gdbus-codegen: Add an extension system which allows hooking codegen

The extension points are chosen by what libdex needs to generate future
based method calls.

gio/gdbus-2.0/codegen/codegen_main.py

@@ -26,6 +26,8 @@
 import argparse
 import os
 import sys
+import importlib.util
+import traceback
 from contextlib import contextmanager
 
 from . import config
@@ -38,6 +40,16 @@ from . import codegen_rst
 from .utils import print_error, print_warning
 
 
+def import_from_path(module_name, file_path):
+    spec = importlib.util.spec_from_file_location(module_name, file_path)
+    if not spec:
+        raise Exception("Not a Python file")
+    module = importlib.util.module_from_spec(spec)
+    sys.modules[module_name] = module
+    spec.loader.exec_module(module)
+    return module
+
+
 def find_arg(arg_list, arg_name):
     for a in arg_list:
         if a.name == arg_name:
@@ -274,6 +286,12 @@ def codegen_main():
         help="Additional define required for decorator specified by "
         "--symbol-decorator",
     )
+    arg_parser.add_argument(
+        "--extension-path",
+        metavar="EXTENSION_PATH",
+        default="",
+        help="Path to a gdbus-codegen Python extension file (unstable API)",
+    )
 
     group = arg_parser.add_mutually_exclusive_group()
     group.add_argument(
@@ -305,6 +323,20 @@ def codegen_main():
 
     args = arg_parser.parse_args()
 
+    codegen_ext = {}
+    if args.extension_path:
+        try:
+            codegen_ext = import_from_path("GDBusCodegenExt", args.extension_path)
+            codegen_ext.init(
+                args,
+                {
+                    "version": 1,
+                },
+            )
+        except Exception:
+            print_warning(traceback.format_exc())
+            print_error("Loading extension ‘{}’ failed".format(args.extension_path))
+
     if len(args.xml_files) > 0:
         print_warning(
             'The "--xml-files" option is deprecated; use positional arguments instead'
@@ -479,6 +511,7 @@ def codegen_main():
                 args.symbol_decorator,
                 args.symbol_decorator_header,
                 outfile,
+                codegen_ext,
             )
             gen.generate()
 
@@ -494,6 +527,7 @@ def codegen_main():
                 glib_min_required,
                 args.symbol_decorator_define,
                 outfile,
+                codegen_ext,
             )
             gen.generate()