Allow event node handle to be garbage collected (Minestom#1835)

(cherry picked from commit 79ce957)

src/main/java/net/minestom/server/event/EventNodeImpl.java

@@ -8,6 +8,7 @@
 import org.jetbrains.annotations.NotNull;
 import org.jetbrains.annotations.Nullable;
 
+import java.lang.ref.WeakReference;
 import java.util.*;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.CopyOnWriteArrayList;
@@ -445,22 +446,24 @@ void invalidate() {
             final var mappedNodeCache = node.registeredMappedNode;
             if (mappedNodeCache.isEmpty()) return null;
             Set<EventFilter<E, ?>> filters = new HashSet<>(mappedNodeCache.size());
-            Map<Object, Handle<E>> handlers = new WeakHashMap<>(mappedNodeCache.size());
+            Map<Object, WeakReference<Handle<E>>> handlers = new WeakHashMap<>(mappedNodeCache.size());
+
             // Retrieve all filters used to retrieve potential handlers
             for (var mappedEntry : mappedNodeCache.entrySet()) {
                 final EventNodeImpl<E> mappedNode = mappedEntry.getValue();
                 final Handle<E> handle = (Handle<E>) mappedNode.getHandle(eventType);
                 if (!handle.hasListener()) continue; // Implicit update
                 filters.add(mappedNode.filter);
-                handlers.put(mappedEntry.getKey(), handle);
+                handlers.put(mappedEntry.getKey(), new WeakReference<>(handle));
             }
             // If at least one mapped node listen to this handle type,
             // loop through them and forward to mapped node if there is a match
             if (filters.isEmpty()) return null;
             final EventFilter<E, ?>[] filterList = filters.toArray(EventFilter[]::new);
             final BiConsumer<EventFilter<E, ?>, E> mapper = (filter, event) -> {
                 final Object handler = filter.castHandler(event);
-                final Handle<E> handle = handlers.get(handler);
+                final WeakReference<Handle<E>> handleRef = handlers.get(handler);
+                final Handle<E> handle = handleRef != null ? handleRef.get() : null;
                 if (handle != null) handle.call(event);
             };
             // Specialize the consumer depending on the number of filters to avoid looping

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

@@ -1,13 +1,16 @@
 package net.minestom.server.instance;
 
 import net.minestom.server.coordinate.Pos;
+import net.minestom.server.event.instance.InstanceTickEvent;
 import net.minestom.server.event.player.PlayerMoveEvent;
 import net.minestom.server.event.player.PlayerTickEvent;
+import net.minestom.server.world.DimensionType;
 import net.minestom.testing.Env;
 import net.minestom.testing.EnvTest;
 import org.junit.jupiter.api.Test;
 
 import java.lang.ref.WeakReference;
+import java.util.UUID;
 
 import static net.minestom.testing.TestUtils.waitUntilCleared;
 
@@ -80,4 +83,23 @@ public void chunkGC(Env env) {
         chunk = null;
         waitUntilCleared(ref);
     }
+
+    @Test
+    public void testGCWithEventsLambda(Env env) {
+        var ref = new WeakReference<>(new InstanceContainer(UUID.randomUUID(), DimensionType.OVERWORLD));
+        env.process().instance().registerInstance(ref.get());
+
+        tmp(ref.get());
+
+        ref.get().tick(0);
+        env.process().instance().unregisterInstance(ref.get());
+
+        waitUntilCleared(ref);
+    }
+
+    private void tmp(InstanceContainer instanceContainer) {
+        instanceContainer.eventNode().addListener(InstanceTickEvent.class, (e) -> {
+            var uuid = instanceContainer.getUniqueId();
+        });
+    }
 }