Tag Lifecycle Events (#10993)

* wip tags

* use generics in tag registrars

* comment out varargs methods for now

* split up patch

* cache loaded service provider

* finish renames

* use builderWithExpectedSize

* finalize

patches/api/0443-Add-Lifecycle-Event-system.patch

@@ -8,7 +8,7 @@ meant for managing resources across reloads and from points in the
 PluginBootstrap.
 
 diff --git a/src/main/java/io/papermc/paper/plugin/bootstrap/BootstrapContext.java b/src/main/java/io/papermc/paper/plugin/bootstrap/BootstrapContext.java
-index 08f2050356acaf74e3210416760e3873c2dafd2c..37dfdcfcbd14947e0550e7528aca68f452e53eb6 100644
+index 70d5f9802f90605a5120ff2a000a2e9395f0aecc..c319f3bbfabdd39ee571c59d7bfc144c58232abc 100644
 --- a/src/main/java/io/papermc/paper/plugin/bootstrap/BootstrapContext.java
 +++ b/src/main/java/io/papermc/paper/plugin/bootstrap/BootstrapContext.java
 @@ -1,6 +1,9 @@
@@ -447,23 +447,27 @@ index 0000000000000000000000000000000000000000..92ea0374079a228ccc59c00fcf58abff
 +}
 diff --git a/src/main/java/io/papermc/paper/plugin/lifecycle/event/types/LifecycleEventTypeProvider.java b/src/main/java/io/papermc/paper/plugin/lifecycle/event/types/LifecycleEventTypeProvider.java
 new file mode 100644
-index 0000000000000000000000000000000000000000..1588f6943a909bed053a952e650e043c44028c2d
+index 0000000000000000000000000000000000000000..3b17f056d162dd42cae0d33cbbb22fece82fe525
 --- /dev/null
 +++ b/src/main/java/io/papermc/paper/plugin/lifecycle/event/types/LifecycleEventTypeProvider.java
-@@ -0,0 +1,18 @@
+@@ -0,0 +1,22 @@
 +package io.papermc.paper.plugin.lifecycle.event.types;
 +
 +import io.papermc.paper.plugin.lifecycle.event.LifecycleEvent;
 +import io.papermc.paper.plugin.lifecycle.event.LifecycleEventOwner;
++import java.util.Optional;
 +import java.util.ServiceLoader;
 +import org.jetbrains.annotations.ApiStatus;
 +
 +@ApiStatus.Internal
 +interface LifecycleEventTypeProvider {
 +
-+    LifecycleEventTypeProvider PROVIDER = ServiceLoader.load(LifecycleEventTypeProvider.class)
-+        .findFirst()
-+        .orElseThrow();
++    Optional<LifecycleEventTypeProvider> INSTANCE = ServiceLoader.load(LifecycleEventTypeProvider.class)
++        .findFirst();
++
++    static LifecycleEventTypeProvider provider() {
++        return INSTANCE.orElseThrow();
++    }
 +
 +    <O extends LifecycleEventOwner, E extends LifecycleEvent> LifecycleEventType.Monitorable<O, E> monitor(String name, Class<? extends O> ownerType);
 +
@@ -471,7 +475,7 @@ index 0000000000000000000000000000000000000000..1588f6943a909bed053a952e650e043c
 +}
 diff --git a/src/main/java/io/papermc/paper/plugin/lifecycle/event/types/LifecycleEvents.java b/src/main/java/io/papermc/paper/plugin/lifecycle/event/types/LifecycleEvents.java
 new file mode 100644
-index 0000000000000000000000000000000000000000..304f978e40e1759bb19704cc5cec399500905195
+index 0000000000000000000000000000000000000000..29476a1a82b3526ac03f21011d297d01462d153e
 --- /dev/null
 +++ b/src/main/java/io/papermc/paper/plugin/lifecycle/event/types/LifecycleEvents.java
 @@ -0,0 +1,52 @@
@@ -494,33 +498,33 @@ index 0000000000000000000000000000000000000000..304f978e40e1759bb19704cc5cec3995
 +
 +    //<editor-fold desc="helper methods" defaultstate="collapsed">
 +    @ApiStatus.Internal
-+    private static <E extends LifecycleEvent> LifecycleEventType.Monitorable<Plugin, E> plugin(final String name) {
++    static <E extends LifecycleEvent> LifecycleEventType.Monitorable<Plugin, E> plugin(final String name) {
 +        return monitor(name, Plugin.class);
 +    }
 +
 +    @ApiStatus.Internal
-+    private static <E extends LifecycleEvent> LifecycleEventType.Prioritizable<Plugin, E> pluginPrioritized(final String name) {
++    static <E extends LifecycleEvent> LifecycleEventType.Prioritizable<Plugin, E> pluginPrioritized(final String name) {
 +        return prioritized(name, Plugin.class);
 +    }
 +
 +    @ApiStatus.Internal
-+    private static <E extends LifecycleEvent> LifecycleEventType.Monitorable<BootstrapContext, E> bootstrap(final String name) {
++    static <E extends LifecycleEvent> LifecycleEventType.Monitorable<BootstrapContext, E> bootstrap(final String name) {
 +        return monitor(name, BootstrapContext.class);
 +    }
 +
 +    @ApiStatus.Internal
-+    private static <E extends LifecycleEvent> LifecycleEventType.Prioritizable<BootstrapContext, E> bootstrapPrioritized(final String name) {
++    static <E extends LifecycleEvent> LifecycleEventType.Prioritizable<BootstrapContext, E> bootstrapPrioritized(final String name) {
 +        return prioritized(name, BootstrapContext.class);
 +    }
 +
 +    @ApiStatus.Internal
-+    private static <O extends LifecycleEventOwner, E extends LifecycleEvent, O2 extends O> LifecycleEventType.Monitorable<O, E> monitor(final String name, final Class<O2> ownerType) {
-+        return LifecycleEventTypeProvider.PROVIDER.monitor(name, ownerType);
++    static <O extends LifecycleEventOwner, E extends LifecycleEvent, O2 extends O> LifecycleEventType.Monitorable<O, E> monitor(final String name, final Class<O2> ownerType) {
++        return LifecycleEventTypeProvider.provider().monitor(name, ownerType);
 +    }
 +
 +    @ApiStatus.Internal
-+    private static <O extends LifecycleEventOwner, E extends LifecycleEvent> LifecycleEventType.Prioritizable<O, E> prioritized(final String name, final Class<? extends O> ownerType) {
-+        return LifecycleEventTypeProvider.PROVIDER.prioritized(name, ownerType);
++    static <O extends LifecycleEventOwner, E extends LifecycleEvent> LifecycleEventType.Prioritizable<O, E> prioritized(final String name, final Class<? extends O> ownerType) {
++        return LifecycleEventTypeProvider.provider().prioritized(name, ownerType);
 +    }
 +    //</editor-fold>
 +

