Threadsafe World.c(int i, int j)

src/main/java/net/minecraft/server/World.java

@@ -286,13 +286,18 @@ public class World implements IBlockAccess {
     Chunk lastChunkAccessed;
     int lastXAccessed = Integer.MIN_VALUE;
     int lastZAccessed = Integer.MIN_VALUE;
+    final Object chunkLock = new Object();
     public Chunk c(int i, int j) {
+        Chunk result = null;
+        synchronized (chunkLock) {
             if (lastChunkAccessed == null || lastXAccessed != i || lastZAccessed != j) {
                 lastXAccessed = i;
                 lastZAccessed = j;
                 lastChunkAccessed = this.G.b(i, j);
             }
-        return lastChunkAccessed;
+            result = lastChunkAccessed;
+        }
+        return result;
     }
     // CraftBukkit end