PaperMC/paper-server/patches/sources/net/minecraft/util/SpawnUtil.java.patch

--- a/net/minecraft/util/SpawnUtil.java
+++ b/net/minecraft/util/SpawnUtil.java
@@ -16,6 +_,7 @@
 import net.minecraft.world.level.block.state.BlockState;
 
 public class SpawnUtil {
+
     public static <T extends Mob> Optional<T> trySpawnMob(
         EntityType<T> entityType,
         EntitySpawnReason spawnReason,
@@ -27,6 +_,24 @@
         SpawnUtil.Strategy strategy,
         boolean checkCollision
     ) {
+        // CraftBukkit start
+        return trySpawnMob(entityType, spawnReason, level, pos, attempts, range, yOffset, strategy, checkCollision, org.bukkit.event.entity.CreatureSpawnEvent.SpawnReason.DEFAULT, null); // Paper - pre creature spawn event
+    }
+
+    public static <T extends Mob> Optional<T> trySpawnMob(
+        EntityType<T> entityType,
+        EntitySpawnReason spawnReason,
+        ServerLevel level,
+        BlockPos pos,
+        int attempts,
+        int range,
+        int yOffset,
+        SpawnUtil.Strategy strategy,
+        boolean checkCollision,
+        org.bukkit.event.entity.CreatureSpawnEvent.SpawnReason reason,
+        @javax.annotation.Nullable Runnable onAbort // Paper - pre creature spawn event
+    ) {
+        // CraftBukkit end
         BlockPos.MutableBlockPos mutableBlockPos = pos.mutable();
 
         for (int i = 0; i < attempts; i++) {
@@ -39,15 +_,32 @@
                     !checkCollision
                         || level.noCollision(entityType.getSpawnAABB(mutableBlockPos.getX() + 0.5, mutableBlockPos.getY(), mutableBlockPos.getZ() + 0.5))
                 )) {
+                // Paper start - PreCreatureSpawnEvent
+                final com.destroystokyo.paper.event.entity.PreCreatureSpawnEvent event = new com.destroystokyo.paper.event.entity.PreCreatureSpawnEvent(
+                    io.papermc.paper.util.MCUtil.toLocation(level, pos),
+                    org.bukkit.craftbukkit.entity.CraftEntityType.minecraftToBukkit(entityType),
+                    reason
+                );
+                if (!event.callEvent()) {
+                    if (event.shouldAbortSpawn()) {
+                        if (onAbort != null) {
+                            onAbort.run();
+                        }
+                        return Optional.empty();
+                    }
+                    break;
+                }
+                // Paper end - PreCreatureSpawnEvent
                 T mob = (T)entityType.create(level, null, mutableBlockPos, spawnReason, false, false);
                 if (mob != null) {
                     if (mob.checkSpawnRules(level, spawnReason) && mob.checkSpawnObstruction(level)) {
-                        level.addFreshEntityWithPassengers(mob);
+                        level.addFreshEntityWithPassengers(mob, reason); // CraftBukkit
+                        if (mob.isRemoved()) return Optional.empty(); // CraftBukkit
                         mob.playAmbientSound();
                         return Optional.of(mob);
                     }
 
-                    mob.discard();
+                    mob.discard(null); // CraftBukkit - add Bukkit remove cause
                 }
             }
         }