Paper/paper-server/patches/sources/net/minecraft/server/level/ServerChunkCache.java.patch

--- a/net/minecraft/server/level/ServerChunkCache.java
+++ b/net/minecraft/server/level/ServerChunkCache.java
@@ -95,6 +95,16 @@
         this.clearCache();
     }
 
+    // CraftBukkit start - properly implement isChunkLoaded
+    public boolean isChunkLoaded(int chunkX, int chunkZ) {
+        ChunkHolder chunk = this.chunkMap.getUpdatingChunkIfPresent(ChunkPos.asLong(chunkX, chunkZ));
+        if (chunk == null) {
+            return false;
+        }
+        return chunk.getFullChunkNow() != null;
+    }
+    // CraftBukkit end
+
     @Override
     public ThreadedLevelLightEngine getLightEngine() {
         return this.lightEngine;
@@ -138,7 +148,7 @@
                 if (k == this.lastChunkPos[l] && leastStatus == this.lastChunkStatus[l]) {
                     ChunkAccess ichunkaccess = this.lastChunk[l];
 
-                    if (ichunkaccess != null || !create) {
+                    if (ichunkaccess != null) { // CraftBukkit - the chunk can become accessible in the meantime TODO for non-null chunks it might also make sense to check that the chunk's state hasn't changed in the meantime
                         return ichunkaccess;
                     }
                 }
@@ -151,7 +161,7 @@
             Objects.requireNonNull(completablefuture);
             chunkproviderserver_b.managedBlock(completablefuture::isDone);
             ChunkResult<ChunkAccess> chunkresult = (ChunkResult) completablefuture.join();
-            ChunkAccess ichunkaccess1 = (ChunkAccess) chunkresult.orElse((Object) null);
+            ChunkAccess ichunkaccess1 = (ChunkAccess) chunkresult.orElse(null); // CraftBukkit - decompile error
 
             if (ichunkaccess1 == null && create) {
                 throw (IllegalStateException) Util.pauseInIde(new IllegalStateException("Chunk not there when requested: " + chunkresult.getError()));
@@ -231,7 +241,15 @@
         int l = ChunkLevel.byStatus(leastStatus);
         ChunkHolder playerchunk = this.getVisibleChunkIfPresent(k);
 
-        if (create) {
+        // CraftBukkit start - don't add new ticket for currently unloading chunk
+        boolean currentlyUnloading = false;
+        if (playerchunk != null) {
+            FullChunkStatus oldChunkState = ChunkLevel.fullStatus(playerchunk.oldTicketLevel);
+            FullChunkStatus currentChunkState = ChunkLevel.fullStatus(playerchunk.getTicketLevel());
+            currentlyUnloading = (oldChunkState.isOrAfter(FullChunkStatus.FULL) && !currentChunkState.isOrAfter(FullChunkStatus.FULL));
+        }
+        if (create && !currentlyUnloading) {
+            // CraftBukkit end
             this.distanceManager.addTicket(TicketType.UNKNOWN, chunkcoordintpair, l, chunkcoordintpair);
             if (this.chunkAbsent(playerchunk, l)) {
                 ProfilerFiller gameprofilerfiller = Profiler.get();
@@ -250,7 +268,7 @@
     }
 
     private boolean chunkAbsent(@Nullable ChunkHolder holder, int maxLevel) {
-        return holder == null || holder.getTicketLevel() > maxLevel;
+        return holder == null || holder.oldTicketLevel > maxLevel; // CraftBukkit using oldTicketLevel for isLoaded checks
     }
 
     @Override
@@ -309,11 +327,33 @@
 
     @Override
     public void close() throws IOException {
-        this.save(true);
+        // CraftBukkit start
+        this.close(true);
+    }
+
+    public void close(boolean save) throws IOException {
+        if (save) {
+            this.save(true);
+        }
+        // CraftBukkit end
         this.dataStorage.close();
         this.lightEngine.close();
         this.chunkMap.close();
+    }
+
+    // CraftBukkit start - modelled on below
+    public void purgeUnload() {
+        ProfilerFiller gameprofilerfiller = Profiler.get();
+
+        gameprofilerfiller.push("purge");
+        this.distanceManager.purgeStaleTickets();
+        this.runDistanceManagerUpdates();
+        gameprofilerfiller.popPush("unload");
+        this.chunkMap.tick(() -> true);
+        gameprofilerfiller.pop();
+        this.clearCache();
     }
+    // CraftBukkit end
 
     @Override
     public void tick(BooleanSupplier shouldKeepTicking, boolean tickChunks) {
@@ -401,14 +441,14 @@
 
         this.lastSpawnState = spawnercreature_d;
         profiler.popPush("spawnAndTick");
-        boolean flag = this.level.getGameRules().getBoolean(GameRules.RULE_DOMOBSPAWNING);
+        boolean flag = this.level.getGameRules().getBoolean(GameRules.RULE_DOMOBSPAWNING) && !this.level.players().isEmpty(); // CraftBukkit
         int k = this.level.getGameRules().getInt(GameRules.RULE_RANDOMTICKING);
         List list1;
 
         if (flag && (this.spawnEnemies || this.spawnFriendlies)) {
-            boolean flag1 = this.level.getLevelData().getGameTime() % 400L == 0L;
+            boolean flag1 = this.level.ticksPerSpawnCategory.getLong(org.bukkit.entity.SpawnCategory.ANIMAL) != 0L && this.level.getLevelData().getGameTime() % this.level.ticksPerSpawnCategory.getLong(org.bukkit.entity.SpawnCategory.ANIMAL) == 0L; // CraftBukkit
 
-            list1 = NaturalSpawner.getFilteredSpawningCategories(spawnercreature_d, this.spawnFriendlies, this.spawnEnemies, flag1);
+            list1 = NaturalSpawner.getFilteredSpawningCategories(spawnercreature_d, this.spawnFriendlies, this.spawnEnemies, flag1, this.level); // CraftBukkit
         } else {
             list1 = List.of();
         }
@@ -541,10 +581,16 @@
 
     @Override
     public void setSpawnSettings(boolean spawnMonsters) {
-        this.spawnEnemies = spawnMonsters;
-        this.spawnFriendlies = this.spawnFriendlies;
+        // CraftBukkit start
+        this.setSpawnSettings(spawnMonsters, this.spawnFriendlies);
     }
 
+    public void setSpawnSettings(boolean flag, boolean spawnFriendlies) {
+        this.spawnEnemies = flag;
+        this.spawnFriendlies = spawnFriendlies;
+        // CraftBukkit end
+    }
+
     public String getChunkDebugData(ChunkPos pos) {
         return this.chunkMap.getChunkDebugData(pos);
     }
@@ -618,14 +664,20 @@
         }
 
         @Override
-        protected boolean pollTask() {
+        // CraftBukkit start - process pending Chunk loadCallback() and unloadCallback() after each run task
+        public boolean pollTask() {
+        try {
             if (ServerChunkCache.this.runDistanceManagerUpdates()) {
                 return true;
             } else {
                 ServerChunkCache.this.lightEngine.tryScheduleUpdate();
                 return super.pollTask();
             }
+        } finally {
+            ServerChunkCache.this.chunkMap.callbackExecutor.run();
         }
+        // CraftBukkit end
+        }
     }
 
     private static record ChunkAndHolder(LevelChunk chunk, ChunkHolder holder) {