papermc/Spigot-Server-Patches/0382-Synchronize-DataPaletteBlock-instead-of-ReentrantLoc.patch

From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Aikar <aikar@aikar.co>
Date: Fri, 29 May 2020 20:29:02 -0400
Subject: [PATCH] Synchronize DataPaletteBlock instead of ReentrantLock

Mojang has flaws in their logic about chunks being concurrently
wrote to. So we constantly see crashes around multiple threads writing.

Additionally, java has optimized synchronization so well that its
in many times faster than trying to manage read wrote locks for low
contention situations.

And this is extremely a low contention situation.

diff --git a/src/main/java/net/minecraft/server/DataPaletteBlock.java b/src/main/java/net/minecraft/server/DataPaletteBlock.java
index 3586fe065f21fbf1e71b602c372a690ef603f377..44310efd63b0a460e9dcdc1c8ad0abec78052061 100644
--- a/src/main/java/net/minecraft/server/DataPaletteBlock.java
+++ b/src/main/java/net/minecraft/server/DataPaletteBlock.java
@@ -24,7 +24,7 @@ public class DataPaletteBlock<T> implements DataPaletteExpandable<T> {
     private int i; private int getBitsPerObject() { return this.i; } // Paper - OBFHELPER
     private final ReentrantLock j = new ReentrantLock();
 
-    public void a() {
+    public void a() { /* // Paper start - disable this - use proper synchronization
         if (this.j.isLocked() && !this.j.isHeldByCurrentThread()) {
             String s = (String) Thread.getAllStackTraces().keySet().stream().filter(Objects::nonNull).map((thread) -> {
                 return thread.getName() + ": \n\tat " + (String) Arrays.stream(thread.getStackTrace()).map(Object::toString).collect(Collectors.joining("\n\tat "));
@@ -36,11 +36,11 @@ public class DataPaletteBlock<T> implements DataPaletteExpandable<T> {
             throw new ReportedException(crashreport);
         } else {
             this.j.lock();
-        }
+        } */ // Paper end
     }
 
     public void b() {
-        this.j.unlock();
+        //this.j.unlock(); // Paper - disable this
     }
 
     public DataPaletteBlock(DataPalette<T> datapalette, RegistryBlockID<T> registryblockid, Function<NBTTagCompound, T> function, Function<T, NBTTagCompound> function1, T t0) {
@@ -76,7 +76,7 @@ public class DataPaletteBlock<T> implements DataPaletteExpandable<T> {
     }
 
     @Override
-    public int onResize(int i, T t0) {
+    public synchronized int onResize(int i, T t0) { // Paper - synchronize
         this.a();
         DataBits databits = this.a;
         DataPalette<T> datapalette = this.h;
@@ -99,18 +99,18 @@ public class DataPaletteBlock<T> implements DataPaletteExpandable<T> {
     }
 
     public T setBlock(int i, int j, int k, T t0) {
-        this.a();
-        T t1 = this.a(b(i, j, k), t0);
+        //this.a(); // Paper - remove to reduce ops - synchronize handled below
+        return this.a(b(i, j, k), t0); // Paper
 
-        this.b();
-        return t1;
+        //this.b(); // Paper
+        //return t1; // PAper
     }
 
     public T b(int i, int j, int k, T t0) {
         return this.a(b(i, j, k), t0);
     }
 
-    protected T a(int i, T t0) {
+    protected synchronized T a(int i, T t0) { // Paper - synchronize - writes
         int j = this.h.a(t0);
         int k = this.a.a(i, j);
         T t1 = this.h.a(k);
@@ -135,7 +135,7 @@ public class DataPaletteBlock<T> implements DataPaletteExpandable<T> {
     }
 
     public void writeDataPaletteBlock(PacketDataSerializer packetDataSerializer) { this.b(packetDataSerializer); } // Paper - OBFHELPER
-    public void b(PacketDataSerializer packetdataserializer) {
+    public synchronized void b(PacketDataSerializer packetdataserializer) { // Paper - synchronize
         this.a();
         packetdataserializer.writeByte(this.i);
         this.h.b(packetdataserializer);
@@ -143,7 +143,7 @@ public class DataPaletteBlock<T> implements DataPaletteExpandable<T> {
         this.b();
     }
 
-    public void a(NBTTagList nbttaglist, long[] along) {
+    public synchronized void a(NBTTagList nbttaglist, long[] along) { // Paper - synchronize
         this.a();
         int i = Math.max(4, MathHelper.d(nbttaglist.size()));
 
@@ -176,7 +176,7 @@ public class DataPaletteBlock<T> implements DataPaletteExpandable<T> {
         this.b();
     }
 
-    public void a(NBTTagCompound nbttagcompound, String s, String s1) {
+    public synchronized void a(NBTTagCompound nbttagcompound, String s, String s1) { // Paper - synchronize
         this.a();
         DataPaletteHash<T> datapalettehash = new DataPaletteHash<>(this.d, this.i, this.c, this.e, this.f);
         T t0 = this.g;