patches/api/0468-Brigadier-based-command-API.patch

@@ -1540,7 +1540,7 @@ index 0000000000000000000000000000000000000000..159b691e7a1a7066f3e706e80d75ca8f
 +}
 diff --git a/src/main/java/io/papermc/paper/command/brigadier/argument/VanillaArgumentProvider.java b/src/main/java/io/papermc/paper/command/brigadier/argument/VanillaArgumentProvider.java
 new file mode 100644
-index 0000000000000000000000000000000000000000..e8c1c0f781bfc852af4a0f01452566ae994dc285
+index 0000000000000000000000000000000000000000..a36a36a46b5d71489e1b3e183d48e6e83721ebdf
 --- /dev/null
 +++ b/src/main/java/io/papermc/paper/command/brigadier/argument/VanillaArgumentProvider.java
 @@ -0,0 +1,107 @@
@@ -1587,8 +1587,8 @@ index 0000000000000000000000000000000000000000..e8c1c0f781bfc852af4a0f01452566ae
 +        .findFirst();
 +
 +    static VanillaArgumentProvider provider() {
-+            return PROVIDER.orElseThrow();
-+        }
++        return PROVIDER.orElseThrow();
++    }
 +
 +    ArgumentType<EntitySelectorArgumentResolver> entity();
 +
@@ -1914,7 +1914,7 @@ index 0000000000000000000000000000000000000000..906ce6eff30ebd9ec3010ce03b471418
 +public interface SelectorArgumentResolver<T> extends ArgumentResolver<T> {
 +}
 diff --git a/src/main/java/io/papermc/paper/plugin/lifecycle/event/types/LifecycleEvents.java b/src/main/java/io/papermc/paper/plugin/lifecycle/event/types/LifecycleEvents.java
-index 304f978e40e1759bb19704cc5cec399500905195..1fab48593c567fe05b085ac6e12dc22556cf0b92 100644
+index 29476a1a82b3526ac03f21011d297d01462d153e..dae4935d8e9f46c52f700927a43a9202dac13df6 100644
 --- a/src/main/java/io/papermc/paper/plugin/lifecycle/event/types/LifecycleEvents.java
 +++ b/src/main/java/io/papermc/paper/plugin/lifecycle/event/types/LifecycleEvents.java
 @@ -1,9 +1,11 @@
@@ -1942,7 +1942,7 @@ index 304f978e40e1759bb19704cc5cec399500905195..1fab48593c567fe05b085ac6e12dc225
 +
      //<editor-fold desc="helper methods" defaultstate="collapsed">
      @ApiStatus.Internal
-     private static <E extends LifecycleEvent> LifecycleEventType.Monitorable<Plugin, E> plugin(final String name) {
+     static <E extends LifecycleEvent> LifecycleEventType.Monitorable<Plugin, E> plugin(final String name) {
 diff --git a/src/main/java/org/bukkit/command/Command.java b/src/main/java/org/bukkit/command/Command.java
 index 03d2643d166824458c88a49f20270e93b14f3988..0a26fffe9b1e5080b5639767a03af11006108b4a 100644
 --- a/src/main/java/org/bukkit/command/Command.java
@@ -2046,7 +2046,7 @@ index 3ec32b46264cfff857b50129b5e0fa5584943ec6..bdfe68b386b5ca2878475e548d3c9a38
  
          return true;
 diff --git a/src/main/java/org/bukkit/event/server/TabCompleteEvent.java b/src/main/java/org/bukkit/event/server/TabCompleteEvent.java
-index b43c3cb5c88eada186d6f81712c244aaa18fb53e..ea26cb90d988d693f26e37229fbdee975c0b11f4 100644
+index 6465e290c090d82986352d5ab7ba5dc65bd3dc17..c71c122ccc4775d030688f7b8df0b4feb49136f4 100644
 --- a/src/main/java/org/bukkit/event/server/TabCompleteEvent.java
 +++ b/src/main/java/org/bukkit/event/server/TabCompleteEvent.java
 @@ -18,6 +18,8 @@ import org.jetbrains.annotations.NotNull;

