From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Caleb Bassham <caleb.bassham@gmail.com>
Date: Fri, 28 Sep 2018 02:32:19 -0500
Subject: [PATCH] Call player spectator target events and improve
 implementation

Use a proper teleport for teleporting to entities in different
worlds.

Implementation improvements authored by Spottedleaf <Spottedleaf@users.noreply.github.com>
Validate that the target entity is valid and deny spectate
requests from frozen players.

Also, make sure the entity is spawned to the client before
sending the camera packet. If the entity isn't spawned clientside
when it receives the camera packet, then the client will not
spectate the target entity.

Co-authored-by: Spottedleaf <Spottedleaf@users.noreply.github.com>

diff --git a/src/main/java/net/minecraft/server/level/ServerPlayer.java b/src/main/java/net/minecraft/server/level/ServerPlayer.java
index d5670af087fbcc9ebc18d4779373487b704cf810..529d4ec405c76ed2fce5d147d41251513d5293f2 100644
--- a/src/main/java/net/minecraft/server/level/ServerPlayer.java
+++ b/src/main/java/net/minecraft/server/level/ServerPlayer.java
@@ -1824,14 +1824,58 @@ public class ServerPlayer extends Player {
     }
 
     public void setCamera(Entity entity) {
+        // Paper start - Add PlayerStartSpectatingEntityEvent and PlayerStopSpectatingEntity Event and improve implementation
         Entity entity1 = this.getCamera();
 
-        this.camera = (Entity) (entity == null ? this : entity);
-        if (entity1 != this.camera) {
-            this.connection.send(new ClientboundSetCameraPacket(this.camera));
-            this.connection.b(this.camera.getX(), this.camera.getY(), this.camera.getZ(), this.getYRot(), this.getXRot(), TeleportCause.SPECTATE); // CraftBukkit
+        if (entity == null) {
+            entity = this;
         }
 
+        if (entity1 == entity) return; // new spec target is the current spec target
+
+        if (entity == this) {
+            com.destroystokyo.paper.event.player.PlayerStopSpectatingEntityEvent playerStopSpectatingEntityEvent = new com.destroystokyo.paper.event.player.PlayerStopSpectatingEntityEvent(this.getBukkitEntity(), entity1.getBukkitEntity());
+
+            if (!playerStopSpectatingEntityEvent.callEvent()) {
+                return;
+            }
+        } else {
+            com.destroystokyo.paper.event.player.PlayerStartSpectatingEntityEvent playerStartSpectatingEntityEvent = new com.destroystokyo.paper.event.player.PlayerStartSpectatingEntityEvent(this.getBukkitEntity(), entity1.getBukkitEntity(), entity.getBukkitEntity());
+
+            if (!playerStartSpectatingEntityEvent.callEvent()) {
+                return;
+            }
+        }
+        // Validate
+        if (entity != this) {
+            if (entity.isRemoved() || !entity.valid || entity.level == null) {
+                MinecraftServer.LOGGER.info("Blocking player " + this + " from spectating invalid entity " + entity);
+                return;
+            }
+            if (this.isImmobile()) {
+                // use debug: clients might maliciously spam this
+                MinecraftServer.LOGGER.debug("Blocking frozen player " + this + " from spectating entity " + entity);
+                return;
+            }
+        }
+
+        this.camera = entity; // only set after validating state
+
+        if (entity != this) {
+            // Make sure we're in the right place
+            this.ejectPassengers(); // teleport can fail if we have passengers...
+            this.getBukkitEntity().teleport(new Location(entity.getCommandSenderWorld().getWorld(), entity.getX(), entity.getY(), entity.getZ(), this.getYRot(), this.getXRot()), TeleportCause.SPECTATE); // Correctly handle cross-world entities from api calls by using CB teleport
+
+            // Make sure we're tracking the entity before sending
+            ChunkMap.TrackedEntity tracker = ((ServerLevel)entity.level).getChunkSource().chunkMap.entityMap.get(entity.getId());
+            if (tracker != null) { // dumb plugins...
+                tracker.updatePlayer(this);
+            }
+        } else {
+            this.connection.teleport(this.camera.getX(), this.camera.getY(), this.camera.getZ(), this.getYRot(), this.getXRot(), TeleportCause.SPECTATE); // CraftBukkit
+        }
+        this.connection.send(new ClientboundSetCameraPacket(entity));
+        // Paper end
     }
 
     @Override