Failing scenario: Fastify with React Router

[daniel-ac-martin]

I've encountered a scenario in which this library fails; Fastify with React Router (framework). It fails by returning an empty body.

I've done some debugging, and it seems to be caused by the code in:

serverless-http/lib/provider/aws/format-response.js

Lines 34 to 45 in 6c71eda

	if (headers['transfer-encoding'] === 'chunked' || response.chunkedEncoding) {
	const raw = Response.body(response).toString().split('\r\n');
	const parsed = [];
	for (let i = 0; i < raw.length; i +=2) {
	const size = parseInt(raw[i], 16);
	const value = raw[i + 1];
	if (value) {
	parsed.push(value.substring(0, size));
	}
	}
	body = parsed.join('')
	}

In this particular scenario the situation is as follows:

1. response.chunkedEncoding is false
2. headers['transfer-encoding'] is 'chunked'
3. The body is NOT chunked:
   • Parsing will fail
   • raw.length is 1
   • raw[0] is the actual body and so it not an integer (parseInt fails)

I've also noticed that headers['transfer-encoding'] is NOT removed; my understanding is that it should be.

I can make a PR with a suggested fix, but it probably needs input from someone with deeper knowledge that I have. For example, I don't know if the scenario I've described is bizarre in some way or if it is a bug in either Fastify or React Router.

[daniel-ac-martin]

Related previous discussion: #142

[daniel-ac-martin]

To add a bit more information on this failing scenario, it seems to come about when streaming to the http.ServerResponse object. In my case, this is happening due to using the writeReadableStreamToWritable function defined here: https://github.com/remix-run/react-router/blob/main/packages/react-router-node/stream.ts#L4

I'm not sure what sets the 'transfer-encoding' header, Fastify or Node.js.

I can avoid this issue by avoiding that function and not streaming into http.ServerResponse, but there might be other ways to trigger this scenario.