Support reloadable registries

generator/src/main/java/org/spongepowered/vanilla/generator/GeneratorMain.java

@@ -175,7 +175,6 @@ private static Pair<RegistryAccess.Frozen, ReloadableServerResources> loadVanill
         );
 
 
-        final RegistryAccess.Frozen compositeRegistries = withDimensions.getAccessForLoading(RegistryLayer.RELOADABLE);
         final var resourcesFuture = ReloadableServerResources.loadResources(
             resourceManager,
             withDimensions,
@@ -207,7 +206,7 @@ private static Pair<RegistryAccess.Frozen, ReloadableServerResources> loadVanill
         Logger.info("Datapack load complete");
 
         return Pair.of(
-            compositeRegistries,
+            resources.fullRegistries().get(),
             resources
         );
     }

src/main/java/org/spongepowered/common/registry/RegistryHolderLogic.java

@@ -220,4 +220,15 @@ public void freezeSpongeRootRegistry() {
     public void freezeSpongeDynamicRegistries() {
         this.roots.get(RegistryRoots.SPONGE).forEach(net.minecraft.core.Registry::freeze);
     }
+
+    public void reload(final RegistryAccess access) {
+        final WritableRegistry root = new MappedRegistry<>(
+            net.minecraft.resources.ResourceKey.createRegistryKey((ResourceLocation) (Object) RegistryRoots.MINECRAFT),
+            Lifecycle.experimental()
+        );
+
+        access.registries().forEach(entry -> root.register(entry.key(), entry.value(), RegistrationInfo.BUILT_IN));
+        root.freeze();
+        this.roots.put((ResourceKey) (Object) ResourceLocation.withDefaultNamespace("root"), root);
+    }
 }

src/mixins/java/org/spongepowered/common/mixin/api/minecraft/server/MinecraftServerMixin_API.java

@@ -170,7 +170,7 @@ public abstract class MinecraftServerMixin_API implements SpongeServer, SpongeRe
         this.api$playerDataHandler = new SpongePlayerDataManager(this);
         this.api$teleportHelper = new SpongeTeleportHelper();
         this.api$mapStorage = new SpongeMapStorage();
-        this.api$registryHolder = new RegistryHolderLogic($$3.registries().compositeAccess());
+        this.api$registryHolder = new RegistryHolderLogic($$3.dataPackResources().fullRegistries().get());
         this.api$userManager = new SpongeUserManager((MinecraftServer) (Object) this);
 
         this.api$dataPackManager = new SpongeDataPackManager((MinecraftServer) (Object) this, this.storageSource.getLevelPath(LevelResource.DATAPACK_DIR));

src/mixins/java/org/spongepowered/common/mixin/core/server/MinecraftServerMixin.java

@@ -33,6 +33,7 @@
 import net.minecraft.obfuscate.DontObfuscate;
 import net.minecraft.resources.ResourceKey;
 import net.minecraft.server.MinecraftServer;
+import net.minecraft.server.ReloadableServerRegistries;
 import net.minecraft.server.level.ServerLevel;
 import net.minecraft.server.packs.repository.PackRepository;
 import net.minecraft.server.players.GameProfileCache;
@@ -61,6 +62,7 @@
 import org.spongepowered.asm.mixin.Overwrite;
 import org.spongepowered.asm.mixin.Shadow;
 import org.spongepowered.asm.mixin.injection.At;
+import org.spongepowered.asm.mixin.injection.Coerce;
 import org.spongepowered.asm.mixin.injection.Constant;
 import org.spongepowered.asm.mixin.injection.Inject;
 import org.spongepowered.asm.mixin.injection.ModifyConstant;
@@ -81,6 +83,7 @@
 import org.spongepowered.common.config.inheritable.WorldConfig;
 import org.spongepowered.common.datapack.SpongeDataPackManager;
 import org.spongepowered.common.event.tracking.PhaseTracker;
+import org.spongepowered.common.registry.SpongeRegistryHolder;
 import org.spongepowered.common.service.server.SpongeServerScopedServiceProvider;
 
 import java.io.IOException;
@@ -114,6 +117,7 @@ public abstract class MinecraftServerMixin implements SpongeServer, MinecraftSer
     @Shadow public abstract WorldData shadow$getWorldData();
     @Shadow protected abstract void loadLevel(); // has overrides!
     @Shadow public abstract boolean shadow$haveTime();
+    @Shadow public abstract ReloadableServerRegistries.Holder shadow$reloadableRegistries();
     @Shadow private volatile boolean isSaving;
     // @formatter:on
 
@@ -387,6 +391,13 @@ public void setDifficulty(final Difficulty difficulty, final boolean forceDiffic
         this.shadow$getPackRepository().reload();
     }
 
+    // Targets lambda where ReloadableResources is set to server because at
+    // this point reloaded resources become "valid" in terms of registries
+    @Inject(method = "lambda$reloadResources$29", at = @At(value = "TAIL"))
+    private void impl$handleRegistriesReload(final Collection<String> $$0x, final @Coerce Object reloadableResources, final CallbackInfo ci) {
+        ((SpongeRegistryHolder) this).registryHolder().reload(this.shadow$reloadableRegistries().get());
+    }
+
     @Override
     public String toString() {
         return this.getClass().getSimpleName();