patches/api/0475-Registry-Modification-API.patch

@@ -773,10 +773,10 @@ index 0000000000000000000000000000000000000000..a49d328e95f7fda6567ee6c4f5f1878a
 +}
 diff --git a/src/main/java/io/papermc/paper/registry/tag/TagKeyImpl.java b/src/main/java/io/papermc/paper/registry/tag/TagKeyImpl.java
 new file mode 100644
-index 0000000000000000000000000000000000000000..11d19e339c7c62f2eb4467277552c27e4e83069c
+index 0000000000000000000000000000000000000000..5fde17e97eddcd1d01ee39694c9159d0ff2a09e3
 --- /dev/null
 +++ b/src/main/java/io/papermc/paper/registry/tag/TagKeyImpl.java
-@@ -0,0 +1,12 @@
+@@ -0,0 +1,17 @@
 +package io.papermc.paper.registry.tag;
 +
 +import io.papermc.paper.registry.RegistryKey;
@@ -788,9 +788,14 @@ index 0000000000000000000000000000000000000000..11d19e339c7c62f2eb4467277552c27e
 +@ApiStatus.Internal
 +@DefaultQualifier(NonNull.class)
 +record TagKeyImpl<T>(RegistryKey<T> registryKey, Key key) implements TagKey<T> {
++
++    @Override
++    public String toString() {
++        return "#" + this.key + " (in " + this.registryKey + ")";
++    }
 +}
 diff --git a/src/main/java/org/bukkit/Registry.java b/src/main/java/org/bukkit/Registry.java
-index cd1e38debbec745dd13cd895327f544dcf42594d..c0428edee6ecbbd646c0e58da39a7ca476914fd1 100644
+index 5d21459e9128c515508a2b4b2265d9824e10d9d5..20015393f91af405c99db2635a471fb6ff19e4bf 100644
 --- a/src/main/java/org/bukkit/Registry.java
 +++ b/src/main/java/org/bukkit/Registry.java
 @@ -358,6 +358,27 @@ public interface Registry<T extends Keyed> extends Iterable<T> {

