SPIGOT-6186: Canceling a CreatureSpawnEvent​ results in a "Unable to summon entity due to duplicate UUIDs" error

By: md_5 <git@md-5.net>

paper-server/nms-patches/EntityTypes.patch

@@ -18,7 +18,7 @@
          return (EntityTypes) IRegistry.a((IRegistry) IRegistry.ENTITY_TYPE, s, (Object) entitytypes_builder.a(s));
      }
  
-@@ -169,10 +169,17 @@
+@@ -169,10 +169,18 @@
  
      @Nullable
      public T spawnCreature(WorldServer worldserver, @Nullable NBTTagCompound nbttagcompound, @Nullable IChatBaseComponent ichatbasecomponent, @Nullable EntityHuman entityhuman, BlockPosition blockposition, EnumMobSpawn enummobspawn, boolean flag, boolean flag1) {
@@ -32,12 +32,13 @@
  
          if (t0 != null) {
 -            worldserver.addAllEntities(t0);
-+            return worldserver.addAllEntities(t0, spawnReason) ? t0 : null; // Don't return an entity when CreatureSpawnEvent is canceled
++            worldserver.addAllEntities(t0, spawnReason);
++            return !t0.dead ? t0 : null; // Don't return an entity when CreatureSpawnEvent is canceled
 +            // CraftBukkit end
          }
  
          return t0;
-@@ -208,7 +215,7 @@
+@@ -208,7 +216,7 @@
                  t0.setCustomName(ichatbasecomponent);
              }
  
@@ -46,7 +47,7 @@
              return t0;
          }
      }
-@@ -351,7 +358,7 @@
+@@ -351,7 +359,7 @@
              }
  
              return entity;
@@ -55,7 +56,7 @@
      }
  
      private static Optional<Entity> b(NBTTagCompound nbttagcompound, World world) {
-@@ -403,7 +410,7 @@
+@@ -403,7 +411,7 @@
              this.g = enumcreaturetype == EnumCreatureType.CREATURE || enumcreaturetype == EnumCreatureType.MISC;
          }