From 7e6256031e8db371c47b33f612150c7c0f9aee17 Mon Sep 17 00:00:00 2001
From: Aikar <aikar@aikar.co>
Date: Wed, 17 Oct 2018 22:31:54 -0400
Subject: [PATCH] Ensure chunk neighbor counts do not get desynced

Mojang was not checking that the chunk did not overwrite, or
was successfully removed.

We're seeing odd reports in #1561 that indicates issues around
this are having problems.

diff --git a/src/main/java/net/minecraft/server/ChunkMap.java b/src/main/java/net/minecraft/server/ChunkMap.java
index 39ac032b0b..1fb0770015 100644
--- a/src/main/java/net/minecraft/server/ChunkMap.java
+++ b/src/main/java/net/minecraft/server/ChunkMap.java
@@ -17,6 +17,7 @@ public class ChunkMap extends Long2ObjectOpenHashMap<Chunk> {
         chunk.world.timings.syncChunkLoadPostTimer.startTiming(); // Paper
         lastChunkByPos = chunk; // Paper
         Chunk chunk1 = (Chunk) super.put(i, chunk);
+        if (chunk1 == null) { // Paper - we should never be overwriting chunks
         ChunkCoordIntPair chunkcoordintpair = new ChunkCoordIntPair(i);
 
         for (int j = chunkcoordintpair.x - 1; j <= chunkcoordintpair.x + 1; ++j) {
@@ -47,7 +48,11 @@ public class ChunkMap extends Long2ObjectOpenHashMap<Chunk> {
                     chunk.setNeighborLoaded(x, z);
                 }
             }
+            // Paper start
+        } } else {
+            a.error("Overwrote existing chunk! (" + chunk.world.getWorld().getName() + ":" + chunk.locX+"," + chunk.locZ + ")", new IllegalStateException());
         }
+        // Paper end
         // Paper start - if this is a spare chunk (not part of any players view distance), go ahead and queue it for unload.
         if (!((WorldServer)chunk.world).getPlayerChunkMap().isChunkInUse(chunk.locX, chunk.locZ)) {
             if (chunk.world.paperConfig.delayChunkUnloadsBy > 0) {
@@ -69,6 +74,7 @@ public class ChunkMap extends Long2ObjectOpenHashMap<Chunk> {
 
     public Chunk a(long i) {
         Chunk chunk = (Chunk) super.remove(i);
+        if (chunk != null) { // Paper - don't decrement if we didn't remove anything
         ChunkCoordIntPair chunkcoordintpair = new ChunkCoordIntPair(i);
 
         for (int j = chunkcoordintpair.x - 1; j <= chunkcoordintpair.x + 1; ++j) {
@@ -84,6 +90,7 @@ public class ChunkMap extends Long2ObjectOpenHashMap<Chunk> {
         }
 
         // Paper start
+        } // close if (chunk != null)
         if (lastChunkByPos != null && i == lastChunkByPos.chunkKey) {
             lastChunkByPos = null;
         }
-- 
2.19.1