Fix data race all_type_info_populate in free-threading mode

Description:
- fixed data race all_type_info_populate in free-threading mode
- added test

For example, we have 2 threads entering `all_type_info`.
Both enter `all_type_info_get_cache`` function and
there is a first one which inserts a tuple (type, empty_vector) to the map
and second is waiting. Inserting thread gets the (iter_to_key, True) and non-inserting thread
after waiting gets (iter_to_key, False).
Inserting thread than will add a weakref and will then call into `all_type_info_populate`.
However, non-inserting thread is not entering `if (ins.second) {` clause and
returns `ins.first->second;`` which is just empty_vector.
Finally, non-inserting thread is failing the check in `allocate_layout`:
```c++
if (n_types == 0) {
    pybind11_fail(
        "instance allocation failed: new instance has no pybind11-registered base types");
}
```

include/pybind11/detail/type_caster_base.h

@@ -117,7 +117,6 @@ PYBIND11_NOINLINE void all_type_info_populate(PyTypeObject *t, std::vector<type_
     for (handle parent : reinterpret_borrow<tuple>(t->tp_bases)) {
         check.push_back((PyTypeObject *) parent.ptr());
     }
-
     auto const &type_dict = get_internals().registered_types_py;
     for (size_t i = 0; i < check.size(); i++) {
         auto *type = check[i];
@@ -177,11 +176,6 @@ PYBIND11_NOINLINE void all_type_info_populate(PyTypeObject *t, std::vector<type_
  */
 inline const std::vector<detail::type_info *> &all_type_info(PyTypeObject *type) {
     auto ins = all_type_info_get_cache(type);
-    if (ins.second) {
-        // New cache entry: populate it
-        all_type_info_populate(type, ins.first->second);
-    }
-
     return ins.first->second;
 }
 

include/pybind11/pybind11.h

@@ -2326,13 +2326,20 @@ keep_alive_impl(size_t Nurse, size_t Patient, function_call &call, handle ret) {
 inline std::pair<decltype(internals::registered_types_py)::iterator, bool>
 all_type_info_get_cache(PyTypeObject *type) {
     auto res = with_internals([type](internals &internals) {
-        return internals
-            .registered_types_py
+        auto ins = internals
+                       .registered_types_py
 #ifdef __cpp_lib_unordered_map_try_emplace
-            .try_emplace(type);
+                       .try_emplace(type);
 #else
-            .emplace(type, std::vector<detail::type_info *>());
+                       .emplace(type, std::vector<detail::type_info *>());
 #endif
+        if (ins.second) {
+            // In free-threading this method should be called
+            // under pymutex lock to avoid other threads
+            // continue running with empty ins.first->second
+            all_type_info_populate(type, ins.first->second);
+        }
+        return ins;
     });
     if (res.second) {
         // New cache entry created; set up a weak reference to automatically remove it if the type

tests/pybind11_tests.cpp

@@ -128,4 +128,9 @@ PYBIND11_MODULE(pybind11_tests, m, py::mod_gil_not_used()) {
     for (const auto &initializer : initializers()) {
         initializer(m);
     }
+
+    py::class_<TestContext>(m, "TestContext")
+        .def(py::init<>(&TestContext::createNewContextForInit))
+        .def("__enter__", &TestContext::contextEnter)
+        .def("__exit__", &TestContext::contextExit);
 }

tests/pybind11_tests.h

@@ -96,3 +96,23 @@ void ignoreOldStyleInitWarnings(F &&body) {
     )",
              py::dict(py::arg("body") = py::cpp_function(body)));
 }
+
+class TestContext {
+public:
+    TestContext() = delete;
+    TestContext(const TestContext &) = delete;
+    TestContext(TestContext &&) = delete;
+    static TestContext *createNewContextForInit() { return new TestContext("new-context"); }
+
+    pybind11::object contextEnter() {
+        py::object contextObj = py::cast(*this);
+        return contextObj;
+    }
+    void contextExit(const pybind11::object & /*excType*/,
+                     const pybind11::object & /*excVal*/,
+                     const pybind11::object & /*excTb*/) {}
+
+private:
+    TestContext(std::string context) : context(context) {}
+    std::string context;
+};

tests/test_class.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import sys
 from unittest import mock
 
 import pytest
@@ -501,3 +502,45 @@ def test_pr4220_tripped_over_this():
         m.Empty0().get_msg()
         == "This is really only meant to exercise successful compilation."
     )
+
+
+@pytest.mark.skipif(sys.platform.startswith("emscripten"), reason="Requires threads")
+def test_all_type_info_multithreaded():
+    # Test data race in all_type_info method in free-threading mode.
+    # For example, we have 2 threads entering `all_type_info`.
+    # Both enter `all_type_info_get_cache`` function and
+    # there is a first one which inserts a tuple (type, empty_vector) to the map
+    # and second is waiting. Inserting thread gets the (iter_to_key, True) and non-inserting thread
+    # after waiting gets (iter_to_key, False).
+    # Inserting thread than will add a weakref and will then call into `all_type_info_populate`.
+    # However, non-inserting thread is not entering `if (ins.second) {` clause and
+    # returns `ins.first->second;`` which is just empty_vector.
+    # Finally, non-inserting thread is failing the check in `allocate_layout`:
+    # if (n_types == 0) {
+    #     pybind11_fail(
+    #         "instance allocation failed: new instance has no pybind11-registered base types");
+    # }
+    import threading
+
+    from pybind11_tests import TestContext
+
+    class Context(TestContext):
+        def __init__(self, *args, **kwargs):
+            super().__init__(*args, **kwargs)
+
+    num_runs = 4
+    num_threads = 5
+    barrier = threading.Barrier(num_threads)
+
+    def func():
+        barrier.wait()
+        with Context():
+            pass
+
+    for _ in range(num_runs):
+        threads = [threading.Thread(target=func) for _ in range(num_threads)]
+        for thread in threads:
+            thread.start()
+
+        for thread in threads:
+            thread.join()