[3.14] gh-137422: Fix race condition in PyImport_AddModuleRef (gh-141822) (gh-141830)

(cherry picked from commit 2d50dd242e)

Co-authored-by: Sam Gross <colesbury@gmail.com>

Python/import.c

@@ -3,6 +3,7 @@
 #include "Python.h"
 #include "pycore_audit.h"         // _PySys_Audit()
 #include "pycore_ceval.h"
+#include "pycore_critical_section.h"  // Py_BEGIN_CRITICAL_SECTION()
 #include "pycore_hashtable.h"     // _Py_hashtable_new_full()
 #include "pycore_import.h"        // _PyImport_BootstrapImp()
 #include "pycore_initconfig.h"    // _PyStatus_OK()
@@ -309,13 +310,8 @@ PyImport_GetModule(PyObject *name)
    if not, create a new one and insert it in the modules dictionary. */
 
 static PyObject *
-import_add_module(PyThreadState *tstate, PyObject *name)
+import_add_module_lock_held(PyObject *modules, PyObject *name)
 {
-    PyObject *modules = get_modules_dict(tstate, false);
-    if (modules == NULL) {
-        return NULL;
-    }
-
     PyObject *m;
     if (PyMapping_GetOptionalItem(modules, name, &m) < 0) {
         return NULL;
@@ -335,6 +331,21 @@ import_add_module(PyThreadState *tstate, PyObject *name)
     return m;
 }
 
+static PyObject *
+import_add_module(PyThreadState *tstate, PyObject *name)
+{
+    PyObject *modules = get_modules_dict(tstate, false);
+    if (modules == NULL) {
+        return NULL;
+    }
+
+    PyObject *m;
+    Py_BEGIN_CRITICAL_SECTION(modules);
+    m = import_add_module_lock_held(modules, name);
+    Py_END_CRITICAL_SECTION();
+    return m;
+}
+
 PyObject *
 PyImport_AddModuleRef(const char *name)
 {