Fix errors where Monster::enemy references garbage memory

Source/monster.cpp

@@ -442,10 +442,11 @@ void ClrAllMonsters()
 		monster.position.old = { 0, 0 };
 		monster.direction = static_cast<Direction>(GenerateRnd(8));
 		monster.animInfo = {};
-		monster.flags = 0;
+		monster.flags = MFLAG_NO_ENEMY;
 		monster.isInvalid = false;
-		monster.enemy = GenerateRnd(gbActivePlayers);
-		monster.enemyPosition = Players[monster.enemy].position.future;
+		monster.enemy = 0;
+		monster.enemyPosition = {};
+		DiscardRandomValues(1);
 	}
 }
 
@@ -581,13 +582,21 @@ tl::expected<void, std::string> LoadDiabMonsts()
 
 void DeleteMonster(size_t activeIndex)
 {
-	const Monster &monster = Monsters[ActiveMonsters[activeIndex]];
+	const unsigned monsterId = ActiveMonsters[activeIndex];
+	const Monster &monster = Monsters[monsterId];
 	if ((monster.flags & MFLAG_BERSERK) != 0) {
 		AddUnLight(monster.lightId);
 	}
 
 	ActiveMonsterCount--;
 	std::swap(ActiveMonsters[activeIndex], ActiveMonsters[ActiveMonsterCount]); // This ensures alive monsters are before ActiveMonsterCount in the array and any deleted monster after
+
+	for (size_t i = 0; i < ActiveMonsterCount; i++) {
+		Monster &activeMonster = Monsters[ActiveMonsters[i]];
+		if ((activeMonster.flags & MFLAG_TARGETS_MONSTER) != 0 && activeMonster.enemy == monsterId) {
+			activeMonster.flags |= MFLAG_NO_ENEMY;
+		}
+	}
 }
 
 void NewMonsterAnim(Monster &monster, MonsterGraphic graphic, Direction md, AnimationDistributionFlags flags = AnimationDistributionFlags::None, int8_t numSkippedFrames = 0, int8_t distributeFramesBeforeFrame = 0)
@@ -1212,10 +1221,12 @@ void MonsterAttackPlayer(Monster &monster, Player &player, int hit, int minDam,
 
 void MonsterAttackEnemy(Monster &monster, int hit, int minDam, int maxDam)
 {
-	if ((monster.flags & MFLAG_TARGETS_MONSTER) != 0)
-		MonsterAttackMonster(monster, Monsters[monster.enemy], hit, minDam, maxDam);
-	else
-		MonsterAttackPlayer(monster, Players[monster.enemy], hit, minDam, maxDam);
+	if ((monster.flags & MFLAG_NO_ENEMY) == 0) {
+		if ((monster.flags & MFLAG_TARGETS_MONSTER) != 0)
+			MonsterAttackMonster(monster, Monsters[monster.enemy], hit, minDam, maxDam);
+		else
+			MonsterAttackPlayer(monster, Players[monster.enemy], hit, minDam, maxDam);
+	}
 }
 
 bool MonsterAttack(Monster &monster)
@@ -4056,6 +4067,20 @@ void DeleteMonsterList()
 	}
 }
 
+void RemoveEnemyReferences(const Player &player)
+{
+	if (&player == MyPlayer || !player.isOnActiveLevel())
+		return;
+
+	const size_t playerId = player.getId();
+	for (size_t i = 0; i < ActiveMonsterCount; i++) {
+		Monster &activeMonster = Monsters[ActiveMonsters[i]];
+		if ((activeMonster.flags & MFLAG_TARGETS_MONSTER) == 0 && activeMonster.enemy == playerId) {
+			activeMonster.flags |= MFLAG_NO_ENEMY;
+		}
+	}
+}
+
 void ProcessMonsters()
 {
 	DeleteMonsterList();
@@ -4096,20 +4121,21 @@ void ProcessMonsters()
 			UpdateEnemy(monster);
 		}
 
