PaperMC/nms-patches/SpawnerCreature.patch

--- a/net/minecraft/server/SpawnerCreature.java
+++ b/net/minecraft/server/SpawnerCreature.java
@@ -7,6 +7,10 @@
 import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
 
+// CraftBukkit start
+import org.bukkit.event.entity.CreatureSpawnEvent.SpawnReason;
+// CraftBukkit end
+
 public final class SpawnerCreature {
 
     private static final Logger LOGGER = LogManager.getLogger();
@@ -88,9 +92,12 @@
                                                         entityinsentient.setPositionRotation((double) f, (double) k, (double) f1, worldserver.random.nextFloat() * 360.0F, 0.0F);
                                                         if ((d0 <= 16384.0D || !entityinsentient.isTypeNotPersistent(d0)) && entityinsentient.a((GeneratorAccess) worldserver, EnumMobSpawn.NATURAL) && entityinsentient.a((IWorldReader) worldserver)) {
                                                             groupdataentity = entityinsentient.prepare(worldserver, worldserver.getDamageScaler(new BlockPosition(entityinsentient)), EnumMobSpawn.NATURAL, groupdataentity, (NBTTagCompound) null);
-                                                            ++i;
-                                                            ++i2;
-                                                            worldserver.addEntity(entityinsentient);
+                                                            // CraftBukkit start
+                                                            if (worldserver.addEntity(entityinsentient, SpawnReason.NATURAL)) {
+                                                                ++i;
+                                                                ++i2;
+                                                            }
+                                                            // CraftBukkit end
                                                             if (i >= entityinsentient.getMaxSpawnGroup()) {
                                                                 return;
                                                             }
@@ -216,7 +223,7 @@
 
                                 if (entityinsentient.a(generatoraccess, EnumMobSpawn.CHUNK_GENERATION) && entityinsentient.a((IWorldReader) generatoraccess)) {
                                     groupdataentity = entityinsentient.prepare(generatoraccess, generatoraccess.getDamageScaler(new BlockPosition(entityinsentient)), EnumMobSpawn.CHUNK_GENERATION, groupdataentity, (NBTTagCompound) null);
-                                    generatoraccess.addEntity(entityinsentient);
+                                    generatoraccess.addEntity(entityinsentient, SpawnReason.CHUNK_GEN); // CraftBukkit
                                     flag = true;
                                 }
                             }