PaperMC/paper-server/nms-patches/net/minecraft/world/entity/animal/EntityDolphin.patch

--- a/net/minecraft/world/entity/animal/EntityDolphin.java
+++ b/net/minecraft/world/entity/animal/EntityDolphin.java
@@ -233,6 +233,12 @@
             ItemStack itemstack = entityitem.getItem();
 
             if (this.canHoldItem(itemstack)) {
+                // CraftBukkit start - call EntityPickupItemEvent
+                if (org.bukkit.craftbukkit.event.CraftEventFactory.callEntityPickupItemEvent(this, entityitem, 0, false).isCancelled()) {
+                    return;
+                }
+                itemstack = entityitem.getItem(); // update ItemStack from event
+                // CraftBukkit end
                 this.onItemPickup(entityitem);
                 this.setItemSlot(EnumItemSlot.MAINHAND, itemstack);
                 this.handDropChances[EnumItemSlot.MAINHAND.getIndex()] = 2.0F;
@@ -391,7 +397,7 @@
 
         @Override
         public boolean canUse() {
-            return this.dolphin.gotFish() && this.dolphin.getAirSupply() >= 100;
+            return this.dolphin.gotFish() && this.dolphin.getAirSupply() >= 100 && this.dolphin.level.getWorld().canGenerateStructures(); // MC-151364, SPIGOT-5494: hangs if generate-structures=false
         }
 
         @Override
@@ -501,7 +507,7 @@
 
         @Override
         public void start() {
-            this.player.addEffect(new MobEffect(MobEffects.DOLPHINS_GRACE, 100), this.dolphin);
+            this.player.addEffect(new MobEffect(MobEffects.DOLPHINS_GRACE, 100), this.dolphin, org.bukkit.event.entity.EntityPotionEffectEvent.Cause.DOLPHIN); // CraftBukkit
         }
 
         @Override
@@ -520,7 +526,7 @@
             }
 
             if (this.player.isSwimming() && this.player.level.random.nextInt(6) == 0) {
-                this.player.addEffect(new MobEffect(MobEffects.DOLPHINS_GRACE, 100), this.dolphin);
+                this.player.addEffect(new MobEffect(MobEffects.DOLPHINS_GRACE, 100), this.dolphin, org.bukkit.event.entity.EntityPotionEffectEvent.Cause.DOLPHIN); // CraftBukkit
             }
 
         }