"Error while running event TaskList::on_gui_checked_state_changed (ID 3)" in multiplayer

[martinvonwittich]

Description

I'm playing in multiplayer with a friend (I was hosting, and he had joined my game), and we had a strange TaskList issue today concerning the entry "[Mall] Big Electric Miner" in our task list:

• My friend saw the task as uncompleted, and asked me if this really wasn't completed yet, because he was certain that our mall was building big electric miners.
• I was confused, because as far as I was concerned, this task was completed, and it did show as completed in my game.
• My friend attempted to mark the task as completed, which crashed Factorio on my computer with the following error message:

1979.050 Error MainLoop.cpp:1382: Exception at tick 42243974: The mod Task List (0.4.6) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event TaskList::on_gui_checked_state_changed (ID 3)
Gui element with name 69 already present in the parent element.
stack traceback:
    [C]: in function 'add'
    __flib__/gui.lua:85: in function 'recursive_build'
    __flib__/gui.lua:163: in function 'add'
    __TaskList__/scripts/gui/tasks/index.lua:134: in function 'add_task'
    __TaskList__/scripts/task.lua:61: in function 'callback'
    __TaskList__/scripts/task.lua:104: in function 'update_guis'
    __TaskList__/scripts/task.lua:59: in function 'toggle_completed'
    __TaskList__/scripts/gui/tasks/actions.lua:79: in function 'handler'
    __TaskList__/scripts/gui/tasks/index.lua:95: in function 'dispatch'
    __TaskList__/control.lua:109: in function <__TaskList__/control.lua:104>

• My friend then saved the game on his side (I wasn't able to due to the crash), and hosted the game in turn.
• I tried to mark a different task as completed (IIRC, not sure what I did exactly), which immediately crashed the game on his computer (not sure if it was exactly the same error message, and of course I have only access to my own log).
• From then on we kinda kept our hands off the task list, afraid we might crash the game again :)

Now in single player, I'm unfortunately unable to crash the game in this way, but the task is still peculiar, maybe this can help figuring out the issue?

• I can see the task in the list when I have the checkbox "show completed" enabled.
• I can check and uncheck the task as completed, but unchecking it doesn't truly make the task uncompleted - contrary to all other tasks, it won't move upwards to the other uncompleted tasks.
• When attempting to edit the task, the edit UI is empty, as if I were creating a new task instead of editing one. The edit UI also has no delete button. Attempting to "change" the task by inputting a new title will just create a new task.
• Attempting to create a subtask will create a new top-level task instead.

The save game: https://clip.iserv.eu/t/ELI3Wj9NiWgakVPiiD4EDdizWjaqVuWn9h3fDi9R2lM/tasklist%20bug.zip (beware, the link is only valid for 30 days)

factorio-previous.log

Reproduction

I cannot reproduce the crash on my own, but in theory, it might be reproducible in a multiplayer game as follows:

1. Host a multiplayer game with the save.
2. Have another player join the game.
3. Have the other player attempt to mark "[Mall] Big Electric Miner" as (un)completed.
4. This has crashed the game for us.

Without multiplayer, at least the peculiar behavior of the task is reproducible:

1. Load the save.
2. Attempt to edit the task "[Mall] Big Electric Miner".
3. Note how the task isn't really editable - it's in the list, but attempt to edit it gives the "new task" UI. Marking the task as uncompleted won't actually move it to the top to the other uncompleted tasks.

[martinvonwittich]

Up to a minute ago I believed that this might be some kind of a strange desync bug - as if the task list on his side had a different state from the task list on my side. I assumed that my game had a task list in a consistent state (where the entry was marked as completed and still fine), and that he had a corrupted task list (where the entry wasn't marked as completed). I thought the crash occurred when he attempted to mark the entry as completed because then this conflict might trigger the crash.

To verify my assumption, I located my last save game right before the crash, and attempted to mark the entry as uncompleted. Turns out, my task list is apparently not in a consistent state, because the attempt crashed my game immediately, even in singleplayer :)

I've uploaded this savegame too: https://clip.iserv.eu/t/OiMld8oXVLy0dqz24jGgSzp7TfrNB4fPmsN6l-Di7fA/tasklist%20bug%20reproducible.zip

The issue is reproducible with this save game like follows:

1. Load the save game.
2. Open the task list.
3. Show completed tasks.
4. Attempt to uncheck "[Mall] Big Electric Miner" -> crash

[raiguard]

Unfortunately, I am unable to reproduce the crash. This is the second issue to come up with regard to the GUI becoming desynced from the task state, and unfortunately, without a reproduction to get it into that state, there isn't much I can do to fix it.

This mod is a dumpster fire code-wise and I have little motivation to rewrite it.

Edit: I managed to reproduce it!

[raiguard]

I am no closer to figuring out how the issue occurs in the first place, but I have applied a band-aid that should prevent this crash in the future.