gh-109521: Fix obscure cases handling in PyImport_GetImporter() (GH-109522)

PyImport_GetImporter() now sets RuntimeError if it fails to get sys.path_hooks
or sys.path_importer_cache or they are not list and dict correspondingly.

Previously it could return NULL without setting error in obscure cases,
crash or raise SystemError if these attributes have wrong type.

Author: serhiy-storchaka

Misc/NEWS.d/next/C API/2023-09-17-21-47-31.gh-issue-109521.JDF6i9.rst

@@ -0,0 +1,5 @@
+:c:func:`PyImport_GetImporter` now sets RuntimeError if it fails to get
+:data:`sys.path_hooks` or :data:`sys.path_importer_cache` or they are not
+list and dict correspondingly. Previously it could return NULL without
+setting error in obscure cases, crash or raise SystemError if these
+attributes have wrong type.

Python/import.c

@@ -2363,9 +2363,14 @@ get_path_importer(PyThreadState *tstate, PyObject *path_importer_cache,
     PyObject *importer;
     Py_ssize_t j, nhooks;
 
-    /* These conditions are the caller's responsibility: */
-    assert(PyList_Check(path_hooks));
-    assert(PyDict_Check(path_importer_cache));
+    if (!PyList_Check(path_hooks)) {
+        PyErr_SetString(PyExc_RuntimeError, "sys.path_hooks is not a list");
+        return NULL;
+    }
+    if (!PyDict_Check(path_importer_cache)) {
+        PyErr_SetString(PyExc_RuntimeError, "sys.path_importer_cache is not a dict");
+        return NULL;
+    }
 
     nhooks = PyList_Size(path_hooks);
     if (nhooks < 0)
@@ -2408,11 +2413,22 @@ PyImport_GetImporter(PyObject *path)
 {
     PyThreadState *tstate = _PyThreadState_GET();
     PyObject *path_importer_cache = PySys_GetObject("path_importer_cache");
+    if (path_importer_cache == NULL) {
+        PyErr_SetString(PyExc_RuntimeError, "lost sys.path_importer_cache");
+        return NULL;
+    }
+    Py_INCREF(path_importer_cache);
     PyObject *path_hooks = PySys_GetObject("path_hooks");
-    if (path_importer_cache == NULL || path_hooks == NULL) {
+    if (path_hooks == NULL) {
+        PyErr_SetString(PyExc_RuntimeError, "lost sys.path_hooks");
+        Py_DECREF(path_importer_cache);
         return NULL;
     }
-    return get_path_importer(tstate, path_importer_cache, path_hooks, path);
+    Py_INCREF(path_hooks);
+    PyObject *importer = get_path_importer(tstate, path_importer_cache, path_hooks, path);
+    Py_DECREF(path_hooks);
+    Py_DECREF(path_importer_cache);
+    return importer;
 }