[BUG] SkinUploadSocket.onClose throws JsonSyntaxException when close reason is a JSON primitive

[CristianVGdev]

SkinUploadSocket.onClose assumes the WebSocket close reason is always a JsonObject.

That assumption is not always valid. In some cases the close reason is a JSON primitive or plain string, but the code still attempts to deserialize it directly as JsonObject, which causes Gson to throw.

Exception observed:

com.google.gson.JsonSyntaxException: Expected a com.google.gson.JsonObject but was com.google.gson.JsonPrimitive; at path $

Stack trace:

[21:07:33] [WebSocketConnectReadThread-92/ERROR]: [floodgate] [debug] Got an error
com.google.gson.JsonSyntaxException: Expected a com.google.gson.JsonObject but was com.google.gson.JsonPrimitive; at path $
	at com.google.gson.internal.bind.TypeAdapters$32$1.read(TypeAdapters.java:923)
	at com.google.gson.Gson.fromJson(Gson.java:1358)
	at com.google.gson.Gson.fromJson(Gson.java:1259)
	at com.google.gson.Gson.fromJson(Gson.java:1169)
	at com.google.gson.Gson.fromJson(Gson.java:1106)
	at org.geysermc.floodgate.skin.SkinUploadSocket.onClose(SkinUploadSocket.java:155)
	at org.java_websocket.client.WebSocketClient.onWebsocketClose(WebSocketClient.java:688)
	at org.java_websocket.WebSocketImpl.closeConnection(WebSocketImpl.java:557)
	at org.java_websocket.WebSocketImpl.eot(WebSocketImpl.java:612)
	at org.java_websocket.client.WebSocketClient.run(WebSocketClient.java:546)
	at java.base/java.lang.Thread.run(Thread.java:1474)

The problem is the unconditional deserialization as JsonObject. If the close reason is not an object (for example a primitive JSON value), Gson throws and the close handler fails.

The close reason format is not guaranteed to always be a JSON object, so this path should handle it defensively.

A safer approach would be:

• parse the payload as JsonElement
• check isJsonObject() before accessing object fields
• otherwise ignore or log the raw reason

Environment

1. Floodgate: Build #130
2. Geyser: Build #1093
3. Server software: Spigot
4. Server version: 1.21.11 (4596)
5. Java: 25