bpo-45211: Remember the stdlib dir during startup. (gh-28586)

During runtime startup we figure out the stdlib dir but currently throw that information away. This change preserves it and exposes it via PyConfig.stdlib_dir, _Py_GetStdlibDir(), and sys._stdlib_dir.

https://bugs.python.org/issue45211

Python/initconfig.c

@@ -669,6 +669,7 @@ PyConfig_Clear(PyConfig *config)
     _PyWideStringList_Clear(&config->xoptions);
     _PyWideStringList_Clear(&config->module_search_paths);
     config->module_search_paths_set = 0;
+    CLEAR(config->stdlib_dir);
 
     CLEAR(config->executable);
     CLEAR(config->base_executable);
@@ -909,6 +910,7 @@ _PyConfig_Copy(PyConfig *config, const PyConfig *config2)
     COPY_WSTRLIST(xoptions);
     COPY_WSTRLIST(module_search_paths);
     COPY_ATTR(module_search_paths_set);
+    COPY_WSTR_ATTR(stdlib_dir);
 
     COPY_WSTR_ATTR(executable);
     COPY_WSTR_ATTR(base_executable);
@@ -1015,6 +1017,7 @@ _PyConfig_AsDict(const PyConfig *config)
     SET_ITEM_WSTR(home);
     SET_ITEM_INT(module_search_paths_set);
     SET_ITEM_WSTRLIST(module_search_paths);
+    SET_ITEM_WSTR(stdlib_dir);
     SET_ITEM_WSTR(executable);
     SET_ITEM_WSTR(base_executable);
     SET_ITEM_WSTR(prefix);
@@ -1318,6 +1321,7 @@ _PyConfig_FromDict(PyConfig *config, PyObject *dict)
     // Path configuration output
     GET_UINT(module_search_paths_set);
     GET_WSTRLIST(module_search_paths);
+    GET_WSTR_OPT(stdlib_dir);
     GET_WSTR_OPT(executable);
     GET_WSTR_OPT(base_executable);
     GET_WSTR_OPT(prefix);
@@ -3094,6 +3098,7 @@ _Py_DumpPathConfig(PyThreadState *tstate)
     PySys_WriteStderr("  environment = %i\n", config->use_environment);
     PySys_WriteStderr("  user site = %i\n", config->user_site_directory);
     PySys_WriteStderr("  import site = %i\n", config->site_import);
+    DUMP_CONFIG("stdlib dir", stdlib_dir);
 #undef DUMP_CONFIG
 
 #define DUMP_SYS(NAME) \

Python/pathconfig.c

@@ -54,6 +54,7 @@ pathconfig_clear(_PyPathConfig *config)
     CLEAR(config->program_full_path);
     CLEAR(config->prefix);
     CLEAR(config->exec_prefix);
+    CLEAR(config->stdlib_dir);
     CLEAR(config->module_search_path);
     CLEAR(config->program_name);
     CLEAR(config->home);
@@ -83,6 +84,7 @@ pathconfig_copy(_PyPathConfig *config, const _PyPathConfig *config2)
     COPY_ATTR(prefix);
     COPY_ATTR(exec_prefix);
     COPY_ATTR(module_search_path);
+    COPY_ATTR(stdlib_dir);
     COPY_ATTR(program_name);
     COPY_ATTR(home);
 #ifdef MS_WINDOWS
@@ -167,6 +169,7 @@ pathconfig_set_from_config(_PyPathConfig *pathconfig, const PyConfig *config)
     COPY_CONFIG(program_full_path, executable);
     COPY_CONFIG(prefix, prefix);
     COPY_CONFIG(exec_prefix, exec_prefix);
+    COPY_CONFIG(stdlib_dir, stdlib_dir);
     COPY_CONFIG(program_name, program_name);
     COPY_CONFIG(home, home);
 #ifdef MS_WINDOWS
@@ -218,6 +221,7 @@ _PyPathConfig_AsDict(void)
     SET_ITEM_STR(prefix);
     SET_ITEM_STR(exec_prefix);
     SET_ITEM_STR(module_search_path);
+    SET_ITEM_STR(stdlib_dir);
     SET_ITEM_STR(program_name);
     SET_ITEM_STR(home);
 #ifdef MS_WINDOWS
@@ -311,6 +315,7 @@ config_init_module_search_paths(PyConfig *config, _PyPathConfig *pathconfig)
 
