From d4150f3df14622ffc4ad49ff655ba6aaa23114bb 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/ChunkProviderServer.java b/src/main/java/net/minecraft/server/ChunkProviderServer.java
index 15bd3cacb3..2d4d7250f2 100644
--- a/src/main/java/net/minecraft/server/ChunkProviderServer.java
+++ b/src/main/java/net/minecraft/server/ChunkProviderServer.java
@@ -1019,6 +1019,7 @@ public class ChunkProviderServer extends IChunkProvider {
                         // we do not want to use this.executeNext as that also processes chunk loads and might count against task counter.
                         // We also have already ticked the distance manager above too.
                         if (server.chunksTasksRan < 200 && now - lastChunkTask > 100000 && super.executeNext()) {
+                            ChunkProviderServer.this.playerChunkMap.chunkLoadConversionCallbackExecutor.run(); // run immediately after a task is potentially queued
                             ChunkProviderServer.this.lightEngine.queueUpdate();
                             server.chunksTasksRan++;
                             lastChunkTask = now;
@@ -1042,7 +1043,11 @@ public class ChunkProviderServer extends IChunkProvider {
                 return true;
             } else {
                 ChunkProviderServer.this.lightEngine.queueUpdate();
-                return super.executeNext() || execChunkTask; // Paper
+                // Paper start - Add chunk load conversion callback executor to prevent deadlock due to recursion in the chunk task queue sorter
+                boolean executed = super.executeNext();
+                ChunkProviderServer.this.playerChunkMap.chunkLoadConversionCallbackExecutor.run(); // run immediately after a task is potentially queued
+                return executed || execChunkTask;
+                // Paper end - Add chunk load conversion callback executor to prevent deadlock due to recursion in the chunk task queue sorter
             }
         } finally {
             playerChunkMap.callbackExecutor.run();
diff --git a/src/main/java/net/minecraft/server/PlayerChunkMap.java b/src/main/java/net/minecraft/server/PlayerChunkMap.java
index c38d31fafe..e19342eb89 100644
--- a/src/main/java/net/minecraft/server/PlayerChunkMap.java
+++ b/src/main/java/net/minecraft/server/PlayerChunkMap.java
@@ -92,6 +92,7 @@ public class PlayerChunkMap extends IChunkLoader implements PlayerChunk.d {
         @Override
         public void execute(Runnable runnable) {
             if (queued != null) {
+                MinecraftServer.LOGGER.fatal("Failed to schedule runnable", new IllegalStateException("Already queued")); // Paper - make sure this is printed
                 throw new IllegalStateException("Already queued");
             }
             queued = runnable;
@@ -108,6 +109,8 @@ public class PlayerChunkMap extends IChunkLoader implements PlayerChunk.d {
     };
     // CraftBukkit end
 
+    final CallbackExecutor chunkLoadConversionCallbackExecutor = new CallbackExecutor(); // Paper
+
     // Paper start - distance maps
     private final com.destroystokyo.paper.util.misc.PooledLinkedHashSets<EntityPlayer> pooledLinkedPlayerHashSets = new com.destroystokyo.paper.util.misc.PooledLinkedHashSets<>();
 
@@ -1002,7 +1005,7 @@ public class PlayerChunkMap extends IChunkLoader implements PlayerChunk.d {
                 return Either.left(chunk);
             });
         }, (runnable) -> {
-            this.mailboxMain.a(ChunkTaskQueueSorter.a(playerchunk, runnable)); // CraftBukkit - decompile error
+            this.mailboxMain.a(ChunkTaskQueueSorter.a(playerchunk, () -> PlayerChunkMap.this.chunkLoadConversionCallbackExecutor.execute(runnable))); // CraftBukkit - decompile error // 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) -> {
-- 
2.25.1