Make sure the chunk conversion task is executed immediately

Appending to the tail of the chunk tasks leaves a
window for the chunk to be moved to a
non-ticking status.

Additionally, use CB's callback executor so we
can ensure that we are not incorrectly
scheduling.

Spigot-Server-Patches/0483-Fix-Chunk-Post-Processing-deadlock-risk.patch

@@ -1,4 +1,4 @@
-From 0ad488560e10639c5fb359999c8b16487780fd21 Mon Sep 17 00:00:00 2001
+From 5895b7d41d8065551cc27a225ccf2c5494d3b491 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
@@ -24,19 +24,61 @@ 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 ba6bdc40a..a72f821f2 100644
+--- a/src/main/java/net/minecraft/server/ChunkProviderServer.java
++++ b/src/main/java/net/minecraft/server/ChunkProviderServer.java
+@@ -938,6 +938,7 @@ 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;
+@@ -961,7 +962,11 @@ 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 c38d31faf..12639dfb9 100644
+index c38d31faf..e19342eb8 100644
 --- a/src/main/java/net/minecraft/server/PlayerChunkMap.java
 +++ b/src/main/java/net/minecraft/server/PlayerChunkMap.java
-@@ -1002,7 +1002,7 @@ public class PlayerChunkMap extends IChunkLoader implements PlayerChunk.d {
+@@ -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, () -> this.executor.addTask(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.
++            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
+2.26.0