Crafting and building vetoes

A mod may refuse a craft or a placement before the game acts on it. Interactions with entities, from opening a chest to dismantling a piece, have their own filter. A filter that panics turns the mod off and lets the action through.

Crafting

load.on_craft(|craft| ...) runs on each player’s craft request in the server’s world, with the game’s CraftRecipeEvent: the crafting_operator_id, the crafting_station_id (None for crafting by hand), the recipe_id and the recipe_amount. Returning false refuses it: nothing is made and nothing is taken.

fn init(load: &mut enshwick::Load<'_>) -> Result<(), enshwick::Off> {
    let world = load.world()?;
    // Only administrators craft by hand.
    load.on_craft(move |craft| {
        craft.crafting_station_id.is_some()
            || world.player(craft.crafting_operator_id).is_some_and(|p| p.is_admin())
    })
}

The loader screens the requests on the game thread at the start of the next step, before the crafting system reads them, so World reads work in the filter. A refused request reaches the crafting system empty, with no crafter and no amount.

Building

load.on_build(|placement| ...) runs after the game’s cursor check allows a placement, with a Placement, Enshwick’s name, since the game’s check has none: the building player, the item placed and its position in metres. Returning false refuses it the way the game refuses a placement without building permission: the building systems do not place it.

fn init(load: &mut enshwick::Load<'_>) -> Result<(), enshwick::Off> {
    // Nothing is built in a ring around the world's centre.
    load.on_build(|placement| {
        let [x, _, z] = placement.position;
        (x * x + z * z).sqrt() > 50.0
    })
}

The server runs the cursor check on the cursor each player’s client sends, and the client runs it for its own prediction, so the filter runs on both sides, on the cursor systems’ worker threads, on every frame a player aims to build. Keep it quick: no allocation, no lock, no log, and no world reads, which belong on the game thread. The server’s answer decides. A client that runs the mod too shows the cursor as not allowed, so give the same answer on both sides: base it on what both know, the item and the place, or on join values the server shares. A client without the mod shows the placement as allowed, and the server places nothing.