Paper/patches/server/1034-Add-debug-for-chunk-system-unload-crash.patch

From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Spottedleaf <Spottedleaf@users.noreply.github.com>
Date: Wed, 19 Jun 2024 10:52:07 -0700
Subject: [PATCH] Add debug for chunk system unload crash

Somehow, a chunkholder is present in the unload queue after
it has been unloaded. It is likely that this is a result of
adding the chunk holder to the unload queue while it is
unloading. However, that should not be possible.

To find out where it is being added to the unload queue, track
the last stacktrace which adds to the unload queue and check
on chunk holder remove if the holder is present in the unload queue
and log the stacktrace.

diff --git a/src/main/java/ca/spottedleaf/moonrise/patches/chunk_system/scheduling/NewChunkHolder.java b/src/main/java/ca/spottedleaf/moonrise/patches/chunk_system/scheduling/NewChunkHolder.java
index d5fc5756ea960096ff23376a6b7ac68a2a462d22..0715514c8bb49c805b04f42ff37903a7d590aaa8 100644
--- a/src/main/java/ca/spottedleaf/moonrise/patches/chunk_system/scheduling/NewChunkHolder.java
+++ b/src/main/java/ca/spottedleaf/moonrise/patches/chunk_system/scheduling/NewChunkHolder.java
@@ -748,9 +748,22 @@ public final class NewChunkHolder {
 
     /** Unloaded from chunk map */
     private boolean unloaded;
+    private Throwable lastUnloadAdd;
 
     void markUnloaded() {
         this.unloaded = true;
+        if (this.inUnloadQueue) {
+            if (this.lastUnloadAdd != null) {
+                LOGGER.error("Unloaded chunkholder " + this.toString() + " while in the unload queue", this.lastUnloadAdd);
+            } else {
+                // should never happen
+                LOGGER.error("Unloaded chunkholder " + this.toString() + " while in the unload queue without a throwable");
+            }
+
+            // prevent crash by removing (note: we hold scheduling lock here)
+            this.inUnloadQueue = false;
+            this.scheduler.chunkHolderManager.unloadQueue.removeChunk(this.chunkX, this.chunkZ);
+        }
     }
 
     private boolean inUnloadQueue = false;
@@ -768,12 +781,14 @@ public final class NewChunkHolder {
             // ensure in unload queue
             if (!this.inUnloadQueue) {
                 this.inUnloadQueue = true;
+                this.lastUnloadAdd = new Throwable();
                 this.scheduler.chunkHolderManager.unloadQueue.addChunk(this.chunkX, this.chunkZ);
             }
         } else {
             // ensure not in unload queue
             if (this.inUnloadQueue) {
                 this.inUnloadQueue = false;
+                this.lastUnloadAdd = null;
                 this.scheduler.chunkHolderManager.unloadQueue.removeChunk(this.chunkX, this.chunkZ);
             }
         }