fix: synchronize entity removal when needed

worldedit-bukkit/src/main/java/com/sk89q/worldedit/bukkit/BukkitEntity.java

@@ -19,6 +19,7 @@
 
 package com.sk89q.worldedit.bukkit;
 
+import com.fastasyncworldedit.core.util.TaskManager;
 import com.sk89q.worldedit.bukkit.adapter.BukkitImplAdapter;
 import com.sk89q.worldedit.entity.BaseEntity;
 import com.sk89q.worldedit.entity.Entity;
@@ -110,17 +111,21 @@ public BaseEntity getState() {
 
     @Override
     public boolean remove() {
-        org.bukkit.entity.Entity entity = entityRef.get();
-        if (entity != null) {
-            try {
-                entity.remove();
-            } catch (UnsupportedOperationException e) {
-                return false;
+        // synchronize the whole method, not just the remove operation as we always need to synchronize and
+        // can make sure the entity reference was not invalidated in the few milliseconds between the next available tick (lol)
+        return TaskManager.taskManager().sync(() -> {
+            org.bukkit.entity.Entity entity = entityRef.get();
+            if (entity != null) {
+                try {
+                    entity.remove();
+                } catch (UnsupportedOperationException e) {
+                    return false;
+                }
+                return entity.isDead();
+            } else {
+                return true;
             }
-            return entity.isDead();
-        } else {
-            return true;
-        }
+        });
     }
 
     @SuppressWarnings("unchecked")

worldedit-core/src/main/java/com/sk89q/worldedit/command/ClipboardCommands.java

@@ -36,6 +36,7 @@
 import com.fastasyncworldedit.core.util.ImgurUtility;
 import com.fastasyncworldedit.core.util.MainUtil;
 import com.fastasyncworldedit.core.util.MaskTraverser;
+import com.fastasyncworldedit.core.util.TaskManager;
 import com.fastasyncworldedit.core.util.task.RunnableVal;
 import com.google.common.collect.Lists;
 import com.sk89q.worldedit.EditSession;
@@ -98,6 +99,7 @@
 import java.net.URI;
 import java.net.URL;
 import java.nio.file.Files;
+import java.util.Collection;
 import java.util.HashSet;
 import java.util.List;
 import java.util.Set;
@@ -479,7 +481,14 @@ public void place(
                     .apply(region.getMaximumPoint().subtract(region.getMinimumPoint()).toVector3())
                     .toBlockPoint());
             if (removeEntities) {
-                editSession.getEntities(new CuboidRegion(realTo, max)).forEach(Entity::remove);
+                // Collect entities on the current (command) thread, in case it benefits from asynchronous retrieval (unlikely)
+                final Collection<? extends Entity> entities = editSession.getEntities(new CuboidRegion(realTo, max));
+                // BukkitEntity#remove is synchronized, but it makes more sense to synchronize here beforehand in case many
+                // entities are affected. BukkitEntity will not synchronize if it's already called on the main thread.
+                TaskManager.taskManager().sync(() -> {
+                    entities.forEach(Entity::remove);
+                    return true;
+                });
             }
             if (selectPasted || onlySelect) {
                 RegionSelector selector = new CuboidRegionSelector(world, realTo, max);