Core/AI: Fix crashes caused by charmed Creatures having null AI for 1 map update tick (#27434)

Implement using ScheduledChangeAI instead of nullptr to signal a required AI change

5 files changed, 86 insertions, 4 deletions

diff --git a/src/server/game/AI/CoreAI/ScheduledChangeAI.cpp b/src/server/game/AI/CoreAI/ScheduledChangeAI.cpp
new file mode 100644
index 00000000000..c4f513fcfa6
--- /dev/null
+++ b/src/server/game/AI/CoreAI/ScheduledChangeAI.cpp
@@ -0,0 +1,21 @@
+/*
+ * This file is part of the TrinityCore Project. See AUTHORS file for Copyright information
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the
+ * Free Software Foundation; either version 2 of the License, or (at your
+ * option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "ScheduledChangeAI.h"
+
+ScheduledChangeAI::ScheduledChangeAI(Creature* creature): CreatureAI(creature)
+{ }
diff --git a/src/server/game/AI/CoreAI/ScheduledChangeAI.h b/src/server/game/AI/CoreAI/ScheduledChangeAI.h
new file mode 100644
index 00000000000..79b8344eab0
--- /dev/null
+++ b/src/server/game/AI/CoreAI/ScheduledChangeAI.h
@@ -0,0 +1,40 @@
+/*
+ * This file is part of the TrinityCore Project. See AUTHORS file for Copyright information
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the
+ * Free Software Foundation; either version 2 of the License, or (at your
+ * option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
+ * more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#ifndef TRINITY_SCHEDULEDCHANGEAI_H
+#define TRINITY_SCHEDULEDCHANGEAI_H
+
+#include "CreatureAI.h"
+
+class TC_GAME_API ScheduledChangeAI final : public CreatureAI
+{
+    public:
+        explicit ScheduledChangeAI(Creature* creature);
+
+        void MoveInLineOfSight(Unit*) override { }
+        void AttackStart(Unit*) override { }
+        void JustStartedThreateningMe(Unit*) override { }
+        void JustEnteredCombat(Unit*) override { }
+        void UpdateAI(uint32) override { }
+        void JustAppeared() override { }
+        void EnterEvadeMode(EvadeReason /*why*/) override { }
+        void OnCharmed(bool /*isNew*/) override { }
+
+        static int Permissible(Creature const* /*creature*/) { return PERMIT_BASE_NO; }
+};
+
+#endif
diff --git a/src/server/game/AI/CreatureAIRegistry.cpp b/src/server/game/AI/CreatureAIRegistry.cpp
index 2ae35369966..50b9c5b34c8 100644
--- a/src/server/game/AI/CreatureAIRegistry.cpp
+++ b/src/server/game/AI/CreatureAIRegistry.cpp
@@ -23,6 +23,7 @@
 #include "PassiveAI.h"
 #include "PetAI.h"
 #include "ReactorAI.h"
+#include "ScheduledChangeAI.h"
 #include "SmartAI.h"
 #include "TotemAI.h"
 
@@ -46,6 +47,7 @@ namespace AIRegistry
         (new CreatureAIFactory<TurretAI>("TurretAI"))->RegisterSelf();
         (new CreatureAIFactory<VehicleAI>("VehicleAI"))->RegisterSelf();
         (new CreatureAIFactory<SmartAI>("SmartAI"))->RegisterSelf();
+        (new CreatureAIFactory<ScheduledChangeAI, false>("ScheduledChangeAI"))->RegisterSelf();
 
         (new GameObjectAIFactory<NullGameObjectAI>("NullGameObjectAI"))->RegisterSelf();
         (new GameObjectAIFactory<GameObjectAI>("GameObjectAI"))->RegisterSelf();
diff --git a/src/server/game/Entities/Unit/Unit.cpp b/src/server/game/Entities/Unit/Unit.cpp
index eda7b604fe6..278087e4890 100644
--- a/src/server/game/Entities/Unit/Unit.cpp
+++ b/src/server/game/Entities/Unit/Unit.cpp
@@ -60,6 +60,7 @@
 #include "PlayerAI.h"
 #include "QuestDef.h"
 #include "ReputationMgr.h"
+#include "ScheduledChangeAI.h"
 #include "SpellAuraEffects.h"
 #include "SpellAuras.h"
 #include "Spell.h"
@@ -491,7 +492,7 @@ void Unit::Update(uint32 p_time)
     // All position info based actions have been executed, reset info
     _positionUpdateInfo.Reset();
 
-    if (!GetAI() && (GetTypeId() != TYPEID_PLAYER || (IsCharmed() && GetCharmerGUID().IsCreature())))
+    if (HasScheduledAIChange() && (GetTypeId() != TYPEID_PLAYER || (IsCharmed() && GetCharmerGUID().IsCreature())))
         UpdateCharmAI();
     RefreshAI();
 }
@@ -9524,11 +9525,11 @@ void Unit::ScheduleAIChange()
     bool const charmed = IsCharmed();
 
     if (charmed)
-        PushAI(nullptr);
+        PushAI(GetScheduledChangeAI());
     else
     {
         RestoreDisabledAI();
-        PushAI(nullptr); //This could actually be PopAI() to get the previous AI but it's required atm to trigger UpdateCharmAI()
+        PushAI(GetScheduledChangeAI()); //This could actually be PopAI() to get the previous AI but it's required atm to trigger UpdateCharmAI()
     }
 }
 
@@ -9536,10 +9537,26 @@ void Unit::RestoreDisabledAI()
 {
     // Keep popping the stack until we either reach the bottom or find a valid AI
     while (PopAI())
-        if (GetTopAI())
+        if (GetTopAI() && dynamic_cast<ScheduledChangeAI*>(GetTopAI()) == nullptr)
             return;
 }
 
+UnitAI* Unit::GetScheduledChangeAI()
+{
+    if (Creature* creature = ToCreature())
+        return new ScheduledChangeAI(creature);
+    else
+        return nullptr;
+}
+
+bool Unit::HasScheduledAIChange() const
+{
+    if (UnitAI* ai = GetAI())
+        return dynamic_cast<ScheduledChangeAI*>(ai) != nullptr;
+    else
+        return true;
+}
+
 void Unit::AddToWorld()
 {
     if (IsInWorld())
diff --git a/src/server/game/Entities/Unit/Unit.h b/src/server/game/Entities/Unit/Unit.h
index ea63c22ea3a..07f46b6a7c7 100644
--- a/src/server/game/Entities/Unit/Unit.h
+++ b/src/server/game/Entities/Unit/Unit.h
@@ -804,6 +804,8 @@ class TC_GAME_API Unit : public WorldObject
         void SetAI(UnitAI* newAI);
         UnitAI* GetTopAI() const { return i_AIs.empty() ? nullptr : i_AIs.top().get(); }
         void RefreshAI();
+        UnitAI* GetScheduledChangeAI();
+        bool HasScheduledAIChange() const;
     public:
 
         void AddToWorld() override;