fix: lock the translator list when adding the local exception translator

include/pybind11/detail/internals-inl.h

@@ -143,6 +143,11 @@ PYBIND11_INLINE void translate_local_exception(std::exception_ptr p) {
 
 PYBIND11_INLINE void check_internals_local_exception_translator(internals *internals_ptr) {
     if (internals_ptr) {
+#    if defined(Py_GIL_DISABLED) && PY_VERSION_HEX >= 0x030E00C1
+        // The pre-3.14rc1 fallback mutex can deadlock on reentrant imports.
+        // Not with_exception_translators(): that calls get_internals(), which runs this hook.
+        pycritical_section lock(internals_ptr->exception_translator_mutex);
+#    endif
         for (auto et : internals_ptr->registered_exception_translators) {
             if (et == &translate_local_exception) {
                 return;

tests/CMakeLists.txt

@@ -257,7 +257,8 @@ tests_extra_targets(
   "pybind11_cross_module_tests")
 
 # And add additional targets for other tests.
-tests_extra_targets("test_exceptions.py" "cross_module_interleaved_error_already_set")
+tests_extra_targets("test_exceptions.py"
+                    "cross_module_interleaved_error_already_set;mod_concurrent_import")
 tests_extra_targets("test_gil_scoped.py" "cross_module_gil_utils")
 tests_extra_targets("test_cpp_conduit.py"
                     "exo_planet_pybind11;exo_planet_c_api;home_planet_very_lonely_traveler")

tests/mod_concurrent_import.cpp

@@ -0,0 +1,10 @@
+// Copyright (c) 2026 The pybind Community.
+
+#include <pybind11/pybind11.h>
+
+namespace py = pybind11;
+
+// test_exceptions.py needs a module whose *first* import happens inside a test, so nothing else
+// may import this one.
+
+PYBIND11_MODULE(mod_concurrent_import, , py::mod_gil_not_used()) {}

tests/test_exceptions.cpp

@@ -424,4 +424,8 @@ TEST_SUBMODULE(exceptions, m) {
             PyErr_SetObject(PyExc_Exception, exc_inst.ptr());
         }
     });
+
+    // The on-fetch hook (check_internals_local_exception_translator) only runs when a module is
+    // imported; test_local_translator_registration_vs_internals_fetch needs to keep running it.
+    m.def("ensure_internals", []() { py::detail::ensure_internals(); });
 }

tests/test_exceptions.py

@@ -1,7 +1,9 @@
 from __future__ import annotations
 
+import importlib
 import re
 import sys
+import threading
 
 import pytest
 from custom_exceptions import PythonMyException7
@@ -442,3 +444,31 @@ def test_return_exception_void():
     with pytest.raises(TypeError) as excinfo:
         m.return_exception_void()
     assert "Exception" in str(excinfo.value)
+
+
+@pytest.mark.skipif("not env.PY_GIL_DISABLED", reason="Only relevant without the GIL")
+def test_local_translator_registration_vs_internals_fetch():
+    # Nothing to assert: the race only shows up under TSan, and only with libc++ or
+    # MSVC (the hook isn't built with libstdc++).
+    if "mod_concurrent_import" in sys.modules:
+        pytest.skip("The first import can only be exercised once per process")
+
+    stop = threading.Event()
+    errors = []
+
+    def ensure_internals_loop():
+        try:
+            while not stop.is_set():
+                m.ensure_internals()
+        except Exception as e:
+            errors.append(e)
+
+    reader = threading.Thread(target=ensure_internals_loop)
+    reader.start()
+    try:
+        importlib.import_module("mod_concurrent_import")
+    finally:
+        stop.set()
+        reader.join()
+    if errors:
+        raise errors[0]