Fix the first #setInstance being called in the main update thread

src/main/java/net/minestom/server/UpdateManager.java

@@ -70,7 +70,7 @@ public final class UpdateManager {
                     futures = threadProvider.update(time);
                 }
 
-                // Waiting players update (newly connected waiting to get into the server)
+                // Waiting players update (newly connected clients waiting to get into the server)
                 entityManager.updateWaitingPlayers();
 
                 // Keep Alive Handling

src/main/java/net/minestom/server/entity/Entity.java

@@ -20,6 +20,7 @@ import net.minestom.server.instance.InstanceManager;
 import net.minestom.server.instance.WorldBorder;
 import net.minestom.server.instance.block.CustomBlock;
 import net.minestom.server.network.packet.server.play.*;
+import net.minestom.server.thread.ThreadProvider;
 import net.minestom.server.utils.ArrayUtils;
 import net.minestom.server.utils.BlockPosition;
 import net.minestom.server.utils.Position;
@@ -33,6 +34,7 @@ import net.minestom.server.utils.validate.Check;
 
 import java.util.*;
 import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ConcurrentLinkedQueue;
 import java.util.concurrent.CopyOnWriteArraySet;
 import java.util.concurrent.atomic.AtomicInteger;
 import java.util.function.Consumer;
@@ -114,7 +116,8 @@ public abstract class Entity implements Viewable, EventHandler, DataContainer {
     protected boolean noGravity;
     protected Pose pose = Pose.STANDING;
 
-    protected final List<Consumer<Entity>> nextTick = Collections.synchronizedList(new ArrayList<>());
+    // list of scheduled tasks to be executed during the next entity tick
+    protected final ConcurrentLinkedQueue<Consumer<Entity>> nextTick = new ConcurrentLinkedQueue<>();
 
     // Tick related
     private long ticks;
@@ -134,8 +137,14 @@ public abstract class Entity implements Viewable, EventHandler, DataContainer {
         setVelocityUpdatePeriod(5);
     }
 
+    /**
+     * Schedule a task to be run during the next entity tick
+     * It ensures that the task will be executed in the same thread as the entity (depending of the {@link ThreadProvider})
+     *
+     * @param callback the task to execute during the next entity tick
+     */
     public void scheduleNextTick(Consumer<Entity> callback) {
-        nextTick.add(callback);
+        this.nextTick.add(callback);
     }
 
     public Entity(EntityType entityType) {
@@ -353,12 +362,14 @@ public abstract class Entity implements Viewable, EventHandler, DataContainer {
             return;
         }
 
-        synchronized (nextTick) {
-            for (final Consumer<Entity> e : nextTick) {
-                e.accept(this);
+        // scheduled tasks
+        if (!nextTick.isEmpty()) {
+            Consumer<Entity> callback;
+            while ((callback = nextTick.poll()) != null) {
+                callback.accept(this);
             }
-            nextTick.clear();
         }
+
         // Synchronization with updated fields in #getPosition()
         {
             // X/Y/Z axis

src/main/java/net/minestom/server/entity/EntityManager.java

@@ -36,17 +36,20 @@ public final class EntityManager {
 
             Check.notNull(spawningInstance, "You need to specify a spawning instance in the PlayerLoginEvent");
 
-            waitingPlayer.setInstance(spawningInstance);
+            {
+                final Player finalWaitingPlayer = waitingPlayer;
+                spawningInstance.scheduleNextTick(instance -> finalWaitingPlayer.setInstance(instance));
+            }
         }
     }
 
     /**
-     * Call the player initialization callbacks and the event {@link PlayerPreLoginEvent}
-     * If the player hasn't been kicked, add him to the waiting list
+     * Call the player initialization callbacks and the event {@link PlayerPreLoginEvent}.
+     * If the {@link Player} hasn't been kicked, add him to the waiting list.
      * <p>
-     * Can be considered as a pre-init thing
+     * Can be considered as a pre-init thing.
      *
-     * @param player the player to add
+     * @param player the {@link Player} to add
      */
     public void addWaitingPlayer(Player player) {
 

src/main/java/net/minestom/server/entity/Player.java

@@ -176,8 +176,10 @@ public class Player extends LivingEntity implements CommandSender {
     }
 
     /**
-     * Used when the player is created
-     * Init the player and spawn him
+     * Used when the player is created.
+     * Init the player and spawn him.
+     * <p>
+     * WARNING: executed in the main update thread
      */
     protected void init() {
         JoinGamePacket joinGamePacket = new JoinGamePacket();

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

@@ -938,8 +938,8 @@ public abstract class Instance implements BlockModifier, EventHandler, DataConta
      * @param time the current time
      */
     public void tick(long time) {
-        {
-            // scheduled tasks
+        // scheduled tasks
+        if (!nextTick.isEmpty()) {
             Consumer<Instance> callback;
             while ((callback = nextTick.poll()) != null) {
                 callback.accept(this);