Fix CustomPacketPayload for MC 1.21.11+ / Paper 26 (Id record, DiscardedPayload, serialize)

[rkfsociety]

Summary

Improves ProtocolLib compatibility with CustomPacketPayload on Minecraft 1.21.11+ and Paper 26.x (including Folia): correct key extraction, body serialization, outbound payloads that the vanilla network codec accepts, and a small updater robustness fix.

Problems

1. Key lookup — The interface often no longer exposes a no-arg method that returns Identifier / ResourceLocation directly. getId() returns the nested Id record; the Mojang identifier comes from a nested accessor (e.g. id()). The previous fuzzy match failed during <clinit> with ExceptionInInitializerError.
2. Obfuscated / remapped runtimes — The nested type may not have simple name Id, so resolving CustomPacketPayload$Id by name alone is unreliable. The PR uses the return type of the bridge method (e.g. getId()) and getDeclaredConstructors for the record constructor.
3. Serialize / write method — There is often no void *(ByteBuf) on the interface. Serialization is a single-arg void method whose parameter must accept the real FriendlyByteBuf (or compatible) instance returned by MinecraftReflection.getPacketDataSerializer. The PR falls back to a BFS over the type hierarchy and matches parameters with paramType.isAssignableFrom(actualSerializerClass).
4. Outbound CUSTOM_PAYLOAD encoding — The play codec path casts payloads to net.minecraft.network.protocol.common.custom.DiscardedPayload. A ByteBuddy-generated CustomPacketPayload implementation is not a DiscardedPayload, which caused ClassCastException when sending (e.g. plugin channels). When DiscardedPayload(identifier, byte[]) exists (Paper 26+), newHandle() uses that constructor instead of ByteBuddy.
5. SpigotUpdater — NoClassDefFoundError for SpigotUpdater$SpigotUpdateRunnable in some setups. The inner runnable was moved to a top-level class SpigotUpdateRunnable.

Changes (files)

Area	Change
CustomPacketPayloadWrapper.java	Chained key accessor; optional DiscardedPayload path; broader serialize + Id record resolution; byte[] fallback when reading unknown payloads
SpigotUpdateRunnable.java	New file; SpigotUpdater uses new SpigotUpdateRunnable(this)
SpigotUpdater.java	Remove inner class; add setRemoteVersion; trim imports

Testing

• ./gradlew compileJava, ./gradlew shadowJar — OK
• Paper 26.1.2 (build 60, Folia): login / plugin messaging; no CustomPacketPayloadWrapper static init failures; no ClassCastException on outbound clientbound/minecraft:custom_payload

Notes for reviewers

• When DiscardedPayload(id, byte[]) is absent (older versions), behavior falls back to the existing ByteBuddy ProtocolLibCustomPacketPayload path.
• The Id record + ByteBuddy branch remains for versions that need getId() → Id but do not yet use the DiscardedPayload outbound constructor.

[Copilot]

Pull request overview

Improves ProtocolLib’s compatibility with modern CustomPacketPayload implementations (MC 1.21.11+ / Paper 26.x) by hardening key extraction and payload serialization, and adjusts outbound payload construction to satisfy newer network codec expectations. Also refactors the Spigot updater worker into a top-level class to avoid runtime classloading issues.

Changes:

• Resolve CustomPacketPayload keys via chained accessors (supporting Id record patterns) and more robust nested type/constructor discovery.
• Serialize payload bodies via a concrete-class method lookup when the interface no longer exposes a suitable write(...) method; add a byte[] reflection fallback for unknown payloads.
• Prefer constructing DiscardedPayload(id, byte[]) when present to avoid outbound codec ClassCastException; move SpigotUpdateRunnable to a top-level class and add setRemoteVersion.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
src/main/java/com/comphenix/protocol/wrappers/CustomPacketPayloadWrapper.java	Updates key/Id resolution, serialization lookup strategy, and outbound handle construction (including DiscardedPayload path).
src/main/java/com/comphenix/protocol/updater/SpigotUpdateRunnable.java	Introduces top-level update worker runnable used by SpigotUpdater.
src/main/java/com/comphenix/protocol/updater/SpigotUpdater.java	Switches to the top-level runnable and exposes a setter for remoteVersion.

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