bpo-38304: Add PyConfig.struct_size (GH-16451) (GH-16453)

Add a new struct_size field to PyPreConfig and PyConfig structures to
allow to modify these structures in the future without breaking the
backward compatibility.

* Replace private _config_version field with public struct_size field
  in PyPreConfig and PyConfig.
* Public PyPreConfig_InitIsolatedConfig() and
  PyPreConfig_InitPythonConfig()
  return type becomes PyStatus, instead of void.
* Internal _PyConfig_InitCompatConfig(),
  _PyPreConfig_InitCompatConfig(), _PyPreConfig_InitFromConfig(),
  _PyPreConfig_InitFromPreConfig() return type becomes PyStatus,
  instead of void.
* Remove _Py_CONFIG_VERSION
* Update the Initialization Configuration documentation.

(cherry picked from commit 441b10cf2855955c86565f8d59e72c2efc0f0a57)
diff --git a/Python/frozenmain.c b/Python/frozenmain.c
index c56938a..76309e9 100644
--- a/Python/frozenmain.c
+++ b/Python/frozenmain.c
@@ -40,6 +40,7 @@
     }
 
     PyConfig config;
+    config.struct_size = sizeof(PyConfig);
     status = PyConfig_InitPythonConfig(&config);
     if (PyStatus_Exception(status)) {
         PyConfig_Clear(&config);
diff --git a/Python/initconfig.c b/Python/initconfig.c
index a527271..dbc9de5 100644
--- a/Python/initconfig.c
+++ b/Python/initconfig.c
@@ -528,6 +528,18 @@
      ? _PyStatus_ERR("cannot decode " NAME) \
      : _PyStatus_NO_MEMORY())
 
+
+static PyStatus
+config_check_struct_size(const PyConfig *config)
+{
+    if (config->struct_size != sizeof(PyConfig)) {
+        return _PyStatus_ERR("unsupported PyConfig structure size "
+                             "(Python version mismatch?)");
+    }
+    return _PyStatus_OK();
+}
+
+
 /* Free memory allocated in config, but don't clear all attributes */
 void
 PyConfig_Clear(PyConfig *config)
@@ -568,12 +580,19 @@
 }
 
 
-void
+PyStatus
 _PyConfig_InitCompatConfig(PyConfig *config)
 {
+    size_t struct_size = config->struct_size;
     memset(config, 0, sizeof(*config));
+    config->struct_size = struct_size;
 
-    config->_config_version = _Py_CONFIG_VERSION;
+    PyStatus status = config_check_struct_size(config);
+    if (_PyStatus_EXCEPTION(status)) {
+        _PyStatus_UPDATE_FUNC(status);
+        return status;
+    }
+
     config->_config_init = (int)_PyConfig_INIT_COMPAT;
     config->isolated = -1;
     config->use_environment = -1;
@@ -603,13 +622,17 @@
 #ifdef MS_WINDOWS
     config->legacy_windows_stdio = -1;
 #endif
+    return _PyStatus_OK();
 }
 
 
-static void
+static PyStatus
 config_init_defaults(PyConfig *config)
 {
-    _PyConfig_InitCompatConfig(config);
+    PyStatus status = _PyConfig_InitCompatConfig(config);
+    if (_PyStatus_EXCEPTION(status)) {
+        return status;
+    }
 
     config->isolated = 0;
     config->use_environment = 1;
@@ -628,13 +651,18 @@
 #ifdef MS_WINDOWS
     config->legacy_windows_stdio = 0;
 #endif
+    return _PyStatus_OK();
 }
 
