Fix furnace not cooking when hopper inserts item

GROUPS.md

@@ -133,7 +133,9 @@ These groups are used mostly for informational purposes
 * `container`: Node is a container which physically stores items within and has at least 1 inventory
    * `container=2`: Has one inventory with list name `"main"`. Items can be placed and taken freely
    * `container=3`: Same as `container=2`, but shulker boxes can not be inserted
-   * `container=4`: Furnace-like, has lists `"src"`, `"fuel"` and `"dst"`
+   * `container=4`: Furnace-like, has lists `"src"`, `"fuel"` and `"dst"`.
+                    It is expected that this also reacts on `on_timer`;
+                    the node timer must be started from other mods when they add into `"src"` or `"fuel"`
    * `container=1`: Other/unspecified container type
 * `spawn_egg=1`: Spawn egg
 

mods/ITEMS/mcl_hoppers/init.lua

@@ -308,6 +308,10 @@ minetest.register_abm({
 		end
 		if slot_id then
 			mcl_util.move_item_container(pos, "main", slot_id, downpos)
+			if g == 4 then
+				-- Start furnace's timer function, it will sort out whether furnace can burn or not.
+				minetest.get_node_timer(downpos):start(1.0)
+			end
 		end
 	end,
 })
@@ -366,6 +370,8 @@ minetest.register_abm({
 			if slot_id then
 				mcl_util.move_item_container(pos, "main", slot_id, front, "fuel")
 			end
+			-- Start furnace's timer function, it will sort out whether furnace can burn or not.
+			minetest.get_node_timer(front):start(1.0)
 		end
 	end
 })