bdiff: add version to help detect breaking binary changes

Previously, we have no way to detect if a compiled .so file could be used or
not, and blindly load it if it exists. Usually we carefully maintain
compatibility of .so and fallback to pure code gracefully. But if we stick
to the rules, certain nice changes will be impossible to make in a clean
way.

This patch adds a "version" constant to the module so we can detect
inconsistency and take appropriate actions (warn, abort, fallback to pure,
run make automatically) in module loader.

mercurial/bdiff_module.c

@@ -192,6 +192,8 b' static PyMethodDef methods[] = {'
 	{NULL, NULL}
 };
 
+static const int version = 1;
+
 #ifdef IS_PY3K
 static struct PyModuleDef bdiff_module = {
 	PyModuleDef_HEAD_INIT,
@@ -203,11 +205,16 b' static struct PyModuleDef bdiff_module ='
 
 PyMODINIT_FUNC PyInit_bdiff(void)
 {
-	return PyModule_Create(&bdiff_module);
+	PyObject *m;
+	m = PyModule_Create(&bdiff_module);
+	PyModule_AddIntConstant(m, "version", version);
+	return m;
 }
 #else
 PyMODINIT_FUNC initbdiff(void)
 {
-	Py_InitModule3("bdiff", methods, mdiff_doc);
+	PyObject *m;
+	m = Py_InitModule3("bdiff", methods, mdiff_doc);
+	PyModule_AddIntConstant(m, "version", version);
 }
 #endif