 
 PyStatus
 PyConfig_InitPythonConfig(PyConfig *config)
 {
-    config_init_defaults(config);
+    PyStatus status = config_init_defaults(config);
+    if (_PyStatus_EXCEPTION(status)) {
+        _PyStatus_UPDATE_FUNC(status);
+        return status;
+    }
 
     config->_config_init = (int)_PyConfig_INIT_PYTHON;
     config->configure_c_stdio = 1;
@@ -647,7 +675,11 @@
 PyStatus
 PyConfig_InitIsolatedConfig(PyConfig *config)
 {
-    config_init_defaults(config);
+    PyStatus status = config_init_defaults(config);
+    if (_PyStatus_EXCEPTION(status)) {
+        _PyStatus_UPDATE_FUNC(status);
+        return status;
+    }
 
     config->_config_init = (int)_PyConfig_INIT_ISOLATED;
     config->isolated = 1;
@@ -742,6 +774,19 @@
 _PyConfig_Copy(PyConfig *config, const PyConfig *config2)
 {
     PyStatus status;
+
+    status = config_check_struct_size(config);
+    if (_PyStatus_EXCEPTION(status)) {
+        _PyStatus_UPDATE_FUNC(status);
+        return status;
+    }
+
+    status = config_check_struct_size(config2);
+    if (_PyStatus_EXCEPTION(status)) {
+        _PyStatus_UPDATE_FUNC(status);
+        return status;
+    }
+
     PyConfig_Clear(config);
 
 #define COPY_ATTR(ATTR) config->ATTR = config2->ATTR
@@ -2199,7 +2244,12 @@
     }
 
     PyPreConfig preconfig;
-    _PyPreConfig_InitFromPreConfig(&preconfig, &_PyRuntime.preconfig);
+    preconfig.struct_size = sizeof(PyPreConfig);
+
+    status = _PyPreConfig_InitFromPreConfig(&preconfig, &_PyRuntime.preconfig);
+    if (_PyStatus_EXCEPTION(status)) {
+        return status;
+    }
 
     _PyPreConfig_GetConfig(&preconfig, config);
 
@@ -2338,6 +2388,12 @@
     PyStatus status;
     PyWideStringList orig_argv = PyWideStringList_INIT;
 
+    status = config_check_struct_size(config);
+    if (_PyStatus_EXCEPTION(status)) {
+        _PyStatus_UPDATE_FUNC(status);
+        return status;
+    }
+
     status = _Py_PreInitializeFromConfig(config, NULL);
     if (_PyStatus_EXCEPTION(status)) {
         return status;
diff --git a/Python/pathconfig.c b/Python/pathconfig.c
index d5b8b1a..41ef7d2 100644
--- a/Python/pathconfig.c
+++ b/Python/pathconfig.c
@@ -434,7 +434,12 @@
 {
     PyStatus status;
     PyConfig config;
-    _PyConfig_InitCompatConfig(&config);
+    config.struct_size = sizeof(PyConfig);
+
+    status = _PyConfig_InitCompatConfig(&config);
+    if (_PyStatus_EXCEPTION(status)) {
+        goto done;
+    }
 
     /* Call _PyConfig_InitPathConfig() */
     status = PyConfig_Read(&config);
diff --git a/Python/preconfig.c b/Python/preconfig.c
index 8be6533..d18b01d 100644
--- a/Python/preconfig.c
+++ b/Python/preconfig.c
@@ -269,12 +269,30 @@
 /* --- PyPreConfig ----------------------------------------------- */
 
 
-void
+static PyStatus
+preconfig_check_struct_size(PyPreConfig *config)
+{
+    if (config->struct_size != sizeof(PyPreConfig)) {
+        return _PyStatus_ERR("unsupported PyPreConfig structure size "
+                             "(Python version mismatch?)");
+    }
+    return _PyStatus_OK();
+}
+
+
+PyStatus
 _PyPreConfig_InitCompatConfig(PyPreConfig *config)
 {
+    size_t struct_size = config->struct_size;
     memset(config, 0, sizeof(*config));
+    config->struct_size = struct_size;
 
-    config->_config_version = _Py_CONFIG_VERSION;
+    PyStatus status = preconfig_check_struct_size(config);
+    if (_PyStatus_EXCEPTION(status)) {
+        _PyStatus_UPDATE_FUNC(status);
+        return status;
+    }
+
     config->_config_init = (int)_PyConfig_INIT_COMPAT;
     config->parse_argv = 0;
     config->isolated = -1;
@@ -295,13 +313,18 @@
 #ifdef MS_WINDOWS
     config->legacy_windows_fs_encoding = -1;
 #endif
+    return _PyStatus_OK();
 }
 
