api: allow nvim_buf_attach from lua using callbacks

src/nvim/lua/executor.c

@@ -363,6 +363,33 @@ static int nlua_getenv(lua_State *lstate)
 }
 #endif
 
+/// add the value to the registry
+LuaRef nlua_ref(lua_State *lstate, int index)
+{
+  lua_pushvalue(lstate, index);
+  return luaL_ref(lstate, LUA_REGISTRYINDEX);
+}
+
+/// remove the value from the registry
+void nlua_unref(lua_State *lstate, LuaRef ref)
+{
+  if (ref > 0) {
+    luaL_unref(lstate, LUA_REGISTRYINDEX, ref);
+  }
+}
+
+void executor_free_luaref(LuaRef ref)
+{
+  lua_State *const lstate = nlua_enter();
+  nlua_unref(lstate, ref);
+}
+
+/// push a value referenced in the regirstry
+void nlua_pushref(lua_State *lstate, LuaRef ref)
+{
+  lua_rawgeti(lstate, LUA_REGISTRYINDEX, ref);
+}
+
 /// Evaluate lua string
 ///
 /// Used for luaeval().
@@ -451,9 +478,29 @@ Object executor_exec_lua_api(const String str, const Array args, Error *err)
     return NIL;
   }
 
-  return nlua_pop_Object(lstate, err);
+  return nlua_pop_Object(lstate, false, err);
 }
 
+Object executor_exec_lua_cb(LuaRef ref, const char *name, Array args)
+{
+  lua_State *const lstate = nlua_enter();
+  nlua_pushref(lstate, ref);
+  lua_pushstring(lstate, name);
+  for (size_t i = 0; i < args.size; i++) {
+    nlua_push_Object(lstate, args.items[i]);
+  }
+
+  if (lua_pcall(lstate, (int)args.size+1, 1, 0)) {
+    // TODO(bfredl): callbacks:s might not always be msg-safe, for instance
+    // lua callbacks for redraw events. Later on let the caller deal with the
+    // error instead.
+    nlua_error(lstate, _("Error executing lua callback: %.*s"));
+    return NIL;
+  }
+  Error err = ERROR_INIT;
+
+  return nlua_pop_Object(lstate, false, &err);
+}
 
 /// Run lua string
 ///