Paper/patches/server/0404-Fix-Chunk-Post-Processing-deadlock-risk.patch

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 c82345c74e0e6246f304283c2e06e2d1fcfe53ca..d3094c8a6a906737d4ca56a76ff40b3586961680 100644
--- a/src/main/java/net/minecraft/server/level/ChunkMap.java
+++ b/src/main/java/net/minecraft/server/level/ChunkMap.java
@@ -189,6 +189,7 @@ 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
@@ -1028,7 +1029,7 @@ 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 751454ad5a2c374c01ff360535428db36c0aa1b3..e4f5f564892836a2e925f419e8fcb60c70b21a47 100644
--- a/src/main/java/net/minecraft/server/level/ServerChunkCache.java
+++ b/src/main/java/net/minecraft/server/level/ServerChunkCache.java
@@ -985,6 +985,7 @@ 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