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Spigot still maintains some partial implementation of "tick skipping", a practice in which the MinecraftServer.currentTick field is updated not by an increment of one per actual tick, but instead set to System.currentTimeMillis() / 50. This behaviour means that the tracked tick may "skip" a tick value in case a previous tick took more than the expected 50ms. To compensate for this in important paths, spigot/craftbukkit implements "wall-time". Instead of incrementing/decrementing ticks on block entities/entities by one for each call to their tick() method, they instead increment/decrement important values, like an ItemEntity's age or pickupDelay, by the difference of `currentTick - lastTick`, where `lastTick` is the value of `currentTick` during the last tick() call. These "fixes" however do not play nicely with minecraft's simulation distance as entities/block entities implementing the above behaviour would "catch up" their values when moving from a non-ticking chunk to a ticking one as their `lastTick` value remains stuck on the last tick in a ticking chunk and hence lead to a large "catch up" once ticked again. Paper completely removes the "tick skipping" behaviour (See patch "Further-improve-server-tick-loop"), making the above precautions completely unnecessary, which also rids paper of the previous described incompatibility with non-ticking chunks.
30 lines
1.8 KiB
Diff
30 lines
1.8 KiB
Diff
From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
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From: Spottedleaf <Spottedleaf@users.noreply.github.com>
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Date: Fri, 12 Jun 2020 16:51:39 -0700
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Subject: [PATCH] Prevent position desync causing tp exploit
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Caused the server to revert to the player's overworld coordinates
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after teleporting into the end.
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Sidenote: The underlying issue is that the move call can teleport
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entities and do other things like kill the entity. In the future,
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to fix all exploits derieved from this usually unexpected
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behaviour, we need to move all of this dangerous logic outside
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of the move call and into an appropriate place in the tick method.
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diff --git a/src/main/java/net/minecraft/server/network/ServerGamePacketListenerImpl.java b/src/main/java/net/minecraft/server/network/ServerGamePacketListenerImpl.java
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index c6bcde25b476ef2362f469bd7cba82ce97cb300c..3455781a47fbececab33406e829ceb392c72a113 100644
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--- a/src/main/java/net/minecraft/server/network/ServerGamePacketListenerImpl.java
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+++ b/src/main/java/net/minecraft/server/network/ServerGamePacketListenerImpl.java
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@@ -1349,6 +1349,11 @@ public class ServerGamePacketListenerImpl extends ServerCommonPacketListenerImpl
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this.player.move(MoverType.PLAYER, new Vec3(d6, d7, d8));
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this.player.onGround = packet.isOnGround(); // CraftBukkit - SPIGOT-5810, SPIGOT-5835, SPIGOT-6828: reset by this.player.move
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+ // Paper start - prevent position desync
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+ if (this.awaitingPositionFromClient != null) {
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+ return; // ... thanks Mojang for letting move calls teleport across dimensions.
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+ }
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+ // Paper end - prevent position desync
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double d11 = d7;
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d6 = d0 - this.player.getX();
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