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 boundaries 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 reproducible 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 eb0fbf2126e7c9978f46c92b6999f00c5aea9a58..bfd0398e4bca5458d2913715b742b3c5a0b076f8 100644
--- a/src/main/java/net/minecraft/server/level/ChunkMap.java
+++ b/src/main/java/net/minecraft/server/level/ChunkMap.java
@@ -182,6 +182,7 @@ 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<>();
 
@@ -1021,16 +1022,15 @@ public class ChunkMap extends ChunkStorage implements ChunkHolder.PlayerProvider
         });
         CompletableFuture<Either<LevelChunk, ChunkHolder.ChunkLoadingFailure>> completablefuture1 = completablefuture.thenApplyAsync((either) -> {
             return either.mapLeft((list) -> {
-                return (LevelChunk) list.get(list.size() / 2);
-            });
-        }, (runnable) -> {
-            this.mainThreadMailbox.tell(ChunkTaskPriorityQueueSorter.message(holder, runnable));
-        }).thenApplyAsync((either) -> {
-            return either.ifLeft((chunk) -> {
+                // Paper start - revert 1.18.2 diff
+                final LevelChunk chunk = (LevelChunk) list.get(list.size() / 2);
                 chunk.postProcessGeneration();
                 this.level.startTickingChunk(chunk);
+                return chunk;
             });
-        }, this.mainThreadExecutor);
+        }, (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.
+        }); // Paper end - revert 1.18.2 diff
 
         completablefuture1.thenAcceptAsync((either) -> {
             either.ifLeft((chunk) -> {
diff --git a/src/main/java/net/minecraft/server/level/ServerChunkCache.java b/src/main/java/net/minecraft/server/level/ServerChunkCache.java
index 4581a95a792e8eb157c8577547cb6f76a8e8f883..55b8f9dc3ad2b282e83c912ad8bf46b31cbd3765 100644
--- a/src/main/java/net/minecraft/server/level/ServerChunkCache.java
+++ b/src/main/java/net/minecraft/server/level/ServerChunkCache.java
@@ -1138,6 +1138,7 @@ 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