Block free (b:0 / s:0) shops; break pre-existing ones on interact

Free shops — a buy or sell price of exactly 0 — are an abuse vector. Disallow
them and clean up any that already exist.

- PreShopCreation/FreePriceChecker (NORMAL, after PriceChecker normalises the
  price line): rejects creation with INVALID_PRICE if the buy or sell price is
  0. An unoffered side (NO_PRICE / -1) is unaffected, so non-zero buy-only or
  sell-only shops still work.
- PreTransaction/FreeShopBreaker (LOWEST): the first time anyone interacts with
  a pre-existing free shop, cancel the trade with INVALID_SHOP (sends the
  "invalid shop" message via ErrorMessageSender) and destroy the sign
  (ShopDestroyedEvent → de-register, then breakNaturally).
- Register both in ChestShop.registerEvents().

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: ParadauxIO

plugin/src/main/java/com/Acrobot/ChestShop/ChestShop.java

@@ -392,6 +392,7 @@ private void registerPreShopCreationEvents() {
         registerEvent(new com.Acrobot.ChestShop.Listeners.PreShopCreation.PermissionChecker());
         registerEvent(new com.Acrobot.ChestShop.Listeners.PreShopCreation.ErrorMessageSender());
         registerEvent(new PriceChecker());
+        registerEvent(new FreePriceChecker()); // DC: block free (b:0 / s:0) shops
         registerEvent(new QuantityChecker());
         registerEvent(new TerrainChecker());
     }
@@ -412,6 +413,7 @@ private void registerPreTransactionEvents() {
 
         registerEvent(new InvalidNameIgnorer());
         registerEvent(new CreativeModeIgnorer());
+        registerEvent(new FreeShopBreaker()); // DC: break pre-existing free (b:0 / s:0) shops on interact
         registerEvent(new ErrorMessageSender());
         registerEvent(new PermissionChecker());
         registerEvent(new PriceValidator());

plugin/src/main/java/com/Acrobot/ChestShop/Listeners/PreShopCreation/FreePriceChecker.java

@@ -0,0 +1,35 @@
+package com.Acrobot.ChestShop.Listeners.PreShopCreation;
+
+import com.Acrobot.Breeze.Utils.PriceUtil;
+import com.Acrobot.ChestShop.Events.PreShopCreationEvent;
+import com.Acrobot.ChestShop.Signs.ChestShopSign;
+import org.bukkit.event.EventHandler;
+import org.bukkit.event.EventPriority;
+import org.bukkit.event.Listener;
+
+import java.math.BigDecimal;
+
+import static com.Acrobot.ChestShop.Events.PreShopCreationEvent.CreationOutcome.INVALID_PRICE;
+
+/**
+ * DemocracyCraft: free shops are not allowed — a buy or sell price of exactly 0
+ * (the {@code b:0} / {@code s:0} case). Runs after {@link PriceChecker} (LOWEST)
+ * has normalised the price line, reads the resulting buy/sell prices, and
+ * rejects creation if either offered side is priced at 0. An <em>unoffered</em>
+ * side ({@link PriceUtil#NO_PRICE} / -1) is fine — a buy-only or sell-only shop
+ * with a non-zero price is unaffected.
+ */
+public class FreePriceChecker implements Listener {
+
+    @EventHandler(priority = EventPriority.NORMAL)
+    public static void onPreShopCreation(PreShopCreationEvent event) {
+        String price = ChestShopSign.getPrice(event.getSignLines());
+        if (isFree(PriceUtil.getExactBuyPrice(price)) || isFree(PriceUtil.getExactSellPrice(price))) {
+            event.setOutcome(INVALID_PRICE);
+        }
+    }
+
+    private static boolean isFree(BigDecimal price) {
+        return price.compareTo(PriceUtil.FREE) == 0;
+    }
+}

plugin/src/main/java/com/Acrobot/ChestShop/Listeners/PreTransaction/FreeShopBreaker.java

@@ -0,0 +1,53 @@
+package com.Acrobot.ChestShop.Listeners.PreTransaction;
+
+import com.Acrobot.Breeze.Utils.PriceUtil;
+import com.Acrobot.ChestShop.Events.PreTransactionEvent;
+import com.Acrobot.ChestShop.Listeners.Block.Break.SignBreak;
+import com.Acrobot.ChestShop.Signs.ChestShopSign;
+import org.bukkit.block.Sign;
+import org.bukkit.event.EventHandler;
+import org.bukkit.event.EventPriority;
+import org.bukkit.event.Listener;
+
+import java.math.BigDecimal;
+
+import static com.Acrobot.ChestShop.Events.PreTransactionEvent.TransactionOutcome.INVALID_SHOP;
+
+/**
+ * DemocracyCraft: free shops (a buy or sell price of exactly 0 — the {@code b:0}
+ * / {@code s:0} case) are blocked at creation by {@link com.Acrobot.ChestShop.Listeners.PreShopCreation.FreePriceChecker}.
+ * Any that pre-date that rule are cleaned up the first time someone interacts
+ * with them: the transaction is cancelled with {@code INVALID_SHOP} — which
+ * sends the "invalid shop" message via the PreTransaction ErrorMessageSender —
+ * and the sign is broken (firing a ShopDestroyedEvent so the shop is
+ * de-registered, then removing the block).
+ */
+public class FreeShopBreaker implements Listener {
+
+    @EventHandler(priority = EventPriority.LOWEST)
+    public static void onPreTransaction(PreTransactionEvent event) {
+        if (event.isCancelled()) {
+            return;
+        }
+
+        Sign sign = event.getSign();
+        if (sign == null) {
+            return;
+        }
+
+        String price = ChestShopSign.getPrice(sign);
+        if (!isFree(PriceUtil.getExactBuyPrice(price)) && !isFree(PriceUtil.getExactSellPrice(price))) {
+            return;
+        }
+
+        // Cancel the trade (INVALID_SHOP → "invalid shop" message to the client)
+        // and destroy the offending shop.
+        event.setCancelled(INVALID_SHOP);
+        SignBreak.sendShopDestroyedEvent(sign, event.getClient());
+        sign.getBlock().breakNaturally();
+    }
+
+    private static boolean isFree(BigDecimal price) {
+        return price.compareTo(PriceUtil.FREE) == 0;
+    }
+}