooks != NULL
+                                  && PyList_GET_SIZE(interp->audit_hooks) > 0);
     return (interp->runtime->audit_hooks.head
-            || interp->audit_hooks
+            || interp_has_audit_hooks
             || PyDTrace_AUDIT_ENABLED());
 }
 
@@ -310,7 +314,8 @@ sys_audit_tstate(PyThreadState *ts, const char *event,
     }
 
     /* Call interpreter hooks */
-    if (is->audit_hooks) {
+    PyObject *audit_hooks = is->audit_hooks;
+    if (audit_hooks != NULL && PyList_GET_SIZE(audit_hooks) > 0) {
         eventName = PyUnicode_FromString(event);
         if (!eventName) {
             goto exit;
@@ -540,15 +545,7 @@ sys_addaudithook_impl(PyObject *module, PyObject *hook)
     }
 
     PyInterpreterState *interp = tstate->interp;
-    if (interp->audit_hooks == NULL) {
-        interp->audit_hooks = PyList_New(0);
-        if (interp->audit_hooks == NULL) {
-            return NULL;
-        }
-        /* Avoid having our list of hooks show up in the GC module */
-        PyObject_GC_UnTrack(interp->audit_hooks);
-    }
-
+    assert(interp->audit_hooks != NULL);
     if (PyList_Append(interp->audit_hooks, hook) < 0) {
         return NULL;
     }
@@ -4367,6 +4364,13 @@ _PySys_Create(PyThreadState *tstate, PyObject **sysmod_p)
 
     PyInterpreterState *interp = tstate->interp;
 
+    PyObject *audit_hooks = PyList_New(0);
+    if (audit_hooks == NULL) {
+        goto error;
+    }
+    _PyObject_GC_UNTRACK(audit_hooks);
+    interp->audit_hooks = audit_hooks;
+
     PyObject *modules = _PyImport_InitModules(interp);
     if (modules == NULL) {
         goto error;