fix: preserve instance tag on #copy() (fix #112)

src/main/java/net/minestom/server/instance/Instance.java

@@ -93,7 +93,7 @@ public abstract class Instance implements Block.Getter, Block.Setter,
     protected UUID uniqueId;
 
     // instance custom data
-    private final TagHandler tagHandler = TagHandler.newHandler();
+    protected TagHandler tagHandler = TagHandler.newHandler();
     private final Scheduler scheduler = Scheduler.newScheduler();
     private final EventNode<InstanceEvent> eventNode;
 

src/main/java/net/minestom/server/instance/InstanceContainer.java

@@ -516,7 +516,8 @@ public class InstanceContainer extends Instance {
     public synchronized InstanceContainer copy() {
         InstanceContainer copiedInstance = new InstanceContainer(UUID.randomUUID(), getDimensionType());
         copiedInstance.srcInstance = this;
-        copiedInstance.lastBlockChangeTime = lastBlockChangeTime;
+        copiedInstance.tagHandler = this.tagHandler.copy();
+        copiedInstance.lastBlockChangeTime = this.lastBlockChangeTime;
         for (Chunk chunk : chunks.values()) {
             final int chunkX = chunk.getChunkX();
             final int chunkZ = chunk.getChunkZ();

src/test/java/net/minestom/server/instance/InstanceContainerTest.java

@@ -0,0 +1,28 @@
+package net.minestom.server.instance;
+
+import net.minestom.server.tag.Tag;
+import net.minestom.server.world.DimensionType;
+import net.minestom.server.world.DimensionTypeManager;
+import org.junit.jupiter.api.Test;
+
+import java.util.UUID;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class InstanceContainerTest {
+
+    static {
+        new DimensionTypeManager().addDimension(DimensionType.OVERWORLD);
+    }
+
+    @Test
+    public void copyPreservesTag() {
+        var tag = Tag.String("test");
+        var instance = new InstanceContainer(UUID.randomUUID(), DimensionType.OVERWORLD);
+        instance.setTag(tag, "123");
+
+        var copyInstance = instance.copy();
+        var result = copyInstance.getTag(tag);
+        assertEquals("123", result);
+    }
+}

src/test/java/net/minestom/server/tag/TagTransientTest.java

@@ -29,4 +29,25 @@ public class TagTransientTest {
         assertEquals("abcdef", result);
     }
 
+    @Test
+    public void tagHandlerCopyPreservesTransient() {
+        var tagHandler = TagHandler.newHandler();
+        Tag<String> tag = Tag.Transient("a");
+        tagHandler.setTag(tag, "abcdef");
+
+        var copyHandler = tagHandler.copy();
+        var result = copyHandler.getTag(tag);
+        assertEquals("abcdef", result);
+    }
+
+    @Test
+    public void asCompoundDoesNotPreserveTransient() {
+        var tagHandler = TagHandler.newHandler();
+        Tag<String> tag = Tag.Transient("a");
+        tagHandler.setTag(tag, "abcdef");
+
+        var compound = tagHandler.asCompound();
+        assertNull(compound.get("a"));
+    }
+
 }