Fix incorrect invulnerability damage reduction

Fixes incorrect spigot handling of the invulnerability damage
reduction applied when an already invulnerable entity is damaged with a
larger damage amount than the initial damage.
Vanilla still damages entities even if invulnerable if the damage to be
applied is larger than the previous damage taken. In that case, vanilla
applies the difference between the previous damage taken and the
proposed damage.

Spigot's damage modifier API takes over the computation of damage
reducing effects, however spigot invokes this handling with the initial
damage before computing the difference to the previous damage amount.
This leads to the reduction values to generally be larger than expected,
as they are computed on the not-yet-reduced value.
Spigot applies these reductions after calling the EntityDamageEvent and
*then* subtracts the previous damage point, leading to the final damage
amount being smaller than expected.

This patch cannot simply call the EntityDamageEvent with the reduced
damage, as that would lead to EntityDamageEvent#getDamage() returning
the already reduced damage, which breaks its method contract.
Instead, this patch makes use of the DamageModifier API, implementing
the last-damage-reduction as a DamageModifier.

patches/api/0496-Fix-incorrect-invulnerability-damage-reduction.patch

@@ -0,0 +1,40 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Bjarne Koll <lynxplay101@gmail.com>
+Date: Mon, 28 Oct 2024 15:58:50 +0100
+Subject: [PATCH] Fix incorrect invulnerability damage reduction
+
+Fixes incorrect spigot handling of the invulnerability damage
+reduction applied when an already invulnerable entity is damaged with a
+larger damage amount than the initial damage.
+Vanilla still damages entities even if invulnerable if the damage to be
+applied is larger than the previous damage taken. In that case, vanilla
+applies the difference between the previous damage taken and the
+proposed damage.
+
+Spigot's damage modifier API takes over the computation of damage
+reducing effects, however spigot invokes this handling with the initial
+damage before computing the difference to the previous damage amount.
+This leads to the reduction values to generally be larger than expected,
+as they are computed on the not-yet-reduced value.
+Spigot applies these reductions after calling the EntityDamageEvent and
+*then* subtracts the previous damage point, leading to the final damage
+amount being smaller than expected.
+
+This patch cannot simply call the EntityDamageEvent with the reduced
+damage, as that would lead to EntityDamageEvent#getDamage() returning
+the already reduced damage, which breaks its method contract.
+Instead, this patch makes use of the DamageModifier API, implementing
+the last-damage-reduction as a DamageModifier.
+
+diff --git a/src/main/java/org/bukkit/event/entity/EntityDamageEvent.java b/src/main/java/org/bukkit/event/entity/EntityDamageEvent.java
+index ef5b2a0f18c1c126db0b0c4a4d2a57483680665a..da04d15af08d3dfa9c92ebfb03667ab01c528c0e 100644
+--- a/src/main/java/org/bukkit/event/entity/EntityDamageEvent.java
++++ b/src/main/java/org/bukkit/event/entity/EntityDamageEvent.java
+@@ -247,6 +247,7 @@ public class EntityDamageEvent extends EntityEvent implements Cancellable {
+          * raw {@link EntityDamageEvent#getDamage()}.
+          */
+         BASE,
++        INVULNERABILITY_REDUCTION, // Paper - fix invulnerability reduction in EntityDamageEvent
+         /**
+          * This represents the damage increased by freezing status.
+          */

