Fix spectator camera implementation

There are several problems that were solved:

1. Spectating failed to follow players through cross-region/cross-world
   teleporting.
2. Inability to set camera to off-region entity
3. Various crashes involved with off-region camera entity
4. Spectator does not always follow entities through portals

We fix #1 by sending ClientboundSetCameraPacket when the spectated
entity becomes in tracked, as the client appears to be unable to
correctly handle when the camera entity is removed, even temporarily.

We fix #2 by allowing off-region entities that are not dead.
However, this required the teleportation code in both setCamera
and the ServerPlayer#tick method to be modified to correctly
handle off-region camera targets.

We fix #3 by correctly teleporting to off-region locations
when needed.

We fix #4 by following the Bukkit entity. This allows spectator
mode to properly follow non-player entities through portals.

Issues #1 (cross-world) and #4 are probably present in Paper/Vanilla, however
solving #2 and #3 required a solution for these.

Fixes https://github.com/PaperMC/Folia/issues/324

folia-server/minecraft-patches/sources/io/papermc/paper/threadedregions/TeleportUtils.java.patch

@@ -1,6 +1,6 @@
 --- /dev/null
 +++ b/io/papermc/paper/threadedregions/TeleportUtils.java
-@@ -1,0 +_,70 @@
+@@ -1,0 +_,82 @@
 +package io.papermc.paper.threadedregions;
 +
 +import ca.spottedleaf.concurrentutil.completable.CallbackCompletable;
@@ -13,8 +13,14 @@
 +
 +public final class TeleportUtils {
 +
-+    public static void teleport(final Entity from, final boolean useFromRootVehicle, final Entity to, final Float yaw, final Float pitch,
-+                                final long teleportFlags, final PlayerTeleportEvent.TeleportCause cause, final Consumer<Entity> onComplete) {
++    public static <T extends Entity> void teleport(final T from, final boolean useFromRootVehicle, final Entity to, final Float yaw, final Float pitch,
++                                                   final long teleportFlags, final PlayerTeleportEvent.TeleportCause cause, final Consumer<Entity> onComplete) {
++        teleport(from, useFromRootVehicle, to, yaw, pitch, teleportFlags, cause, onComplete, null);
++    }
++
++    public static <T extends Entity> void teleport(final T from, final boolean useFromRootVehicle, final Entity to, final Float yaw, final Float pitch,
++                                                   final long teleportFlags, final PlayerTeleportEvent.TeleportCause cause, final Consumer<Entity> onComplete,
++                                                   final java.util.function.Predicate<T> preTeleport) {
 +        // retrieve coordinates
 +        final CallbackCompletable<Location> positionCompletable = new CallbackCompletable<>();
 +
@@ -27,10 +33,16 @@
 +                    return;
 +                }
 +                final boolean scheduled = from.getBukkitEntity().taskScheduler.schedule(
-+                    (final Entity realFrom) -> {
++                    (final T realFrom) -> {
 +                        final Vec3 pos = new Vec3(
 +                            loc.getX(), loc.getY(), loc.getZ()
 +                        );
++                        if (preTeleport != null && !preTeleport.test(realFrom)) {
++                            if (onComplete != null) {
++                                onComplete.accept(null);
++                            }
++                            return;
++                        }
 +                        (useFromRootVehicle ? realFrom.getRootVehicle() : realFrom).teleportAsync(
 +                            ((CraftWorld)loc.getWorld()).getHandle(), pos, null, null, null,
 +                            cause, teleportFlags, onComplete

folia-server/minecraft-patches/sources/net/minecraft/server/level/ServerPlayer.java.patch

@@ -188,6 +188,31 @@
                              } else {
                                  LOGGER.warn("Failed to spawn player ender pearl in level ({}), skipping", optional.get());
                              }
+@@ -817,12 +_,23 @@
+ 
+         Entity camera = this.getCamera();
+         if (camera != this) {
+-            if (camera.isAlive()) {
++            if (camera.canBeSpectated()) { // Folia - region threading - replace removed check
++                if (ca.spottedleaf.moonrise.common.util.TickThread.isTickThreadFor(camera) && !camera.isRemoved()) { // Folia - region threading
+                 this.absMoveTo(camera.getX(), camera.getY(), camera.getZ(), camera.getYRot(), camera.getXRot());
+                 this.serverLevel().getChunkSource().move(this);
+                 if (this.wantsToStopRiding()) {
+                     this.setCamera(this);
+                 }
++                } else { // Folia start - region threading
++                    Entity realCamera = camera.getBukkitEntity().getHandleRaw();
++                    if (realCamera != camera) {
++                        this.setCamera(this);
++                        this.setCamera(realCamera);
++                    } else {
++                        this.teleportToCameraOffRegion();
++                    }
++                }
++                // Folia end - region threading
+             } else {
+                 this.setCamera(this);
+             }
 @@ -1357,9 +_,332 @@
          }
      }
@@ -521,18 +546,66 @@
          if (this.isSleeping()) return null; // CraftBukkit - SPIGOT-3154
          if (this.isRemoved()) {
              return null;
-@@ -2398,6 +_,11 @@
+@@ -2397,7 +_,30 @@
+         return (Entity)(this.camera == null ? this : this.camera);
      }
  