 
-void
+PyStatus
 PyPreConfig_InitPythonConfig(PyPreConfig *config)
 {
-    _PyPreConfig_InitCompatConfig(config);
+    PyStatus status = _PyPreConfig_InitCompatConfig(config);
+    if (_PyStatus_EXCEPTION(status)) {
+        _PyStatus_UPDATE_FUNC(status);
+        return status;
+    }
 
     config->_config_init = (int)_PyConfig_INIT_PYTHON;
     config->isolated = 0;
@@ -316,13 +339,18 @@
 #ifdef MS_WINDOWS
     config->legacy_windows_fs_encoding = 0;
 #endif
+    return _PyStatus_OK();
 }
 
 
-void
+PyStatus
 PyPreConfig_InitIsolatedConfig(PyPreConfig *config)
 {
-    _PyPreConfig_InitCompatConfig(config);
+    PyStatus status = _PyPreConfig_InitCompatConfig(config);
+    if (_PyStatus_EXCEPTION(status)) {
+        _PyStatus_UPDATE_FUNC(status);
+        return status;
+    }
 
     config->_config_init = (int)_PyConfig_INIT_ISOLATED;
     config->configure_locale = 0;
@@ -333,41 +361,55 @@
 #ifdef MS_WINDOWS
     config->legacy_windows_fs_encoding = 0;
 #endif
+    return _PyStatus_OK();
 }
 
 
-void
+PyStatus
 _PyPreConfig_InitFromPreConfig(PyPreConfig *config,
                                const PyPreConfig *config2)
 {
-    PyPreConfig_InitPythonConfig(config);
+    PyStatus status = PyPreConfig_InitPythonConfig(config);
+    if (_PyStatus_EXCEPTION(status)) {
+        return status;
+    }
+
     preconfig_copy(config, config2);
+    return _PyStatus_OK();
 }
 
 
-void
+PyStatus
 _PyPreConfig_InitFromConfig(PyPreConfig *preconfig, const PyConfig *config)
 {
+    PyStatus status;
     _PyConfigInitEnum config_init = (_PyConfigInitEnum)config->_config_init;
     switch (config_init) {
     case _PyConfig_INIT_PYTHON:
-        PyPreConfig_InitPythonConfig(preconfig);
+        status = PyPreConfig_InitPythonConfig(preconfig);
         break;
     case _PyConfig_INIT_ISOLATED:
-        PyPreConfig_InitIsolatedConfig(preconfig);
+        status = PyPreConfig_InitIsolatedConfig(preconfig);
         break;
     case _PyConfig_INIT_COMPAT:
     default:
-        _PyPreConfig_InitCompatConfig(preconfig);
+        status = _PyPreConfig_InitCompatConfig(preconfig);
     }
+
+    if (_PyStatus_EXCEPTION(status)) {
+        return status;
+    }
+
     _PyPreConfig_GetConfig(preconfig, config);
+    return _PyStatus_OK();
 }
 
 
 static void
 preconfig_copy(PyPreConfig *config, const PyPreConfig *config2)
 {
-    assert(config2->_config_version == _Py_CONFIG_VERSION);
+    assert(config->struct_size == sizeof(PyPreConfig));
+
 #define COPY_ATTR(ATTR) config->ATTR = config2->ATTR
 
     COPY_ATTR(_config_init);
@@ -787,6 +829,12 @@
         return status;
     }
 
+    status = preconfig_check_struct_size(config);
+    if (_PyStatus_EXCEPTION(status)) {
+        _PyStatus_UPDATE_FUNC(status);
+        return status;
+    }
+
     preconfig_get_global_vars(config);
 
     /* Copy LC_CTYPE locale, since it's modified later */
@@ -801,7 +849,12 @@
 
     /* Save the config to be able to restore it if encodings change */
     PyPreConfig save_config;
-    _PyPreConfig_InitFromPreConfig(&save_config, config);
+    save_config.struct_size = sizeof(PyPreConfig);
+
+    status = _PyPreConfig_InitFromPreConfig(&save_config, config);
+    if (_PyStatus_EXCEPTION(status)) {
+        return status;
+    }
 
