From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Aikar <aikar@aikar.co>
Date: Sat, 18 Apr 2020 04:36:11 -0400
Subject: [PATCH] Fix Chunk Post Processing deadlock risk

See: https://gist.github.com/aikar/dd22bbd2a3d78a2fd3d92e95e9f28dc6

as part of post processing a chunk, we can call ChunkConverter.

ChunkConverter then kicks off major physics updates, and when blocks
that have connections across chunk boundries occur, a recursive risk
can occur where A updates a block that triggers a physics request.

That physics request may trigger a chunk request, that then enqueues
a task into the Mailbox ChunkTaskQueueSorter.

If anything requests that same chunk that is in the middle of conversion,
it's mailbox queue is going to be held up, so the subsequent chunk request
will be unable to proceed.

We delay post processing of Chunk.A() 1 "pass" by re stuffing it back into
the executor so that the mailbox ChunkQueue is now considered empty.

This successfully fixed a reoccurring and highly reproduceable crash
for heightmaps.

diff --git a/src/main/java/net/minecraft/server/level/ChunkMap.java b/src/main/java/net/minecraft/server/level/ChunkMap.java
index b5459099b368ff6c8e62d254196350c2189e52a9..e7b29f632ee7a2fa77a191b25256134dae620739 100644
--- a/src/main/java/net/minecraft/server/level/ChunkMap.java
+++ b/src/main/java/net/minecraft/server/level/ChunkMap.java
@@ -195,6 +195,7 @@ public class ChunkMap extends ChunkStorage implements ChunkHolder.PlayerProvider
     };
     // CraftBukkit end
 
+    final CallbackExecutor chunkLoadConversionCallbackExecutor = new CallbackExecutor(); // Paper
     // Paper start - distance maps
     private final com.destroystokyo.paper.util.misc.PooledLinkedHashSets<ServerPlayer> pooledLinkedPlayerHashSets = new com.destroystokyo.paper.util.misc.PooledLinkedHashSets<>();
     // Paper start - no-tick view distance
@@ -1091,7 +1092,7 @@ public class ChunkMap extends ChunkStorage implements ChunkHolder.PlayerProvider
                 return Either.left(chunk);
             });
         }, (runnable) -> {
-            this.mainThreadMailbox.tell(ChunkTaskPriorityQueueSorter.message(holder, runnable));
+            this.mainThreadMailbox.tell(ChunkTaskPriorityQueueSorter.message(holder, () -> ChunkMap.this.chunkLoadConversionCallbackExecutor.execute(runnable))); // Paper - delay running Chunk post processing until outside of the sorter to prevent a deadlock scenario when post processing causes another chunk request.
         });
 
         completablefuture1.thenAcceptAsync((either) -> {
diff --git a/src/main/java/net/minecraft/server/level/ServerChunkCache.java b/src/main/java/net/minecraft/server/level/ServerChunkCache.java
index 0765eefbba823570149f076a0d4d6aba1aa4e863..5009a782d80d5c358acb5a412d63a567fd7db3ab 100644
--- a/src/main/java/net/minecraft/server/level/ServerChunkCache.java
+++ b/src/main/java/net/minecraft/server/level/ServerChunkCache.java
@@ -1014,6 +1014,7 @@ public class ServerChunkCache extends ChunkSource {
                 return super.pollTask() || execChunkTask; // Paper
             }
         } finally {
+            chunkMap.chunkLoadConversionCallbackExecutor.run(); // Paper - Add chunk load conversion callback executor to prevent deadlock due to recursion in the chunk task queue sorter
             chunkMap.callbackExecutor.run();
         }
         // CraftBukkit end