++    // Folia start - region threading
++    private void teleportToCameraOffRegion() {
++        Entity cameraFinal = this.camera;
++        // use the task scheduler, as we don't know where the caller is invoking from
++        if (this != cameraFinal) {
++            this.getBukkitEntity().taskScheduler.schedule((final ServerPlayer newPlayer) -> {
++                io.papermc.paper.threadedregions.TeleportUtils.teleport(
++                        newPlayer, false, cameraFinal, null, null, 0L,
++                        org.bukkit.event.player.PlayerTeleportEvent.TeleportCause.SPECTATE, null,
++                        (final ServerPlayer newerPlayer) -> {
++                            return newerPlayer.camera == cameraFinal;
++                        }
++                );
++            }, null, 1L);
++        } // else: do not bother teleporting to self
++    }
++    // Folia end - region threading
++
      public void setCamera(@Nullable Entity entityToSpectate) {
 +        // Folia start - region threading
-+        if (entityToSpectate != null && !ca.spottedleaf.moonrise.common.util.TickThread.isTickThreadFor(entityToSpectate)) {
++        if (entityToSpectate != null && (entityToSpectate != this && !entityToSpectate.canBeSpectated())) {
 +            return;
 +        }
 +        // Folia end - region threading
          Entity camera = this.getCamera();
          this.camera = (Entity)(entityToSpectate == null ? this : entityToSpectate);
          if (camera != this.camera) {
+@@ -2416,16 +_,19 @@
+                 }
+             }
+             // Paper end - Add PlayerStartSpectatingEntityEvent and PlayerStopSpectatingEntity
+-            if (this.camera.level() instanceof ServerLevel serverLevel) {
+-                this.teleportTo(serverLevel, this.camera.getX(), this.camera.getY(), this.camera.getZ(), Set.of(), this.getYRot(), this.getXRot(), false, org.bukkit.event.player.PlayerTeleportEvent.TeleportCause.SPECTATE); // CraftBukkit
+-            }
+-
+-            if (entityToSpectate != null) {
+-                this.serverLevel().getChunkSource().move(this);
+-            }
+-
++            // Folia - region threading - move down
++
++            // Folia - region threading - not needed
++
++            // Folia start - region threading - handle camera setting better
++            if (this.camera == this
++                || (ca.spottedleaf.moonrise.common.util.TickThread.isTickThreadFor(this.camera) && this.camera.moonrise$getTrackedEntity() != null
++                    && this.camera.moonrise$getTrackedEntity().seenBy.contains(this.connection))) {
++                // Folia end - region threading - handle camera setting better
+             this.connection.send(new ClientboundSetCameraPacket(this.camera));
+-            this.connection.resetPosition();
++            } // Folia - region threading - handle camera setting better
++            //this.connection.resetPosition(); // Folia - region threading - not needed
++            this.teleportToCameraOffRegion(); // Folia - region threading - moved down
+         }
+     }
+ 
 @@ -2896,11 +_,11 @@
      }
  

folia-server/minecraft-patches/sources/net/minecraft/world/entity/Entity.java.patch

@@ -27,7 +27,7 @@
      private EntityDimensions dimensions;
      private float eyeHeight;
      public boolean isInPowderSnow;
-@@ -525,6 +_,19 @@
+@@ -525,6 +_,23 @@
          }
      }
      // Paper end - optimise entity tracker
@@ -43,6 +43,10 @@
 +            this.pistonDeltasGameTime += fromRedstoneTimeOffset;
 +        }
 +    }
++
++    public boolean canBeSpectated() {
++        return !this.getBukkitEntity().taskScheduler.isRetired();
++    }
 +    // Folia end - region ticking
  
      public Entity(EntityType<?> entityType, Level level) {
@@ -992,6 +996,32 @@
      protected void removeAfterChangingDimensions() {
          this.setRemoved(Entity.RemovalReason.CHANGED_DIMENSION, null); // CraftBukkit - add Bukkit remove cause
          if (this instanceof Leashable leashable && leashable.isLeashed()) { // Paper - only call if it is leashed
+@@ -4246,6 +_,12 @@
+     }
+ 
+     public void startSeenByPlayer(ServerPlayer serverPlayer) {
++        // Folia start - region threading
++        if (serverPlayer.getCamera() == this) {
++            // set camera again
++            serverPlayer.connection.send(new net.minecraft.network.protocol.game.ClientboundSetCameraPacket(this));
++        }
++        // Folia end - region threading
+     }
+ 
+     public void stopSeenByPlayer(ServerPlayer serverPlayer) {
+@@ -4255,6 +_,12 @@
+             new io.papermc.paper.event.player.PlayerUntrackEntityEvent(serverPlayer.getBukkitEntity(), this.getBukkitEntity()).callEvent();
+         }
+         // Paper end - entity tracking events
++        // Folia start - region threading
++        if (serverPlayer.getCamera() == this) {
++            // unset camera, the player tick method should TP us close enough again to invoke startSeenByPlayer
++            serverPlayer.connection.send(new net.minecraft.network.protocol.game.ClientboundSetCameraPacket(serverPlayer));
++        }
++        // Folia end - region threading
+     }
+ 
+     public float rotate(Rotation transformRotation) {
 @@ -4790,7 +_,8 @@
              }
          }

folia-server/minecraft-patches/sources/net/minecraft/world/entity/LivingEntity.java.patch

@@ -9,7 +9,7 @@
      protected int autoSpinAttackTicks;
      protected float autoSpinAttackDmg;
      @Nullable
-@@ -307,6 +_,21 @@
+@@ -307,6 +_,26 @@
          return this.getYHeadRot();
      }
      // CraftBukkit end
@@ -23,6 +23,11 @@
 +    }
 +
 +    @Override
++    public boolean canBeSpectated() {
++        return super.canBeSpectated() && this.getHealth() > 0.0F;
++    }
++
++    @Override
 +    protected void resetStoredPositions() {
 +        super.resetStoredPositions();
 +        this.lastClimbablePos = Optional.empty();