Rare house corruption issue

[crudelios]

https://github.com/bvschaik/julius/blob/bae63eb689737a720445f614ead61f41f86cbb9e/src/figuretype/migrant.c#L282

Buckle up because the above line causes rare corruptions.

So, as you may notice above, the code for a homeless entering a house does not check for the house state, only if the building type is a house.

On some rare occasions (it requires a very specific set of circumstances that I'll describe below) that will cause corruptions.

The requirements are:

• You have 4 house tiles that are eligible for merging
• All 4 tiles are already small tents but one of them has at least room for one person
• A homeless is created elsewhere and selects one of the houses as the destination before the houses merge
• The houses do merge
• The selected house id of the homeless is not the same as the remaining house id after the merge (as the other 3 1x1's were deleted by the game)
• Very soon after the merge, the homeless guy enteres the house - specifically, before building_update_state is called

If all the conditions are met, the homeless will enter a house with the state of BUILDING_STATE_DELETED_BY_GAME.

However, upon entering, the map building tiles will be updated to point to the zombie building id.

Also, when building_update_state is called, the building will finally be deleted and the tile will point to a non existent building.

This will obviously corrupt the city, but only briefly: if the 2x2 evolves the tile reference will be updated again so the issue should fix itself.

Anyway, it's a pretty interesting and rare bug.

This is causing crashes in Augustus however, due to the different way memory is handled there.

[bvschaik]

Interesting one. What do you suggest to do to fix the issue without breaking compatibility? Or should we leave it as-is for Julius since it fixes itself in the end?

[crudelios]

A simple check for the building state when the homeless arrives should prevent the issue in the future. If it's not a house or it's not BUILDING_STATE_IN_USE when the homeless arrives, simply kill him.

[crudelios]

Here's the relevant changed I used to fix the issue in Augustus:

https://github.com/Keriew/augustus/commit/b51cfaf864579ed0e48ec8de01fcf5c821de1b23#diff-17376d420a8b1b25d89314eda333aa555795549d6346f9097210f902626f7bc7

[crudelios]

@bvschaik Do you want me to fix this issue?

[crudelios]

This was unexpected. The change is failing the tests!

I'll have to revert them.

[crudelios]

Given that the fix changes the save results, I'm thinking we simply don't fix it.

[bvschaik]

I was afraid it would. I agree with not fixing this.