bpo-30598: _PySys_EndInit() now duplicates warnoptions

Fix a reference in subinterpreters, like test_callbacks_leak() of test_atexit. warnoptions is a list used to pass options from the command line to the sys module constructor. Before this change, the list was shared by multiple interpreter which is not the expected behaviour. Each interpreter should have their own independent mutable world. This change duplicates the list in each interpreter. So each interpreter owns its own list, so each interpreter can clear its own list.
python · vstinner · Jun 8, 2017 · Jun 8, 2017 · Jun 8, 2017 · 0e2d42849806ca77f6ed67e417482bd037dabe0f
commit 0e2d42849806ca77f6ed67e417482bd037dabe0f
diff --git a/Python/sysmodule.c b/Python/sysmodule.c
@@ -2136,10 +2136,10 @@ _PySys_EndInit(PyObject *sysdict)
        if (warnoptions == NULL)
            return -1;
    }
-    else {
-        Py_INCREF(warnoptions);
-    }
-    SET_SYS_FROM_STRING_BORROW_INT_RESULT("warnoptions", warnoptions);
+
+    SET_SYS_FROM_STRING_INT_RESULT("warnoptions",
+                                   PyList_GetSlice(warnoptions,
+                                                   0, Py_SIZE(warnoptions)));

    SET_SYS_FROM_STRING_BORROW_INT_RESULT("_xoptions", get_xoptions());