patches/server/1065-Fix-incorrect-invulnerability-damage-reduction.patch

@@ -0,0 +1,115 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Bjarne Koll <lynxplay101@gmail.com>
+Date: Mon, 28 Oct 2024 15:59:44 +0100
+Subject: [PATCH] Fix incorrect invulnerability damage reduction
+
+Fixes incorrect spigot handling of the invulnerability damage
+reduction applied when an already invulnerable entity is damaged with a
+larger damage amount than the initial damage.
+Vanilla still damages entities even if invulnerable if the damage to be
+applied is larger than the previous damage taken. In that case, vanilla
+applies the difference between the previous damage taken and the
+proposed damage.
+
+Spigot's damage modifier API takes over the computation of damage
+reducing effects, however spigot invokes this handling with the initial
+damage before computing the difference to the previous damage amount.
+This leads to the reduction values to generally be larger than expected,
+as they are computed on the not-yet-reduced value.
+Spigot applies these reductions after calling the EntityDamageEvent and
+*then* subtracts the previous damage point, leading to the final damage
+amount being smaller than expected.
+
+This patch cannot simply call the EntityDamageEvent with the reduced
+damage, as that would lead to EntityDamageEvent#getDamage() returning
+the already reduced damage, which breaks its method contract.
+Instead, this patch makes use of the DamageModifier API, implementing
+the last-damage-reduction as a DamageModifier.
+
+diff --git a/src/main/java/net/minecraft/world/entity/LivingEntity.java b/src/main/java/net/minecraft/world/entity/LivingEntity.java
+index e86314de8d908a0c3e9f17d3e163c11180cf3f59..c3d50a8448e95e3eccb494d8a678554d52f6ec72 100644
+--- a/src/main/java/net/minecraft/world/entity/LivingEntity.java
++++ b/src/main/java/net/minecraft/world/entity/LivingEntity.java
+@@ -1505,12 +1505,12 @@ public abstract class LivingEntity extends Entity implements Attackable {
+                 }
+ 
+                 // Paper start - only call damage event when actuallyHurt will be called - move call logic down
+-                event = this.handleEntityDamage(source, amount);
++                event = this.handleEntityDamage(source, amount, this.lastHurt); // Paper - fix invulnerability reduction in EntityDamageEvent - pass lastDamage reduction
+                 amount = computeAmountFromEntityDamageEvent(event);
+                 // Paper end - only call damage event when actuallyHurt will be called - move call logic down
+ 
+                 // CraftBukkit start
+-                if (!this.actuallyHurt(world, source, (float) event.getFinalDamage() - this.lastHurt, event)) {
++                if (!this.actuallyHurt(world, source, (float) event.getFinalDamage(), event)) { // Paper - fix invulnerability reduction in EntityDamageEvent - no longer subtract lastHurt, that is part of the damage event calc now
+                     return false;
+                 }
+                 if (this instanceof ServerPlayer && event.getDamage() == 0 && originalAmount == 0) return false; // Paper - revert to vanilla damage - players are not affected by damage that is 0 - skip damage if the vanilla damage is 0 and was not modified by plugins in the event.
+@@ -1519,7 +1519,7 @@ public abstract class LivingEntity extends Entity implements Attackable {
+                 flag1 = false;
+             } else {
+                 // Paper start - only call damage event when actuallyHurt will be called - move call logic down
+-                event = this.handleEntityDamage(source, amount);
++                event = this.handleEntityDamage(source, amount, 0); // Paper - fix invulnerability reduction in EntityDamageEvent - pass lastDamage reduction (none in this branch)
+                 amount = computeAmountFromEntityDamageEvent(event);
+                 // Paper end - only call damage event when actuallyHurt will be called - move call logic down
+                 // CraftBukkit start
+@@ -2322,8 +2322,19 @@ public abstract class LivingEntity extends Entity implements Attackable {
+     }
+ 
+     // CraftBukkit start
+-    private EntityDamageEvent handleEntityDamage(final DamageSource damagesource, float f) {
++    private EntityDamageEvent handleEntityDamage(final DamageSource damagesource, float f, final float invulnerabilityRelatedLastDamage) { // Paper - fix invulnerability reduction in EntityDamageEvent
+         float originalDamage = f;
++        // Paper start - fix invulnerability reduction in EntityDamageEvent
++        final com.google.common.base.Function<Double, Double> invulnerabilityReductionEquation = d -> {
++            if (invulnerabilityRelatedLastDamage == 0) return 0D; // no last damage, no reduction
++            // last damage existed, this means the reduction *technically* is (new damage - last damage).
++            // If the event damage was changed to something less than invul damage, hard lock it at 0.