     /* Set LC_CTYPE to the user preferred locale */
     if (config->configure_locale) {
@@ -923,7 +976,12 @@
 _PyPreConfig_Write(const PyPreConfig *src_config)
 {
     PyPreConfig config;
-    _PyPreConfig_InitFromPreConfig(&config, src_config);
+    config.struct_size = sizeof(PyPreConfig);
+
+    PyStatus status = _PyPreConfig_InitFromPreConfig(&config, src_config);
+    if (_PyStatus_EXCEPTION(status)) {
+        return status;
+    }
 
     if (_PyRuntime.core_initialized) {
         /* bpo-34008: Calling this functions after Py_Initialize() ignores
diff --git a/Python/pylifecycle.c b/Python/pylifecycle.c
index 42a062e..c154d06 100644
--- a/Python/pylifecycle.c
+++ b/Python/pylifecycle.c
@@ -731,7 +731,12 @@
     runtime->preinitializing = 1;
 
     PyPreConfig config;
-    _PyPreConfig_InitFromPreConfig(&config, src_config);
+    config.struct_size = sizeof(PyPreConfig);
+
+    status = _PyPreConfig_InitFromPreConfig(&config, src_config);
+    if (_PyStatus_EXCEPTION(status)) {
+        return status;
+    }
 
     status = _PyPreConfig_Read(&config, args);
     if (_PyStatus_EXCEPTION(status)) {
@@ -790,7 +795,12 @@
     }
 
     PyPreConfig preconfig;
-    _PyPreConfig_InitFromConfig(&preconfig, config);
+    preconfig.struct_size = sizeof(PyPreConfig);
+
+    status = _PyPreConfig_InitFromConfig(&preconfig, config);
+    if (_PyStatus_EXCEPTION(status)) {
+        return status;
+    }
 
     if (!config->parse_argv) {
         return Py_PreInitialize(&preconfig);
@@ -838,7 +848,12 @@
     }
 
     PyConfig config;
-    _PyConfig_InitCompatConfig(&config);
+    config.struct_size = sizeof(PyConfig);
+
+    status = _PyConfig_InitCompatConfig(&config);
+    if (_PyStatus_EXCEPTION(status)) {
+        goto done;
+    }
 
     status = _PyConfig_Copy(&config, src_config);
     if (_PyStatus_EXCEPTION(status)) {
@@ -1061,7 +1076,13 @@
     }
 
     PyConfig config;
-    _PyConfig_InitCompatConfig(&config);
+    config.struct_size = sizeof(PyConfig);
+
+    status = _PyConfig_InitCompatConfig(&config);
+    if (_PyStatus_EXCEPTION(status)) {
+        Py_ExitStatusException(status);
+    }
+
     config.install_signal_handlers = install_sigs;
 
     status = Py_InitializeFromConfig(&config);
diff --git a/Python/pystate.c b/Python/pystate.c
index f2924a8..f0662fa 100644
--- a/Python/pystate.c
+++ b/Python/pystate.c
@@ -60,7 +60,12 @@
 
     _PyGC_Initialize(&runtime->gc);
     _PyEval_Initialize(&runtime->ceval);
-    PyPreConfig_InitPythonConfig(&runtime->preconfig);
+
+    runtime->preconfig.struct_size = sizeof(PyPreConfig);
+    PyStatus status = PyPreConfig_InitPythonConfig(&runtime->preconfig);
+    if (_PyStatus_EXCEPTION(status)) {
+        return status;
+    }
 
     runtime->gilstate.check_enabled = 1;
 
@@ -205,6 +210,7 @@
     interp->id_refcount = -1;
     interp->check_interval = 100;
 
+    interp->config.struct_size = sizeof(PyConfig);
     PyStatus status = PyConfig_InitPythonConfig(&interp->config);
     if (_PyStatus_EXCEPTION(status)) {
         /* Don't report status to caller: PyConfig_InitPythonConfig()