Handle HTTP handshakes for WebSocket connections

[kettanaito]

Scope

Improves an existing behavior

Compatibility

• This is a breaking change

Feature description

• Context in How to test WebSocket using `ws` lib? #2414

Summary

Some third-party WebSocket clients, like ws, may integrate a manual HTTP handshake for WebSocket connections. If such occur, Interceptors should handle them appropriately.

I am raising this in MSW because Interceptors may be too low-level of a library to handle this. Effectively, where we do this doesn't matter. Just has to be accounted for.

Proposal

The handshake handling itself isn't that complicated:

http.get(theSameUrlAsWsLink, ({ request }) => {
  if (request.headers.get('upgrade') === 'websocket') {
    return new HttpResponse(null, {
      status: 101,
      headers: {
        'Upgrade': 'websocket',
        'Connection': 'Upgrade',
        'Sec-WebSocket-Accept': 's3pPLMBiTxaQ9kYGzzhZRbK+xOo=',
      },
    })
  }
})

What's complicated is the ergonomics of a handler (ws link) that unfolds into multiple handlers (ws + http.get). MSW doesn't have a concept of a compound handler like that at the moment, and I would hate to ask people to spread ...ws.link() in their handlers.