patches/api/0489-Tag-Lifecycle-Events.patch

@@ -0,0 +1,406 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Jake Potrebic <jake.m.potrebic@gmail.com>
+Date: Thu, 20 Jun 2024 09:40:53 -0700
+Subject: [PATCH] Tag Lifecycle Events
+
+
+diff --git a/src/main/java/io/papermc/paper/plugin/lifecycle/event/types/LifecycleEventTypeProvider.java b/src/main/java/io/papermc/paper/plugin/lifecycle/event/types/LifecycleEventTypeProvider.java
+index 3b17f056d162dd42cae0d33cbbb22fece82fe525..a52ab9e09420768d89385c881fb7a4c424e5b8d6 100644
+--- a/src/main/java/io/papermc/paper/plugin/lifecycle/event/types/LifecycleEventTypeProvider.java
++++ b/src/main/java/io/papermc/paper/plugin/lifecycle/event/types/LifecycleEventTypeProvider.java
+@@ -19,4 +19,6 @@ interface LifecycleEventTypeProvider {
+     <O extends LifecycleEventOwner, E extends LifecycleEvent> LifecycleEventType.Monitorable<O, E> monitor(String name, Class<? extends O> ownerType);
+ 
+     <O extends LifecycleEventOwner, E extends LifecycleEvent> LifecycleEventType.Prioritizable<O, E> prioritized(String name, Class<? extends O> ownerType);
++
++    TagEventTypeProvider tagProvider();
+ }
+diff --git a/src/main/java/io/papermc/paper/plugin/lifecycle/event/types/LifecycleEvents.java b/src/main/java/io/papermc/paper/plugin/lifecycle/event/types/LifecycleEvents.java
+index dae4935d8e9f46c52f700927a43a9202dac13df6..720fe2546015838708ce794c291ca187cf7bca9c 100644
+--- a/src/main/java/io/papermc/paper/plugin/lifecycle/event/types/LifecycleEvents.java
++++ b/src/main/java/io/papermc/paper/plugin/lifecycle/event/types/LifecycleEvents.java
+@@ -24,6 +24,12 @@ public final class LifecycleEvents {
+      */
+     public static final LifecycleEventType.Prioritizable<LifecycleEventOwner, ReloadableRegistrarEvent<Commands>> COMMANDS = prioritized("commands", LifecycleEventOwner.class);
+ 
++    /**
++     * These events are for registering tags to the server's tag system. You can register a handler for these events
++     * only in {@link io.papermc.paper.plugin.bootstrap.PluginBootstrap#bootstrap(BootstrapContext)}.
++     */
++    public static final TagEventTypeProvider TAGS = LifecycleEventTypeProvider.provider().tagProvider();
++
+     //<editor-fold desc="helper methods" defaultstate="collapsed">
+     @ApiStatus.Internal
+     static <E extends LifecycleEvent> LifecycleEventType.Monitorable<Plugin, E> plugin(final String name) {
+diff --git a/src/main/java/io/papermc/paper/plugin/lifecycle/event/types/TagEventTypeProvider.java b/src/main/java/io/papermc/paper/plugin/lifecycle/event/types/TagEventTypeProvider.java
+new file mode 100644
+index 0000000000000000000000000000000000000000..61c5f6cc6986193eab596ee55e1a414afcf4fbd1
+--- /dev/null
++++ b/src/main/java/io/papermc/paper/plugin/lifecycle/event/types/TagEventTypeProvider.java
+@@ -0,0 +1,40 @@
++package io.papermc.paper.plugin.lifecycle.event.types;
++
++import io.papermc.paper.plugin.bootstrap.BootstrapContext;
++import io.papermc.paper.plugin.lifecycle.event.registrar.ReloadableRegistrarEvent;
++import io.papermc.paper.registry.RegistryKey;
++import io.papermc.paper.tag.PostFlattenTagRegistrar;
++import io.papermc.paper.tag.PreFlattenTagRegistrar;
++import org.checkerframework.checker.nullness.qual.NonNull;
++import org.jetbrains.annotations.ApiStatus;
++
++/**
++ * Provides event types for tag registration.
++ *
++ * @see PreFlattenTagRegistrar
