Fix block entity memory leak

src/mixins/java/org/spongepowered/common/mixin/core/server/level/ServerLevelMixin.java

@@ -55,6 +55,7 @@
 import net.minecraft.world.level.storage.WorldData;
 import net.minecraft.world.phys.Vec2;
 import net.minecraft.world.phys.Vec3;
+import org.objectweb.asm.Opcodes;
 import org.spongepowered.api.ResourceKey;
 import org.spongepowered.api.Sponge;
 import org.spongepowered.api.block.BlockSnapshot;
@@ -128,6 +129,7 @@ public abstract class ServerLevelMixin extends LevelMixin implements ServerLevel
 
     // @formatter:off
     @Shadow @Final private ServerLevelData serverLevelData;
+    @Shadow private int emptyTime;
 
     @Shadow @Nonnull public abstract MinecraftServer shadow$getServer();
     @Shadow protected abstract void shadow$saveLevelData();
@@ -441,6 +443,25 @@ public void save(@Nullable final ProgressListener progress, final boolean flush,
         this.impl$recentTickTimes[this.shadow$getServer().getTickCount() % 100] = postTickTime - this.impl$preTickTime;
     }
 
+    @Inject(
+        method = "tick",
+        at = @At(
+            value = "FIELD",
+            target = "Lnet/minecraft/server/level/ServerLevel;emptyTime:I",
+            opcode = Opcodes.PUTFIELD,
+            shift = At.Shift.AFTER
+        )
+    )
+    private void impl$unloadBlockEntities(final BooleanSupplier param0, final CallbackInfo ci) {
+        // This code fixes block entity memory leak
+        // https://github.com/SpongePowered/Sponge/pull/3689
+        if (this.emptyTime >= 300 && !this.blockEntitiesToUnload.isEmpty()) {
+            this.tickableBlockEntities.removeAll(this.blockEntitiesToUnload);
+            this.blockEntityList.removeAll(this.blockEntitiesToUnload);
+            this.blockEntitiesToUnload.clear();
+        }
+    }
+
     private void impl$setWorldOnBorder() {
         ((WorldBorderBridge) this.shadow$getWorldBorder()).bridge$setAssociatedWorld(this.bridge$getKey());
     }

src/mixins/java/org/spongepowered/common/mixin/core/world/level/LevelMixin.java

@@ -66,6 +66,7 @@
 import org.spongepowered.common.util.DataUtil;
 import org.spongepowered.math.vector.Vector3d;
 
+import java.util.List;
 import java.util.function.Predicate;
 
 @Mixin(net.minecraft.world.level.Level.class)
@@ -77,6 +78,9 @@ public abstract class LevelMixin implements LevelBridge, LevelAccessor {
     @Shadow protected float rainLevel;
     @Shadow protected float oThunderLevel;
     @Shadow protected float thunderLevel;
+    @Shadow @Final public List<BlockEntity> blockEntityList;
+    @Shadow @Final public List<BlockEntity> tickableBlockEntities;
+    @Shadow @Final protected List<BlockEntity> blockEntitiesToUnload;
 
     @Shadow public abstract LevelData shadow$getLevelData();
     @Shadow public abstract void shadow$updateSkyBrightness();