Rewrite chunk system (#8177)

Patch documentation to come

Issues with the old system that are fixed now:
- World generation does not scale with cpu cores effectively.
- Relies on the main thread for scheduling and maintaining chunk state, dropping chunk load/generate rates at lower tps.
- Unreliable prioritisation of chunk gen/load calls that block the main thread.
- Shutdown logic is utterly unreliable, as it has to wait for all chunks to unload - is it guaranteed that the chunk system is in a state on shutdown that it can reliably do this? Watchdog shutdown also typically failed due to thread checks, which is now resolved.
- Saving of data is not unified (i.e can save chunk data without saving entity data, poses problems for desync if shutdown is really abnormal.
- Entities are not loaded with chunks. This caused quite a bit of headache for Chunk#getEntities API, but now the new chunk system loads entities with chunks so that they are ready whenever the chunk loads in. Effectively brings the behavior back to 1.16 era, but still storing entities in their own separate regionfiles.

The above list is not complete. The patch documentation will complete it.

New chunk system hard relies on starlight and dataconverter, and most importantly the new concurrent utilities in ConcurrentUtil.

Some of the old async chunk i/o interface (i.e the old file io thread reroutes _some_ calls to the new file io thread) is kept for plugin compat reasons. It will be removed in the next major version of minecraft.

The old legacy chunk system patches have been moved to the removed folder in case we need them again.

patches/server/Optimise-nearby-player-lookups.patch

@@ -1,5 +1,5 @@
 From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
-From: Spottedleaf <spottedleaf@spottedleaf.dev>
+From: Spottedleaf <Spottedleaf@users.noreply.github.com>
 Date: Thu, 27 Aug 2020 16:22:52 -0700
 Subject: [PATCH] Optimise nearby player lookups
 
@@ -43,27 +43,26 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
 --- a/src/main/java/net/minecraft/server/level/ChunkMap.java
 +++ b/src/main/java/net/minecraft/server/level/ChunkMap.java
 @@ -0,0 +0,0 @@ public class ChunkMap extends ChunkStorage implements ChunkHolder.PlayerProvider
-     public final com.destroystokyo.paper.util.misc.PlayerAreaMap playerMobDistanceMap; // Paper
      public final ReferenceOpenHashSet<ChunkHolder> needsChangeBroadcasting = new ReferenceOpenHashSet<>();
  
+     // Paper - rewrite chunk system
 +    // Paper start - optimise checkDespawn
 +    public static final int GENERAL_AREA_MAP_SQUARE_RADIUS = 40;
 +    public static final double GENERAL_AREA_MAP_ACCEPTABLE_SEARCH_RANGE = 16.0 * (GENERAL_AREA_MAP_SQUARE_RADIUS - 1);
 +    public static final double GENERAL_AREA_MAP_ACCEPTABLE_SEARCH_RANGE_SQUARED = GENERAL_AREA_MAP_ACCEPTABLE_SEARCH_RANGE * GENERAL_AREA_MAP_ACCEPTABLE_SEARCH_RANGE;
 +    public final com.destroystokyo.paper.util.misc.PlayerAreaMap playerGeneralAreaMap;
 +    // Paper end - optimise checkDespawn
-+
-     // CraftBukkit start - recursion-safe executor for Chunk loadCallback() and unloadCallback()
-     public final CallbackExecutor callbackExecutor = new CallbackExecutor();
-     public static final class CallbackExecutor implements java.util.concurrent.Executor, Runnable {
+ 
+     // Paper start - distance maps
+     private final com.destroystokyo.paper.util.misc.PooledLinkedHashSets<ServerPlayer> pooledLinkedPlayerHashSets = new com.destroystokyo.paper.util.misc.PooledLinkedHashSets<>();
 @@ -0,0 +0,0 @@ public class ChunkMap extends ChunkStorage implements ChunkHolder.PlayerProvider
+             trackMap.add(player, chunkX, chunkZ, Math.min(trackRange, net.minecraft.server.ChunkSystem.getSendViewDistance(player)));
+         }
          // Paper end - use distance map to optimise entity tracker
-         // Note: players need to be explicitly added to distance maps before they can be updated
-         this.playerChunkTickRangeMap.add(player, chunkX, chunkZ, DistanceManager.MOB_SPAWN_RANGE); // Paper - optimise ChunkMap#anyPlayerCloseEnoughForSpawning
 +        this.playerGeneralAreaMap.add(player, chunkX, chunkZ, GENERAL_AREA_MAP_SQUARE_RADIUS); // Paper - optimise checkDespawn
-         // Paper start - per player mob spawning
-         if (this.playerMobDistanceMap != null) {
-             this.playerMobDistanceMap.add(player, chunkX, chunkZ, this.distanceManager.getSimulationDistance());
+     }
+ 
+     void removePlayerFromDistanceMaps(ServerPlayer player) {
 @@ -0,0 +0,0 @@ public class ChunkMap extends ChunkStorage implements ChunkHolder.PlayerProvider
          this.playerMobSpawnMap.remove(player);
          this.playerChunkTickRangeMap.remove(player);
@@ -73,13 +72,13 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
          if (this.playerMobDistanceMap != null) {
              this.playerMobDistanceMap.remove(player);
 @@ -0,0 +0,0 @@ public class ChunkMap extends ChunkStorage implements ChunkHolder.PlayerProvider
+             trackMap.update(player, chunkX, chunkZ, Math.min(trackRange, net.minecraft.server.ChunkSystem.getSendViewDistance(player)));
          }
          // Paper end - use distance map to optimise entity tracker
-         this.playerChunkTickRangeMap.update(player, chunkX, chunkZ, DistanceManager.MOB_SPAWN_RANGE); // Paper - optimise ChunkMap#anyPlayerCloseEnoughForSpawning
 +        this.playerGeneralAreaMap.update(player, chunkX, chunkZ, GENERAL_AREA_MAP_SQUARE_RADIUS); // Paper - optimise checkDespawn
-         // Paper start - per player mob spawning
-         if (this.playerMobDistanceMap != null) {
-             this.playerMobDistanceMap.update(player, chunkX, chunkZ, this.distanceManager.getSimulationDistance());
+     }
+     // Paper end
+     // Paper start
 @@ -0,0 +0,0 @@ public class ChunkMap extends ChunkStorage implements ChunkHolder.PlayerProvider
                  }
              });