++ * @see PostFlattenTagRegistrar
++ */
++@ApiStatus.Experimental
++@ApiStatus.NonExtendable
++public interface TagEventTypeProvider {
++
++    /**
++     * Get a prioritizable, reloadable registrar event for tags before they are flattened.
++     *
++     * @param registryKey the registry key for the tag type
++     * @return the registry event type
++     * @param <T> the type of value in the tag
++     * @see PreFlattenTagRegistrar
++     */
++    <T> LifecycleEventType.@NonNull Prioritizable<BootstrapContext, ReloadableRegistrarEvent<PreFlattenTagRegistrar<T>>> preFlatten(@NonNull RegistryKey<T> registryKey);
++
++    /**
++     * Get a prioritizable, reloadable registrar event for tags after they are flattened.
++     *
++     * @param registryKey the registry key for the tag type
++     * @return the registry event type
++     * @param <T> the type of value in the tag
++     * @see PostFlattenTagRegistrar
++     */
++    <T> LifecycleEventType.@NonNull Prioritizable<BootstrapContext, ReloadableRegistrarEvent<PostFlattenTagRegistrar<T>>> postFlatten(@NonNull RegistryKey<T> registryKey);
++}
+diff --git a/src/main/java/io/papermc/paper/tag/PostFlattenTagRegistrar.java b/src/main/java/io/papermc/paper/tag/PostFlattenTagRegistrar.java
+new file mode 100644
+index 0000000000000000000000000000000000000000..32d79dac18c402c3d95edb642eef1563eddf5e9e
+--- /dev/null
++++ b/src/main/java/io/papermc/paper/tag/PostFlattenTagRegistrar.java
+@@ -0,0 +1,103 @@
++package io.papermc.paper.tag;
++
++import io.papermc.paper.plugin.lifecycle.event.registrar.Registrar;
++import io.papermc.paper.registry.RegistryKey;
++import io.papermc.paper.registry.TypedKey;
++import io.papermc.paper.registry.tag.TagKey;
++import java.util.Collection;
++import java.util.Map;
++import org.checkerframework.checker.nullness.qual.NonNull;
++import org.jetbrains.annotations.ApiStatus;
++import org.jetbrains.annotations.Contract;
++import org.jetbrains.annotations.Unmodifiable;
++
++/**
++ * Registrar for tags after they have been flattened. Flattened
++ * tags are tags which have any nested tags resolved to the tagged
++ * values the nested tags point to. This registrar, being a post-flatten
++ * registrar, allows for modification after that flattening has happened, when
++ * tags only point to individual entries and not other nested tags.
++ * <p>
++ * An example of a custom enchant being registered to the vanilla
++ * {@code #minecraft:in_enchanting_table} tag.
++ * <pre>{@code
++ * class YourBootstrapClass implements PluginBootstrap {
++ *
++ *     @Override
++ *     public void bootstrap(@NotNull BootstrapContext context) {
++ *         LifecycleEventManager<BootstrapContext> manager = context.getLifecycleManager();
++ *         manager.registerEventHandler(LifecycleEvents.TAGS.postFlatten(RegistryKey.ENCHANTMENT), event -> {
++ *             final PostFlattenTagRegistrar<Enchantment> registrar = event.registrar();
++ *             registrar.addToTag(
++ *                 EnchantmentTagKeys.IN_ENCHANTING_TABLE,
++ *                 Set.of(CUSTOM_ENCHANT)
++ *             );
++ *         });
++ *     }
++ * }
++ * }</pre>
++ *
++ * @param <T> the type of value in the tag
++ * @see PreFlattenTagRegistrar
++ */
++@ApiStatus.Experimental
++@ApiStatus.NonExtendable
++public interface PostFlattenTagRegistrar<T> extends Registrar {
++
++    /**
