Add a python-driven subinterpreter test

Author: b-pass

tests/CMakeLists.txt

@@ -148,6 +148,7 @@ set(PYBIND11_TEST_FILES
     test_exceptions
     test_factory_constructors
     test_gil_scoped
+    test_interpreters.py
     test_iostream
     test_kwargs_and_defaults
     test_local_bindings
@@ -562,6 +563,10 @@ add_custom_target(
   WORKING_DIRECTORY "${CMAKE_CURRENT_BINARY_DIR}"
   USES_TERMINAL)
 
+# This module doesn't get mixed with other test modules because those aren't subinterpreter safe.
+pybind11_add_module(mod_test_interpreters THIN_LTO mod_test_interpreters.cpp)
+add_dependencies(pytest mod_test_interpreters)
+
 if(PYBIND11_TEST_OVERRIDE)
   add_custom_command(
     TARGET pytest

tests/mod_test_interpreters.cpp

@@ -0,0 +1,16 @@
+#include <pybind11/pybind11.h>
+
+namespace py = pybind11;
+
+/* Simple test module/test class to check that the referenced internals data of external pybind11
+ * modules are different across subinterpreters
+ */
+
+PYBIND11_MODULE(mod_test_interpreters,
+                m,
+                py::mod_multi_interpreter_one_gil(),
+                py::mod_gil_not_used(),
+                py::mod_per_interpreter_gil()) {
+    m.def("internals_at",
+          []() { return reinterpret_cast<uintptr_t>(&py::detail::get_internals()); });
+}

tests/test_interpreters.py

@@ -0,0 +1,60 @@
+from __future__ import annotations
+
+import pytest
+import os
+import pickle
+
+def test_interpreters():
+    """Makes sure the internals object differs across subinterpreters"""
+
+    import mod_test_interpreters as m
+
+    i = None
+    try:
+        # 3.14+
+        import interpreters as i
+    except ImportError:
+        try:
+            # 3.13
+            import _interpreters as i
+        except ImportError:
+            try:
+                # 3.12
+                import _xxsubinterpreters as i
+            except ImportError:
+                # <= 3.11
+                pytest.skip("Test requires a the interpreters stdlib module")
+                return
+
+    code = """
+import mod_test_interpreters as m
+import pickle
+with open(pipeo, 'wb') as f:
+    pickle.dump(m.internals_at(), f)
+"""
+
+    interp1 = i.create()
+    interp2 = i.create()
+    try:
+        pipei, pipeo = os.pipe()
+        i.run_string(interp1, code, shared={'pipeo':pipeo})
+        with open(pipei, 'rb') as f:
+            res1 = pickle.load(f)
+
+        pipei, pipeo = os.pipe()
+        i.run_string(interp2, code, shared={'pipeo':pipeo})
+        with open(pipei, 'rb') as f:
+            res2 = pickle.load(f)
+
+        # do this while the two interpreters are active
+        import mod_test_interpreters as m2
+        print(dir(m))
+        print(dir(m2))
+        assert m.internals_at() == m2.internals_at(), "internals should be the same within the main interpreter"
+    finally:
+        i.destroy(interp1)
+        i.destroy(interp2)
+
+    assert res1 != m.internals_at(), "internals should differ from main interpreter"
+    assert res2 != m.internals_at(), "internals should differ from main interpreter"
+    assert res1 != res2, "internals should differ between interpreters"