A Uint8Array payload is sent as a text frame

[Torsten85]

Disclosure: this is an AI finding, made while implementing CRDT document sync
over Socket.IO. The diagnosis and the suggested patch below were produced and
verified by an AI agent; a human reviewed them before filing.

@socket.io/bun-engine@0.1.1 decides whether a payload is binary using
Buffer.isBuffer alone. Any binary payload that is not a Node Buffer — a
plain Uint8Array, a DataView, an ArrayBuffer — falls through to the text
arm and is emitted as PACKET_TYPES.get(type) + String(data).

dist/parser.js:

encodePacket({ type, data }, supportsBinary) {
  if (Buffer.isBuffer(data)) {
    return supportsBinary ? data : "b" + data.toString("base64");
  } else {
    return PACKET_TYPES.get(type) + (data || "");   // <- binary lands here
  }
}

Buffer.isBuffer(new Uint8Array()) is false, so a Socket.IO packet carrying a
Uint8Array attachment goes out as "4" + "1,2,3,...". The client is mid
binary reconstruction and throws:

Error: got plaintext data when reconstructing a packet
    at add (socket.io-parser/build/esm-debug/index.js:132)

The ack never arrives and the emit is lost, with no error on the server side.
That last part is what makes it expensive to find: the sender looks healthy.

Reproduction

Any server-side socket.emit('x', new Uint8Array([1, 2, 3]), ack) while running
on bun-engine. It shows up immediately with a CRDT library — json-crdt's
Model.toBinary() and Patch.toBinary() both return a plain Uint8Array, so
every document and every patch is affected.

Why Node's engine.io is unaffected

engine.io-parser checks data instanceof ArrayBuffer || isView(data), which
covers every typed array. Code that works on the Node adapter therefore breaks
silently when moved to this one.

Suggested fix

Mirror engine.io-parser's check, and normalise before the base64 arm —
toString('base64') on a bare Uint8Array yields "1,2,3", so that arm needs
a real Buffer too:

const binary = Buffer.isBuffer(data)
  ? data
  : data instanceof ArrayBuffer
    ? Buffer.from(data)
    : ArrayBuffer.isView(data)
      ? Buffer.from(data.buffer, data.byteOffset, data.byteLength)
      : null;
if (binary) {
  return supportsBinary ? binary : "b" + binary.toString("base64");
}

Buffer.from(view.buffer, view.byteOffset, view.byteLength) is zero-copy and
correct for a view over a larger buffer, which a bare Buffer.from(view) is
not.

Verified against bun 1.4.0, socket.io@4.8.3, engine.io-client@6.6.6, with
the patch applied and removed.

[darrachequesne]

Hi! Thanks for the report.

This should be fixed by 7264f78, included in 0.1.2.

Could you please check?