++     * Get the registry key for this tag registrar.
++     *
++     * @return the registry key
++     */
++    @NonNull RegistryKey<T> registryKey();
++
++    /**
++     * Get a copy of all tags currently held in this registrar.
++     *
++     * @return an immutable map of all tags
++     */
++    @Contract(value = "-> new", pure = true)
++    @Unmodifiable @NonNull Map<TagKey<T>, Collection<TypedKey<T>>> getAllTags();
++
++    /**
++     * Checks if this registrar has a tag with the given key.
++     *
++     * @param tagKey the key to check for
++     * @return true if the tag exists, false otherwise
++     */
++    @Contract(pure = true)
++    boolean hasTag(@NonNull TagKey<T> tagKey);
++
++    /**
++     * Get the tag with the given key. Use {@link #hasTag(TagKey)} to check
++     * if a tag exists first.
++     *
++     * @param tagKey the key of the tag to get
++     * @return an immutable list of tag entries
++     * @throws java.util.NoSuchElementException if the tag does not exist
++     * @see #hasTag(TagKey)
++     */
++    @Contract(value = "_ -> new", pure = true)
++    @Unmodifiable @NonNull Collection<TypedKey<T>> getTag(@NonNull TagKey<T> tagKey);
++
++    /**
++     * Adds values to the given tag. If the tag does not exist, it will be created.
++     *
++     * @param tagKey the key of the tag to add to
++     * @param values the values to add
++     * @see #setTag(TagKey, Collection)
++     */
++    @Contract(mutates = "this")
++    void addToTag(@NonNull TagKey<T> tagKey, @NonNull Collection<TypedKey<T>> values);
++
++    /**
++     * Sets the values of the given tag. If the tag does not exist, it will be created.
++     * If the tag does exist, it will be overwritten.
++     *
++     * @param tagKey the key of the tag to set
++     * @param values the values to set
++     * @see #addToTag(TagKey, Collection)
++     */
++    @Contract(mutates = "this")
++    void setTag(@NonNull TagKey<T> tagKey, @NonNull Collection<TypedKey<T>> values);
++}
+diff --git a/src/main/java/io/papermc/paper/tag/PreFlattenTagRegistrar.java b/src/main/java/io/papermc/paper/tag/PreFlattenTagRegistrar.java
+new file mode 100644
+index 0000000000000000000000000000000000000000..44f7a9d3c2734fb00d11890ece9dc4ce2fafa796
+--- /dev/null
++++ b/src/main/java/io/papermc/paper/tag/PreFlattenTagRegistrar.java
+@@ -0,0 +1,102 @@
++package io.papermc.paper.tag;
++
++import io.papermc.paper.plugin.lifecycle.event.registrar.Registrar;
++import io.papermc.paper.registry.RegistryKey;
++import io.papermc.paper.registry.tag.TagKey;
++import java.util.Collection;
++import java.util.List;
++import java.util.Map;
++import org.checkerframework.checker.nullness.qual.NonNull;
++import org.jetbrains.annotations.ApiStatus;
++import org.jetbrains.annotations.Contract;
++import org.jetbrains.annotations.Unmodifiable;
++
++/**
++ * Registrar for tags before they are flattened. Flattened
++ * tags are tags which have any nested tags resolved to the tagged
++ * values the nested tags point to. This registrar, being a pre-flatten
++ * registrar, allows for modification before that flattening has happened, when
++ * tags both point to individual entries and other nested tags.
++ * <p>
++ * An example of a tag being created in a pre-flatten registrar:
++ * <pre>{@code
++ * class YourBootstrapClass implements PluginBootstrap {
++ *
++ *     @Override
++ *     public void bootstrap(@NotNull BootstrapContext context) {
