pythongh-124160: Pass main_tstate to update_global_state_for_extension()

Otherwise it'll always return NULL if tstate != main_tstate due to
_Py_IsMainInterpreter() check inside it.

Also, add a regression test that makes sure that `readline` is
importable in case that triggered the crash before.

Programs/_testembed.c

@@ -95,6 +95,14 @@ static void _testembed_Py_Initialize(void)
 }
 
 
+static int test_import_in_subinterpreters(void)
+{
+    _testembed_Py_InitializeFromConfig();
+    PyThreadState_Swap(Py_NewInterpreter());
+    return PyRun_SimpleString("import readline"); // gh-124160
+}
+
+
 /*****************************************************
  * Test repeated initialisation and subinterpreters
  *****************************************************/
@@ -2398,6 +2406,7 @@ static struct TestCase TestCases[] = {
     {"test_repeated_init_exec", test_repeated_init_exec},
     {"test_repeated_simple_init", test_repeated_simple_init},
     {"test_forced_io_encoding", test_forced_io_encoding},
+    {"test_import_in_subinterpreters", test_import_in_subinterpreters},
     {"test_repeated_init_and_subinterpreters", test_repeated_init_and_subinterpreters},
     {"test_repeated_init_and_inittab", test_repeated_init_and_inittab},
     {"test_pre_initialization_api", test_pre_initialization_api},

Python/import.c

@@ -2045,7 +2045,7 @@ import_run_extension(PyThreadState *tstate, PyModInitFunction p0,
                 singlephase.m_init = p0;
             }
             cached = update_global_state_for_extension(
-                    tstate, info->path, info->name, def, &singlephase);
+                    main_tstate, info->path, info->name, def, &singlephase);
             if (cached == NULL) {
                 assert(PyErr_Occurred());
                 goto main_finally;