-		if ((monster.flags & MFLAG_TARGETS_MONSTER) != 0) {
-			assert(monster.enemy >= 0 && monster.enemy < MaxMonsters);
-			// BUGFIX: enemy target may be dead at time of access, thus reading garbage data from `Monsters[monster.enemy].position.future`.
-			monster.position.last = Monsters[monster.enemy].position.future;
-			monster.enemyPosition = monster.position.last;
-		} else {
-			assert(monster.enemy >= 0 && monster.enemy < MAX_PLRS);
-			Player &player = Players[monster.enemy];
-			monster.enemyPosition = player.position.future;
-			if (IsTileVisible(monster.position.tile)) {
-				monster.activeForTicks = UINT8_MAX;
-				monster.position.last = player.position.future;
-			} else if (monster.activeForTicks != 0 && monster.type().type != MT_DIABLO) {
-				monster.activeForTicks--;
+		if ((monster.flags & MFLAG_NO_ENEMY) == 0) {
+			if ((monster.flags & MFLAG_TARGETS_MONSTER) != 0) {
+				assert(monster.enemy >= 0 && monster.enemy < MaxMonsters);
+				monster.position.last = Monsters[monster.enemy].position.future;
+				monster.enemyPosition = monster.position.last;
+			} else {
+				assert(monster.enemy >= 0 && monster.enemy < MAX_PLRS);
+				Player &player = Players[monster.enemy];
+				monster.enemyPosition = player.position.future;
+				if (IsTileVisible(monster.position.tile)) {
+					monster.activeForTicks = UINT8_MAX;
+					monster.position.last = player.position.future;
+				} else if (monster.activeForTicks != 0 && monster.type().type != MT_DIABLO) {
+					monster.activeForTicks--;
+				}
 			}
 		}
 		while (true) {

Source/monster.h

@@ -534,6 +534,7 @@ void PrepDoEnding();
 bool Walk(Monster &monster, Direction md);
 void GolumAi(Monster &monster);
 void DeleteMonsterList();
+void RemoveEnemyReferences(const Player &player);
 void ProcessMonsters();
 void FreeMonsters();
 bool DirOK(const Monster &monster, Direction mdir);

Source/msg.cpp

@@ -34,6 +34,7 @@
 #include "levels/trigs.h"
 #include "lighting.h"
 #include "missiles.h"
+#include "monster.h"
 #include "monsters/validation.hpp"
 #include "nthread.h"
 #include "objects.h"
@@ -2280,6 +2281,11 @@ size_t OnPlayerJoinLevel(const TCmdLocParam2 &message, Player &player)
 	}
 
 	if (player.plractive && &player != MyPlayer) {
+		if (player.isOnActiveLevel()) {
+			RemoveEnemyReferences(player);
+			RemovePlrMissiles(player);
+			FixPlrWalkTags(player);
+		}
 		player.position.tile = position;
 		SetPlayerOld(player);
 		if (isSetLevel)

Source/multi.cpp

@@ -21,6 +21,7 @@
 #include "engine/random.hpp"
 #include "engine/world_tile.hpp"
 #include "menu.h"
+#include "monster.h"
 #include "msg.h"
 #include "nthread.h"
 #include "options.h"
@@ -275,6 +276,7 @@ void PlayerLeftMsg(Player &player, bool left)
 	RemovePortalMissile(player);
 	DeactivatePortal(player);
 	delta_close_portal(player);
+	RemoveEnemyReferences(player);
 	RemovePlrMissiles(player);
 	if (left) {
 		std::string_view pszFmt = _("Player '{:s}' just left the game");

Source/player.cpp

@@ -36,6 +36,7 @@
 #include "loadsave.h"
 #include "minitext.h"
 #include "missiles.h"
+#include "monster.h"
 #include "nthread.h"
 #include "objects.h"
 #include "options.h"
@@ -358,6 +359,7 @@ void InitLevelChange(Player &player)
 {
 	Player &myPlayer = *MyPlayer;
 
+	RemoveEnemyReferences(player);
 	RemovePlrMissiles(player);
 	player.pManaShield = false;
 	player.wReflections = 0;