PaperMC/Spigot-Server-Patches/0526-Workaround-for-Client-Lag-Spikes-MC-162253.patch

From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: MeFisto94 <MeFisto94@users.noreply.github.com>
Date: Tue, 12 May 2020 23:02:43 +0200
Subject: [PATCH] Workaround for Client Lag Spikes (MC-162253)

When crossing certain chunk boundaries, the client needlessly
calculates light maps for chunk neighbours. In some specific map
configurations, these calculations cause a 500ms+ freeze on the Client.

This patch basically serves as a workaround by sending light maps
to the client, so that it doesn't attempt to calculate them.
This mitigates the frametime impact to a minimum (but it's still there).

diff --git a/src/main/java/net/minecraft/server/Chunk.java b/src/main/java/net/minecraft/server/Chunk.java
index c80a55ee53eac128c94d74b78c5641854e974750..d7beb47d9e07707e5875c50932dfe4698cb6ad72 100644
--- a/src/main/java/net/minecraft/server/Chunk.java
+++ b/src/main/java/net/minecraft/server/Chunk.java
@@ -277,7 +277,7 @@ public class Chunk implements IChunkAccess {
 
                     // broadcast
                     Object[] backingSet = inRange.getBackingSet();
-                    Packet[] chunkPackets = new Packet[2];
+                    Packet[] chunkPackets = new Packet[10];
                     for (int index = 0, len = backingSet.length; index < len; ++index) {
                         Object temp = backingSet[index];
                         if (!(temp instanceof EntityPlayer)) {
diff --git a/src/main/java/net/minecraft/server/ChunkSection.java b/src/main/java/net/minecraft/server/ChunkSection.java
index f6df85c6bf27bfa7a16967259a6016c9473201a5..4dcb111c704a495c214827f4ef554fb8e65ca2da 100644
--- a/src/main/java/net/minecraft/server/ChunkSection.java
+++ b/src/main/java/net/minecraft/server/ChunkSection.java
@@ -99,6 +99,7 @@ public class ChunkSection {
         return this.nonEmptyBlockCount == 0;
     }
 
+    public static boolean isEmpty(@Nullable ChunkSection chunksection) { return a(chunksection) ; } // Paper - OBFHELPER
     public static boolean a(@Nullable ChunkSection chunksection) {
         return chunksection == Chunk.a || chunksection.c();
     }
diff --git a/src/main/java/net/minecraft/server/PlayerChunkMap.java b/src/main/java/net/minecraft/server/PlayerChunkMap.java
index 68dd9220bb360c75057919fed0598b415b947e4d..0aa14bfca6e1845eb6e9f5bd4e0e36335fa7f532 100644
--- a/src/main/java/net/minecraft/server/PlayerChunkMap.java
+++ b/src/main/java/net/minecraft/server/PlayerChunkMap.java
@@ -402,7 +402,7 @@ public class PlayerChunkMap extends IChunkLoader implements PlayerChunk.d {
                     player.needsChunkCenterUpdate = false;
                     player.playerConnection.sendPacket(new PacketPlayOutViewCentre(currPosX, currPosZ));
                 }
-                PlayerChunkMap.this.sendChunk(player, new ChunkCoordIntPair(rangeX, rangeZ), new Packet[2], false, true); // unloaded, loaded
+                PlayerChunkMap.this.sendChunk(player, new ChunkCoordIntPair(rangeX, rangeZ), new Packet[10], false, true); // unloaded, loaded
             },
             (EntityPlayer player, int rangeX, int rangeZ, int currPosX, int currPosZ, int prevPosX, int prevPosZ,
              com.destroystokyo.paper.util.misc.PooledLinkedHashSets.PooledObjectLinkedOpenHashSet<EntityPlayer> newState) -> {
@@ -1921,12 +1921,112 @@ public class PlayerChunkMap extends IChunkLoader implements PlayerChunk.d {
 
