Fix virtual lecterns - again (#4494)

core/src/main/java/org/geysermc/geyser/inventory/LecternContainer.java

@@ -26,10 +26,13 @@
 package org.geysermc.geyser.inventory;
 
 import com.github.steveice10.mc.protocol.data.game.inventory.ContainerType;
-import org.cloudburstmc.math.vector.Vector3i;
 import lombok.Getter;
 import lombok.Setter;
+import org.checkerframework.checker.nullness.qual.NonNull;
+import org.cloudburstmc.math.vector.Vector3i;
 import org.cloudburstmc.nbt.NbtMap;
+import org.geysermc.geyser.session.GeyserSession;
+import org.geysermc.geyser.translator.protocol.java.inventory.JavaOpenBookTranslator;
 
 public class LecternContainer extends Container {
     @Getter @Setter
@@ -39,7 +42,34 @@ public class LecternContainer extends Container {
     @Getter @Setter
     private Vector3i position;
 
+    // Sigh. When the lectern container is created, we don't know (yet) if it's fake or not.
+    // So... time for a manual check :/
+    @Getter
+    private boolean isFakeLectern = false;
+
     public LecternContainer(String title, int id, int size, ContainerType containerType, PlayerInventory playerInventory) {
         super(title, id, size, containerType, playerInventory);
     }
+
+    /**
+     * When we are using a fake lectern, the Java server expects us to still be in a player inventory.
+     * We can't use {@link #isUsingRealBlock()} as that may not be determined yet.
+     */
+    @Override
+    public void setItem(int slot, @NonNull GeyserItemStack newItem, GeyserSession session) {
+        if (isFakeLectern) {
+            session.getPlayerInventory().setItem(slot, newItem, session);
+        } else {
+            super.setItem(slot, newItem, session);
+        }
+    }
+
+    /**
+     * This is used ONLY once to set the book of a fake lectern in {@link JavaOpenBookTranslator}.
+     * See {@link LecternContainer#setItem(int, GeyserItemStack, GeyserSession)} as for why this is separate.
+     */
+    public void setFakeLecternBook(GeyserItemStack book, GeyserSession session) {
+        this.isFakeLectern = true;
+        super.setItem(0, book, session);
+    }
 }

core/src/main/java/org/geysermc/geyser/translator/inventory/LecternInventoryTranslator.java

@@ -129,6 +129,11 @@ public class LecternInventoryTranslator extends AbstractBlockInventoryTranslator
 
     @Override
     public void updateSlot(GeyserSession session, Inventory inventory, int slot) {
+        // If we're not in a real lectern, the Java server thinks we are still in the player inventory.
+        if (((LecternContainer) inventory).isFakeLectern()) {
+            InventoryTranslator.PLAYER_INVENTORY_TRANSLATOR.updateSlot(session, session.getPlayerInventory(), slot);
+            return;
+        }
         super.updateSlot(session, inventory, slot);
         if (slot == 0) {
             updateBook(session, inventory, inventory.getItem(0));

core/src/main/java/org/geysermc/geyser/translator/protocol/java/inventory/JavaOpenBookTranslator.java

@@ -30,6 +30,7 @@ import com.github.steveice10.mc.protocol.packet.ingame.clientbound.inventory.Cli
 import com.github.steveice10.mc.protocol.packet.ingame.serverbound.inventory.ServerboundContainerClosePacket;
 import org.geysermc.geyser.inventory.GeyserItemStack;
 import org.geysermc.geyser.inventory.Inventory;
+import org.geysermc.geyser.inventory.LecternContainer;
 import org.geysermc.geyser.item.Items;
 import org.geysermc.geyser.network.GameProtocol;
 import org.geysermc.geyser.session.GeyserSession;
@@ -79,8 +80,8 @@ public class JavaOpenBookTranslator extends PacketTranslator<ClientboundOpenBook
 
             // Should never be null
             Objects.requireNonNull(translator, "lectern translator must exist");
-            Inventory inventory = translator.createInventory("", FAKE_LECTERN_WINDOW_ID, ContainerType.LECTERN , session.getPlayerInventory());
-            inventory.setItem(0, stack, session);
+            Inventory inventory = translator.createInventory("", FAKE_LECTERN_WINDOW_ID, ContainerType.LECTERN, session.getPlayerInventory());
+            ((LecternContainer) inventory).setFakeLecternBook(stack, session);
             InventoryUtils.openInventory(session, inventory);
         }
     }