++            if (d < invulnerabilityRelatedLastDamage) return 0D;
++            return (double) -invulnerabilityRelatedLastDamage;
++        };
++        final float originalInvulnerabilityReduction = invulnerabilityReductionEquation.apply((double) f).floatValue();
++        f += originalInvulnerabilityReduction;
++        // Paper end - fix invulnerability reduction in EntityDamageEvent
+ 
+         com.google.common.base.Function<Double, Double> freezing = new com.google.common.base.Function<Double, Double>() {
+             @Override
+@@ -2400,7 +2411,12 @@ public abstract class LivingEntity extends Entity implements Attackable {
+         };
+         float absorptionModifier = absorption.apply((double) f).floatValue();
+ 
+-        return CraftEventFactory.handleLivingEntityDamageEvent(this, damagesource, originalDamage, freezingModifier, hardHatModifier, blockingModifier, armorModifier, resistanceModifier, magicModifier, absorptionModifier, freezing, hardHat, blocking, armor, resistance, magic, absorption);
++        // Paper start - fix invulnerability reduction in EntityDamageEvent
++        return CraftEventFactory.handleLivingEntityDamageEvent(this, damagesource, originalDamage, freezingModifier, hardHatModifier, blockingModifier, armorModifier, resistanceModifier, magicModifier, absorptionModifier, freezing, hardHat, blocking, armor, resistance, magic, absorption, (damageModifierDoubleMap, damageModifierFunctionMap) -> {
++            damageModifierFunctionMap.put(DamageModifier.INVULNERABILITY_REDUCTION, invulnerabilityReductionEquation);
++            damageModifierDoubleMap.put(DamageModifier.INVULNERABILITY_REDUCTION, (double) originalInvulnerabilityReduction);
++        });
++        // Paper end - fix invulnerability reduction in EntityDamageEvent
+     }
+ 
+     protected boolean actuallyHurt(ServerLevel worldserver, final DamageSource damagesource, float f, final EntityDamageEvent event) { // void -> boolean, add final
+diff --git a/src/main/java/org/bukkit/craftbukkit/event/CraftEventFactory.java b/src/main/java/org/bukkit/craftbukkit/event/CraftEventFactory.java
+index deba03eb37012c638e08e20cd1c98e9db190c790..e37aaf77f94b97b736cc20ef070cefdff0400188 100644
+--- a/src/main/java/org/bukkit/craftbukkit/event/CraftEventFactory.java
++++ b/src/main/java/org/bukkit/craftbukkit/event/CraftEventFactory.java
+@@ -1218,6 +1218,11 @@ public class CraftEventFactory {
+     private static final Function<? super Double, Double> ZERO = Functions.constant(-0.0);
+ 
+     public static EntityDamageEvent handleLivingEntityDamageEvent(Entity damagee, DamageSource source, double rawDamage, double freezingModifier, double hardHatModifier, double blockingModifier, double armorModifier, double resistanceModifier, double magicModifier, double absorptionModifier, Function<Double, Double> freezing, Function<Double, Double> hardHat, Function<Double, Double> blocking, Function<Double, Double> armor, Function<Double, Double> resistance, Function<Double, Double> magic, Function<Double, Double> absorption) {
++    // Paper start - fix invulnerability reduction in EntityDamageEvent
++        return handleLivingEntityDamageEvent(damagee, source, rawDamage, freezingModifier, hardHatModifier, blockingModifier, armorModifier, resistanceModifier, magicModifier, absorptionModifier, freezing, hardHat, blocking, armor, resistance, magic, absorption, null);
++    }
++    public static EntityDamageEvent handleLivingEntityDamageEvent(Entity damagee, DamageSource source, double rawDamage, double freezingModifier, double hardHatModifier, double blockingModifier, double armorModifier, double resistanceModifier, double magicModifier, double absorptionModifier, Function<Double, Double> freezing, Function<Double, Double> hardHat, Function<Double, Double> blocking, Function<Double, Double> armor, Function<Double, Double> resistance, Function<Double, Double> magic, Function<Double, Double> absorption, java.util.function.BiConsumer<Map<DamageModifier, Double>, Map<DamageModifier, Function<? super Double, Double>>> callback) {
++    // Paper end - fix invulnerability reduction in EntityDamageEvent
+         Map<DamageModifier, Double> modifiers = new EnumMap<>(DamageModifier.class);
+         Map<DamageModifier, Function<? super Double, Double>> modifierFunctions = new EnumMap<>(DamageModifier.class);
+         modifiers.put(DamageModifier.BASE, rawDamage);
+@@ -1242,6 +1247,7 @@ public class CraftEventFactory {
+         modifierFunctions.put(DamageModifier.MAGIC, magic);
+         modifiers.put(DamageModifier.ABSORPTION, absorptionModifier);
+         modifierFunctions.put(DamageModifier.ABSORPTION, absorption);
++        if (callback != null) callback.accept(modifiers, modifierFunctions); // Paper - fix invulnerability reduction in EntityDamageEvent
+         return CraftEventFactory.handleEntityDamageEvent(damagee, source, modifiers, modifierFunctions);
+     }
+