++ *         LifecycleEventManager<BootstrapContext> manager = context.getLifecycleManager();
++ *         manager.registerEventHandler(LifecycleEvents.TAGS.preFlatten(RegistryKey.ITEM), event -> {
++ *             final PreFlattenTagRegistrar<ItemType> registrar = event.registrar();
++ *             registrar.setTag(AXE_PICKAXE, Set.of(
++ *                 TagEntry.tagEntry(ItemTypeTagKeys.PICKAXES),
++ *                 TagEntry.tagEntry(ItemTypeTagKeys.AXES)
++ *             ));
++ *         });
++ *     }
++ * }
++ * }</pre>
++ *
++ * @param <T> the type of value in the tag
++ * @see PostFlattenTagRegistrar
++ */
++@ApiStatus.Experimental
++@ApiStatus.NonExtendable
++public interface PreFlattenTagRegistrar<T> extends Registrar {
++
++    /**
++     * Get the registry key for this tag registrar.
++     *
++     * @return the registry key
++     */
++    @NonNull RegistryKey<T> registryKey();
++
++    /**
++     * Get a copy of all tags currently held in this registrar.
++     *
++     * @return an immutable map of all tags
++     */
++    @Contract(value = "-> new", pure = true)
++    @Unmodifiable @NonNull Map<TagKey<T>, Collection<TagEntry<T>>> getAllTags();
++
++    /**
++     * Checks if this registrar has a tag with the given key.
++     *
++     * @param tagKey the key to check for
++     * @return true if the tag exists, false otherwise
++     */
++    @Contract(pure = true)
++    boolean hasTag(@NonNull TagKey<T> tagKey);
++
++    /**
++     * Get the tag with the given key. Use {@link #hasTag(TagKey)} to check
++     * if a tag exists first.
++     *
++     * @param tagKey the key of the tag to get
++     * @return an immutable list of tag entries
++     * @throws java.util.NoSuchElementException if the tag does not exist
++     * @see #hasTag(TagKey)
++     */
++    @Contract(value = "_ -> new", pure = true)
++    @Unmodifiable @NonNull List<TagEntry<T>> getTag(@NonNull TagKey<T> tagKey);
++
++    /**
++     * Adds entries to the given tag. If the tag does not exist, it will be created.
++     *
++     * @param tagKey the key of the tag to add to
++     * @param entries the entries to add
++     * @see #setTag(TagKey, Collection)
++     */
++    @Contract(mutates = "this")
++    void addToTag(@NonNull TagKey<T> tagKey, @NonNull Collection<TagEntry<T>> entries);
++
++    /**
++     * Sets the entries of the given tag. If the tag does not exist, it will be created.
++     * If the tag does exist, it will be overwritten.
++     *
++     * @param tagKey the key of the tag to set
++     * @param entries the entries to set
++     * @see #addToTag(TagKey, Collection)
++     */
++    @Contract(mutates = "this")
++    void setTag(@NonNull TagKey<T> tagKey, @NonNull Collection<TagEntry<T>> entries);
++}
+diff --git a/src/main/java/io/papermc/paper/tag/TagEntry.java b/src/main/java/io/papermc/paper/tag/TagEntry.java
+new file mode 100644
+index 0000000000000000000000000000000000000000..436b701f1231354a78a9e0ab04ffd9097343ecfc
+--- /dev/null
++++ b/src/main/java/io/papermc/paper/tag/TagEntry.java
+@@ -0,0 +1,89 @@
++package io.papermc.paper.tag;
++
++import io.papermc.paper.registry.TypedKey;
++import io.papermc.paper.registry.tag.TagKey;
++import net.kyori.adventure.key.Keyed;
++import org.checkerframework.checker.nullness.qual.NonNull;
++import org.jetbrains.annotations.ApiStatus;
