fix(regression): support embedded submodule (#5650)

* tests: add test for embedded submodule

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

* tests: adjust

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

* Update test_interpreter.cpp

* Update test_interpreter.cpp

* Update test_interpreter.cpp

* Update test_interpreter.cpp

* Update pybind11.h

* fix: end instead of endif

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

* Update include/pybind11/pybind11.h

---------

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

Author: henryiii

include/pybind11/pybind11.h

@@ -1342,13 +1342,18 @@ class module_ : public object {
         // GraalPy doesn't support PyModule_GetFilenameObject,
         // so getting by attribute (see PR #5584)
         handle this_module = m_ptr;
-        result.attr("__file__") = this_module.attr("__file__");
+        if (object this_file = getattr(this_module, "__file__", none())) {
+            result.attr("__file__") = this_file;
+        }
 #else
         handle this_file = PyModule_GetFilenameObject(m_ptr);
-        if (!this_file) {
+        if (this_file) {
+            result.attr("__file__") = this_file;
+        } else if (PyErr_ExceptionMatches(PyExc_SystemError) != 0) {
+            PyErr_Clear();
+        } else {
             throw error_already_set();
         }
-        result.attr("__file__") = this_file;
 #endif
         attr(name) = result;
         return result;

tests/test_embed/test_interpreter.cpp

@@ -61,6 +61,9 @@ PYBIND11_EMBEDDED_MODULE(widget_module, m) {
         .def_property_readonly("the_message", &Widget::the_message);
 
     m.def("add", [](int i, int j) { return i + j; });
+
+    auto sub = m.def_submodule("sub");
+    sub.def("add", [](int i, int j) { return i + j; });
 }
 
 PYBIND11_EMBEDDED_MODULE(trampoline_module, m) {
@@ -316,6 +319,9 @@ TEST_CASE("Restart the interpreter") {
     auto cpp_module = py::module_::import("widget_module");
     REQUIRE(cpp_module.attr("add")(1, 2).cast<int>() == 3);
 
+    // Also verify submodules work
+    REQUIRE(cpp_module.attr("sub").attr("add")(1, 41).cast<int>() == 42);
+
     // C++ type information is reloaded and can be used in python modules.
     auto py_module = py::module_::import("test_interpreter");
     auto py_widget = py_module.attr("DerivedWidget")("Hello after restart");