Add EntityConstructEvent

paper-api/src/main/java/io/papermc/paper/event/entity/EntityConstructEvent.java

@@ -0,0 +1,63 @@
+package io.papermc.paper.event.entity;
+
+import org.bukkit.block.Block;
+import org.bukkit.entity.Entity;
+import org.bukkit.entity.Golem;
+import org.bukkit.event.Cancellable;
+import org.bukkit.event.HandlerList;
+import org.bukkit.event.entity.EntityEvent;
+import org.bukkit.event.entity.EntitySpawnEvent;
+import org.jetbrains.annotations.ApiStatus;
+import org.jetbrains.annotations.Unmodifiable;
+import org.jspecify.annotations.NullMarked;
+import java.util.List;
+
+/**
+ * Called just before an {@link Entity} spawns due to a pattern of blocks being constructed (golems, the wither, etc.)
+ * <br>
+ * Note: This event is fired before {@link EntitySpawnEvent}, before the entity is added to the world,
+ * the success of this event does not guarantee the entity will actually spawn.
+ */
+@NullMarked
+public class EntityConstructEvent extends EntityEvent implements Cancellable {
+
+    private static final HandlerList HANDLER_LIST = new HandlerList();
+
+    private final List<Block> blocks;
+    private boolean cancelled;
+
+    @ApiStatus.Internal
+    public EntityConstructEvent(Entity entity, List<Block> blocks) {
+        super(entity);
+        this.blocks = List.copyOf(blocks);
+    }
+
+    /**
+     * Get an immutable list of the blocks required for this construction, including
+     * any required air blocks.
+     * 
+     * @return the blocks
+     */
+    public @Unmodifiable List<Block> getBlocks() {
+        return blocks;
+    }
+
+    @Override
+    public void setCancelled(final boolean cancel) {
+        this.cancelled = cancel;
+    }
+
+    @Override
+    public boolean isCancelled() {
+        return cancelled;
+    }
+
+    @Override
+    public HandlerList getHandlers() {
+        return HANDLER_LIST;
+    }
+
+    public static HandlerList getHandlerList() {
+        return HANDLER_LIST;
+    }
+}

paper-server/patches/sources/net/minecraft/world/level/block/CarvedPumpkinBlock.java.patch

@@ -8,7 +8,7 @@
                  this.replaceCopperBlockWithChest(level, copperGolemMatch);
                  copperGolem.spawn(this.getWeatherStateFromPattern(copperGolemMatch));
              }
-@@ -105,9 +_,22 @@
+@@ -105,9 +_,25 @@
      }
 
      private static void spawnGolemInWorld(final Level level, final BlockPattern.BlockPatternMatch match, final Entity golem, final BlockPos spawnPos) {
@@ -17,6 +17,9 @@
          golem.snapTo(spawnPos.getX() + 0.5, spawnPos.getY() + 0.05, spawnPos.getZ() + 0.5, 0.0F, 0.0F);
 -        level.addFreshEntity(golem);
 +        // Paper start
++        if (!new io.papermc.paper.event.entity.EntityConstructEvent(golem.getBukkitEntity(), org.bukkit.craftbukkit.block.CraftBlock.getMatchingBlocks(level, match)).callEvent()) {
++            return;
++        }
 +        org.bukkit.event.entity.CreatureSpawnEvent.SpawnReason spawnReason;
 +        if (golem.getType() == net.minecraft.world.entity.EntityType.SNOW_GOLEM) {
 +            spawnReason = org.bukkit.event.entity.CreatureSpawnEvent.SpawnReason.BUILD_SNOWMAN;

paper-server/patches/sources/net/minecraft/world/level/block/WitherSkullBlock.java.patch

@@ -17,10 +17,15 @@
                          BlockPos spawnPos = match.getBlock(1, 2, 0).getPos();
                          witherBoss.snapTo(
                              spawnPos.getX() + 0.5,
-@@ -68,12 +_,18 @@
+@@ -68,12 +_,23 @@
                          );
                          witherBoss.yBodyRot = match.getForwards().getAxis() == Direction.Axis.X ? 0.0F : 90.0F;
                          witherBoss.makeInvulnerable();
++                        // Paper start
++                        if (!new io.papermc.paper.event.entity.EntityConstructEvent(witherBoss.getBukkitEntity(), org.bukkit.craftbukkit.block.CraftBlock.getMatchingBlocks(level, match)).callEvent()) {
++                            return;
++                        }
++                        // Paper end
 +                        // CraftBukkit start
 +                        if (!level.addFreshEntity(witherBoss, org.bukkit.event.entity.CreatureSpawnEvent.SpawnReason.BUILD_WITHER)) {
 +                            return;

paper-server/src/main/java/org/bukkit/craftbukkit/block/CraftBlock.java

@@ -23,6 +23,8 @@
 import net.minecraft.world.level.block.RedStoneWireBlock;
 import net.minecraft.world.level.block.SaplingBlock;
 import net.minecraft.world.level.block.state.BlockState;
+import net.minecraft.world.level.block.state.pattern.BlockInWorld;
+import net.minecraft.world.level.block.state.pattern.BlockPattern;
 import net.minecraft.world.level.redstone.Redstone;
 import net.minecraft.world.phys.AABB;
 import net.minecraft.world.phys.BlockHitResult;
@@ -297,6 +299,17 @@ public static BlockFace notchToBlockFace(@Nullable Direction direction) {
         };
     }
 
+    public static List<Block> getMatchingBlocks(LevelAccessor level, BlockPattern.BlockPatternMatch match) {
+        List<Block> blocks = new ArrayList<>();
+        for (int x = 0; x < match.getWidth(); x++) {
+            for (int y = 0; y < match.getHeight(); y++) {
+                BlockInWorld block = match.getBlock(x, y, 0);
+                blocks.add(at(level, block.getPos()));
+            }
+        }
+        return blocks;
+    }
+
     @Override
     public org.bukkit.block.BlockState getState() {
         return CraftBlockStates.getBlockState(this);