++import org.jetbrains.annotations.Contract;
++
++/**
++ * An entry is a pre-flattened tag. Represents
++ * either an individual registry entry or a whole tag.
++ *
++ * @param <T> the type of value in the tag
++ * @see PreFlattenTagRegistrar
++ */
++@ApiStatus.Experimental
++@ApiStatus.NonExtendable
++public interface TagEntry<T> extends Keyed {
++
++    /**
++     * Create required tag entry for a single value.
++     *
++     * @param entryKey the key of the entry
++     * @return a new tag entry for a value
++     * @param <T> the type of value
++     */
++    @Contract(value = "_ -> new", pure = true)
++    static <T> @NonNull TagEntry<T> valueEntry(final @NonNull TypedKey<T> entryKey) {
++        return valueEntry(entryKey, true);
++    }
++
++    /**
++     * Create tag entry for a single value.
++     *
++     * @param entryKey the key of the entry
++     * @param isRequired if this entry is required (see {@link #isRequired()})
++     * @return a new tag entry for a value
++     * @param <T> the type of value
++     */
++    @Contract(value = "_, _ -> new", pure = true)
++    static <T> @NonNull TagEntry<T> valueEntry(final @NonNull TypedKey<T> entryKey, final boolean isRequired) {
++        return new TagEntryImpl<>(entryKey.key(), false, isRequired);
++    }
++
++    /**
++     * Create a required tag entry for a nested tag.
++     *
++     * @param tagKey they key for the tag
++     * @return a new tag entry for a tag
++     * @param <T> the type of value
++     */
++    @Contract(value = "_ -> new", pure = true)
++    static <T> @NonNull TagEntry<T> tagEntry(final @NonNull TagKey<T> tagKey) {
++        return tagEntry(tagKey, true);
++    }
++
++    /**
++     * Create a tag entry for a nested tag.
++     *
++     * @param tagKey they key for the tag
++     * @param isRequired if this entry is required (see {@link #isRequired()})
++     * @return a new tag entry for a tag
++     * @param <T> the type of value
++     */
++    @Contract(value = "_, _ -> new", pure = true)
++    static <T> @NonNull TagEntry<T> tagEntry(final @NonNull TagKey<T> tagKey, final boolean isRequired) {
++        return new TagEntryImpl<>(tagKey.key(), true, isRequired);
++    }
++
++    /**
++     * Returns if this entry represents a tag.
++     *
++     * @return true if this entry is a tag, false if it is an individual entry
++     */
++    @Contract(pure = true)
++    boolean isTag();
++
++    /**
++     * Returns if this entry is required. If an entry is required,
++     * the value or tag must exist on the server in order for the tag
++     * to load correctly. A missing value will prevent the tag holding
++     * that missing value from being created.
++     *
++     * @return true if this entry is required, false if it is optional
++     */
++    @Contract(pure = true)
++    boolean isRequired();
++}
+diff --git a/src/main/java/io/papermc/paper/tag/TagEntryImpl.java b/src/main/java/io/papermc/paper/tag/TagEntryImpl.java
+new file mode 100644
+index 0000000000000000000000000000000000000000..c0ffd2b9f814d92d0d9b370dfa7e352f1241f614
+--- /dev/null
++++ b/src/main/java/io/papermc/paper/tag/TagEntryImpl.java
+@@ -0,0 +1,8 @@
++package io.papermc.paper.tag;
++
++import net.kyori.adventure.key.Key;
++import org.jetbrains.annotations.ApiStatus;
++
++@ApiStatus.Internal
++record TagEntryImpl<T>(Key key, boolean isTag, boolean isRequired) implements TagEntry<T> {
++}