get values for builders in a safer way

paper-server/patches/sources/net/minecraft/core/RegistrySetBuilder.java.patch

@@ -12,3 +12,17 @@
              public ResourceKey<? extends Registry<? extends T>> key() {
                  return registryKey;
              }
+@@ -121,6 +_,13 @@
+                     public <T> Optional<RegistryOps.RegistryInfo<T>> lookup(ResourceKey<? extends Registry<? extends T>> registryKey) {
+                         return getEntry(registryKey).map(Entry::opsInfo);
+                     }
++
++                    // Paper start - add method to get the value for pre-filling builders in the reg mod API
++                    @Override
++                    public HolderLookup.Provider lookupForValueCopyViaBuilders() {
++                        throw new UnsupportedOperationException("Not implemented");
++                    }
++                    // Paper end - add method to get the value for pre-filling builders in the reg mod API
+                 });
+             }
+         };

paper-server/patches/sources/net/minecraft/resources/RegistryDataLoader.java.patch

@@ -1,5 +1,27 @@
 --- a/net/minecraft/resources/RegistryDataLoader.java
 +++ b/net/minecraft/resources/RegistryDataLoader.java
+@@ -179,11 +_,21 @@
+         final Map<ResourceKey<? extends Registry<?>>, RegistryOps.RegistryInfo<?>> map = new HashMap<>();
+         registryLookups.forEach(registryLookup -> map.put(registryLookup.key(), createInfoForContextRegistry((HolderLookup.RegistryLookup<?>)registryLookup)));
+         loaders.forEach(loader -> map.put(loader.registry.key(), createInfoForNewRegistry(loader.registry)));
++        // this creates a HolderLookup.Provider to be used exclusively to obtain real instances of values to pre-populate builders
++        // for the Registry Modification API. This concatenates the context registry lookups and the empty registries about to be filled.
++        final HolderLookup.Provider providerForBuilders = HolderLookup.Provider.create(java.util.stream.Stream.concat(registryLookups.stream(), loaders.stream().map(Loader::registry))); // Paper - add method to get the value for pre-filling builders in the reg mod API
+         return new RegistryOps.RegistryInfoLookup() {
+             @Override
+             public <T> Optional<RegistryOps.RegistryInfo<T>> lookup(ResourceKey<? extends Registry<? extends T>> registryKey) {
+                 return Optional.ofNullable((RegistryOps.RegistryInfo<T>)map.get(registryKey));
+             }
++
++            // Paper start - add method to get the value for pre-filling builders in the reg mod API
++            @Override
++            public HolderLookup.Provider lookupForValueCopyViaBuilders() {
++                return providerForBuilders;
++            }
++            // Paper end - add method to get the value for pre-filling builders in the reg mod API
+         };
+     }
+
 @@ -247,13 +_,14 @@
          RegistryOps<JsonElement> ops,
          ResourceKey<E> resourceKey,

paper-server/patches/sources/net/minecraft/resources/RegistryOps.java.patch

@@ -0,0 +1,24 @@
+--- a/net/minecraft/resources/RegistryOps.java
++++ b/net/minecraft/resources/RegistryOps.java
+@@ -117,6 +_,13 @@
+         public int hashCode() {
+             return this.lookupProvider.hashCode();
+         }
++
++        // Paper start - add method to get the value for pre-filling builders in the reg mod API
++        @Override
++        public HolderLookup.Provider lookupForValueCopyViaBuilders() {
++            return this.lookupProvider;
++        }
++        // Paper end - add method to get the value for pre-filling builders in the reg mod API
+     }
+
+     public record RegistryInfo<T>(HolderOwner<T> owner, HolderGetter<T> getter, Lifecycle elementsLifecycle) {
+@@ -127,5 +_,7 @@
+
+     public interface RegistryInfoLookup {
+         <T> Optional<RegistryOps.RegistryInfo<T>> lookup(ResourceKey<? extends Registry<? extends T>> registryKey);
++
++        HolderLookup.Provider lookupForValueCopyViaBuilders(); // Paper - add method to get the value for pre-filling builders in the reg mod API
+     }
+ }

paper-server/src/main/java/io/papermc/paper/registry/data/util/Conversions.java

@@ -7,13 +7,10 @@
 import io.papermc.paper.registry.PaperRegistryBuilder;
 import io.papermc.paper.registry.PaperRegistryBuilderFactory;
 import io.papermc.paper.registry.entry.RegistryEntryMeta;
-import java.util.Optional;
 import java.util.function.Consumer;
-import java.util.function.Function;
 import net.kyori.adventure.text.Component;
 import net.minecraft.core.Holder;
 import net.minecraft.core.HolderLookup;
-import net.minecraft.core.MappedRegistry;
 import net.minecraft.core.Registry;
 import net.minecraft.core.RegistryAccess;
 import net.minecraft.resources.RegistryOps;
@@ -87,14 +84,8 @@ public <M, A extends Keyed, B extends PaperRegistryBuilder<M, A>> Holder<M> crea
         final ResourceKey<? extends Registry<M>> registryKey,
         final RegistryEntryMeta.Buildable<M, A, B> buildableMeta
     ) {
-        final RegistryOps.RegistryInfo<M> info = this.lookup.lookup(registryKey).orElseThrow();
-        final Function<ResourceKey<M>, Optional<M>> existingValueGetter;
-        if (info.getter() instanceof final HolderLookup.RegistryLookup<M> registryLookup) {
-            existingValueGetter = registryLookup::getValueForCopying;
-        } else {
-            existingValueGetter = ((MappedRegistry<M>) info.owner())::getValueForCopying;
-        }
-        return new PaperRegistryBuilderFactory<>(registryKey, this, buildableMeta.builderFiller(), existingValueGetter);
+        final HolderLookup.RegistryLookup<M> lookupForBuilders = this.lookup.lookupForValueCopyViaBuilders().lookupOrThrow(registryKey);
+        return new PaperRegistryBuilderFactory<>(registryKey, this, buildableMeta.builderFiller(), lookupForBuilders::getValueForCopying);
     }
 
 }