     }
 
+    // Paper start
+    private static int getLightMask(final Chunk chunk) {
+        final ChunkSection[] chunkSections = chunk.getSections();
+        int mask = 0;
+
+        for (int i = 0; i < chunkSections.length; ++i) {
+            /*
+
+
+Lightmasks have 18 bits, from the -1 (void) section until the 17th (air) section.
+Sections go from 0..16. Now whenever a section is not empty, it can potentially change lighting for the section itself, the section below and the section above, hence the bitmask 111b, which is 7d.
+
+             */
+            mask |= (ChunkSection.isEmpty(chunkSections[i]) ? 0 : 7) << i;
+        }
+
+        return mask;
+    }
+
+    private static int getCeilingLightMask(final Chunk chunk) {
+        int mask = getLightMask(chunk);
+
+        /*
+         It is similar to get highest bit, it would turn an 001010 into an 001111 so basically the highest bit and all below.
+         We then invert this, so we'd have 110000 and compare that to the "main" chunk.
+         This is because the bug only appears when the current chunks lightmaps are higher than those of the neighbors, thus we can omit sending neighbors which are lower than the current chunks lights.
+
+         so TLDR is that getCeilingLightMask returns a light mask with all bits set below the highest affected section. We could also count the number of leading zeros and invert them, somehow.
+         @TODO: Implement Leafs suggestion
+         either use Integer#numberOfLeadingZeros or document what this bithack is supposed to be doing then
+         */
+        mask |= mask >> 1;
+        mask |= mask >> 2;
+        mask |= mask >> 4;
+        mask |= mask >> 8;
+        mask |= mask >> 16;
+
+        return mask;
+    }
+    // Paper end
+
     final void sendChunk(EntityPlayer entityplayer, Packet<?>[] apacket, Chunk chunk) { this.a(entityplayer, apacket, chunk); } // Paper - OBFHELPER
     private void a(EntityPlayer entityplayer, Packet<?>[] apacket, Chunk chunk) {
         if (apacket[0] == null) {
+            // Paper start - add 8 for light fix workaround
+            if (apacket.length != 10) { // in case Plugins call sendChunk, resize
+                apacket = new Packet[10];
+            }
+            // Paper end
             apacket[0] = new PacketPlayOutMapChunk(chunk, 65535);
             apacket[1] = new PacketPlayOutLightUpdate(chunk.getPos(), this.lightEngine);
+
+            // Paper start - Fix MC-162253
+            final int lightMask = getLightMask(chunk);
+            int i = 1;
+            for (int x = -1; x <= 1; x++) {
+                for (int z = -1; z <= 1; z++) {
+                    if (x == 0 && z == 0) {
+                        continue;
+                    }
+
+                    ++i;
+
+                    if (!chunk.isNeighbourLoaded(x, z)) {
+                        continue;
+                    }
+
+                    final Chunk neighbor = chunk.getRelativeNeighbourIfLoaded(x, z);
+                    final int updateLightMask = lightMask & ~getCeilingLightMask(neighbor);
+
+                    if (updateLightMask == 0) {
+                        continue;
+                    }
+
+                    apacket[i] = new PacketPlayOutLightUpdate(new ChunkCoordIntPair(chunk.getPos().x + x, chunk.getPos().z + z), lightEngine, updateLightMask, 0);
+                }
+            }
+        }
+
+        final int viewDistance = playerViewDistanceBroadcastMap.getLastViewDistance(entityplayer);
+        final long lastPosition = playerViewDistanceBroadcastMap.getLastCoordinate(entityplayer);
+
+        int j = 1;
+        for (int x = -1; x <= 1; x++) {
+            for (int z = -1; z <= 1; z++) {
+                if (x == 0 && z == 0) {
+                    continue;
+                }
+
+                ++j;
+
+                Packet<?> packet = apacket[j];
+                if (packet == null) {
+                    continue;
+                }
+
+                final int distX = Math.abs(MCUtil.getCoordinateX(lastPosition) - (chunk.getPos().x + x));
+                final int distZ = Math.abs(MCUtil.getCoordinateZ(lastPosition) - (chunk.getPos().z + z));
+
+                if (Math.max(distX, distZ) > viewDistance) {
+                    continue;
+                }
+                entityplayer.playerConnection.sendPacket(packet);
+            }
         }
+        // Paper end - Fix MC-162253
 
         entityplayer.a(chunk.getPos(), apacket[0], apacket[1]);
         PacketDebug.a(this.world, chunk.getPos());