bpo-36356: Fix _PyCoreConfig_Read() (GH-12454)

Don't override parameters which are already set by the user.

Python/coreconfig.c

@@ -961,6 +961,7 @@ config_read_env_vars(_PyCoreConfig *config)
         config->malloc_stats = 1;
     }
 
+    if (config->module_search_path_env == NULL) {
         wchar_t *path;
         int res = _PyCoreConfig_GetEnvDup(config, &path,
                                           L"PYTHONPATH", "PYTHONPATH");
@@ -968,6 +969,7 @@ config_read_env_vars(_PyCoreConfig *config)
             return DECODE_LOCALE_ERR("PYTHONPATH", res);
         }
         config->module_search_path_env = path;
+    }
 
     if (config->use_hash_seed < 0) {
         _PyInitError err = config_init_hash_seed(config);
@@ -1689,6 +1691,7 @@ config_parse_cmdline(_PyCoreConfig *config, _PyCmdline *cmdline,
         }
 
         if (c == 'c') {
+            if (config->run_command == NULL) {
                 /* -c is the last option; following arguments
                    that look like options are left for the
                    command to interpret. */
@@ -1701,6 +1704,7 @@ config_parse_cmdline(_PyCoreConfig *config, _PyCmdline *cmdline,
                 command[len - 2] = '\n';
                 command[len - 1] = 0;
                 config->run_command = command;
+            }
             break;
         }
 
@@ -1708,10 +1712,12 @@ config_parse_cmdline(_PyCoreConfig *config, _PyCmdline *cmdline,
             /* -m is the last option; following arguments
                that look like options are left for the
                module to interpret. */
+            if (config->run_module == NULL) {
                 config->run_module = _PyMem_RawWcsdup(_PyOS_optarg);
                 if (config->run_module == NULL) {
                     return _Py_INIT_NO_MEMORY();
                 }
+            }
             break;
         }
 
@@ -1825,7 +1831,8 @@ config_parse_cmdline(_PyCoreConfig *config, _PyCmdline *cmdline,
 
     if (config->run_command == NULL && config->run_module == NULL
         && _PyOS_optind < cmdline->argv.length
-        && wcscmp(cmdline->argv.items[_PyOS_optind], L"-") != 0)
+        && wcscmp(cmdline->argv.items[_PyOS_optind], L"-") != 0
+        && config->run_filename == NULL)
     {
         config->run_filename = _PyMem_RawWcsdup(cmdline->argv.items[_PyOS_optind]);
         if (config->run_filename == NULL) {
@@ -2032,10 +2039,12 @@ config_from_cmdline(_PyCoreConfig *config, _PyCmdline *cmdline,
 
     _PyCoreConfig_GetGlobalConfig(config);
 
+    if (config->program == NULL) {
         err = config_init_program(config, cmdline);
         if (_Py_INIT_FAILED(err)) {
             return err;
         }
+    }
 
     err = config_parse_cmdline(config, cmdline, &need_usage);
     if (_Py_INIT_FAILED(err)) {