bpo-32030: Add _PyCoreConfig_Copy() (#4874)

Each interpreter now has its core_config and main_config copy:

* Add _PyCoreConfig_Copy() and _PyMainInterpreterConfig_Copy()
* Move _PyCoreConfig_Read(), _PyCoreConfig_Clear() and
  _PyMainInterpreterConfig_Clear() from Python/pylifecycle.c to
  Modules/main.c
* Fix _Py_InitializeEx_Private(): call _PyCoreConfig_ReadEnv() before
  _Py_InitializeCore()

Modules/main.c

@@ -1914,6 +1914,154 @@ pymain_parse_cmdline_envvars(_PyMain *pymain)
 }
 
 
+/* Read configuration settings from standard locations
+ *
+ * This function doesn't make any changes to the interpreter state - it
+ * merely populates any missing configuration settings. This allows an
+ * embedding application to completely override a config option by
+ * setting it before calling this function, or else modify the default
+ * setting before passing the fully populated config to Py_EndInitialization.
+ *
+ * More advanced selective initialization tricks are possible by calling
+ * this function multiple times with various preconfigured settings.
+ */
+
+_PyInitError
+_PyCoreConfig_Read(_PyCoreConfig *config)
+{
+    if (config->program_name == NULL) {
+#ifdef MS_WINDOWS
+        const wchar_t *program_name = L"python";
+#else
+        const wchar_t *program_name = L"python3";
+#endif
+        config->program_name = _PyMem_RawWcsdup(program_name);
+        if (config->program_name == NULL) {
+            return _Py_INIT_NO_MEMORY();
+        }
+    }
+
+    return _Py_INIT_OK();
+}
+
+
+void
+_PyCoreConfig_Clear(_PyCoreConfig *config)
+{
+#define CLEAR(ATTR) \
+    do { \
+        PyMem_RawFree(ATTR); \
+        ATTR = NULL; \
+    } while (0)
+
+    CLEAR(config->module_search_path_env);
+    CLEAR(config->home);
+    CLEAR(config->program_name);
+#undef CLEAR
+}
+
+
+int
+_PyCoreConfig_Copy(_PyCoreConfig *config, const _PyCoreConfig *config2)
+{
+    _PyCoreConfig_Clear(config);
+
+#define COPY_ATTR(ATTR) config->ATTR = config2->ATTR
+    COPY_ATTR(ignore_environment);
+    COPY_ATTR(use_hash_seed);
+    COPY_ATTR(hash_seed);
+    COPY_ATTR(_disable_importlib);
+    COPY_ATTR(allocator);
+    COPY_ATTR(dev_mode);
+    COPY_ATTR(faulthandler);
+    COPY_ATTR(tracemalloc);
+    COPY_ATTR(import_time);
+    COPY_ATTR(show_ref_count);
+    COPY_ATTR(show_alloc_count);
+    COPY_ATTR(dump_refs);
+    COPY_ATTR(malloc_stats);
+    COPY_ATTR(utf8_mode);
+#undef COPY_ATTR
+
+#define COPY_STR_ATTR(ATTR) \
+    do { \
+        if (config2->ATTR != NULL) { \
+            config->ATTR = _PyMem_RawWcsdup(config2->ATTR); \
+            if (config->ATTR == NULL) { \
+                return -1; \
+            } \
+        } \
+    } while (0)
+
+    COPY_STR_ATTR(module_search_path_env);
+    COPY_STR_ATTR(home);
+    COPY_STR_ATTR(program_name);
+#undef COPY_STR_ATTR
+    return 0;
+}
+
+
+void
+_PyMainInterpreterConfig_Clear(_PyMainInterpreterConfig *config)
+{
+    Py_CLEAR(config->argv);
+    Py_CLEAR(config->module_search_path);
+    Py_CLEAR(config->warnoptions);
+    Py_CLEAR(config->xoptions);
+}
+
+
+static PyObject*
+config_copy_attr(PyObject *obj)
+{
+    if (PyUnicode_Check(obj)) {
+        Py_INCREF(obj);
+        return obj;
+    }
+    else if (PyList_Check(obj)) {
+        return PyList_GetSlice(obj, 0, Py_SIZE(obj));
+    }
+    else if (PyDict_Check(obj)) {
+        /* The dict type is used for xoptions. Make the assumption that keys
+           and values are immutables */
+        return PyDict_Copy(obj);
+    }
+    else {
+        PyErr_Format(PyExc_TypeError,
+                     "cannot copy config attribute of type %.200s",
+                     Py_TYPE(obj)->tp_name);
+        return NULL;
+    }
+}
+
+
+int
+_PyMainInterpreterConfig_Copy(_PyMainInterpreterConfig *config,
+                              const _PyMainInterpreterConfig *config2)
+{
+    _PyMainInterpreterConfig_Clear(config);
+
+#define COPY_ATTR(ATTR) \
+    do { \
+        if (config2->ATTR != NULL) { \
+            config->ATTR = config_copy_attr(config2->ATTR); \
+            if (config->ATTR == NULL) { \
+                return -1; \
+            } \
+        } \
+    } while (0)
+
+    COPY_ATTR(argv);
+    COPY_ATTR(module_search_path);
+    COPY_ATTR(warnoptions);
+    COPY_ATTR(xoptions);
+#undef COPY_ATTR
+    return 0;
+}
+
+
+
+
 static PyObject *
 config_create_path_list(const wchar_t *path, wchar_t delim)
 {