From 8dc8aeace5fb3b0df212085c7ac4eb23fd8e3024 Mon Sep 17 00:00:00 2001
From: Joshua Barretto <joshua.s.barretto@gmail.com>
Date: Thu, 7 Oct 2021 14:23:55 +0100
Subject: [PATCH] Added minimum server-side chunk loading

---
 server/src/lib.rs             |  8 +++++++-
 server/src/sys/msg/terrain.rs | 19 +++++++++++++++++++
 server/src/sys/terrain.rs     |  4 +---
 3 files changed, 27 insertions(+), 4 deletions(-)

diff --git a/server/src/lib.rs b/server/src/lib.rs
index 704b4c7a17..bd0ba1567b 100644
--- a/server/src/lib.rs
+++ b/server/src/lib.rs
@@ -96,7 +96,7 @@ use prometheus_hyper::Server as PrometheusServer;
 use specs::{join::Join, Builder, Entity as EcsEntity, Entity, SystemData, WorldExt};
 use std::{
     i32,
-    ops::{Deref, DerefMut},
+    ops::{Deref, DerefMut, Range},
     sync::Arc,
     time::{Duration, Instant},
 };
@@ -133,6 +133,12 @@ impl Default for SpawnPoint {
     fn default() -> Self { Self(Vec3::new(0.0, 0.0, 256.0)) }
 }
 
+// This is the minimum chunk range that is kept loaded around each player
+// server-side. This is independent of the client's view distance and exists to
+// avoid exploits such as small view distance chunk reloading and also to keep
+// various mechanics working fluidly (i.e: not unloading nearby entities).
+pub const MIN_VD: Range<u32> = 5..6;
+
 // Tick count used for throttling network updates
 // Note this doesn't account for dt (so update rate changes with tick rate)
 #[derive(Copy, Clone, Default)]
diff --git a/server/src/sys/msg/terrain.rs b/server/src/sys/msg/terrain.rs
index 91fa83680a..43e6cc4b9a 100644
--- a/server/src/sys/msg/terrain.rs
+++ b/server/src/sys/msg/terrain.rs
@@ -2,6 +2,7 @@ use crate::{client::Client, metrics::NetworkRequestMetrics, presence::Presence};
 use common::{
     comp::Pos,
     event::{EventBus, ServerEvent},
+    spiral::Spiral2d,
     terrain::{TerrainChunkSize, TerrainGrid},
     vol::RectVolSize,
 };
@@ -103,6 +104,24 @@ impl<'a> System<'a> for Sys {
                     }
                     Ok(())
                 });
+
+                // Load a minimum radius of chunks around each player.
+                // This is used to prevent view distance reloading exploits and make sure that
+                // entity simulation occurs within a minimum radius around the
+                // player.
+                if let Some(pos) = positions.get(entity) {
+                    let player_chunk = pos
+                        .0
+                        .xy()
+                        .map2(TerrainChunkSize::RECT_SIZE, |e, sz| e as i32 / sz as i32);
+                    for rpos in Spiral2d::new().take((crate::MIN_VD.start as usize + 1).pow(2)) {
+                        let key = player_chunk + rpos;
+                        if terrain.get_key(key).is_none() {
+                            events.push(ServerEvent::ChunkRequest(entity, key));
+                        }
+                    }
+                }
+
                 events
             })
             .flatten()
diff --git a/server/src/sys/terrain.rs b/server/src/sys/terrain.rs
index 6d9751e892..f8e7f1ae56 100644
--- a/server/src/sys/terrain.rs
+++ b/server/src/sys/terrain.rs
@@ -495,15 +495,13 @@ pub fn chunk_in_vd(
     terrain: &TerrainGrid,
     vd: u32,
 ) -> bool {
-    const MINIMUM_UNLOAD_DIST: u32 = 4;
-
     let player_chunk_pos = terrain.pos_key(player_pos.map(|e| e as i32));
 
     let adjusted_dist_sqr = (player_chunk_pos - chunk_pos)
         .map(|e: i32| (e.abs() as u32).saturating_sub(2))
         .magnitude_squared();
 
-    adjusted_dist_sqr <= vd.max(MINIMUM_UNLOAD_DIST).pow(2)
+    adjusted_dist_sqr <= vd.max(crate::MIN_VD.end).pow(2)
 }
 
 fn is_spawn_chunk(chunk_pos: Vec2<i32>, spawn_pos: SpawnPoint, terrain: &TerrainGrid) -> bool {