Response Headers Get Unexpectedly Modified

[FlorianWendelborn]

There's a small bug/unexpected behavior in this function.

Minimal Repro

const headers = { cors: { 'Access-Control-Allow-Origin': '*' } }

require('http').createServer(
	require('spirit').node.adapter(r => r.path === '/bug' ? {
		status: 200,
		headers: headers.cors,
	} : {
		status: 200,
		headers: headers.cors,
		body: 'Not Empty',
	})
).listen(8080)

Request 1: curl "http://127.0.0.1:8080"

HTTP/1.1 200 OK
Access-Control-Allow-Origin: *
Content-Length: 9
Date: Wed, 18 Apr 2018 21:06:35 GMT
Connection: close

Not Empty

Request 2: curl "http://127.0.0.1:8080/bug"

HTTP/1.1 200 OK
Access-Control-Allow-Origin: *
Content-Length: 0
Date: Wed, 18 Apr 2018 21:07:05 GMT
Connection: close

Retry request 1: curl "http://127.0.0.1:8080"

HTTP/1.1 200 OK
Access-Control-Allow-Origin: *
Content-Length: 0
Date: Wed, 18 Apr 2018 21:07:30 GMT
Connection: close

What exactly happens?

Spirit tries to add Content-Length: 0 to the response headers, but those also get added to headers.cors, making all bodies that will ever be sent vanish.