Binary events sent to handlers have a 0x4 byte prepended to the binary data (v4 server)

[kelnos]

Describe the bug

When socketioxide is acting as a v4 server, and a client sends a binary event, the Vec<u8> that is exposed to the handler has a 0x4 byte prepended to the otherwise-correct data.

To Reproduce

See https://github.com/kelnos/socketioxide-binary-test for a reproducer; README contains instructions for running it.

Expected behavior

Binary data should appear in the socketioxide handler as it was sent by the client, not with an extra 0x4 byte in front of the data.

Versions (please complete the following information):

• Socketioxide version: 0.8.0 or 0.10.2
• Http lib: warp 0.3 or axum 0.7.4
• Socket.io client version: Java 1.0.2, JS 2.5.0

Additional context

There are no issues when socketioxide is acting as a v5 server.

[kelnos]

This issue happens only when the client connects using the WebSocket transport. The HTTP polling transport is fine.

[kelnos]

After tracing through the Java socket.io client, I do see that the binary packet encoder prepends a byte for the packet type, which is MESSAGE in this case, which has the value 4. I also dug through the Java socket.io server's code as well as the JS engine.io-parser code, and I see where it parses incoming WS binary frames, and pulls off the first byte as the packet type.

So that seems to be what's happening here. I guess the current EIO version doesn't have that packet type byte at the front of the WS frame, but EIO v3 does? And socketioxide doesn't appear to differentiate?