    - exec_prefix
    - module_search_path
+   - stdlib_dir
    - prefix
    - program_full_path
 
@@ -401,6 +406,7 @@ config_init_pathconfig(PyConfig *config, int compute_path_config)
     COPY_ATTR(program_full_path, executable);
     COPY_ATTR(prefix, prefix);
     COPY_ATTR(exec_prefix, exec_prefix);
+    COPY_ATTR(stdlib_dir, stdlib_dir);
 
 #undef COPY_ATTR
 
@@ -486,16 +492,25 @@ Py_SetPath(const wchar_t *path)
 
     PyMem_RawFree(_Py_path_config.prefix);
     PyMem_RawFree(_Py_path_config.exec_prefix);
+    PyMem_RawFree(_Py_path_config.stdlib_dir);
     PyMem_RawFree(_Py_path_config.module_search_path);
 
     _Py_path_config.prefix = _PyMem_RawWcsdup(L"");
     _Py_path_config.exec_prefix = _PyMem_RawWcsdup(L"");
+    // XXX Copy this from the new module_search_path?
+    if (_Py_path_config.home != NULL) {
+        _Py_path_config.stdlib_dir = _PyMem_RawWcsdup(_Py_path_config.home);
+    }
+    else {
+        _Py_path_config.stdlib_dir = _PyMem_RawWcsdup(L"");
+    }
     _Py_path_config.module_search_path = _PyMem_RawWcsdup(path);
 
     PyMem_SetAllocator(PYMEM_DOMAIN_RAW, &old_alloc);
 
     if (_Py_path_config.prefix == NULL
         || _Py_path_config.exec_prefix == NULL
+        || _Py_path_config.stdlib_dir == NULL
         || _Py_path_config.module_search_path == NULL)
     {
         path_out_of_memory(__func__);
@@ -515,10 +530,13 @@ Py_SetPythonHome(const wchar_t *home)
 
     PyMem_RawFree(_Py_path_config.home);
     _Py_path_config.home = _PyMem_RawWcsdup(home);
+    if (_Py_path_config.home != NULL) {
+        _Py_path_config.stdlib_dir = _PyMem_RawWcsdup(home);
+    }
 
     PyMem_SetAllocator(PYMEM_DOMAIN_RAW, &old_alloc);
 
-    if (_Py_path_config.home == NULL) {
+    if (_Py_path_config.home == NULL || _Py_path_config.stdlib_dir == NULL) {
         path_out_of_memory(__func__);
     }
 }
@@ -572,6 +590,17 @@ Py_GetPath(void)
 }
 
 
+wchar_t *
+_Py_GetStdlibDir(void)
+{
+    wchar_t *stdlib_dir = _Py_path_config.stdlib_dir;
+    if (stdlib_dir != NULL && stdlib_dir[0] != L'\0') {
+        return stdlib_dir;
+    }
+    return NULL;
+}
+
+
 wchar_t *
 Py_GetPrefix(void)
 {

Python/sysmodule.c

@@ -2974,6 +2974,14 @@ _PySys_UpdateConfig(PyThreadState *tstate)
 
     SET_SYS("_xoptions", sys_create_xoptions_dict(config));
 
+    const wchar_t *stdlibdir = _Py_GetStdlibDir();
+    if (stdlibdir != NULL) {
+        SET_SYS_FROM_WSTR("_stdlib_dir", stdlibdir);
+    }
+    else {
+        PyDict_SetItemString(sysdict, "_stdlib_dir", Py_None);
+    }
+
 #undef SET_SYS_FROM_WSTR
 #undef COPY_LIST
 #undef COPY_WSTR