Add VoxelManip:close() for explicit free

doc/lua_api.md

@@ -5249,6 +5249,13 @@ Methods
    where the engine will keep the map and the VM in sync automatically.
    * Note: this doesn't do what you think it does and is subject to removal. Don't use it!
 * `get_emerged_area()`: Returns actual emerged minimum and maximum positions.
+* `close()`: Frees the data buffers associated with the VoxelManip object.
+   It will become empty.
+   * Since Lua's garbage collector is not aware of the potentially significant
+     memory behind a VoxelManip, frequent VoxelManip usage can cause the server to
+     run out of RAM. Therefore it's recommend to call this method once you're done
+     with the VoxelManip.
+   * (introduced in 5.13.0)
 
 `VoxelArea`
 -----------

src/script/lua_api/l_vmanip.cpp

@@ -344,6 +344,19 @@ int LuaVoxelManip::l_get_emerged_area(lua_State *L)
 	return 2;
 }
 
+int LuaVoxelManip::l_close(lua_State *L)
+{
+	NO_MAP_LOCK_REQUIRED;
+
+	LuaVoxelManip *o = checkObject<LuaVoxelManip>(L, 1);
+
+	if (o->is_mapgen_vm)
+		throw LuaError("Cannot dispose of mapgen VoxelManip object");
+	o->vm->clear();
+
+	return 0;
+}
+
 LuaVoxelManip::LuaVoxelManip(MMVManip *mmvm, bool is_mg_vm) :
 	is_mapgen_vm(is_mg_vm),
 	vm(mmvm)
@@ -451,5 +464,6 @@ const luaL_Reg LuaVoxelManip::methods[] = {
 	luamethod(LuaVoxelManip, set_param2_data),
 	luamethod(LuaVoxelManip, was_modified),
 	luamethod(LuaVoxelManip, get_emerged_area),
+	luamethod(LuaVoxelManip, close),
 	{0,0}
 };

src/script/lua_api/l_vmanip.h

@@ -45,6 +45,8 @@ private:
 	static int l_was_modified(lua_State *L);
 	static int l_get_emerged_area(lua_State *L);
 
+	static int l_close(lua_State *L);
+
 public:
 	MMVManip *vm = nullptr;